Midterm 1 2E03 (regular flashcards)

Midterm 1 content

Lecture 1 – Introduction

06.01.25

·        “What is real? How do you define real? If you’re talking about what you can feel, what you can smell, what you can taste and see, then real is simply electrical signals interpreted by your brain. This is the world that you know.” – Morpheus, in the matrix

·        The perceptual process

o   Light hits the apple, that light reaches you eye, you process it as an object, you recognize it based on prior experience, so you know what it is, you choose to pick it

o   You perceive how close/far it is and that it is able to be picked

o   1) environmental stimulus 2) light is reflected and transformed  3) receptor processes ** 4) neural processing 5) perception 6) recognition 7) action

o   **transduction: transformation of environmental energy into electrical energy by sensory receptors

o   Are all stimuli external? No, hunger, oxygen levels, blood pressure, fatigue are all interceptors

o   but this course focused on external stimuli

·        receptor processes

o   sensory receptors: cells specialised to respond to environmental energy, with each sensory systems receptors specialised to respond to a specific type of energy

·        Neural processing

o   Takes place in the interconnected circuits of neurons like the retina and in much more complex circuits within the brain. Each sense sends signals to different areas of the brain

o   Signals are transmitted from the brain and changed along the wa

o   Intracortical processing transforms sensory data into percepts of sight sound taste, smell and touch

·        Perception to action

o   Electrical signals are transformed into conscious awareness = recognition = involved motor activities (behavioural response)

·        Perception depends on experience

o   It is a cycle

o   E.g. you move closer and realize it’s a ball, or actually a rotten apple

·        The difference between sensation and perception

o   Sensation:

Lecture 2 – The biology of perception

7.01.25

·        Chapter 1  - Sensory Neuroscience and the Biology of Perception

·        Why measure perception?

o   Physical stimuli are “real” and can be measured

o   Perception is a private experience

o  

o   Can we measure perception? Hotly debated among psychologists

o   Why try? May offer clues about the nature of the brain, the way it processes information, and ultimately, the biological processes that lead to sensation and perception

·        Methods used to study sensation and perception

o   Threshold: what is the faintest sound that you can hear

o   Scaling – measuring private experience:  rate your pain on a scale of 1-10

o   Signal detection theory – measuring difficult decisions: did you see a light or was it your imagination?

o   Sensory neuroscience: how do sensory receptors and nerves underlie our perceptual experience?

o   Neuroimaging: what parts of the brain are activated during certain tasks?

o   Computation models: can we create models of sensory systems that adapt and learn, like human?

·        A very brief history of neuroscience

o   While weber and Fechner were initiating the experimental study of perception (1800s), physiologists were hard at work learning how the senses and brain operate

§  Charles Darwin: Theory of evolution. Suggested continuity in the structure and function of senses and nervous system of model organisms and humans

§  Johannes muller: Doctrine of specific nerve energies. We are only aware of the activity in our nerves, and we cannot be directly aware of the world itself; it is most important which nerve is stimulated (not how)

§  Hermann von Helmholtz: first to effectively measure how fast neurons transmit their signals (i.e. neurons obey normal rules of physics and chemistry)

§  Santiago Ramon y Cajal: made the most spectacular drawing s of neurons and their connections ever created. Suggested that neurons are discrete entities that do not actually touch on another (neuron doctrine)

§  Sir Charles Sherrington: coined the term synapse

§  Otto Loewi: discovered the first neurotransmitter (acetylcholine) to prove that (most) neurons communicate using chemical, not electrical, signals

§  Sir Alan Hodgkin and Sir Andrew Huxley: discovered the ionic basis of the resting potential and the action potential

·        The sensory system provides AFFERENT information

o  

·        Transduction is the first step in the sensory process

o   All sensation begins by the process of transduction, the conversion of external energy into an electrical signal (the receptor potential)         

§  Always mediated by opening/closing ion channels!

o  

·        How do receptors cells “sense” environmental signals?

o   Proteins on their plasma membranes respond to environmental signals

o  

o   Ligand-gated channel

§  Ligand: ion or molecule that binds reversibly

§  Opens a channel pore that allows specific ions to pass through

o   G-protein coupled receptor

§  Ligand binds G-protein coupled receptor (GPCR)

§  GPCR activates a G-protein

§  G-protein initiates a signal cascade that ultimately open ion channels

o   Stretch/Pressure-gated channel

§  Deformation of the plasm membrane causes the protein to change conformation

§  Opens a channel pore that allows specific ions to pass

§  E.g. touch system, in response to pressure

·        The receptor potential is a change in the membrane potential of the sensory receptor

o   E.g.

o    Stimulus: pressure on skin

o   Response: deformation of stretch-gated receptor causes movement of ions across the membrane of the sensory neuron

o   **Neurotransmitters can be ligands, but so can other things

·        Receptor potentials can depolarize cells to the action potential threshold

o  

o   Can investigate what a neuron encodes by identifying the stimulus that gets a neuron to fire (i.e. the stimulus that allows the neuron to reach threshold)

§  For example, some neurons in the visual cortex fire best to horizontal lines

·        How does the signal pass from one neuron to the next

o   Synaptic transmission

o   Sensory signals are relayed to other neurons via synaptic transmission

o   Law of dynamic polarization: basically, only ever transmit in one direction

o   Neurotransmitter: chemical messenger packed into vesicles; bind to specific receptors on postsynaptic cells

·        Chemical synaptic transmission involves a series of steps

o   Receptors:

§  Specialized proteins with high specificity for neurotransmitters

§  Their activation either directly or indirectly changes the membrane potential of the postsynaptic cell

§ 

§  1. AP arrives at terminal

§  2. Depolarization opens voltage-gated CA2+ channels

§  CA2+ entry triggers vesicle fusion

§  4. Neurotransmitters diffuse across synaptic cleft and binds to receptors

§  5. Response in postsynaptic cell

·        Usually included changes in Na, K, Cl, or CA permeability

§  Response is terminated by removal of NT from cleft

·        6. Broken down by enzymes

·        7. Reuptake into presynaptic terminal or y surrounding glial cells

·        8. Diffusion out of synaptic cleft

·        The binding of neurotransmitter to post-synaptic receptors causes the synaptic potential

o  

o   Momentary ion flux causes a change in membrane potential

o   Excitatory: bring membrane potential above threshold = cause firing

o   Inhibitory: bring membrane potential below threshold = inhibit firing

·        Sensory neurons have a unique structure

o  

o   Peripheral end is gathering information eg. by skin

o   Central end is in spinal cord

·        Principles of sensory coding

o   Stimulus location

§  Topographical relationships are maintained from the sensory organ to the primary cortical site

§  Receptive field: each neuron in the cortical areas for touch of vision is stimulated only if a particular site on the corresponding sensory surface (skin or retina) is stimulated

o   Intensity

§  Absolute threshold is largely determined by minimum receptor potential

§  More intense stimulus = larger receptor potential = greater AP frequency) to a point)

o   Duration

§  Adaptation: continued exposure to a stimulus leads to a reduced awareness, often attributed to adaptation of sensory receptors

o   Modality

§  Sensory receptors are only sensitive to a specific type of energy (recall Muller’s doctrine of specific nerve energies)

§  E.g. different types of touch: pressure, pain, temperature

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·        Neuroanatomy

·        Axes of the human nervous system

o   The long axis of the human CNS has a bend in in; forebrain is tilted forward

o   Must consider this when applying anatomical terms

o    

·        Anatomical and functional brain regions

o   The brain is divided into 4 lobes

§  Gyri (singular, gyrus): crests of folded cortical tissue

§  Sulci (singular, sulcus): grooves that divide gyri

o   Different brain areas are dedicated to particular tasks

o  

·        The cerebral cortex is the ultimate destination for all sensory signals

o   All sensory inputs first arrive at a primary receiving area int eh cerebral cortex (darkened regions in the image on the right)

o   Then, info is transmitted to other cortical areas (association areas)

§  Info from more than one sense is combined = polysensory

o   Operations that take place in the cortex ultimately produce perceptual experiences

·        A look inside the brain

o   Cortex

§  Sheet-like arrays of neurons

§  The cerebral cortex is a thin layer of neural tissue that covers the entire cerebrum

·        Mostly made up of 6 layers (neocortex)

·        V.s. phylogenetically older paleocortex and archicortex (3 layers, e..g hippocampus

o   White matter

§  Axon tracts and commissures (i.e. tracts that cross brain midline)

o   Nuclei (nucleus)

§  Local accumulation of neurons that have roughly similar connection and functions

o  

·        Subcortical structures are relay sites

o   Thalamus

§  Large, paired structure

§  Composed of anatomically distinct nuclei

§  All sensory signals (except smell) travel via the thalamus en route to the cortex

o   Spinal cord

§  Transmits sensory and motor information to and from the brain

o   Brainstem

§  Somatosensory, auditory and taste signals enter the brain via cranial nerves attached to the brainstem

§  Relays motor info from the brain

o  

·        The cranial nerves

o   12 pairs pf nerves that pass through opening sin the skull

o   Cranial nerves I (olfactory), II (optic) and VIII (vestibulocochlear) are exclusively dedicated to sensory information

o   3 cranial nerves are dedicated to muscle that move the eyes (III, IV, VI) (motor)

o   Remaining 6 are exclusively motor, of both motor and sensory

o  

·        Anatomy of the spinal cord

o   Ganglion (ganglia)

§  Local accumulations of neurons and glia in the PNS

o   Segmental/spinal/peripheral nerves (31 pairs)

