lecture 26 - nervous system 6: vision & the brain

Question 1

what are rods connected to?

Answer 1

bipolar cells

each bipolar cell is collecting information from several rod cells

bipolar cells operate vertically through the retina connecting the optic nerve and the photoreceptors

Question 2

what do the horizontal connections in the retina do?

Answer 2

allow interactions between outputs of photoreceptors across the retina

allow the activity of neighbouring clusters of rods to influence another neighbouring cluster of rods

horizontal and acramine cells

Question 3

what neurotransmitter is released by photoreceptors in the dark?

Answer 3

glutamate

it excites some bipolar cells but inhibits others
in turn bipolar cells excite or inhibit ganglion cells which give rise to nerve fibres

Question 4

what is the ganglion cell receptive field?

Answer 4

small patch of retina where light excites that ganglion cell to fire impulses

ganglion cells have centre-surround organisation because of the horizontal connections
• on centre / off surround
• off centre / on surround

Question 5

what are the 2 types of centre-surround responses of rod ganglion cells?

Answer 5

on centre type

off centre type

Question 6

on centre type response of rod ganglion cells

Answer 6

light shone on centre excites ganglion cell

light shone on surround inhibits ganglion cell

when light shone on both there is a weak response from the ganglion cell

Question 7

off centre type response of rod ganglion cells

Answer 7

light shone on centre inhibits ganglion cell

light shone on surround excites ganglion cell

when light shone on both there is a weak response from the ganglion cell

Question 8

what centre-surround responses do contrast sensitive rod ganglion cells have?

Answer 8

off centre retinal ganglion cell output

centre activated by dark and inhibited by light

surround activated by light and inhibited by dark

Question 9

response of contrast sensitive rod ganglion cells

Answer 9

if the cell is illuminated evenly, excitation and inhibition cancel each other out

if we cover some of the surround, inhibition is stronger that excitation so no firing

if we cover the centre, strong excitation of centre with incomplete inhibition of surround, lots of firing

if whole ganglion is dark, strong excitation of centre balanced by inhibition of surround

Question 10

benefits of centre-surround organisation of rod ganglion cells

Answer 10

helps to emphasise contrast at edges of visual objects

important for identifying shapes

basis for some visual illusions

Question 11

cone ganglion cells and centre-surround organisation

Answer 11

centre and surround fed by different cone types

2 types:
• red : green
• blue : yellow

white light (includes both red and green) gives weak firing - excitation and inhibition balanced

Question 12

red : green centre-surround organisation of cone ganglion cells

Answer 12

red centre - excitation
green surround - inhibition

just red centre - all excitation and no inhibition, large output

all red - reduced output, some inhibition from the surround even though its sensitive to green

red centre, green surround - weak response, surround inhibited by green

Question 13

the visual pathway

Answer 13

optic nerves leave the eye and travel back to brain

enter above pituitary gland at the optic chasm where there is crossing over

fibres continue into brain and synapse in the lateral geniculate body in the thalamus

nerve cells then give rise to optic radiations that leave the thalamus and go the visual cortex called the occipital lobe

Question 14

visual field mapping

Answer 14

fibres from each half retina (nasal, temporal) in each eye take different routes to generate left and right visual fields

left eye temporal and right eye nasal (red) go to left LGN and cortex - represent right visual field

right eye temporal and left eye nasal (blue) go to right LGN and cortex – represent left visual field

fibres from nasal retina’s cross at the optic chiasm

visual field overlap is important as allows the stereoscopic vision

Question 15

what is stereoscopic vision?

Answer 15

the perception of depth and 3D structure

Question 16

where is the human primary visual cortex found?

Answer 16

in the occipital lobe at the back of the brain

it is along the calcarine fissure

Question 17

cortical processing

Answer 17

neurones in the visual cortex have more selective responses than ganglion cells and lateral geniculate neurone (LGNs)

• prefer bars and egdes
• prefer specific orientations
• prefer a specific direction of movement

Question 18

what are the 2 different visual pathways in the cortex?

Answer 18

the where pathway

the what pathway

Question 19

visual areas in the cortex

Answer 19

surrounding the primary visual cortex (V1) are many other visual areas: V2, MT, V4, IT

as we go further away from V1 their responses become more complex

MT sensitive to moving stimuli & objects you recognise as they’re moving

IT sensitive to faces & not to any other type of shape

Question 20

the what pathway

Answer 20

begins at V1 and carries in ventrally

this pathway is concerned with what the object is

Question 21

the where pathway

Answer 21

begins at V1 and goes dorsally

concerned about where the stimulus is coming from