PSY 325 - Exam 1

Human Senses

• Vision (sight)

• Audition (hearing)

• Tactile Perception (touch)

• Olfaction (smell)

• Gustation (taste)

Sensation

converting physical features of the environment into electrochemical signals in the body

Perception

the psychological representation of these sensory signals that is used to identify aspects of the world, store them in memory, and influence thought and action

From Stimulus to Perception

• stimulus energy (light, sound, smell, etc.) → sensory receptors (eyes, ears, nose, etc) → neural impulses → brain (visual, auditory, olfactory areas)

• sensation begins during the transition from stimulus energy to sensory receptors and continues up until neural impulses are generated

• perception covers the transition from neural impulses to the brain processing regions

3 levels for studying perception

1. computational

2. algorithmic

3. hardware implementation

Computational

• what is the goal of the computation

• what is the object that I am looking at?

• overall analysis

Algorithmic

• how can the computational theory be implemented?

• What are the edges of this object?

• steps and processes along the way

Hardware implementation

• how is the algorithm realized physically

• what is the biology of the object

• parts, neurons, and circuits that allow for it

Psychophysics

• the study of the relationship between physical stimuli and the sensations and perceptions evoked by them

• Gustav Fechner (1801-1887) coined this in the book Elements of Psychophysics

Goals of Psychophysics

1. How intense must a stimulus be for the stimulus to be detectable by a human observer?

2. How different must two stimuli be for the difference to be detectable?

3. What is the scaling of perceptual experience?

Absolute Threshold

• the minimum intensity of a physical stimulus that can be detected

• everyone will have a slightly different absolute threshold

Methods for finding someone’s threshold

• method of adjustment

• method of constant stimuli

• staircase method

Method of Adjustment (absolute)

• present the stimulus far above (or below) the threshold and have the participant gradually adjust the intensity

• stop adjusting when they are barely able to hear the stimulus/just start being able to hear the stimulus

Pros and Cons of Method of Adjustment

• Pros: quick, there is a method of comparison (difference between the methods and absolute)

• Cons: inaccurate, order effects

Method of Constant Stimuli (absolute)

• pre-specify a range of stimuli to test and then present them in random order

• the participant responds “yes” or “no” if they heard the stimulus

Pros and Cons of Method of Constant Stimuli

• Pros: sound scale is randomized which gets rid of order effects from method of adjustment, more accurate

• Cons: time consuming and inefficient

Staircase Method

• start at an intensity that is near where you expect the threshold to be

• if the participant hears it, decrease the stimulus intensity

• if the participant doesn’t hear it, increase the stimulus intensity

• we can focus on the area around the threshold and not waste time testing the extremes

• greater precision and less time consuming

• used in medical settings and in research

Double Staircase Method

• converging staircases from opposing directions that eventually converge, tone based on tone heard TWO TRIALS AGO

• increases validity and ensures accuracy

Difference Threshold

• how much would you need to change the intensity of a stimulus, so that a subject perceives it as different than another

• measured as a JND or Just Noticeable Difference

Method of Adjustment (Difference)

both stimuli start at the same intensity and the participant adjusts the comparison stimulus until it is noticeable less/greater than the standard

Method of Constant Stimuli (Difference)

systematically vary the comparison stimulus and ask whether the participant can see (or hear) a difference

Computing JND

• participants should say the comparison is more intense than the standard 50% of the time when they are equal

• take the point on the curve where 75% of the responses were “yes” and the point where 25% of the responses were “yes” and divide the difference by 2

Function Steepness and JNDs

• more steep = smaller JND

• less steep = greater JND

Weber’s Law

• our perceptions are not linear (phone flashlight demo), stimulus intensity matters!

• JNDs will increase as stimulus intensity increases

• relationship differs among perceptual dimensions (brightness, heaviness, etc.)

• JND = kI (k = Weber fraction which varies by domain and dimension, I = stimulus intensity)

Fechner’s Law

• allows us to predict what someone’s perceived intensity of a stimulus is based on absolute threshold

• makes the assumption that just one unit of JND is equal to one unit of perceived intensity

• depends on validity of Weber’s law

• works for brightness and loudness (JNDs increase as intensity increases), but does not account for electric shock or perception in some other dimensions

• S = k ln(I/I0) (S = perceived intensity, k = Weber fraction, I = stimulus intensity, I0 = absolute threshold)

Magnitude Estimation (S.S. Stevens)

• participant assigns a standard intensity stimulus a value of 100 and then rates a wide variety of other stimuli (all above threshold)