§  Bundles of peripheral axons ensheathed by glial cells

o  

Lecture 3 – Neuroimaging, Computational Methods and Thresholds

09.01.25

·        Methods used to study the nervous system

·        Electrophysiological recording

o   Intracellular recording: measure voltage changes across the cell membrane

§  Compare voltage inside versus outside

§  Signal amplitude = 1-100 mV

§  Can record really small changes in electrical potentials

o   Extracellular recording: measure voltage changes just outside the cell

§  Compare activity near the cell to activity at some distant (inactive) place

§  Signal amplitude = 10- 500 uV

§  Can record receptor/synaptic potentials, only bigger changes of action potentials

o  

o   Pros and cons

§  Intra does smaller potentials

§  Extracellular doesn’t damage the cell

§  We can’t use this in humans its very invasive and involves drilling into the skull, sticking probes in the brain

§  Con: only one neuron at a time

§  Pro: very high temporal and spatial resolution

·        Neuroimaging: a set of methods that generate images of the structure and/or function of the brain

o   Investigate thousands or millions of neurons at once

o   Can examine the brain in healthy, living humans

o   Electroencephalography (EEG): measures electrical activity through dozens of electrodes placed on the scalp

§  Different scalp electrodes record from different parts of the brain

§  Can roughly locate populations of neurons that respond to a stimulus (e.g. a flash of light)     

·        The average activity resulting from many responses ot the same stimulus is called an event-related potential (ERP)

§ 

§  Pros and cons

·        Lower spatial resolution, not as much detail e.g. rough localization to a few millimeters

·        Pro: high temporal resolution, milliseconds

·        E.g. how activity flows through the brain over time

·        Pro: not invasive

o   Magnetoencephalography (MEG) measures changes in tiny magnetic fields across populations of many neurons in the brain

§  Magnetic field changes accompany small electrical changes during neuronal firing

·        Eg. since neurons have flow electricity, there is also a small magnetic field created e.g. right hand rule

§  MEG instrument is called a superconducting quantum interference device (SQUID)

§  MEG can localize populations of active neurons

§ 

§  Pro/Con

·        VERY costly, expensive device and dedicated, special room

·        Slightly better spatial resolution, especially better for deeper structure because its not relying on scalp sensors; much better for deeper, subcortical structures

o   Magnetic resonance imaging (MRI): a patient is placed in a large, powerful magnet that produces a strong magnetic field that influences how atoms spin

§  A radiofrequency current pulsed through the patient causes the atoms to spin out of equilibrium

§  When the pulse is turned off, MRI sensors detect the energy released as atoms realign with the magnetic field

§  MRI tells us a bout water rich (i.e. soft) tissues

§  Get a snapshot of the brain from a living person =structural information

§  Can reconstruct 3D images

§ 

§  Pro/con

·         MEG is activity; vs MRI is structural information

·        Costly (compared to like an x-ray)

·        Pro: doesn’t use radiation

·        Better pictures of soft tissue vs x-ray

·        Very uncomfortable: can’t move, claustrophobic, very loud; makes it hard to implement for many populations

·        Because its loud, its hard to present auditory stimulus so it can’t be used for audition

o   Function MRI (fMRI): magnetic pulses pick up evidence of demand for more oxygen in the brain, creating a blood oxygen level-dependent (BOLD) signal

§  More active areas need more blood (oxygen)

§  Can record the activity of the living brain =functional information

§  Stimulus evoked activity minus baseline = change caused by stimulus (i.e. subtractive)

§  Pro/cons

·        Low temporal resolution; because recording blood flow; neurons have to use up energy, then vascular system needs to supply more blood; so there is a delay

·        Indirect measure; blood flow response to neuron activity

·        Very helpful for subcortical structures

·        non-invasive

§ 

o   Positron emission tomography (PET): a small amount of tracer (a biologically active, radioactive material) is injected into the patient’s bloodstream (2-deoxy-D-glucose, 2DG)

§  Specialized camera detects the radiation emitted from brain regions using more of the tracer (i.e. metabolically active areas)

§  E.g. type of glucose that the brain can use =where is it directed during various tasks

§ 

§  Pro/con

·         Poor spatial resolution

·        Can use auditory stimulus

·        Can look at deep structures

Modeling as a Method

·        Mathematical models use mathematical language, concepts and equations to closely mimic psychology and neuronal processes with mathematical precision

o   Example: H&Hs model described how action potentials in neurons are initiated and propagated

o  

·        Computation models use mathematical language and equations to describe steps in physiological and/or neural processes (often implemented on a computer)

·       

o   The real world is more structure, redundant and predictable vs in a field of random noise, knowing one spot tells you nothing about its neighbour

·        Computational models

o   Efficient coding models: assume that sensory systems become tuned to predictability in natural environments in ways that economically encode predictable sensory inputs while highlighting inputs that are less predictable

§  Like how computers store and compress data

§  Compress redundant information, keep the bits that you care about

§ 

o   Bayesian models: employ Bayesian statistical models – which assume that earlier observations should bias expectations for future events – to build a model of the world (sensory inputs)

§  Models predict future events (predictive coding). If predictions don’t match inputs (prediction error), the model is adjusted to improve future predictions

§  What you experience in the world might help you understand/predict the future

o   Artificial neural networks: comprised of layers of heavily interconnected computational units analogous to neurons massively connected with one anther through their axons, dendrites and synapses; Strength of connections can increase or decrease with experience akin to learning

§  Includes AI, machine learning, neural networks, deep learning

§ 

§  Deep neural networks: have many ayers of units (nodes) with millions of connections; very good at taking lots of info and classifying it into categories

·        This is the AI technology responsible for google home or fcial recognition software

§  A neural network has

·        Inputs

·        Weights: how important is this input to the outcome?

·        Threshold: minimum output of a single node in order for data to be sent to the next layer

·        Output

§  Deep neural networks are feed-forward and some can also be trained through feedback

§ 

·         

Thresholds and the Dawn of Psychophysics

·        Classical psychophysics

o   Pioneered by german physicist and philosopher Gustav Fechner

o   Considered the true father of experimental psychology

o   Pioneered psychophysics: the study of quantitative relationships between physical stimuli and psychological experiences

·        How can we describe the relationship between mind and matter?

o   Why relate physical stimuli to perceptual experience using mathematical models?

§  If we can quantify what the standard is, we can identify when people may be experiencing deviations (e.g. usually hear X sound, or see at X distance)

o   Function: mathematical description of how one variable is related to another; generally expressed as a formula

o  

o   Why not start at origin?

§   If there’s no stimulus, there’s nothing to detect

§   The gap represents the threshold; minimum value of stimulus before it is detected

·        Classical psychophysics is centred on the idea of thresholds

o   Absolute threshold: minimum stimulus level required to be registered by the brain as a sensory event; where the function begins

§  Subthreshold: below the level of detection

§  Suprathreshold: above the level of detection

§  Examples:

·       

·        How can we measure thresholds?

o   1. Method of adjustment

§  Turn knob until you can just barely see light, hear sound

o   2. Method of limits

o   3. Method of constant stimuli

o  

Lecture 4 – Psychophysics

13.01.25

·        How can we describe the relationship between mind and matter?

o   Function: mathematical description of how one variable is related to another; generally expressed as a formula

o  

o   They do not begin at the origin because of threshold before stimulus is able to be perceived

·        Classical psycho physics is centered on the idea of thresholds    

o   Absolute threshold: minimum stimulus level required to be registered by the brain as a sensory event; where the function begins

o   Subthreshold: below the level of detection

o   Suprathreshold: above the level of detection

o  

·        How can we measure thresholds

o   1. Method of adjustment

§ 

§  Knob – adjust just until you can just detect it

o   2. Method of limits

§ 

§  Yes or no; progressively dimmer, say when you can’t detect it anymore

§  Can be decreasing or increasing

o   3. Method of constant stimuli

§ 

§    Different intensities are presented; out of order; say whether participant can detect it

o   Why might methods 1 & 2 be less accurate than 3?

§  Participant can’t predict based on previous stimuli, so their responses are more accurate

o   Why do we need to repeat these measures over and over?

·        What type of data does a psychophysics experiment generate?

o  

o   Threshold is taken as intensity that gets response 50% of the time

o   Why don’t you always detect at 9.5?

§  Attention can vary; can be bored or locked in

§  Physical factors: eg variability in physical stimulus on computer screen

§  Biological systems aren’t perfect; many neurons have some baseline level of firing; how much increase before attributed to stimulus

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·        What determines the shape of the psychometric function?

o   Absolute threshold gives only a starting point

o   To determine the rest of the function, need to have an idea of:

§  What the slope is at suprathreshold levels, and

§  How the slope changes with increasing intensity

o   Difference threshold (or just noticeable difference, JND) delta I (Ernst Weber):

§  How much does a stimulus need to change in order to produce a detectable change in perception?

§ 

·        How to find the difference threshold?              

o   Experiment

§  Present subject with 2 stimuli and ask which one is heavier (i.e. greater stimulus intensity) then repeat (e.g. 50x)

§  Calculate % of times the subject said the target was heavier than the reference

o   2 alternative forced choice: two stimuli are presented side by side and subjects must make a comparative judgement

§  MUST pick between its heavier or its lighter

o   What does it mean if the subject says that the target is heavier (more insense) 50% of the time

§  è to the person it seems like the same = perceptual equivalence point = they have no idea

o  

·        What if we change the weight of the standard

o   If the target is bigger, you need more to tell that there is a difference

·        Describing Weber’s observation mathematically

o  

o   What happens to the functions as a result of increasing the standard light intensity?

