PL3102 CA1 + Finals

What are examples of glia cells?

• Oligodendrocytes

• Astrocytes

• Microglia

• Schwann cells

How do neurons create a functional circuit?

• Brain neuron

• Intrinsic neuron

• Sensory neuron/Motor neuron

• Muscle

Vice Versa

What is an afferent and efferent signal respectively? What is an interneuron?

Afferent signal: Carrying information towards the brain

Efferent signal: Carrying information away from the brain

Interneuron: Form local connections between neurons within a single structure of the brain

How do you measure the electrical activity of a neuron?

Intracellular/single cell recording

What are these electrical signals within a neuron? What are they a result of?

• They are changes in the voltage of the cell membrane which travel down the axon

• They are a result of the movement of ions between the inside and outsidet of the cell

What is the cell membrane of a neuron made up of? What is its function?

Phospholipid bilayer

Function: To regulate what kind of molecules can enter the cell

At rest, is the inside of the neuron more negatively or positively charged? Are there more Na+/K+ inside/out the cell?

Negatively charged

More Na+ outside the cell

More K+ inside the cell

What is the resting potential of a neuron?

• It is the electrical gradient of the cell membrane at rest: the difference in charge between the inside and outside of the cell

• This electrical gradient is related to the concentration of ions inside vs outside the cell

• -70mV

What causes ions to move across the cell membrane?

1. Electrical gradient

   • At rest, the inside of the cell is more negatively charged

   • Sodium and potassium are positively charged. Opposite electrical charges attract, so the electrical gradient at rest compels these ions to move into the cell

2. Concentration gradients

   • At rest, there are more Na+ ions outside the cell and more K+ ions inside the cell

   • Ions will tend to diffuse from an area of higher concentration to an area of lower concentration

What maintains the resting potential? (Why doesn’t the cell membrane neutralise itself?")

1. Sodium channels are closed at rest

• Therefore, Na+ ions are unable to follow the electrical and concentration gradient to enter the cell

• K+ channels are open but there is no nett movement as K+ ions leaving the cell due to the chemical gradient are balanced by ions entering the cell due to the electrical gradient

• Sodium-potassium pump

  • Actively transports sodium out of the cell and potassium into the cell

  • 1 ATP = 3 Na+, 2 K+

  • Actively transport involves ATP because it is moving the ions against its concentration gradient

Explain the phases of the electrical potential graph of an AP

1. Baseline electrical state of the cell (resting membrane potential) (-70mV)

2. Initial, weak depolarisation of the cell membrane (caused by synaptic input from other neurons)

3. Triggers a rapid large depolarisation of the cell membrane

   • Due to the opening of voltage-gated Na+ channels

   • The channels will only open when the membrane potential crosses a certain threshold

   • Sodium rushes into the cell once channels open

   • This makes the inside more positively charged

4. Followed by rapid repolarisation of the cell membrane (overshooting the baseline state)

   • Due to inactivation and closure of Na+ channels and the opening of voltage-gated K+ channels

   • Potassium exits the cell once channels open

   • This makes the inside negatively-charged again

5. Gradual return to baseline state

   • Due to closure of K+ channels and sodium-potassium pump acting to restore the resting potential

How does the signal (AP) travel?

• The AP propagates down the axon due to the diffusion of ions that enter during the depolarisation phase

  • Adjacent channels open in reaction to the increased intracellular charge

  • This leads to the chain reaction of the channels opening one after another

What is the absolute refractory period?

It is a period following an AP in which the neuron cannot fire another AP

• This is due to the closure and inactivation of voltage-gated sodium channels

• APs rely on the opening of voltage-gated Na+ channels for the rapid depolarisation of the cell

What is the relative refractory period?

It is a period following an AP in which it is less likely for the neuron to fire another AP

• Because the neuron’s electrical potential overshot the baseline state of the cell following the rapid repolarisation of the cell

• The cell membrane is now hyperpolarised

• Therefore, stronger input is needed to reach the threshold to trigger a new AP

What is the importance of the refractory period?

• It is to prevent the AP moving backwards along the axon

What is Myelin?

• It is made up of the membrane of glia cells e.g. oligodendrocyte (it wraps around the axon body)

• It is an insulating sheath around an axon

• It increases the speed at which the AP travels down the axon

• APs jump between the gaps in the myelin sheath (called Nodes of Ranvier)

Why is the synpase important?