• brightness and electric shock have opposing curves

• we can estimate perceived intensity from based on actual intensity

• perception for many things does not scale linearly, there is an exponential relationship that just noticeable difference is different at larger and smaller intensities

Stevens’s Power Law

S = cI^n (S = perceived intensity, c = constant based on units used, I = intensity, n = dimension-specific exponent)

Neuron Doctrine (Horace Barlow)

• perception depends on the combination of activity from many specialized neurons; each neuron responds to specific aspects of a stimulus, called trigger features

• each sensory neuron has a job to detect something specific; perception becomes a collective of all detections

The Neuron

• neurons have all of the same components as other cells in the body with two main additions:

• axons and dendrites

axons

carries electrical signals called action potentials away from the cell body

dendrites

receive chemical signals from other neurons

Brain Matter

• gray matter

• white matter

• cerebral spinal fluid (CSF)

Gray matter

cell bodies = processing

White matter

myelin (axons) = sending

cerebral spinal fluid (csf)

• ventricles = cushion/support

• ventricles are empty space where CSF is contained, both increase with an aging brain

Action Potentials

• neurons are in an extracellular fluid with positively and negatively charged ions

• there are different concentrations of positive and negative ions inside and outside of the neuron (gives rise to a voltage)

• the difference inside and outside of the cell in charge is the MEMBRANE POTENTIAL

• neurons have resting potential of about -70mV (more positive outside of the cell)

• ions flow from high to low concentration

• voltage gated ion channels open to allow ions to pass through the membrane

• results in depolarization of the cell; if depolarization passes threshold of -45mV, the polarization reverses and an action potential is fired

• action potentials are all-or-none electrical signals that propagate down the axons

• AP can only travel downstream because ion channels upstream are temporarily deactivates (refractory period)

• this process repeats and continues down the axon

Synaptic Transmission

• when the action potential reaches the end of the axon, the change in electrical potential causes the release of neurotransmitter into the synaptic cleft

• the neurotransmitter then binds with receptors in the dendrites of the postsynaptic neuron, causing ion channels to open and change the membrane potential (in the post synaptic neuron)

• if the postsynaptic cell becomes depolarized enough, it will fire an action potential

EPSP

excitatory post-synaptic potential (increase likelihood of AP - MP moves closer to threshold)

IPSP

inhibitory post-synaptic potential (decrease likelihood of AP - MP moves away from threshold

Lesions and Patient Work

• investigate which brain regions are responsible for cognitive abilities by exploring what happens when they are damaged or removed

• Phineas gage: survive damage to frontal lobe → frontal lobe must play a role in inhibiting impulsive behaviors

• lesions have become much more safe since Phineas Gage, but not usually used in humans (lesions can be artificially generated)

Single Unit Recording

• record rate of action potentials in response to stimulus manipulations

• researchers stick an electrode into a patient’s brain, usually a conscious animal

• records one neuron at a time

• firing rate: spikes/action potentials per second

Pros and Cons of Single Unit Recording

• Pros: good precision of where you are measuring (spatial resolution), and good precision in when measurements are taken (temporal resolution)

• Cons: mainly only used in animals, sometimes in Parkinson’s patients

Electroencephalography (EEG)

• you can record electrical activity by measuring weak signals from inside the brain

• poor spatial resolution as signals must be detected through outer scalp (electrical noise issue); localization is poor, but still possible - requires many trials to reduce issues

• good temporal resolution when enough trials are run for precise timing (can time lock EEG activity to a specific event)

Magnetic Resonance Imaging (MRI)

• person enters a small tight tube for brain imaging

• scanners are usually 3 teslas to:

  • take structural images of the brain

  • fMRI measures blood response that is correlated with brain activity, but is not actually measuring brain activity

Hemodynamic Response

• neural (brain) activity produces increased metabolic demands, which leads to an increase in oxygen-rich hemoglobin

• the relative decrease of deoxyhemoglobin makes up the blood oxygen level dependent (BOLD) signal and is the bases of fMRI

• by measuring blood flow, we can indirectly make inferences about brain activity (BOLD response takes many seconds to happen → good spatial, bad temporal)

Light and electromagnetic energy

• simultaneously both a wave and a string of particles

• wavelength: distance between each wave peak

• amplitude: the height of each peak

Color

represents the visible light portion of the EM spectrum

A string of particles…

• light can be absorbed (black shirt on hot day)

• light can be reflected (shiny surfaces and the sun)

• light can be refracted (eye glasses bend light to revisualize it)

Animals and Eye Structure

different animals have different eye positions for different purposes & evolutionary advantages