§  As stimulus intensity increases, the just noticeable difference increases

§  The curve gets flatter

o   What happens to the difference threshold

§  Need to increase the amount being added to be able to notice a difference

§  Not a constant; dependent on the initial weight (proportional)

o   Weber’s law: delta I = k * I

§  Delta I is a constant proportion (k) of the stimulus intensity

§  Weber’s fraction must be experimentally determined

§ 

§  0.07 =you need to add 7% in order to detect a difference

§  Pitch is the most sensitive sensory dimension

·        How is the JND related to changed in perception?

o   (Weber-)Fechner’s Law: S= k * log(I)

o  

o   Law assumes that all JNDs are perceptually equivalent, i.e. delta I is a “unit of the mind”

o   Our psychological experience increases less quickly than the actual physical stimulus increases

·        Modern psychophysics

o   Debate re: whether stimulus-sensation relationship could be measured continued after Fechner and Weber’s work (mid 1800s)

o   No progress until 1930s when Stanley Smith Stevens proposed a new set of methods for studying perception

§  Believed that you could directly measure sensations (vs Fechner’s indirect approach)

§  Began the area of modern psychophysics

·        Magnitude estimation and the power law

o   Scaling: a general psychophysical procedure to estimate the amount of something related to perception

§  Magnitude estimation: a scaling approach in which subjects provide ratings of their sensations

·        Experiment: subject assigns a number to the standard stimulus (modulus) and provides a relative numerical rating for other stimulus of varying intensities

o   Discovered that relationship between stimulus and sensation can be directly measured by subjected!

§  Subjects’ ratings we consistent with the power law

§ 

·        What is the value of the power exponent (b)?

o   Each sensory experience is related to the specific exponent

o  

o   Nature of the relationship, or “power function” is linear, increasing or decreasing, depending on the exponent

§  Weber and Fechner’s law made all relationships logarithmic; doesn’t hold for exponents greater than 1; they are exponential

§  Most stimuli are logarithmic; pain is best example of exponential

·        What determines the exponent in the power function?

o   Sensory transducer theory: the idea that transduction of the physical stimulus into a biological stimulus is the basis of the power law

o   Using electric shock as an example…

§ 

·        Other scaling techniques

o   Cross-modality matching: compare stimuli from one sensory modality to stimuli of another sensory modality

§  Relationships seem to be similar across individuals (e.g. with “normal” hearing and vision)

o   E.g. electric shock at different values; “turn up sound to match the intensity of the shock”

o   More increase in brightness than in volume

·        Why is this important

o   Just noticeable difference is not only relevant to humans

o   Understanding the perceptual abilities of animals can help us understand the evolution of observable traits

o   E.g. peacock tails; more feathers/extravagance gives reproductive advantage; at what point is additional feathers no longer noticeable to peahen

·        Signal detection theory, SDT

o   Recall: absolute threshold can vary

§  Thus abs. thresh. Depends on likelihood that signal > noise to produce a perceptible event

o   SDT (Tanner and Swets, 1950s): theory that takes into account non-sensory factors that can affect signal detection

§  Uses statistical concepts that take into account cognitive factors (i.e. sources of variability that may influence a subject’s decision-making process

§  Assumes the decision depends on sensitivity of sensory system + judgement by subject

·        Example | detecting signal above noise

o   Radiologist use mammograms to screen women for breast cancer

o   On a mammogram, cancer appears as a fuzzy white region (=signal)

o   There are other fuzzy regions of the mammogram not due to cancer (=noise)

·        Exampke | detecting signla above noise

o   Scenario: you’re in the shower, waiting for a very important phone call

o    The water is making a constant sound that we’ll call noise

o   The sound of the phone ringing is the signal

o    

·        Noise and signal+noise are represented by distribution

o  

o   Magnitude of perception varies, but average loudness of shower noise would equal “x’ (dotted line)

o   When ringtone plays, the signal is added to the noise

o   How loud must the overall sound be before you decide to jump out of the shower?

o   Must decide on a criterion (beta) level for response i.e. an internal threshold set by the observer (cut-off for a yes response)

§  Beta decision is automatic

·        There are 4 possible outcomes in an SDT experiment

o   Signal present + response = HIT

o  

o   In an SDT experiment, we can test the effect of noise alone by giving  the subject a number of trials in which no signal is present

o   Expectation (e.g. if you expect the call, how badly you want to answer it) can affect how sensitively you respond

Lecture 5 – Physical Inputs to Touch

14.01.25

·        There are 4 possible outcomes in an SDT

o  

o   In an SDT experiment, we can test the effect of noise alone by giving the subject a number of trials in which no signal is present

·        How sensitive are you to the ringtone?

o   S+N distribution reflects 1) signal intensity and 2) your detection sensitivity

§  Your sensitivity to a stimulus is illustrated by the separation between the N and S+N distributions

§  By knowing relationship of hits to false alarms, can calculate a sensitivity measure known as d’ (d-prime)

o  

·        Beta can change according to expectations and motivation

o  

o   In what type of scenario would be a subject be motivated to adopt a liberal criterion?

o   Expect stimulus 30% of the time =conservative criterion

o   Expect stimulus 70% of the time =liberal criterion

§  E.. liberal if the phone call is very important you will be very motivated to identify it

§  Give them a dollar every time they are right

§  If you always get spam calls, you will be less motivated

·        The receiver operating characteristic (ROC) curve plots this vs false alarms for a signal of fixed intensity

o   Illustrates the effect of different criterion effects in an SDT experiment

o   Simply changing non-sensory factors can affect signal detection =response bias

o   So, do actual thresholds exist?

§  There is a range or threshold that shifts based on context and cognitive factors, not a fixed absolute threshold to which you respond consistency

o   For a fixed d’, changing you criterion level changes the pattern of hits and false alarms in predictable ways

o  

·        Information in the ROC curve

o   1. Provides estimate of relative sensitivities of different individuals (d’)

§  Can figure out by matching hit and false alarm rates to appropriate ROC curve

§  Increasing d’ increases separation between distributions and the likelihood of a hit

§  What does the straight line represent?

·        Chance performance – 50% correct vs 50% false alarm

·        D’ = 0

·        Not sensitive to the signal at all

§  Consider the case of d’ = 3.0

·        They are incredibly sensitive person

o   2. Provides measure of how non-sensory factors may influence judgements (beta)

§  Can find out where hits/false alarms map on a given ROC

o  

o   Pr(S\n) means probability of identifying a signal when there’s actually just noise

o   Would an ROC curve ever bend into the bottom quadrant – more false alarms then hits?

§  In order for false alarm to be bigger, the signal+noise needs to be less than the noise alone =doesn’t make sense =so no, curve should never fall below chance performance (expect chance isn’t perfect)

Physical Inputs to Touch

·        What is touch?

o   Touch: the sensations caused by stimulation of the skin, muscles, tendons and joints

·        What types of “touch” do you fell?

o   Tactile/mechanical = pressure, vibration; temperature; pain; itch

o   Tactile: sensations caused by mechanical displacement of the skin

§  Need for: to be able to navigate our environment eg. sidewalk vs uneven ground, feel shape and texture of objects; identify and manipulate objects; nonverbal communication

o   Pain: occurs when our body tissue are (potentially) damaged

§  Warning system against danger

o   Thermal sense: perception of temperature

§  Seek/create thermally safe environments

o   Itchiness

§  Detect/avoid irritants

o   Pleasant touch: occurs in response to slow stroking (cuddling, petting); important in bonding

§  Integrates the body with sensory and social environment

o   Kinesthesia: internal sensation arising from muscle, tendons, joints that informs us of the positions and movements of our limbs in space

§  Identify and manipulate objects; act in our world

o   Other internal sense (balance, position); together with kinesthesia == proprioception

§  Balance, coordination

o   Almost always must be in direct contact with an object to perceive it by touch. Exceptions

§  Feel sound if vibrations are very strong

§  Feel sun rays

·        Why do we need touch?

·        The skin is the sense organ for touch

o   The skin is the largest and heaviest sense organ (1.8m^2, 10lbs)

o   Epidermis

§  Outmost layer

§  Protective shield

§  Composed of several sublayers that are constantly replenished

o   Dermis

§  Bulk of skin tissue

§  Contains most touch receptors and nerve endings that generate touch sensations

§ 

·        Touch receptors differ with respect to several features

o   1. Type of stimulation to which the receptor responds. Touch signals are highly specific

§  E.g. mechanical (pressure, vibration), temperature, pain

o   2. Transmission speed, which depends on the afferent fibre

o   3. Rate of adaptation, response to continuous stimulation

o   4. Size of the receptive field: receptors are activated when stimulation is applied to a particular area of the body, which constitutes the receptive field

§  The size of the receptive field is the extent of the body area that elicits a receptor response

·        Touch receptor structure

o   A touch receptor is a pseudo-unipolar neuron

o   The axon – or “nerve fibre” is either myelinated or not

o   The axon may have a specialized ending (capsule) or ion channels that help tune the afferent fibre to a particular feature of touch

o    

o  

·        The speed at which afferent fibres conduct action potentials depends on axon diameter and myelination

o  

o   Different sizes and degrees of myelination

o   Proprioceptors and fastest = most myelination, biggest diameter

o   Describe the relation between axon diameter and myelination and conduction speed

§  More myelin and larger diameter = faster speed

§  Less myelin, smaller diameter = slower

Lecture 6 – Physical Inputs to Touch continued

16.01.25

·        Tactile Receptors (mechanoreceptors) – mechanical stimulation; pressure and vibration

o   Transduction by mechanoreceptors

o   Deformation of the Pacinian corpuscle stretches the membrane of the nerve fibre

o   àOpen stretch-gated ion channels in the membrane

o   àPositively charged ions (cations) flow in and cause membrane depolarization (receptor potential)

o  

·        Types of mechanoreceptors (yellow=axon)

o   Pacinian corpuscle (onion-like) fast changes i.e. vibrations, fly landing on skin

§  Temporal changes in skin deformation

§  High frequency (~250-700 Hz) vibration

§  Fine texture perception

§  Deep(dermis)

o   Ruffini capsule: e.g. hand holding something

§  Sensitive to skin stretch

§  Designed to transmit sustained downward pressure

§  Lateral skin stretch

§  Dermis and deeper tissues

o   Meissner corpuscle: things moving against skin e.g. coffee cup slipping out of hand

§  Designed to transmit low frequency (5~50 Hz) vibration

§  Stable grasp

§  Superficial (junction of dermis and epidermis)

o   Merkel cell: fine detail e.g. braille

§  Sustained pressure, very low frequency (<5Hz)

§  Coarse texture, pattern and form perception, fine details

§  Merkel cell-neurite complex lies in fingerprint ridges

§  Superficial (function of dermis and epidermis)

o   All are found in both hairy and glabrous skin

o  

·        Mechanoreceptors can be classified as either slow or fast adapting

o  

o   Slow adapting (SA) receptors have low temporal resolution =best at transmitting info about unchanging stimuli

o   Fast adapting (FA) receptors have high temporal resolution =best detect stimuli that vary over time; tells when something is changing

·        Mechanoreceptors different with respect to receptive field size

o   Tactile receptive field (RF) is the patch of the body where a stimulus will produce a response (in a given neuron)

§  Small RF: 10-20 mm2; large RF 60mm2 to an entire finger

o   The smaller the RF, the greater the capacity for spatial resolution, i.e. ability to distinguish closely spaced objects

o   What do you think determines receptive field size??

