Week 2: Neural processing and brain substrates

Question 1

Pathway for visual processing

Answer 1

Vision is the most important function for humans

Light –> eye –> retina –> optic nerve –> optic chiasm –> lateral geniculate nucleus –> optic radiation –> the 1st visual cortex (V1)

The image is focused on the retina, which lines the back of the eye.

Question 2

Visual receptors

Answer 2

Rods and cones (outer segment):
- contain light-sensitive chemicals
- visual pigments that react to light and trigger electrical signals
- distributed differently across the retina
- more rods than cones (120 - 6 million)
- peripheral retina
- optic disk –> none –> blind spot
- Fovea has many cones and no rods

Question 3

Rod monochromats

Answer 3

Person without a cone (a genetic defect)

Question 4

Macular degeneration

Answer 4

A condition where the fovea and surrounding are degenerate so the person cannot see whatever they are looking at.

Question 5

Retinitis pigmentosa

Answer 5

A condition where the peripheral retina initially degenerates and causes loss of vision in the periphery (tunnel vision)

Question 6

Focusing

Answer 6

Two-element optical system –> the cornea and the lens

If fails –> Myopia (focus before the retina; near-sightedness) and Hyperopia (focus behind retina; far-sightedness)

Question 7

The cornea

Answer 7

The transparent covering of the front of the eye

~80 percent of the eyes focusing power

it is fixed in place so it can’t adjust it’s focus

Question 8

The lens

Answer 8

20 percent of the eye’s focusing power

can change its shape to adjust the eye’s focus for objects located at different distances

Cilliary muscles - change the thickness of the lens

Question 9

Transduction

Answer 9

When the light is absorbed by light-sensitive visual pigment molecules in the receptors

Question 10

Visual pigments (retinal + opsins)

Answer 10

transduction –> fundamental for sensation and perception

act like filters

our perception is limited by these filters

More intact pigments –> more sensitive –> can see better in the dark

Question 11

Izomerization

Answer 11

Isomerization of the retinal creates a chemical chain reaction –> activates thousands of charged molecules to create electrical signals in receptors –> activation of the receptor

Question 12

Bleaching

Answer 12

Is the seperation of the retinal from the opsin –> visual pigment regeneration (when not in use)

Regeneration much faster in cones

Question 13

Dark adaptation

Answer 13

A lighted place –> a dark place
- Hard to see at first –> see better after some time

Dark adaptation curves for rods and cones
- the cones are more sensitive than the rods at the beginning of dark adaptation

Question 14

Spectral sensitivity

Answer 14

Rods and cones differ in responding to light in different parts of the visual spectrum - different wavelengths

The rods are more sensitive to short-wavelength light than are the cones
rods –> 500nm
cones –> 560nm

Rods are also more sensitive in the dark and the cones in the light.

By measuring the spectral sensitivity curve
- presents one wavelength at a time
- measures the subject’s sensitivity to each wavelength

Question 15

Properties of the rod and cone visual pigments

Answer 15

Determine:
- the increase in sensitivity that occurs in the dark (dark adaptation)
- light sensitivity

• The sensitivity to different wavelengths across the spectrum
  - spectral sensitivity

Question 16

Neural convergence

Answer 16

126 million receptors but only 1 million ganglion cells

more convergences of rods than cones

R to G ratio = 126 to 1 (rod) and 6-1 (cone)

Convergence causes the rods to be more sensitive (to the light) than the cones

Cones, instead, have better acuity (more details)

Question 17

Lateral inhibition and sensation

Answer 17

The capacity of an excited neuron to reduce the activity of its neighbours usually using inhibitory interneurons

This creates a contrast in stimulation that allows increased sensory perception –> clearer signals

Lateral inhibition –> modulating the amount/intensity of incoming information about a specific stimulus

The Chevreul illusion, Mach illusion, Hermann Grid Illusion

Question 18

Mach bands

Answer 18

Light and dark bands create fuzzy borders called mach bands

Question 19

Hermann grid illusion

Answer 19

Four squares of the grid and five receptors under the pattern.

Receptor a is located at the intersection and B, C, D and E have a black square on either side.

The receptors connect to bipolar cells. The response of each of the five receptors are 100. The initial response of the bipolar cells matches the response of the receptors.

Lateral inhibition (-10) travels to bipolar cell A and makes the signal go from 100–> 60.