• Allows communications between neurons

• Allows neurons to integrate signals from different sources and process information

• Forms the basis of cognition

• Mediates the plasticity of our brain

  • The ability of our brain to change over time from experience

  • Allows us to learn new information and form new patterns of behaviour

• Mediates the effects of many drugs on the mind

• Mediates the effect of genes on behaviour

Describe the chemical events that occur at the Synapse

1. The AP arrives at the axon terminal

2. The arrival of the AP opens voltage-gated Ca²+ channels causing Ca²+ ions to enter the axon terminal

3. The Ca²+ ions triggers the synaptic vesicles to fuse with the cell membrane, releasing neurotransmitters into the synaptic cleft

4. The neurotransmitters diffuses across the synaptic cleft and bind to the ligand-gated ion channels on the postsynaptic membrane

5. Binding of neurotransmitter opens the ligand-gated ion channels, resulting in graded potentials → Which can ultimately lead to an AP in the postsynaptic neuron

6. Neurotransmitter levels reduce over time, leading to the closure of the ion channels

What are the different type of receptors that can be found on a neuron cell membrane? What are their differences?

1. Ligand-gated ion channels (ionotropic receptors)

   • Ligand: a molecule that binds to something

   • Triggered by binding of a neurotransmitter

   • Open or close in response to the binding of specific neurotransmitters to their extracellular domains

   • When neurotransmitters bind to these receptors, they induce conformational changes that directly alter the permeability of the channel to specific ions, allowing ions to flow across the cell membrane.

2. Voltage-gated ion channels

   • Activated by changes in the membrane potential

   • Open or close in response to alterations in the electric potential across the cell membrane

   • These channels have specific voltage-sensing domains that respond to changes in membrane potential, causing conformational changes that regulate the opening or closing of the channel pore

3. Metabotropic receptors

   • Indirectly coupled to ion channels via intracellular signaling cascades

   • When neurotransmitters bind to metabotropic receptors, they activate associated G proteins, which then initiate intracellular signaling pathways.

   • The activated G protein will bind to the effector protein

   • This leads to second messenger molecules being produced, activating enzymes that open ion channels

   • These pathways modulate ion channels or other cellular processes through the action of second messenger molecules, leading to slower and longer-lasting effects compared to ionotropic receptors

What is Serotonin and how does an SSRI work?

Serotonin is a neurotransmitter involved in regulating mood amongst other things

SSRIs block the reuptake of serotonin into the presynaptic neuron, so that it remains in the synaptic cleft for longer, enacting its effects

What is Dopamine and what drugs can affect its effects?

Dopamine is a neurotransmitter involved in reward, motor function, and other things

Dopa is a precursor for Dopamine

Levadopa is a precursor for dopa

Antipsychotic medications block the D2 receptor of the postsynaptic neuron preventing dopamine from entering the postsynaptic neuron

MAOIs inhibit degradation of dopamine to DOPAC

Cocaine blocks reuptake of dopamine by the presynaptic neuron (so does methylphenidate and many antidepressants but less strongly)

Where does specificity come from?

Lock-and-Key Mechanism: The interaction between a receptor and its ligand is often described using the "lock-and-key" analogy. In this model, the receptor's binding site (the lock) has a specific shape that only fits the particular neurotransmitter or ligand molecule (the key) that matches its shape and chemical properties. This ensures that only the appropriate ligand can bind to and activate the receptor

What is a temporal summation of signals?

Several impulses from one neuron over time

What is a spatial summation of signals?

Impulses from several neurons at the same time

Where do graded potentials and APs tend to happen in the neuron?

Graded potentials: Dendrites/soma

AP: Axon hillock

What is the Excitatory Postsynaptic Potential (EPSP)?

• A temporary depolarization of the postsynaptic membrane potential caused by the release of neurotransmitters from a presynaptic neuron

• EPSPs occur at chemical synapses where the neurotransmitter released by the presynaptic neuron binds to receptors on the postsynaptic neuron's membrane

What is the Inhibitory Postsynaptic Potential (IPSP)?

• Voltage of the cell membrane becoming hyperpolarised (more negative than the baseline state of a neuron)

• Less likely for an AP to occur

What is the difference between excitatory synapse and inhibitory synapse?

Excitatory synapse:

• An AP in the presynaptic cell that depolarises the membrane of the postsynaptic cell (makes it more likely to fire an AP)

• The depolarisation is caused by sodium ions entering the neuron

Inhibitory synapse:

• An AP in the presynaptic cell that hyperpolarises the membrane of the postsynaptic cell (makes it less likely to fire an AP)

• The hyperpolarisation is caused by chloride ions entering the neuron

What determines whether a neuron fires an AP?

• The dendrites of a given neuron may synapse with axons of many other neurons, some excitatory and others inhibitory

• It depends on the mixture of excitatory and inhibitory signals it currently receives

• Neurons are wired together into circuits/networks that can process signals in complex ways

• Excitation and inhibition interact in complex ways

What does the “all-or-nothing” law mean for APs?

• Synaptic inputs to a neuron can vary in magnitude

• However, there is not such thing as a “weak” AP, it either happens fully or not at all

• “Greater brain activity” = Increased rate of firing (not increased magnitude of AP)

• However, greater excitatory input to a neuron can increase its firing rate (frequency of APs)

What are neurotransmitters?