§  How many dendrites; dendritic arbour; Large rf has a lot of dendrites over a larger area and vice versa

o  

·        Summary of mechanoreceptors properties

o  

·        Thermoreceptors

o   Inform us about changes in skin temperature

o   Are located in epidermal and dermal layers of the skin

o   Afferent fibres that lack specialized endings

o   Conduct via C fibres (small, unmyelinated) and A-delta fibres (small, lightly myelinated)

§ 

o   Warm fibres respond to increases in skin temperature whereas cold fibres respond to decreases in skin temperature

§  Cold: warm = 30:1

o  

·        Thermoreceptor sensitivity

o   We perceive no heat/cold at physiological zero (even though at skin temp of 20-36C there is a considerable amount of thermal energy

§  Thermoreceptors are activated by deviations from physiological zero

o   Very sensitive to local changes in skin temperature but better at detected changes

o    

§  Fire at lot at quick changes create a strong activation vs gradual changes

o   Thermoreceptor activation leads to physiological effects that allow us to adapt to thermal changes in our environment (e.g. shivering, swearing)

o   Do we respond to skin temperature >50C?

§  After this point, pain receptors take over

·        How do thermoreceptors sense heat or cold

o   Ion channels involved in transduction are known as transient receptor potential (TRP) channels

§  Are non-selective cation channels (allow Na and Ca in)

o  

o   Thermally-sensitive TRPs are called thermoTRPs

o   Detect the entire thermal range from non-painful coolness to non-painful warmth

o   Many are also polymodal; respond to more than one stimulus, e.g. temperature and chemicals (e.g. capsaicin or mint)

o  

·        Nociceptors – respond to real or potential danger

o   Specialized sensory receptors that are sensitive to noxious stimuli (stimuli that cause – or have the potential to cause – skin damage)

§  Pain is the unpleasant sensory and emotional consequence of nociceptor activity

o   Nociceptors are free nerve endings in the skin, joints, muscles, internal organs

o   Subtypes activated by different painful stimuli

§  Myelinated a-delta fibres respond to strong pressure or heat [MG1] 

·        Slow and fast adapting

§  Unmyelinated C type respond to intense pressure, intense heat/cold, noxious chemicals (=polymodal)[MG2] 

·        Slowly adapting

o  

·        Candidate noci-transducers are non-selective cation channels

o   thermoTRP channels

§  different from the thermosTRPs that response to painful heat/cold

§  TRPV2 and TRPA1 are candidate nociceptors

o   Acid-sensing ion channels (ASIC)

§  Specifically expressed in nociceptive fibres that innervate skeletal and cardiac muscle (detects internal pain)

§  Thoughts to mediate pain due to pH change (acidity) during low O2

o   Ligand-gated ATP receptors (P2X receptors)

§  Open in response to binding of extracellular ATP

·        Pleasant touch receptors

o   Pleasant touch receptors mediate the emotional properties of nonpainful bodily touch

o   Mediated by unmyelinated C fibres known as C tactile (CT) fibres

§  Separate from pain/itch C-fibres

o   Preferential stimulus = lightly applied, slow-moving (1-10cm/s) mechanical stimulation

§  Optimal stroke rates correspond to speeds that people find pleasant

o   Induce emotional, hormonal, behavioural responses to skin-to-skin contact

o   On hairy skin only (but that’s most of the body)

o  

·        Pleasant touch mediate the EMOTIONAL properties of nonpainful bodily touch

o   People tend to stroke loved ones at ideal speeds

o   Schirmer and gunter (2017): EEG recordings show the CT-targeted stroking makes people more attentive to the emotional content of voices

o   Stroking hairy skin reduced the experience of pain from thermal heat

o   Infants strokes with brushed at fast (30 cm/s) medium (3cm/s) or slow (0.3cm/s) speeds

o   Only medium stroking reduced heart rate

o  

·        THE IMPACT OF C-TACTILE LOW-THERSHOL MECHANORECEPTORS ON AFFECTIVE TOUCH AND SOCIAL INTERACTIONS IN MICE (2022)

o   Got rid of C-tactile fibres; induced social isolation, reduced tactile behaviours

·        Kinesthetic receptors

o   Sensory mechanoreceptors are sensory mechanoreceptors in the muscles, joints and tendons

o   The terminal ends of nerve fibres belonging to sensory neuron

§  Information is transmitted to CNS via A-alpha fibres

§  Endings contain mechanically (stretch)-gated ion channels

o   Kinesthetic receptors sense where our limbs are and what kinds of movements we’re making

§  Muscle spindles convey rate at which muscle fibres are changing in length

·        i.e. the angle formed by a limb and joint

§  golgi tendon organs provide signals about the tension in muscles attached to the tendons

§  joint receptors are activated when a joint is bent at an extreme angle

o   provide information to the motor system -=tell us where our limbs are and what kinds of movements we’re making

o  

·        Case of Ian waterman

o   Lost proprioception: couldn’t feel where his limbs were

o   Relied on sight to move; if he couldn’t see his limbs he couldn’t move

·        From skin to brain

o   Cell bodies of somatosensory neurons lie in the dorsal root ganglion (DRG)

o   Axons of various somatosensory receptors converge into a single spinal nerve

§  Note: 31 pairs (L+R) of spinal nerves that emerge out of the cord

§  Cell bodies lie in the DRG, right next to the spinal cord

§  Action potentials travel from the peripheral end all along the axon to the central end, where they enter the spinal cord at the dorsal horn

o  

·        Signal transfer in the spinal cord

o   Signals enter the dorsal horn, which is organized into multiple layer (laminae)

o   Inputs are organized somatotopically: adjacent areas of the skin are ultimately connected to adjacent areas within a region of the spinal cord

o   The spinal cord is the only channel for transmission of somatosensory and motor info from the bdoy to and from the brain

o   Nerve fibres arising from the skin appear as labeled line, i.e. each fibres type codes a particular touch sensation (stimulus quality)

o  

Lecture 7 – Touch pathways and acuity

20.01.2025

·        From skin to brain

·        Cell bodies of somatosensory neurons lie in the dorsal root ganglion (DRG)

o  

o   Axons of various somatosensory receptors converge into a single spinal nerve

§  Note 31 pairs (L+R) of spinal nerves that emerge out of the cord

o   Cell bodies lie in the DEG, right next to the spinal cord

o   Action potentials travel from the peripheral end all along the axon to the central end, where they enter the spinal cord at the dorsal horn

·        Signal transfer tin the spinal cord

o   Signals enter the dorsal horn, which is organized into multiple layers (laminae)

o   Inputs are organized somatotopically: adjacent areas of the skin are ultimately connected to adjacent areas within a region of the cord

o   The spinal cord is the only channel for transmission of somatosensory (and motor) info from the body to and from the brain

o   Nerve fibres arising from the skin appear as labelled lines, i.e. each fibre type codes particular touch sensation (stimulus quality)

o  

·        Aside – dermatomes

o  

o   The innervation arising from a single dorsal root ganglion and its spinal nerve is called a dermatome. The full set of sensory dermatomes is shown here for a typical adult. Knowledge of this arrangement is particularly important in defining the location of suspected spinal (and other) lesions. The numbers refer to the spinal segments by which each nerve is names

·        Aside – shingles (varicella-zolseter)

o   Maps are valuable as clinical tools in the event of injury or infection of a particular spinal nerve

o   Viral infection that causes a painful rash that most often appears as a stripe of blister

o   Reactivated chickenpox virus travels along nerve pathways to the skin

o  

·        How does the signal get from the primary afferent neuron to the brain?

o   Parallel processing of different touch qualities

o  

·        The dorsal column medial-lemniscal pathway (DCML)

o  

o   1) first-order DRG neurons send their axons up the white matter of the spinal cord =(via dorsal column)

o   2) central terminals of DRG neurons synapse on dorsal column nuclei in the medulla =(via medial lemniscus)

o   3) axons of second order neurons arch over the midline(decussate) to the other side of the brain and ascend to the thalamus  (internal capsule)

o   4) thalamic relay (third-order) neurons project directly to the cerebral cortex (SI)