Three membranes

• sclera

• choroid

• retina

sclera

tough protective covering that makes up the white of your eye and the transparent cornea at the front

choroid

contains most of the blood vessels that supply oxygen and nutrients

retina

made up of neurons, including the photoreceptors

The Cornea

• rigid, transparent membrane at the front of the eye

• sharply refracts the light - performs most of the focusing of light on the retina, but cannot adjust

The Iris and Pupil

the light receiving part of the eye

Pupillary Reflex

• the iris is a muscle that contracts in response to the intensity of light entering the eye

• the pupil gets smaller in bright light and larger in dim light

Three Chambers

• anterior and posterior chambers: filled with aqueous humor (waterish substance that slightly refracts light)

• vitreous chamber: filled with vitreous humor

Floaters

relatively harmless shadows and spots that move across the visual field, clumps of debris that float around inside of the eye (specifically in the vitreous chamber)

Capturing Light

as in the camera example, light is captured through an aperture (pupil) and projected onto a surface (retina)

Human Lens

• allows us to change how we focus light (CAMERA LIKE)

• changes shape depending on distance of focus

• focuses light on one spot

• focal length

Focal Length

• distance from the lens at which the object is in focus

• the farther something is, the lens becomes thinner and does not have to refract light as much (long focal length)

• the closer something is, the lens becomes thicker and must refract more light (short focal length)

Lens Accommodation

• zonule fibers connect the lens to the choroid

• when the ciliary muscles relax, the choroid pulls on the zonule fibers, stretching the lens: this results in a weak lens

• when the ciliary muscles contract, the lens takes on a thicker, rounder shape: this results in a strong lens

Photoreceptors

• two types: rods and cones

• light must travel through all of the retinal layers to reach the photoreceptors

Rods

provide black and white vision in dim light

Cones

provide high acuity color vision in bright light

Spectral sensitivity

• 3 types of cones, 1 type of rod

• S-cones: short wavelength sensitivity

• M-cones: middle wavelength sensitivity

• L-cones: long wavelength sensitivity

• rods fall between S-cones and M-cones on sensitivity

Transduction

• each photoreceptor has photopigments

• photopigments have two possible shapes called isomers

• during photoisomerization, the shape of the photopigment changes from one isomer to another

• this sets in forth a cascade of actions that eventually leads to the depolarization of the photoreceptor

Fovea

• where light from the center of our gaze strikes the retina (center of your vision)

• has the highest acuity

• no rods present and the cones are thinner to fit more in a tight space

Sensory Acuity

• the fineness of discrimination or sensory precision

• acuity is greatest in the central region of the retina, called the fovea

Distribution of photoreceptors

• most cones revolve around the fovea

• rods are farthest from the fovea

• the center of our vision is where we see color best

Optic Disk

• location where the axons of the retinal ganglion cells leave the retina (making up the optic nerve) and travel to the brain

• there are no photoreceptors at this point

Convergence

• retinal ganglion cells are outnumbered by photoreceptors 100 to 1

• many-to-one organization

• a single RGC receives inputs from many other retinal cells; the combination of signals from photoreceptors is called convergence

Spatial Summation

• with a higher degree of convergence, there is a higher firing rate of RGCs, but this results in lower acuity as only one RGC is receiving input

• with a lower degree of convergence, there is lower to baseline firing rate of RGCs, but this results in higher acuity as multiple RGCs are receiving input

Receptive Fields

• the region of a sensory surface in which the presence of a stimulus causes a change in the neuron’s firing rate

• dependent on the size of the dendritic tree and input surface area

• larger convergence = larger receptive field

Relationship between receptive field size & the fovea

• receptive field size increases with distance from fovea

• parasol RGCs: higher convergence, further from fovea → larger receptive field

• midget RGC: lower convergence, closer to fovea → smaller receptive field

RGC Center-Surround Receptive Fields

• on-center: excited by light in the center so action potentials are greater

• off-center: inhibited by light in the center so action potentials are lesser

Lateral inhibition

• each cell is stimulated either through inhibition or excitement by the direct cells to the left and right

• inhibitory horizontal cells are at work

• difference between light stimulation between center & surround creates a difference

  • if these regions both have the same amount of light, they cancel out and are at baseline

• This exaggeration of shade helps us to detect differnces

Dark Adaptation

• the highest level of light that reaches our retina is at least a million times greater than the lowest levels that we can detect

• RGC firing rates range from ~1-100 spikes per second

• Problem: How to represent 1-1,000,000 range with a code of 1-100

• the sensitivity of our RGS (the operating range) changes according to current conditions