• They are chemicals released by neurons that affect other neurons

• Different neurons use different neurotransmitters

What are examples of neurotransmitters? What are their functions?

1. Glutamate: commonly works as an excitatory neurotransmitter, opening ligand-gated sodium ion channels to depolarise the postsynaptic cell

2. GABA: commonly works as an inhibitory neurotransmitter, opening ligand-gated chloride ion channels to hyperpolarise the cell

3. Made up of amino acids

What are neuromodulators?

• Some neurotransmitters are often referred to as neuromodulators

• Unlike the 1-to-1 type of synaptic transmission that neurotransmitters like Glutamate and GABA exhibit, neuromodulators can influence brain function in a more diffuse manner

• They often act via metabotropic receptors rather than ligand-gated ion channels, with slower but more long-lasting effects

• Made up of monoamines (also modified from amino acids)

What are examples of neuromodulators? Where are they produced and what are their functions?

1. Noradrenaline

   • Produced by neurons in the Locus Coeruleus

   • Function: Increase arousal and alertness

2. Serotonin

   • Produced by neurons in the Raphe Nuclei

   • Function: Regulate mood, sleep, etc.

3. Dopamine

   • Produced by neurons in the Substantia Niagra and Ventral Tegmental Area

   • Function: Reward system, motor function, cognition

What neurotransmitters are made up of Neuropeptides?

• Endorphins

• Substacne P

• Neuropeptide Y

How are neurotransmitters synthesised?

• Neurotransmitters are synthesised within the neurons that release them

• Precursors are derived from our diet

  • E.g. Phenylaline → Tyrosine → Dopa → Dopamine → Norepinephrine → Epinephrine

  • E.g. Tryptophan → Serotonin

How is the anatomy of the brain studied?

• Cellular level

• Sub-cellular level

• Whole-brain anatomy

What are methods to the study brain anatomy on a cellular level?

• The neuron through a microscope

• Golgi stain

  • A set of chemicals applied to a piece of brain tissue

What are methods to the study brain anatomy on a sub-cellular level?

• Electron microscopy

  • Passing an electron beam through a tissue sample

What are methods to the study brain anatomy on a whole brain level?

• Post-morterm samples

• CT (computed tomography)

  • “X-ray” for the brain

  • Can visualise denser structures in the body (e.g. bone) but not soft tissue (e.g brain)

  • Therefore, the imaging is a bit low quality and grainy

• CT with contrast dye in bloodstream

  • Able to visualise vascular network in the brain

• MRI (magnetic resonance imaging)

  • Produces images of the brain with high spatial resolution

  • Able to see growth structures of the brain

  • MRI tractography

    • Diffusion-weighted imaging

    • Observing the movement of water molecules in the brain

    • Water travels down the length of the axon

    • This technique allows us to visualise how axons connect to different parts of the brain

How is the function of a brain determined?

1. Effects of damage to the brain

2. Effects of brain stimulation

3. Recording brain activity during behaviour

What are examples of how studying the effects of brain damage has allowed us to understand the function of a brain region?

1. GSW during WW1

   • Loss of vision due to GWS to the occipital cortex

2. Post-morterm examination of people who were unable to speak

   • Damage to the left prefrontal cortex discovered post-mortem in people with impaired speech

   • Broca’s Area: language production

   • Wernicke’s area: language comprehension

3. Stroke patients scanned with MRI/CT

   • Location of damage correlates with difficulty performing a social cognition task (recognising the emotions of people when presented with a part of their facial expression)

What are the limitations of studying clinical cases pertaining to the effects of damage to the brain?

• Limited to what occurs naturally

• Location of damage varies across cases and is unlikely to be confined to a single functional area

Researchers sometimes cause damage to non-human animals to study effects on behaviour, what are examples of techniques they used?

Ablation: Surgical removal of a brain area

Lesion: Localised damage (e.g. chemical injection)

What are examples of how stimulating the brain has allowed us to understand the function of a brain region?

Invasive methods:

1. Deep Brain Stimulation

   • Inserting microelectrodes into the brain of Ps

   • Stimulating specific brain regions and observing their response

Non-invasive methods:

1. Transcranial Magnetic Stimulation (TMS)

   • Limitations: Limited to superficial areas of the brain

   • Doesn’t have good target specificity (Instead of stimulating a single neuron, you end up stimulating the whole area)

   • More diffuse

2. Transcranial Direct Current Stimulation (tDCS)

What are examples of how recording brain activity during behaviour has allowed us to understand the function of a brain region?