·        The spinothalamic pathway

o   1) first order DRG neurons synapse on neurons in the dorsal horn

o   2) Axons of second order dorsal horn neurons cross the midline in the spinal cord and ascent (via spinothalamic tract; anterolateral pathway)

o   3) most of the axons terminate on relay nuclei in the medulla, midbrain, thalamus

o   4these neurons project to the cortex in a diffuse manner

o  

o  

·        Recap: the somatosensory system exhibits features common to sensory systems

o   1) signal transmission is not direct but occurs through a series of relay sites

o   2) different aspects of sensation are coordinated through different pathways that are arranged in parallel fashion (labelled lines) and that display different anatomical and physiological properties

o   Ascending fibres cross over to the opposite side of the brain so that ultimately the left side receives sensory info from right side and vice versa

o   4) afferent fibres are organized in a precise fashion within ascending structures to produce an orderly representation of body structure

·        Touch processing in the cortex

o   Primary somatosensory cortex (S1)

§   Primary cortical area that receives inputs from the thalamus via the internal capsule

§  Located in the postcentral gyrus (i.e. posterior to the central sulcus)

§  Parietal lobe

§  Motor areas of the cortex (control body parts) are just in from of central sulcus

·        Adjacency enhances communication between somatosensory and motor control systems

o   Secondary somatosensory cortex (S2)

§  Receives convergent projections from all areas of S1

·        body representation in S1

o   somatotopic organisation

§  somatotopic map: complete, orderly representation of the body. Adjacent areas on the skin have connection to adjacent areas in the brain

·        contralateral mapping (left neurons for right side of body)

o   non-linear representation (sensory homunculus)

§  cortical neurons also have receptive fields, based on connected neurons

§  receptive field size depends on location in somatotopic map

·        variers within an area (e.g. finger vs trunk)

·        varies across areas (e.g. 3 vs 1)

§  e.g. fingertips have the highest density of mechanoreceptors in the body

o  

·        Canadian heritage minute

o   Wilder penfield

o   Stimulate the brain with electrode, ask patient where they feel pain

·        Columnar organization of the cerebral cortex

o   Cerebral cortex is composed of 6 layers of neurons that are distinct in terms of:

§  Cellular composition

§  Input/output relationships

o   Somatosensory input arrives at layer 4

o  

·        Neural plasticity in the adult cerebral cortex

o   Neural plasticity: changes in the cortical map (neural circuits) can occur in response to physiological changes in sensory and motor function (i.e. experience)

o   Example

§  Monkey trained to use digits 2 and 3 heavily for several months

§  After practice, a larger part of the cortex responds to stimulation of digits 2 and 3

·        Functional re-mapping

o  

o   Normal, sighted volunteers who are blindfolded for 5 days show an increased activity in brain areas associated with vision during somatosensory tasks (discriminating braille patterns)

§  Cross-modal plasticity

·        Use dependent cortical processing from fingertips in touchscreen phone users

o  

o   Used eeg to measure cortical potentials in response to mechanical touch on the thumb, index and middle fingertips of touchscreen users vs non-users

o   Smartphone users have an enhanced thumb sensory representation in the brain

o   The interindividual differences in how the brain processes inputs partly explained by the amount of touchscreen use

·        Limb amputation often leads to phantom limb syndrome

o   Phantom limb: illusion that a missing limb (e.g. after amputation) is still present

§  Usually diminished over time

o   Phantom sensations are often felt when the face is touched

§  Functional reorganization of the somatotopic map after amputation

§  S1 neurons that lost their input are innervated by tactile receptors from the face

o  

·        Ch. 13.2 tactile sensitivity and acuity: what are the smallest details that we can feel?

·        How sensitive are we to mechanical pressure?

o   Esthesiometer: set of calibrated fibres with different diameters, each producing a different force when applied to the skin

§  Smaller diameter = less force

§  Used to determine touch thresholds

o   How could you find the detection threshold?

§   

o   Does a higher threshold mean that that part of the body is more or less sensitive?

§  Higher threshold means less sensitive

Lecture 8 – Tactile Sensitivity and Acuity

21.01.25

·        What are the smallest details that we can feel?

·        How sensitive are we to mechanical pressure?

o   Esthesiometer: set of calibrated fibres with different diameters, each producing a different force when applied to the skin

§  Smaller diameter = less force

§  Used to determine touch thresholds

§  Nylon microfilaments of different diameter and stiffness

o   How could you find the detection threshold?

§  Start with thinnest/softest and increase until they can feel it

§  Method of limits, ascending order

o   Does a higher threshold mean that that part of the body is more or less sensitive?

§    A larger threshold means you’re less sensitive, because it takes more force to be able to detect it

o   Forehead, nose, lip and check very low threshold; highly sensitive

·        How sensitive are we to changes in tactile vibration?

o   Vibrotactile stimulation = change in pressure (short, repetitive) over time

o   Maximal sensitivity occurs at ~200 Hz

o   Overall curve reflects the contributions of different mechano receptor populations (SAI, FAI, FAII) at different levels of vibration

o   Absolute vibratory threshold: the minimum stimulus displaced the skin for it to be detected

o  

§  Lower line = lower threshold = more sensitive

§  Results can be cleanly mapped onto sensitivity of different types of mechanoreceptors

§  i.e. FAII and FAII = fast activating = most sensitive to vibration

§  relationship is true if large areas of the skin are stimulated (fingertip) (large = 6mm diameter as opposed to pinpoint)

o  

o  

o   G = farthest tip of finger, B = palm è fingers get more sensitive from palm to tip

·        How finely can we resolve spatial details

o   Two-point limen/touch threshold: smallest separation of 2 points applied simultaneously to the skin that can still be described

§  Measured by the compass test

§ 

o   Two-point pain threshold: smallest separation of 2 painful stimuli applied simultaneously that can be differentiated

§  Measured by applied small pulses of radiant heat using lasers

o  

§  Blue = 2 point touch

§  Red = 2 point pain

o   How do these compared to detection threshold?

§  Data have generally similar patterns

§  But the face had the lowest detection threshold, vs the fingertips have the lowest touch threshold

·        Coincidence?

o   Determination of 2 closely spaced points instead of just one requires that the brain receives two separate signals

o   Must be detected by separate mechanoreceptors

o   Mechanoreceptors must be small and densely packs!

·        Mechanoreceptor properties and two point discrimination

o   Increase in two point discrimination associated with

§  Smaller mechanoreceptor field size

§  Higher density of mechanoreceptors

o   Signals must no converge on their way to the cortex

o   Spatial acuity is mediated by superficial (i.e. SA-I and possibly FA-I) fibres

o  

o  

·        Factors that affect tactile sensitivity

o   Sex

§  Women tend to have better sense of touch due to smaller finger size (dr. Daniel goldreich, McMaster)

§  Same number of receptors packed into smaller space

o   Age

§  For sighted individuals, tactile acuity decreases with age

§  But for blind individuals: they start out about the same as sighted, but maintained over life

§  Tactile acuity can be maintained across a lifetime with daily attention to tactile stimulation on the fingers

§  Pianists (i.e. others who often use tactile acuity) do decrease some, but less

§ 

§ 

o   Genetics

§  Autism spectrum disorder diagnosis

·        Peripheral mechanosensory neuron dysfunction underlies tactile and behavioural deficits in mouse models of ASDs

·        We report that mice harboring mutations in .. .. . .. .. genes associated with ASDs in humans exhibit altered tactile discrimination and hypersensitivity to gentle touch

·        Social deficits were related to genetic deficit in mechanoreceptors

·        “restoring mecp2 expression exclusively in the somatosensory neurons rescues tactile sensitivity, anxiety-like behaviour, and social deficits

§  Identical twins more similar in acuity than fraternal

§  Big, tall people less sensitive than small

·        Touch Adaptation

o   Adaptation occurs in response to prolonged, steady stimulation

§  E.g. you don’t feel clothes after you’ve been wearing them for a while

o   Promoted by

§  Larger stimulus area

§  Weaker intensity force

§  Stimulating less-sensitive areas of the body

o   Adaptation cannot solely be explained by reduced mechanoreceptor discharge (consider SA fibres)

§  Related to lack of stimulus movement

·        In-class investigations

o    

Lecture 9 – Pain

23.01.25

·        What is pain?

o   The perceptual consequence of nociception

o   Important survival value

§  People who lack nociception are vulnerable to self-harm or even death!

§  Can be congenital (i.e. congenital insensitivity to pain) or acquired (leprosy)

o   A highly subjective state

§  Sensory, emotional and cognitive influences together interact to create our conscious experience of pain

·        Processing of nociceptive signals in the dorsal horn

o   Nociceptive signals arrive at the dorsal horn of the spinal cord

o   Interneurons in the dorsal horn receive information from the brain (aka descending inputs)

o   Interneurons form synapses on neurons that convey nociceptive information to the brain

o  

o   Substantia gelatinosa

·        Gate control theory

o   R. melzack and P. Wall (1988): theory describes how various influences integrate to form pain perception

§  Bottom-up (pain) signals can be blocked via a feedback circuit in the dorsal horn

o  

o   Projection neuron = second-order neuron of anterolateral pathway

o   Inhibitory interneuron acts as a gate to determine how effective nociceptive signals are in activating projection neuron

o   Influences from brain (descending) and A-alpha/A-beta fibres converge upon spinal cord and modulate transmission of incoming pian signals via C fibres =counter-stimulation

§  A-alpha = proprioception, A-beta = tactile; vibration+pressure; mechanoreceptors

§  i.e. rubbing gently around a spot that’s hurt can activate inhibitory neurons and reduce transmission of pain to the brain; non-painful stimulation to counter pain signal

·        Pain is processed by a diverse and distributed network of neurons in the brain

o   Complex cluster of areas that respond to several aspects of pain

§  Discriminative aspects (location, intensity and quality of the noxious stimulus)

·        =blue in diagram

·        SI, SII, thalamus

§  Emotional aspects (unpleasant feeling, fear, anxiety, autonomic system activation that accompanies exposure to noxious stimuli) = orange in diagram

·        Anterior cingulate cortex (ACC): attention allocation, reward anticipation, decision-making, emotion

·        Insula: self-awareness, emotion, homeostatic emotions

·        Prefrontal cortex: cognition and executive control

·        Amygdala: emotion, fear

o    

§  Pain is processed by a lot of regions; lots of central targets

o  

·        Disorders of internal organs are sometimes perceived as cutaneous pain

o   Referred pain: pain that arises in deeper structure of the body is actually felt elsewhere

§  Due to convergence of afferent fibres onto the spinal cord from different parts of the body

o   Ex. pain in “arms or shoulders” due to heart attack =actually due to pain arising from hear muscle

o  

o   This is because some of the second-oder neurons in the dorsal horn receive pain info from organs as well as touch info from the skin, so the brain does’t know which its coming from

·        How is pain measured?

o   Univariate approach

§  Group all dimensions of pain together

§  Measure using standard psychophysical techniques

o   Multivariate approach

§  Separately assesses different dimensions of pain (location, intensity, quality)

§  E.g. mcgill pain questionnaire (MPQ)

o  

§  Absolute threshold: how much heat does it take before you notice = detection threshold

§  Increase heat until they say its painful (point where half of the time they say its painful) = pain threshold

o   Why not use the method of constant stimuli here?