• rod-cone break: rods are more sensitive than cones after about 8 minutes of darkness

• this allows us to still see shapes and figures in the dark with limited range

• when reversed: dark to light - rods become ineffective quickly and you become more dependent on cones

Contralateral Organization

• input from the left visual field goes to the right side of the brain

• input from the right visual field goes to the left side of the brain

Optic Chiasm

• where the neurons making up the optic nerve intersect

• the optic nerves from each eye split in half; RGC axons from the right half of each retina combine to form the right optic track

• RGC axons from the left half of each retina combine to form the left optic tract

Lateral Geniculate Nucleus (LGN)

• part of the thalamus - a relay station for sensory inputs

  • thalamus is large structure in the middle of the brain

• sensory info goes here first and then gets sent out to other parts of the brain (can either allow or prevent info through)

• LGN has many layers that lead to information streams and pathways in the brain (dorsal, ventral, etc.)

• visual gatekeeper

  • thalamus responds stronger when paying attention

LGN Organization

• magnocellular layers (1-2): being dorsal stream, motion

• parvocellular layers (3-6): begin ventral stream, detail and color

• koniocellular layers: between each of the magnocellular and parvocellular layers, color(?), much less understood

Retinotopic Map

• the cells are organized in a way that correspond to areas of the retina

• RGCs with adjacent receptive fields connect to adjacent LGN neurons

• each layer only received input from one eye

• right eye goes to some layers of LGN while left eye goes to others

• Parvocellular layers receive signals from midget RGCs

• Magnocellular layers receive signals from parasol RGCs

• Koniocellular layers receive signals from bistratified RGCs

Primary Visual Cortex (V1)

at the very back and bottom of the brain in the occipital lobe

Hubel and Wiesel

• responsible for figuring out what cells in V1 did

• studied cats

Simple Cells

• dependent on angle, light must be a certain orientation

• responds most strongly to a bar of light with a particular orientation (preferred orientation)

Neural Basis of Tuning

• RGCs send signals to LGN cells one-to-one

• LGN cells have circular center-surround receptive field as do RGCs

What can you tell from a single or multiple cells?

• one orientation leads to one V1 cell firing, but light contrast can also impact firing

• similar firing rates dependent on orientation AND light contrast

• multiple cells help us to increase certainty → population code

• we need to look at many cells in a region to understand what is making each cell fire, relative differences are preserved as we change overall contrast

Complex cells

• orientation selective BUT:

• responds equally well to light bar on dark or dark bar on light

• responds equally well to stimuli at any location in the receptive field

Cortical Columns

small volume of neural tissue ~0.5mm in diameter and 2-4mm tall

Columnar Organization

• sensory neurons that share functional properties often group together in vertical cortical columns (of hundreds to thousands of cells), and functionally similar columns group together

• e.g., stripes that show a preference for inputs from either the left or the right eye (‘ocular dominance columns’)

How can researchers measure columns

• perpendicular

• oblique

Perpendicular

the electrode is placed all the way down the surface, if columns exist, we will find that these cells that it passes through will have similar properties (overlapping receptive fields for example)

Oblique

if you pass through the electrode at an angle, these cells should all have differences as they are from distinct columns

Ocular Dominance Columns

• cells in the column correspond to specific layers of the LGN

  • eyes divided by layers in LGN to LGN being divided by columns in the V1 (layers alternate as to which eye they receive signals from)

Orientation Columns

• the entire thing is organized with all of these layers

• if you take the electrode deeper into the cortex, the tuning changes based on the orientation of the electrode

• the cells V1, there is a strong organization of these cells

Orientation Columns & Visualization

• surface of V1 in a treeshrew

• cortical surface is illuminated with 605nm light and photographed

• blood changes register in the photograph

• images are combined and color-coded to indicate orientation selectivity

What is the goal of retinotopic mapping?

determine whether there is a map of physical space that is encoded in visual cortex

Context of retinotopic mapping

• researchers had previously mapped out visual regions in the non-human primate brain (among others)

• this was impossible to do noninvasively in humans

• until…fMRI!

  • by taking advantage, we can define sub regions of the visual cortex

    • most visual regions have retinotopic map, if we present moving stimuli across the retina, we should see the activity move throughout the map → fMRI

Retinotopic Mapping Hypotheses

• by presenting moving checkerboard stimuli, we should see movements in activity throughout the brain

• importantly, these movements should be independent for each brain region

• visual areas border each other with a mirrored representation

• the exact positioning of these sub regions vary, we can ask more specific questions about that area