1. Intracellular/single cell recording

   • Invasive

   • Used with lab animals, rarely with humans

2. EEG

   • Records from scalp

   • Measures changes by ms

   • Low resolution of location of the signal (Poor spatial resolution)

   • Good temporal resolution (time)

3. PET

   • Measures changes over both time and location

   • But requires exposing brain to radiation

   • Subject is injected with glucose labelled with radioactive atoms

   • PET machine detects gamma rays emitted by the radioactive glucose to track metabolism in the brain

4. fMRI

   • Measures changes over about 1 second

   • Identifies location within 1 to 2 mm

   • Good spatial resolution (location)

   • Poor temporal resolution (electrical recordings not as good)

   • BOLD signal (Blood-Oxygenation Level Dependent signal) measures the level of oxygenated haemoglobin as a proxy of neural activity

   • Based on the principle that when neurons in a region of the brain are active, blood supply increases

What is the CNS made up of?

• Brain

  • Grey matter

  • White matter

  • Nuclei: clusters of cell bodies in the CNS (e.g. Raphe Nuclei)

  • Tracts: bundles of axons in the CNS

  • Caudate Nucleus

  • Corpus Callosum

• Spinal Cord

  • Grey matter: made up of cell bodies & dendrites

  • White matter: Majority of axons

What is the PNS made up of?

• Nerves (bundles of axons in the PNS)

  • Motor nerves

  • Sensory nerves

• Ganglia (bundles of cell bodies in the PNS)

• Autonomic Nervous System

  • Sympathetic Nervous System (fight or flight)

  • Parasympathetic Nervous System (rest and digest)

What are the different anatomical planes?

1. Horizontal

2. Sagittal

3. Coronal

What are the different anatomical landmarks?

1. Left/Right

2. Dorsal/Ventral

3. Anterior/Posterior

4. Lateral/Medial

What is the Cerebral Cortex?

• It is the largest part of the brain in mammals

• Folded sheets of grey matter with axons extending inwards

• At the microscopic level, cells are organised into layers and columns

• Consists of:

  • Gyrus: Peak/bump of the cortex surface

  • Sulcus: Depression/groove in the cortex surface

  • Fissue: a long/deep sulcus

• It is involved in sensory, motor and cognitive processing

How many lobes is the cerebral cortex divided into? What are their names and respective functions? What are other notable structures in the cerebral cortex?

1. Frontal Lobe

   • Executive functions

   • Planning of movements

   • Recent memory

   • Some aspects of emotion

2. Parietal Lobe

   • Body sensations

   • Visuospatial processing

3. Temporal Lobe

   • Hearing

   • Advanced visual processing

4. Occipital Lobe

   • Vision

5. Precentral Gyrus

   • Primary Motor Cortex

6. Prefrontal Cortex

   • Executive functions

   • Working memory

   • Thoughts, actions and emotions

What are the broad divisions of the brain?

1. Forebrain

2. Midbrain

3. Hindbrain

What are the notable structures of the Hindbrain?

1. Brain Stem

   • Medulla and Pons extend from the spinal cord

   • Involved in autonomic functions that are critical for survival

   • E.g. control of breathing, HR, salivation, swallowing, sleep, etc.

2. Cerebellum

   • Control of movement (e.g. posture, coordination)

   • Perhaps cognitive function

What are the notable structures of the Midbrain?

1. Superior and Inferior colliculi

   • Nuclei that process sensory signals from the ears and the eyes

2. Substantia Nigra

   • Contains dopaminergic neurons important to motor control that degenerate in Parkinson’s disease

What are other notable subcortical structures?

1. Thalamus

   • Main source of sensory input to the cortex

   • “Sensory relay point”

2. Hippocampus

   • Memory consolidation

   • Spatial navigation

3. Amygdala

   • Evaluating emotional information (e.g. fear)

4. Ventricles

   • Lateral ventricles (anterior and posterior) contain cerebrospinal fluid

What is perception?

• Your experience of the environment around you provided by your senses

• What you perceive is your own model of the world constructed by your brain

How does the perceptual process work?

1. Environmental stimulus

2. Light is reflected and transformed

3. Receptor processes

   • Rod and cone cells line the back of the eye

   • They convert the light energy into electrical energy and influence what we perceive

4. Neural processing

   • 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

5. Perception

   • “I see something”

6. Recognition

   • “It’s an oak tree”

7. Action

   • “Let’s have a closer look” walks towards tree

Perception doesn’t only rely on sensory input (bottom-up processing) but top-down processes as well (existing knowledge, expectations of the world around us, etc.)

• Visual perception occurs in the brain, not the eyes

Identify these structures

• Pupil

• Cornea

• Lens

• Fovea

• Optic Nerve

• Retina

• Optic Nerve Fibers

• Retina

• Photoreceptors (Rod and cones)

Note: The cornea and lens focus light onto the retina

Identify these structures

• Horizontal cells

• Amacrine cells

• Axons of the Ganglion cells

• Ganglion cells

• Bipolar cells

• Photoreceptors

• Optic Nerve

How does the retina detect light?