Question 20

Receptive field

Answer 20

H. Keffer Hartline (1938, 1940)

Nobel prize (1967)

Recording from a single optic fiber of the frog
* Activation of an optic fiber (ganglion cell)→by a specific area of retina
(hundreds ~ thousands of receptors)
* Receptive fields are overlapping each other (due to inter-cells)
* Activation of one single receptor→activations of several ganglion neurons

Question 21

Centre-surround receptive field

Answer 21

On a receptive field, center and surround area respond differently to the light.

Excitatory centre, inhibitory surround
Inhibitory center and excitatory surround

Question 22

Contextual modulation

Answer 22

Interesting phenomenon of neurons during the visual processing

Enhanced neuronal responses to stimuli placed within the classical receptive field (CRF) that are modulated by stimuli placed outside of the CRF
- condition –> the CRF must be activated before the ERF has any influence

Function –> orientation pop-up, texture-boundary segmentation, detexturization, gain control, recalibration, second-order processing, feature selectivity, color constancy and increased coding efficiency for border extraction.

Question 23

LGN (lateral geniculate nucleus)

Answer 23

Receptive fields were found
- excitatory and inhibitory areas

One of the purposes of the LGN is to regulate neural information as it flows from the retina to the cortex.

The LGN receives more signals from the cortex than from the retina
- feedback to regulate the flow of information that is sent by the LGN to the cortex –> sensory gating and better perception?

Question 24

Visual Cortex

Answer 24

The striate cortex (area V1)

Cells found in the striate cortex with receptive fields
- excitatory and inhibitory areas
- each cell has a specific type of stimulus for the best response (tuning curve)

Retinotoptic organisation

Simple cell vs complex cell

Columnar organisation

Question 25

Simple vs Complex cells (cortical)

Answer 25

Simple cells have excitatory and inhibitory areas arranged side by side. Respond best to small spots of light or bars of a particular orientation

Complex cells respond only when a correctly oriented bar of light moves across the entire receptive field - respond best to a particular direction of movement

Question 26

Optic nerve fiber (ganglion cell)

Answer 26

Center-surround receptive field.

Responds best to small spots, but will also respond to other stimuli

Question 27

Lateral geniculate

Answer 27

Centre-surround receptive fields very similar to the receptive field of a ganglion cell.

Question 28

Feature detection and visual perception

Answer 28

These physiological responses of neurons are the basis of visual perception
- these neurons are acting as feature detectors

Fundamental phenomena
- selective adaptation and selective rearing

Question 29

Selective adaptation

Answer 29

Exposures to the same feature (a specific orientation, or movement) for 1~2 mins
* Decrease the sensitivity
* Increase the threshold
* Both in behavior and physiology
* Provides more stable perception in complicated and changing feature detection
* This help you recognize any familiar object (even when there are some small changes)

Question 30

Selective rearing

Answer 30

Prolonged (days ~ weeks) exposures to the same stimulus (A) during early stage of life make more neurons to respond to the same stimulus (A)
* The Number of neurons responding to the other stimuli decreases * Lose the ability to see it – blinded to the specific orientations

Question 31

V1 –> other brain areas for higher order visual recognition

Answer 31

Neurons respond to body parts or objects

Not much response to simple features

The fusiform face are (FFA) in humans are rich in neurons that respond to faces

Question 32

Neural basis of face perception

Answer 32

2 major visual processes after the 1st visual cortex

• identifying where it is→dorsal visual stream→location
• identifying what it is→ventral visual stream→appearance

Question 33

FACE perception

Answer 33

Occipital face area (OFA)
* Relatively specialized for faces (not bodies or
objects)
* Codes the physical aspects of facial stimuli

Fusiform face area (FFA)
* Relatively specialized for faces (not bodies or
objects)
* Important for computing an invariant facial identity

The superior temporal sulcus (STS)
* Responds to faces and bodies
* Important for action perception and dynamic stimuli (e.g. lip movements)
* Integrates visual and auditory information

Question 34

Body perception

Answer 34

Body shape and size (ex: fat vs thin) and posture

The Extrastriate body area (EBA)

The Fusiform body area (FBA)

The superior temporal sulcus (STS)

Question 35

How does neuronal firing represent feature or object?

Answer 35

Features or objects are represented by the pattern of firing of groups of neurons.

• Sometimes the groups are small (sparse coding), sometimes large (population coding).
• Receptive fields and Neuronal responses are various and flexible