§   It’s the best method for psychometric data bc no expectation for next stimulus

§  But not for pain because you don’t want them to suffer more than they have to by presenting extreme stimuli

§  Especially if all you’re looking for is the threshold

·        Why do people have different pain thresholds?

o   Biopsychosocial model  

§  Biological factors: disease sensitivity, nociception, inflammation, brain function

§  Psychological factors: mood/affect, catastrophizing, stress, coping

§  Social factors: cultural factors, social environment, economic factors, social support

·        Pain sensitization: increased sensitivity; can be cause by peripheral or central changes

o   Peripheral: results from interaction of nociceptors with inflammatory substances released after tissue damage

o   Central: due to increased excitability of neurons at the dorsal horn (neuropathic)

§  E.g. activity in nociceptive afferents that was previously subthreshold now generates Aps in dorsal horn neurons

o   Hyperalgesia: increased or heightened pain to a normally painful stimulus (due to peripheral or central mechanisms)

§  Touching a recently burned area of skin; normal to fell pain after a burn, but hyperalgesia causes nervous system to way overreact in response to something painful

o   Allodynia: stimuli that are normally innocuous (e.g. brushing the skin) causes pain

§  E.g. showering when you have a sunburn

o   Neuropathic pain: a chronic, intensely painful experience that is difficult to treat

§  Occurs when afferent nerve fibres or central pathways are damaged due to injury or disease and pain threshold do not return to pre-injury levels

·        Phantom limb revisited

o   Phantom limb: illusion that a missing limb (e.g. after amputation) is still present

§  Usually diminished over time

o   Phantom sensations are often felt when the face is touched

§  Function reorganization of the somatotopic map after amputation

§  S1 neurons that lost their input are innervated by tactile receptors form the face

o   Phantom pain: a lingering painful sensation (tingling, burning) in the missing limb

§  Example of central pain =nociceptive signals arise in the brain or spinal cord

§  Central pain pathways are still active without peripheral stimulation

·        Diverse methods for moderating pain

o   Pharmacological approaches

§  Analgesics: dampen pain sensations (vs anesthetics)

·        Anesthetics block everything, you don’t feel touch either; analgesic just blocks pain

§  Anti-inflammatory drugs: block pro-inflammatory molecules that activate pain receptors

·        Reduce inflammation

·        Acetaminophen, advil

§  Opiates: activate opioid receptors, which are normally activated endogenous opioids (e.g. endorphins, enkephalins)

·        Differences in pain tolerance may reflect different baseline opioid levels or genetic differences

·        Released during exercise, stress ==> dampen experience of pain

·        Doing things to increase endogenous opioid levels are good to reduce pain

o   Surgical and neurostimulatory approaches

§  Remove part of spinal cord (cordotomy) or part of the frontal lobe (lobotomy)

§  TENS (transcutaneous electrical nerve stimulation) =activates A-beta fibres (benign counter-stimulation)

o   Psychological approaches

§  Counter stimulation or counter irritation

·        Hot water bottle = thermal counter-stimulation

§  Placebo effect =patients proclaim beneficial effects of drugs that contain no medicinal ingredient

·        May actually be due to action of endogenous opioids!

§  Relaxation training, distraction techniques, hypnosis, physical contact with a loved one

·        What about itch?

o   Itch-sensitive touch receptors are called prureceptors

§  C fibres: convey info via the spinothalamic (anterolateral) system

§  Activated by prurigenic (itch-inducing) chemicals like histamine produced by mast cells

o   Interaction between itch and pian

§  Some itch receptors also respond to painful stimuli

§  Pain and itch interact in the spinal cord

·        Do scratching “make it worse”**?

o   Scratching may reduce itchiness!

§  Effect may be at the level of spinal cord and/or the brain

o   Older people may itch/scratch more due to age-related less of Merkel cells

§  Fewer Merkel cells (mechanoreceptors) involved in dampening itch sensations, so they may experience heightened itch

o   ********* except may exacerbate skin conditions

Lecture 10 – Haptic Perception

28.01.25

·        Hot or cold? Perceptual aspects of thermosensation

·        Features of thermosensation

o   Absolute threshold for detecting temp change depends on:

§  Rate of change

§  Size of skin area being stimulated = spatial summation

§  Body location where stimulation is made

·        Sensitivity is like that for pressure

o   Adaptation is a prominent feature of thermal sensation

§  Temps closer to physiological zero are better adapted to

·        Thermosensory adaptation

o  

o  

o   Physiological zero can be shifted =the intermediate temperature (30C) now evokes thermal sensations due to prior adaptation

·        Social touch – perceptual aspects of pleasant touch

·        How do pleasant touch signals reach the brain?

o   Pathways initially follows spinothalamic tract but then diverges at the level of the cortex

§  CT (C-tactile; special class) fibres synapse in lamina II of the dorsal horn =second order neurons decussate (cross midline) and synapse in the thalamus =thalamic neurons project to the insula (links sensory to emotional systems)

§  i.e. different pathways for discriminative touch vs affective touch

o   case et al. (2016)

§  used fMRI to measure brain activity during slow or fast stroking

§  participants rated the pleasantness and intensity of stroking

§  pleasantness rating was related to activity in the anterior cingulate cortex (ACC) and was independent of activity in S1 or insula

o  

·        Social touch affects relationships

o   Mother rats that lick and groom their pups produce offspring that lick and groom their own pups

o   But, if pups from attentive and remote moms are switched at birth, pups inherit the behaviour of their adoptive parents

o   Experience turns licking-and-grooming genes on or off (epigenetics)

o   McGowan et al 2009

§  Examined epigenetic differences in a glucocorticoid receptor between postmortem hippocampus from suicide victims with a history of childhood abuse and those from suicide victims with no history of childhood abuse or controls

§  Decreased levels of glucocorticoid receptor mRNA in suicide victims with a history of childhood abuse

§  Also found increased modification of the promotor region of glucocorticoid receptor

§  Thus, parental care influences the epigenetic regulation of hippocampal neuroendocrine receptor expression.

·        Haptic perception – the perceptual processing of inputs from multiple sensory subsystems, including those in skin, muscles, tendons and joints

o   Active, information-seeking

·        In-class demonstration – haptic perception

o   Active touch was much more effective than passive

o   What procedures were used during active exploration?

o   Rubbing with fingertips = lateral motion – texture

o   Enclosure = “global shape” + size information

o   Unsupported holding = weight

o   Contour following = global shape, exact size

o   Static contact – temperature

o   Pressure – hardness

·        Action for perception: using our hands to actively explore the world of surfaces and objects outside our bodies (e.g. using exploratory procedures)

o   Contrast with perception for action

o   Ability to identify an object is better if we can freely and actively touch the object

§  Why? – James Gibson

§  It is wilful and engages the mind in purposeful exploration = if it’s a mug, I should be able to find handle, if it’s a brush it should have bristles

§  It recruits proprioceptors that provide kinesthetic info

o   Each exploratory procedure is linked ot a specific object property because different procedurres activate different mechanoreceptors

o  

§  E.g. lateral motion activates FAII afferents

·        Haptic object recognition

o   We are very good at haptic object recognition =can usually identify a common object in <5s

o   Information provided is different from the visual system

§  Which object properties are easier to perceive with our sense of touch

·         Material i.e. texture, and temperature

o   These objects are easy to identify visually but are not easily recognized by touch when the lines are raised

o  

·        Haptic algorithm for curvature

o  

·        Haptic object localization

o   Can we locate objects with our sense of touch?

o   First requires that we establish a frame of reference (the coordinate system used to define locations in space)

§  Demo locating the haptic ego-center

§  What can you conclude?

Lecture 11 – Haptic Perception and Olfactory Physiology

30.01.25

·        Haptic Object Localization – beyond the body

o   Can we locate objects with our sense of touch

§  First requires that we establish a frame of reference (the coordinate system used to define locations in space)

§  Locating the haptic ego center

§  What can you conclude?