1. Light strikes the photoreceptors

2. Message is transmitted to the bipolar cells

3. Message is transmitted to the ganglion cells

4. Message is transmitted to the brain via the optic nerve

• The Fovea is the part of our retina that underlies the center of our visual field

  • Predominantly via cone cells

• The rest of the retina is the periphery

  • Predominantly via rod cells

What are the medical conditions that result in deficits in the visual field?

1. Macular degeneration

   • Retina degenerates typically int eh Fovea

   • As a result, patient experiences blindspot at the centre of their visual field

2. Retinitis pigmentosa

   • Retina degenerates typically in the peripheral parts of the retina (initially)

   • As a result, patient experiences blindspot in the periphery of their visual field

3. Akinetopsia

   • Inability to see motion following damage to the cortex

Explain the process of transduction and adaptation to a dark environment

1. When light strikes the photoreceptor cells, retinal absorbs the light and it changes shape

2. This inactivates retinal, becoming unresponsive (‘bleached’)

3. This also triggers chemical events in the cell that change the electrical state of the photoreceptor thereby generating electrical signals in the cell

4. The photopigment needs to regenerate before it can detect light again

5. When we step into a darker environment, the concentration of regenerated photopigment will increase over time

6. The concentration of responsive photopigment determines how sensitive we are to light

7. Both cones and rods adapt to the dark (increasing sensitivity to light)

8. Once adapted, rods are much more sensitive than cones

9. As such, most of our visual perception in the dark arises from rod cells

How do cone cells vary?

They vary in their sensitivity to wavelength:

1. Short-wavelength cones (S) - Blue

2. Medium-wavelength cones (M) - Green

3. Long-wavelength cones (L) - Red

They all detect different colours (wavelengths) and differ from rods

How are the cells in the Fovea wired? What is the vision they provide like?

• Packed with cone cells

• Low convergence of cells (each cone excited a single ganglion cell)

• Highly detailed vision (high spatial resolution)

• Distinguishes among bright lights; responds poorly to dim light

• Good colour vision

How are the cells in the periphery of the retina wired? What is the vision they provide like?

• Packed with rod cells

• High convergence of cells (multiple tods converge on a single ganglion cell)

• Less detailed vision (less spatial resolution)

• Responds to dim lights; poor for distinguishing among bright lights

• Greater sensitivity to faint light

• Poor resolution in the periphery

• Poor colour vision

What is additive colour mixing?

Mixing the three primary colours (red, green, blue) to produce other colours

What is the trichromatic theory of colour vision?

• Colour vision is based on the presence of three types of cone cells in the retina, each sensitive to a different range of wavelengths of light

• These cone cells are most sensitive to short (blue), medium (green), and long (red) wavelengths of light, respectively

• The trichromatic theory explains how our visual system perceives a wide range of colours by combining the signals from these three types of cones in various proportions

• Colour perception depends on the relative response of the three cone types

Why is the relative response of all three cone types important for colour perception?

• The response of a single cone is uninformative about the wavelength, partly because it confounds wavelength with intensity

• The ratio of responses across the three cones to a given wavelength of light remains similar across different intensities

How does context affect our colour perception?

• There is actually no 1-to-1 relationship between wavelength and colour. It depends on the context

• Prior expectations about object colour affect what we perceive (e.g. colour of the strawberry in blue light, red colour is still identifiable)

• Adaptation (seeing faint residues of colours we’ve seen before, “artefacts of our vision”)

  • Function: to normalise our environment as the scenery around us changes

  • Makes our perception more consistent across different environments

• It depends on the current state of our visual system (e.g. adaptation), the visual context (e.g. illumination), and prior expectations

What is colour constancy in our colour perception?

• The perceived colour of an object can be relatively stable across different illumination conditions, despite differences in the wavelengths of light entering our eye

• This requires our visual system to somehow separate the colour of the light source from the colour of objects

How can our visual system separate the colour of the object from the colour of the light source?

• Contextual cues to the lighting conditions

• Adapting to the dominant colour in the environment

• Past experience with familiar obejcts

What are the important features of perception?

1. Location

2. Depth

3. Motion

4. Colour

5. Form

What is the path of the visual pathway?

1. Eyes

2. Optic Nerve

3. Lateral geniculate nucleus in the thalamus

4. Primary visual cortex

5. Other cortical areas

• Signals from retinal ganglion cells are relayed to the primary visual cortex via the thalamus

• Pathways extending further into the cortex enable more complex processing of visual patterns

How does feature selectivity work in the retina and lateral geniculate nucleus (in the thalamus)?

• Ganglion and LGN cells respond to dots of light

• “center-surround” receptive field

  • When light falls within receptive field, it increases excitatory APs in the neuron

  • If the light falls outside of the receptive field, it sends inhibitory signals, thereby decreasing the signals received by the ganglion cells

How does feature selectivity work in V1 (primary visual cortex)?

