The eye and the retina

Question 1

Describe how the intensity of light is related to receptive fields and it’s use:

Answer 1

Each RGC/optic nerve fibre has receptive fields, increasing with intensifying light

In the largest field, the light at the periphery must be more intense at the centre = some synaptic pathways preferred to others

Used for detecting edges of objects + contrast

Question 2

Describe the action of an ON center/OFF surround cell:

Answer 2

Flashing light at centre = increases cells’ response = DP

Flashing light at surround = decreases cells’ response = HP

Flashing a large light covering both centre/surround = little/no response

As excitation from the centre cancels inhibition from surround = lateral inhibition

Question 3

Describe the action of an OFF center/ON surround cell:

Answer 3

Inhibition from a small spot light in centre

Excitation from light in surround

Question 4

Describe the effect that photoreceptors of receptive fields of more than one ganglion cell can have:

Answer 4

Can excite/inhibit post synaptic neurons

As they release glutamate at their synapses

So can either DP/HP a cell depending on the type of receptor on that cell = ionotropic/metabotropic

Question 5

Describe the advantage of centre-surround receptive field organization:

Answer 5

RGC transmit differences in firing rates of cells in centre/surround

Allows them to transmit information about contrast

Question 6

Describe spatial frequency with relation to size of receptive field:

Answer 6

Size of receptive field governs the spatial frequency of information

Small receptive field, stimulated by high spatial frequencies = fine detail

Large receptive field, stimulated by low spatial frequencies = coarse detail

Question 7

Describe summation in the dark:

Answer 7

In dark adaptation, the peripheral opposite the activity field becomes inactive

Since this is a decrease in inhibition between centre and surround, the active field can increase

Resulting in more area for summation

Question 8

Describe the fate of neural signals in ON/OFF-center retinal ganglion cells:

Answer 8

Remain segregated in retina + LGN

Then merge completely in complex cells in primary visual cortex

Question 9

Describe the action of the H cell if light decreases in the surround:

Answer 9

H cell inhibits cone terminal rather than bipolar cell

So central cone inhibited

Identical to what would happen if light increased in centre

Question 10

Describe the action of rods and cones in photopic conditions where they maintain the antagonistic property:

Answer 10

Rod transduction cascade not working

Decreased light on neighbouring cones HP rods by H cells

Their signals feed into the RGC

Adding to the antagonistic surround

Question 11

List the perceptual benefits of lateral inhibition:

Answer 11

Reduced redundancy in signal

Facilitates “constancy”

Increased dynamic range

Question 12

Define sinusoidal grating:

Answer 12

Grating in which the luminance of the image undulates at a regular spatial frequency along an axis

Increasing and decreasing according to the sine curve, y = sin (x)

Question 13

Describe the use of sinusoidal grating stimuli:

Answer 13

Probe visual fields

Characteristics matched mathematically to features in natural scenes

Question 14

Compare parvocellular cells with magnocellular cells, in terms of:

Cell and receptive field size 
Receptive field surround 
Temporal response 
Sensitivity  
Axon 
Wavelength

Answer 14

Parvocellular:
Smaller
Strong receptive field surround
Slow and sustained temporal response
Low sensitivity
Moderate diameter/speed of axons
Reg-green opponent in central retina 

Magnocellular
About 4 times as large
Weaker receptive field surround
Fast and transient temporal response
Big, fat and fast axon
Achromatic wavelength

Question 15

Describe the properties of the contralateral eye:

Answer 15

Small rf centre
Strong surround
R.V low sensitivity
Sustained response
Red-green wavelength sensitive

Question 16

Describe the properties of the ipsilateral eye:

Answer 16

Larger rf centre
Weaker surround
Higher sensitivity
Transient response
Wavelength insensitive

Question 17

Define the visual pathway:

Answer 17

Anatomical pathway by which electrical signals are transmitted to brain that were generated by the retina

Question 18

Describe the visual pathway:

Answer 18

Nerve fibres of RGC collect at optic disc

Before passing out of the orbital bones down the optic nerve to the brain

Nerve fibres meet at the optic chiasm,

Where the nerve fibres of the nasal side from each eye cross over contralaterally

And continue down the optic tract with nerve fibres from the temporal side of the other eye

At end of optic tract = retinal nerve fibres connect with other visual pathway nerves in LGN

Some processing of signals

Before a series of radiating nerve fibres, optic radiation, convey information to the visual cortex in posterior portion of occipital lobe

Question 19

Describe the effect losing the parvocellular pathway:

Answer 19

Reduces acuity as primary role is chromatic processing and visual acuity

And provide input to higher-order pattern processes

Question 20

Describe the effect losing the magnocellular pathway:

Answer 20

Reduces low spatial and high temporal frequency vision

Because of band-pass spatiotemporal characteristic of high temporal frequency sensitivity = detects contrast over a wide range of luminances

And provides input to higher-order pattern and motion processes