·        There is not fixed frame of Ference for the haptic perception of location

·        E.g. left hand to match is consistently too far right; right hand consistently too left

·        Haptic object localization – on the body

o   Localizing stimuli is a complex process that we can do from a very young age

o   Leed, Chinn and Lockman (2019) used vibrating patches to text childrens’ ability to locate stimuli on the skin

o  

§  Very early on they could locate the vibrator on mouth – feeding is very important

§  Become more capable to the rest between months 7 to 17

·        Disorders of touch perception

o   Tactile agnosia: inability to identify objects by touch

§  Caused by lesions of the parietal lobe

§  Must rule out: problems with touch receptors

o   Patient E.C.: lesion to the left inferior parietal lobe

§  Patient could not recognize objects haptically with right hand

§  Could recognize objects with left hand or visually

§  = no cognitive deficit, they still know what an orange feels like, what an orange is called

Olfactory Physiology

·        The chemical senses

o   Olfactory system – detect airborne chemical, perceptual quality = smell

o   Gustatory system – detect chemicals dissolved in saliva, perceptual quality = taste

o   Differ from other senses in several important ways…

·        Function of chemical senses

o   Determine what food are good to eat vs poisonous

§  Sweet  = sugar = good

§  Bitter = spoilt

o   Detect danger i.e. fire/smoke even before we can see it

o   Sexual selection, attracting mates through pheromones

o   Can lead to pleasant emotions e.g. perfume, candles

·        Where are odorants sensed?

o   Odorants: volatile chemicals

§  Perceived as odor

§  Not all volatile chemicals are odorants e.g. we can smell carbon monoxide

§  But all odorants are volatile

o   Ortho-nasal olfaction occurs when we sniff odorant molecules through our nostrils

o   Retro-nasal olfaction occurs when we inhale odorant molecules in our mouth

§  Travel up the back of the mouth into upper nasal cavity

§  When does this occur?              

·        =when we are eating!

·        Olfactory epithelium: the smell sense organ

o  

o   Are these neurons multipolar, bipolar or pseudo-unipolar?

·        Olfactory sensory neurons (OSN) regeneration

o  

o   Dying OSNs are replaces by new sensory neurons!

§  Requires a population of basal cell stem cells

o   Important because unlike other sensory receptor cell, OSNs are not protected by a barrier

o   Nuclei = blue, basal cells and progeny = red, ORN marker = green

·        Olfactory sensory neurons respond to odorants

o   G protein G-olf is specific to olfactory neurons

o   Humans have between 10-20 million OSNs!

o   Odor receptors are G protein coupled receptors. Odorants bind to the external surface

o   Odor receptors are concentrated at the cilia (where the receptor potential is created)

·        G-proteins relay signals from GPCRs to effectors

o  

o   Alpha-subunit has intrinsic GTPase activity =can hydrolyze GTP to GDP

o   Variation 1 – G-beta-gamma can activate effectors

o   Variation 2 – G-alpha can inhibit effectors

·        Olfactory transduction involves GPCRs

o  

·        Neural coding of olfactory signals

o   Neural coding (of odor): the way that the identity, concentration, and pleasurable/aversive values of odorant molecules are represented in a pattern of APs relayed to the brain form the OSNs

o   Labeled-line coding: different OSNs and their associated sensory fibres are responsible for transmitting highly specific information

o   Cross-fibre coding: different qualities of a sensory modality are distinguished by the pattern of nerve discharges across a large population of fibres

o   Which coding mechanism is used by the somatosensory system?

§  Initially, different qualities e.g. identity, type etc. are sent separately = labeled-line

§  Each set of receptors detects a different type of quality

o   The question is, how to distinguish among odorants?

§  Chemical structure, size etc/ not clear for scientists

·        Odorant receptors (ORs) are the largest known gene family

o   ~ 1000 OR genes in humans!

o   Each OSN expressed only one type of OR

o   ORs differ in terms of their specificity

o   Which of these receptors shows the highest degree or specificity?

§  OR-D because it only binds to one molecule, odorant c

§  If receptor D is activated, it must be because odorant c is present

o   Combinatorial coding

o    

o   Aniamls with a better sense of smell have more receptors? Or more types of receptors

·        Odorant receptor specificity

o  

·        Early olfactory processing in the olfactory bulb

o   Incoming olfactory signals converge in glomeruli (singular = glomerulus) tiny collections of sensory nerve endings and the dendrites of postsynaptic cells

§  Humans have ~5500 glomeruli

§  Each OSN projects to only one glomerulus but each glomerulus receives axons from multiple OSNs

o   Unmyelinated axons from OSNs congregate into bundles, pierce cribriform plate and arrive at olfactory bulb

§  Form olfactory nerve (cranial nerve I)

§  Olfaction is ipsilateral: info from right nostril goes to the right olfactory bulb

o  

·        Specificity in the olfactory bulb

o   All OSNs expressing a particular OR type converge on the same glomerulus pair

§  Provides the anatomical basis of the olfactory sensory map!

§  OSNs genetically labelled for a particular OR =axons converge on a small subset of glomeruli in OB

§  Mouse: one glomerulus receives input from about 25000 OSNs and sends output to 25 mitral cells

o  

·        Cells in the olfactory bulb

o   OSNs make excitatory glutamatergic synapses onto

§  Juxta(peri)glomerular cells (not shown)

·        Encircle glomeruli; act as excitatory and inhibitory interneurons

§  Mitral cells and tufted cells

·        Serve as relay centers

·        Axons combine to form the olfactory tract (on per hemisphere) which sends information to the primary olfactory cortex

§  Granule cells

·        Deepest layer in olfactory bulb

·        Extensive network of inhibitory neurons

·        Integrate input from earlier layers to promote specific odorant identification

o  

·        Higher olfactory processing

o  

Lecture 12 – Olfaction

03.02.25

·        Cells in the olfactory bulb

o   OSNs make excitatory glutamatergic synapses onto

§  Juxta(peri)glomerular cells (not shown)

·        Encircle glomeruli; act as excitatory and inhibitory interneurons

§  Mitral cells and tufted cells

·        Serve as relay centers

·        Axons combine to form the olfactory tract (one per hemisphere), which sends information to the primary olfactory cortex

§  Granule cells

·        Deepest layer in olfactory bulb

·        Extensive network of inhibitory neurons

·        Integrate input from earlier layers to promote specific odorant identification

o  

·        Higher olfactory processing

o   MOST INFORMAITN IS IPSILATERAL – STAY ON THE SAME SIDE

o   The thalamus is not a relay site

§  Primary olfactory cortex / aka piriform cortex

·         Major target of the olfactory tract

·        At the junction of frontal and temporal lobes

·        Mitral cells contact pyramidal cells in many regions, but the largest is the piriform cortex. Other regions include the amygdala and entorhinal cortex

·        Responds to variety of odors

·        Sensory pathways of olfactory processing

o   Secondary olfactory cortex

§  Responsible for actual perception and discrimination of odors

§  Where taste-smell integration occurs in flavor perception

o   Hippocampus

§  Though to mediate memory related facets of olfactory experience

o   Amygdala           

§  Though to mediate emotional related facets of olfactory experience

o   Hippocampus + amygdala = both part of the limbic system

o  

olfaction

·        Trigeminal chemoreception – the fell of scent

·        Our experience of odors often ahs a fell to it

o   3^rd chemosensory system for detecting chemical irritants (e.g. vinegar, ethanol, capsaicin in chili peppers              

§  “Specialised component” of the touch-, pain- and temperature-sensing somatosensory pathway in the head and neck

o   Consists of polymodal nociceptive neurons

§  Free nerve endings innervate the oral and nasal chambers including OE

§  Axons make up the trigeminal nerve (cranial nerve V)

o   Chemicals act directly on ion channels on nerve endings to depolarize neurons

§  Aps are transmitted through the brainstem to somatosensory processing areas of the cortex

o  

·        The trigeminal system evokes protective physiological reactions

o   Onions – tearing

o   Pepper – sneezing

o   Capsaicin – increased salivation, tearing sweating, nasal secretion, vasodilation

·        Olfactory psychophysics, identification and adaptation

·        Olfactory psychophysics

o   Detection – identify when it is present

o   Discrimination – identify which one is different

o   Recognition – identify it from a list

o   Identification – identify it on your own

o  

·        Psychophysical methods for detection and discrimination

o   Staircase method: odorant is presented in increasing concentration increments until the participant reports “smelling something” for a few increments

§  Then decreased [odor] until participant reports no detection. Repeat

§  Can fine tune increments to find detect ion (absolute) threshold

§  =METHOD OF LIMITS ASCENDING AND DESCENDING

o   Magnitude estimation task: subject sniffs different concentrations of an odor and assigns a numerical value based on perceived intensity

§  Measured suprathreshold function (i.e. how intensity varies with concentration)

·        Reveals exponent values <1.0

o   Triangle test: subject snuffs 3 odors, two are the same and one is different

§  Must identify which on is different

§  Repeat

§  Measures if subjects can discriminate between odours

o  

o   What might be some problems with these methods?

§  Adaptation – the smell system adapts, so we need to “clean” our nose so that successive presentations are accurate

§  Hard to control the amount they are sniffing – distance and duration/volume

·        Factors that affect detection thresholds

o   Gender

§  Females > males

§  Female smell sensitivity can change through menstrual cycle =hormones

·        (estrogen changes composition of mucus layer in nose)

§  Females have larger olfactory bulbs containing more neurons and glia

§ 

o   Age

§  Age is associated with a decrease in the # of OSNs

§  Right: activation (red) of orbitofrontal cortex (OFC), piriform cortex and amygdala by familiar odors in young and old subjects

§ 

o   Experience

§  People initially anosmic can become sensitive after repeated exposure

·        Factors that TEMPORARILY affect detection thresholds

o   Attention

§  Use more of our brain and can detect more odors when we consciously focus on smelling

o   Alcohol

§  Light use increased olfactory sensitivity, heavy use impairs it

o   Marijuana

§  May stimulate appetite by increasing smell sensitivity (previously shown in mice)

§  One of the reasons why marijuana is prescribed for patients undergoing chemotherapy

·        Odor discrimination vs recognition

o   Healthy person can discriminate thousands of odors

§  Professionals (e.g. wine tasters) even more

o   Takes up to 3x more odorant molecules to recognize an odor

o  

o   WHAT CAN WE CONCLUDE FROM THIS STUDY?