• Cortical cells respond to bars

Simple cells:

• “Simple cells” in V1 respond to edges of a specific orientation

• Some simple cells respond best to vertical edges while other simple cells can prefer horizontal or slanted orientations

• Multiple ganglions’ “receptive field” (circles) make up the “bars/lines” of the simple cell

  • Building a “line detector” with nerve cells

• The response of simple cells can then be put together to form more complex patterns (e.g. objects, faces, etc.)

Complex cells:

• “Complex cells” in V1 respond to both orientation and movement direction

• E.g. when the light detected is moving from R to L (not L to R)

What are location columns and orientation columns?

They are specialised arrangements of neurons that are perpendicular to the surface of the cortex

1. Location columns

   • Characterised by their preference for input from one eye over the other

   • Each location column receives input predominantly from one eye, with adjacent columns processing input from the other eye

2. Orientation columns

   • They are organized within the location column and based on the orientation preference of the neurons within each column

   • Neurons within an orientation column have similar orientation tuning properties, meaning they respond most strongly to visual stimuli oriented in a specific direction (e.g., vertical, horizontal, or diagonal)

What is tiling in regards to vision?

• Wiring between neurons makes them respond to particular visual features

• It is the arrangement of receptive fields of retinal ganglion cells (RGCs) in the retina, which ensures comprehensive coverage of the visual field

• The retina contains different types of RGCs, each with specific receptive field properties, such as size, shape, and sensitivity to light.

Aside from V1, what are the other important parts of our cortex for visual processing?

1. V2: detects more complex patterns and receives signals from V1

2. V3 (largely interior)

3. V4

4. V5: key role in our perception of motion (in the temporal lobe)

5. Inferotemporal cortex

What are the dorsal and ventral pathways for vision?

Dorsal:

• Towards the parietal lobe

• WHERE pathway

• HOW/ACTION pathway

• Damage to the parietal visual areas can produce spatial neglect

• Processing of visual information related to the location of objects

Ventral:

• Towards the temporal lobe

• WHAT pathway

• Advanced visual processing

• Processing of visual information related to object recognition and identification

• Damage to the temporal visual areas can produce agnosia (inability to recognise objects) and prosopagnosia (inability to recognise faces)

• There are neurons selective for face identity

What are the important modules in the ventral pathway?

• Fusiform face area

• Inferotemporal cortex

  • It is a region located in the ventral stream of the visual cortex, specifically in the temporal lobe of the brain

  • It is considered to be a critical area for higher-level visual processing, particularly for the recognition of complex visual stimuli such as objects, faces, and scenes

• Extrastriate Body Area (EBA)

  • Active when we look at parts of the body/entire body

• Parahippocampal Place Area (HPA)

  • Responds to landscapes, buildings, etc.

What is Face Pareidolia? Why do we experience this?

• Face pareidolia is a phenomenon in which individuals perceive faces or facial features in unrelated stimuli, such as inanimate objects, patterns, or random arrangements of shapes

• Faces vary but have a common visual structure

• The same person can look different in multiple angles & lighting

Why?

• Humans possess specialized neural circuitry dedicated to processing and recognizing faces e.g. FFA

  • Importance of faces for social interaction, communication, and survival (biological predisposition)

• Pattern Recognition Mechanisms

  • The human brain is highly adept at detecting and interpreting patterns in sensory input. This includes not only recognizing faces but also detecting shapes, objects, and meaningful configurations in the environment

  • This propensity for pattern recognition can lead to the perception of faces in random or abstract stimuli, even when no actual faces are present.

What is the FFA activated by?

• Real faces

• Face pareidolia

• Mooney faces

• Imagining a face

How do sound waves vary? Describe them

1. Frequency

   • Higher frequency = higher pitch

   • Rapid fluctuations in the density of air molecules in our ear

2. Amplitude

   • Higher amplitude = Higher volume (louder)

     • Loudness is the psychological quality of sound

     • Amplitude is the physical quality of the sound wave

   • Greater change in the air pressure over time, with more intense compression of the air molecules occurring at the peak of the sound wave

3. Timbre

   • The quality that distinguishes between 2 sounds that have the same loudness, pitch, and duration, but which still sound different

   • Factors affecting timbre:

     • The relative intensities of the harmonics that are present

     • The time course of the sound wave (attack & decay)

       • Attack refers to how the sound builds up over time

       • Decay refers to how the sound dissipates over time

What is fundamental frequency and harmonics?