§  Recognition is long-lasting = doesn’t really decline even after a long time

§  E..g you can smell something and it brings you back to your childhood

·        Identification: olfaction and language

o   Identification is harder than recognition

o   The tip-of-the-nose phenomenon: when you sniff something with no visual cues, and you can’t find the words to describe the olfactory experience

o   Why are olfaction and language so disconnected?

§  Because olfactory info isn’t relayed through the thalamus (presumed to process language)?

§  Because processing in different hemispheres (smell = predominantly right, language = predominantly left)?

§  Because the piriform cortex isn’t connected to language processing networks?

·        Factors that affect odor identification

o   Age

§  Number of OSNs that die exceeds number that are regenerated

·        After 85, ~50% of people are anosmic. SO WHAT?

o   If they lose their sense of smell, eating becomes a chore – they stop eating, become malnourished, oversalt their food to taste it

·        Also, young kids perform poorly because of limited experience and learning

o   Sex

o   Genetics

§  Individual variation in how many and which Ors are expressed in our olfactory epithelia

o  

·        How genes affect you sense of smell

o  

o   Humans have <1000 OR genes, 52% of which are pseudogenes (i.e. the proteins coded for don’t get made)

§  Environment may determine which genes are on or off

§  There is huge diversity in the repertoire of functional OR genes among different people!

·        Different people express different functional receptors

·        Different ORs are expressed at different levels

o   More copies = more sensitive to that odorant

o   Can affect odor liking (more intense smells are usually perceived as less pleasant)

·        Olfactory adaptation

o   Reduced awareness of odors (good or bad) after prolonged exposure

§  Temporary reduction in detection threshold and reduced responses to suprathreshold intensities

§  E.g. delicious smell of a bakery disappears a few minutes after entering

o   Short term adaptation is due to receptor adaptation

§  Receptors are internalized and recycled

§  Occurs in <20 min

§  Can be undone quickly

o   Cognitive-emotional factors (innocuous vs harmful) can affect detection thresholds

o   Cross adaptation: the reduction in detection of an odorant following exposure to another odorant =likely due to 2 odorants sharking one or more ORs for transduction

·        Most food preferences are due to different in our olfactory system

Lecture 13 – Olfaction, Olfactory Hedonics

04.02.2025

·        Olfactory adaptation

o   Reduced awareness of odors (good or bad) after prolonged exposure

§  Temporary increase in detection threshold / increased sensitivity and reduced responses in suprathreshold intensities

§  E.g. delicious smell of a bakery disappears a few minutes after entering

o   Short term adaptation is due to receptor adaptation

§  Receptors are internalized and recycled

§  Occurs in <20 mins

§  Can be undone quickly

§ 

o   Cognitive-emotional factors (innocuous vs harmful) can affect detection thresholds

§  E.g. if you think its smoke, you get more sensitive to it, also partly related to attention

o   Cross-adaptation: the reduction in detection of an odorant following exposure to another odorant =likely due to 2 odorants sharing one or more Ors for transduction

§ 

o   Aka gets undone after awhile because receptors get recycled and become available again

·        Olfactory adaptation 2

o   Long term adaptation is due to processing by the olfactory system (brain)

§  Cognitive habituation: the psychological process by which, after long-term exposure to an odor, one no longer has the ability to detect the odor (or has very diminished detection ability)

·        Not due to reduced attention to the stimulus

o   Recovery takes weeks to reverse (de-habituate)

o   Possible mechanism

§  OR internalization with slowed recycling to plasma membrane? = take longer time to recycle?

§  Odorants might be absorbed into the bloodstream after continuous exposure = constantly stimulating ORs? So you’re being stimulated even when you aren’t at home = constant adaption/exposure

§  Cognitive-emotional factors

·        Abnormalities or deficits of olfactory function are known as anosmias

o   Anosmia =total loss of smell sensation

§  Can arise from genetic condition (rare), sinus and viral infections, nasal polyps, head trauma

§  E.g. olfactory bulb doesn’t develop in utero

o   Hyposmia =reduced smell perception

§  Can result from i) infection/inflammation ii) traumatic injury iii) toxin exposure

§  E.g. you have a cold and your nose is stuffed

§  E.g. head injury resulting in some nerves being cut

o   Specific anosmia =inability to smell a particular odor

§  E.g. some people don’t have the genes to smell sulfur in asparagus pee

o   Parosmia =distortions in odor quality

§  covid

o   Phantosmia =odor perception in the absence of airborne odorants

§  Cacosmia =olfactory hallucination of aversive smells

§  Schizophrenia

·        Consequences of anosmia

o   Can’t tell when food is bad, or when things are potentially dangerous == food selection

o   Can’t smell smoke

o   Can’t enjoy food as much, and many events in life revolve around food, enjoying food with other people =become less enjoyable

o   Reduced social interaction =smells’ use in sexual and romantic relationship

o   Lose memories associate with smell

o   == people who lose their sense of smell often fall into clinically depressive states because this system is so closely link to limbic system (emotions)

·        Odor markers for neurodegenerative disease

o   Impaired smell is one of the earliest and most common symptoms of Alzheimer’s and Parkinson’s disease

§  Inability to name odors might be useful for early diagnosis

Olfactory Hedonics

·        Theories of olfactory perception

·        The shape-pattern theory

o   States that odorant molecules have different shapes and olfactory receptors (Ors) have different shapes

o   An odorant will be detected by a specific OR to the extent that the odorant’s molecules fit into that OR

§  Odour creates a unique spatial-temporal pattern of activity in the glomeruli

o   Consistent with combinatorial coding

o   Odour intensity changes with which receptors are activated and therefore our perception

o  

·        Responses of individual glomeruli of the fruit fly to chemically distinct odorants

o   The unique spatial-temporal pattern of glomerular activation that appears for each odorant is called its odor image, i.e. a neural representation of odor identity

o  

o   è many more odors than we have odor receptors

·        Support for the shape-pattern theory

o   Stereo-isomers: molecules that contain all the same atoms but are mirror-image rotations of one another

§  Can smell completely different

§  E..g D-carvone and L-carvone = caraway and spearmint

o   Specific anosmia’s can be explained by missing ORs

·        Inhibitory interneurons modify the patterns created by odours

o   E.g. olfactory bulb response to chocolate will change depending on context

o    

·        Problems with the shape-pattern theory???????

o   E.g. rose smell is a mixture of many chemicals, but can be replicated by just one? How do we reconcile this???

o   Experience changes perception – how does this work with set receptors?

·        So, what is the relationship between odorant structure and odor percept?

o   We don’t know!

o   There is no well-established relationship between the chemical properties of an odorant (e.g. molecular weight, acid-base character, functional group counts) and the perceptual quality of olfaction that it generates

o  

·        A putative odor map

o   Lee et al (2023) used machine learning to create an odour map

o   A molecule’s chemical properties determine where it will sit in the odor map

o   Molecule accurately predicts how the molecule will smell to humans

o  

·        Olfactory hedonics

o   Affective evaluations: doe sit smell good or bad?

·        Olfactory hedonics are influenced by…

o   Familiarity =more familiar = more pleasant

o   Intensity =could increase or decrease pleasantness

§ 

§  E.g. a little fish smell is good but a ton could be unpleasant

o   Genetics =variability in OR genes

o   Upbringing =much evidence that olfactory hedonics are learned

§  Infants and children have different odor preferences from adults

·        E.g. babies don’t care about the smell of poop; it is learned from parents; there are some cultures that don’t care about it

§  A mother’s food choices during pregnancy (e.g. garlic) can influence her child’s smell preferences

§  Smell preferences vary across cultures

·        Different culture like different foods

§  Novel odours can be made to be perceived as good or bad as a function of associating good or bad experiences with it =associative learning

·        e.g. you eat a food and like it but then get sick and don’t like it anymore

·        An evolutionary argument for hedonics

o   Humans are generalist species that can exploit many different habitats

§  Learning is a mechanism by which we acquire odor responses

·        E.g. learned taste aversion: avoidance of a novel food after it has been paired with gastric illness

§  Long term effects are adaptive

§  Research shows that the aversion is to the smell, not taste, of the substance

o   Specialist species (e.g. those that live in a specific habitat) have innate responses to particular odors

§  For them, this is adaptive

§  E.g. California squirrels are innately afraid of the smell of certain snakes

·        Olfaction, associative learning and emotion

o   An odor is liked or disliked because of what it has been associated with in the past

§  Requires recalling a memory

o   One of the most distinctive features of olfaction is the ability to elicit our most emotional and evocative personal memories

§  The olfactory tract is directly connected to the amygdala, critical for emotional learning

§  The orbitofrontal cortex is where we consciously experience and perceive odors

·        It is the “neural locus for assigning affective value”

o   E.g. mint is associated with candy often, vs disliking black tea and tea because it is associated with funerals and since they’re rare she doesn’t encounter them anywhere else... but for most people they’re good or neutral

·        Odor-evoked memory and the truth behind aromatherapy

o   Aromatherapy: the manipulation of odours to influence mood, performance or psychological well-being as well as physiological correlates of emotions (e.g. heart rate, blood pressure, sleep)

o   Some people believe in medicinal powers

o   TRUTH | Orders can elicit beneficial emotional, behavioural, physical effects only if the aroma in question has previously been associated with the corresponding emotional experience

§  i.e. therapeutic effect can call be explained by the emotions associated with the scent

§  the emotions, in turn, can have downstream effects on performance and physiology