Fundamental frequency:

• Most sounds that we hear are more complex sound waves and can be described as a mixture of frequencies

• The waveform has a periodic structure to it

• The fundamental frequency is the overall repetition rate of waveform in 1s

• The fundamental frequency determines the pitch that we hear

Harmonics:

• Musical instruments also tend to produce harmonics

• They are components of the overall waveform that have a frequency that is a whole number multiple of the fundamental frequency

How does our ear detect sound waves?

1. Sound waves are changes in air pressure that propagate through the air around us

2. They arrive at the outer ear and travel down the auditory canal

3. The sound waves collide with the ear drum (tympanic membrane) which is located at the end of the auditory canal

4. The sound waves vibrate the tympanic membrane, which causes three tiny bones, called an anvil, hammer and stirrup, to move and pass on the vibration into the cochlea

5. The cochlea is a snail-shaped structure that consists of chambers filled with fluid

6. When the bones of the middle ear hammer on the oval window, it creates a vibration in that fluid that travels down the length of the cochlea and vibrates a structure in the centre of the cochlea called the basilar membrane

7. The basilar membrane is moved up and down in response to sound waves hitting our ear, this pushes the cilia against the tectorial membrane which causes the cilia to bend.

8. That bending of the cilia causes ion channels in the hair cell to open, depolarising the cell.

9. The depolarization triggers the release of neurotransmitters across the synapse that the hair cell shares with a neuron in the auditory nerve, stimulating action potentials

10. This mechanical bending of the cilia to produce electrical signals is the transaction

How do we detect the frequency of a soundwave?

• The “place method”

• One end of the basilar membrane is called the base and the other end is the apex

• The base of the basilar membrane tends to be narrow and stiff while the apex tends to be wide and floppy

• The frequency of the sound wave is coded in terms of which hair cells are most activated in response to it

• It has been shown that higher-frequency sounds are better able to vibrate at the base of the basilar membrane while lower frequency sounds vibrate at the apex of the basilar membrane

How do you recover hearing for a person whose hair cells get damaged (where their ability to hear gets lost)?

• Cochlear implants

• An array of electrodes is inserted into the chambers of the cochlea to electrically stimulate auditory nerves that originate in different sections of the cochlea

• A microphone is placed in the outer ear and it converts sound waves into a pattern of electrical stimulation for the electrodes in the implant

• This bypasses the functions performed by the middle ear and hair cells, instead electrically stimulating the auditory nerve fibers directly

Describe the auditory pathway in the brain

1. Signals from the cochlea are sent to the cochlea nuclei and superior olive in the brainstem

2. This signal is then passed through the inferior colliculus in the midbrain to the medial geniculate nucleus in the thalamus, and finally to the primary auditory cortex in the temporal cortex

3. Most activity from the left ear arrives at the right auditory cortex and vice versa

4. But there are also ipsilateral projections from the left ear into the left auditory cortex and from the right ear into the right auditory cortex

5. Thus hearing is not as lateralised as vision

What are other brain areas that are involved in auditory processing?

1. Broca’s Area in the left prefrontal cortex

   • Language production

   • Broca’s aphasia: can understand speech but they struggle to produce speech

2. Wernicke’s Area in the left posterior temporal cortex

   • Language comprehension

   • Wernicke’s Aphasia: struggle to understand speech

How does our auditory system localise the source of a sound?

1. Interaural intensity differences

   • The brain compares the intensity of the sound waves arriving at each of our ears as a way of localising where the sound is coming from

   • The head acts as a sound barrier, attenuating the intensity of sounds coming from a location on the other side of the head

   • This means that the sound has greater intensity at the ear facing more directly towards the sound source

   • This works best for high frequency sounds as they are attenuated by the head to a greater extent

   • Sound waves of low frequencies are not attenuated much by the head and so arrive at both ears with the same amplitude regardless of location

   • High frequency sound waves are impeded more by objects that they encounter, while low frequency sound waves are impeded less

2. Comparing the timing of when sound waves reach each ear

   • The ear furthest from the sound will detect the sound later than the ear nearer to the sound source

   • Our brain can compare the timing of when sound waves arrive at each ear as a way of localising where the sound is coming from

   • A lack of interaural timing differences implies that the sound source is located in the central plane

What other systems can affect our perception of sound? What is an example of how this affects our perception of sound?

• Our perception of sound can also be affected by what we see

• We have strong expectations about how particular visual events should sound, such that we can’t help but experience a kind of auditory imagery of the sound that our vision suggests we should be hearing

Example: McGurk effect

• Illustrates how our perception of speech can be affected by how we see a person’s lips move

• We use visual cues to interpret speech, which is generally a useful ability (e.g. helping us to follow a person’s speech in noisy environments)

• Our brain is automatically using the visual information to determine what we consciously hear

• In the McGurk effect, what we see misleads us about the sound that we are hearing

What we hear can also affect what we see

• 2 balls passing each other or bouncing off each other with or without the ‘click’ sound

• What we hear can affect our visual perception of the causal interaction between objects and the trajectories that objects are moving in

• The effect of sound on what we see is likely to be most pronounced when the information is ambiguous

What are the functions of touch?

• It contributes to our awareness of our own body; it provides part of our sense of where our body is located

• The feel of objects helps us to grip them and interact with them

• A social sense; different types of touches experiences different emotions

• How we express and experience attachment in early development

Describe how our skin detects touch. How do these receptors differ from one another?

1. The dermis contains various mechanoreceptors that are responsible for detecting pressure, stretch and vibration of the skin

2. The main mechanoreceptors include Meissner’s corpuscles, Merkel disks, Pacinian corpuscles and Ruffini endings

3. Each of these are specialised receptor endings that connect to a nerve fibre that carries signals back to the spinal cord

4. These mechanical/physical forces on the skin trigger ion channels to open and the production of an actional potential in the nerve attached to the receptor

Difference:

• Different receptors respond to different types of touch due to factors such as the physical structure of the receptor and how deep it is located in the dermis

• E.g. Pacinian Copuscle

  • Many receptors like this are embedded in the skin throughout the body

  • Helps to detect high frequency vibration against the skin

  • Plays a role in our perception of textures → whether a surface feels rough or smooth

How are mechanoreceptors distributed throughout the body?

• The density of mechanoreceptors in our skin varies across our body

• High density of receptors in our fingertips compared to the palm

• Correspondingly, our ability to discriminate fine details of touch is better for our fingertips compared to our palm

• Tactile acuity: the fineness of the details that can be discriminated by touch

• Highest number of mechanoreceptors in hands and lips

Describe the tactile pathway in the brain

1. The mechanoreceptors in the skin connect to nerves that send signals to the spinal cord

2. From there, the signals are passed up the spinal cord to the thalamus, then to the primary somatosensory cortex (S1) at the top of the brain

How are neurons in S1 organised?

• They are organised into a somatosensory map of the body, with the cortical representation of body parts following receptor density and tactile acuity

  • More cortical space is allocated to parts of the body with greater sensitivity to touch

• Different features of the body are organised in a systematic way, where the different fingers are represented near each other, and features of the face are represented near each other

• Represented by a ‘homunculus’ figure

• More cortical space is allocated to parts of the body with greater sensitivity to touch → reflects that much more of S1 is dedicated to processing signals from the hands and lips region

What is an example of how the somatosensory map can change with experience?

• Musicians who have played string instruments for many years

• The hand used to finger the strings has greater cortical representation in the primary somatosensory cortex compared to non-musician controls

• The cortical activity in this study was measured with a technique called MEG, similar to EEG, which measures neural activity non-invasively through the scalp

Describe taste and the mechanisms behind it

• Taste is a chemical sense: the sensation that you get when molecules that are dissolved in our saliva are detected by taste receptors on our tongue

• 5 basic sensations: sweet (sugar), salty (sodium), sour (hydrogen ions (acids)), bitter (alkaloids such as caffeine), umami (msg)

• The overall taste of a substance can generally be described as a mix of the basic tastes

Mechanisms behind taste:

• Taste begins when the chemicals that we put in our mouth interact with the taste buds

• The taste buds are mostly located in structures on the surface of the tongue called papillae

• The papillae are the structures of the tongue surface that produce ridges and valleys, underlying the roughness of the tongue surface

• There are different types of papillae, differing in their shape and location on the togue

• Each papilla contains a number of taste buds, and each taste bud is a ‘garlic-like’ structure that contains multiple taste cells

• The taste cells have tips that extend into the taste pore, to contact with the saliva on the surface of the tongue

• The taste cell connects to nerve fibres, which transmits electrical signals towards the brain

• Transduction occurs when a chemical in our saliva makes contact with the taste cells, causing them to generate electrical signals in the nerve fibres they connect to

• There are different receptor types that respond to molecules that we experience as bitter, sweet, sour and salt

• Stimulation of these receptor sites triggers a number of different chemical reactions within the cell that lead to the movement of charged molecules across the membrane, which creates an electrical signal in the receptor and an action potential in the nerve fiber that the taste cell is connected to

• The taste cells send electrical signals towards the brain via different nerves that connect to different parts of the tongue and mouth

• These pathways connect to the spinal cord then travel up to the thalamus, then into the insula in the primary taste cortex within the lateral sulcus

Explain the individual differences in the function of taste cells

• People differ in the total number of tastebuds they have → giving rise to ‘supertasters’ and ‘nontasters’

• This depends on genetics, age and other things

• E.g. pregnancy increases taste sensitivity → may be an adaptive thing by helping to avoid harmful foods

• Another factor is the sensitivity of the taste receptors themselves

  • Genes may affect the function of taste receptors, influencing how readily the individual can detect compounds in the food that elicit the experience of sweetness or bitterness