visual perception

Question 1

stages of vision?

3

Answer 1

object seen against background

seen by retina upside down in 2D form

seen in upright, 3D colourful form

Question 2

what are the 4 classes of photoreceptors?

Answer 2

3 cone types

1 rod

Question 3

how many classes of ganglion cells are there?

Answer 3

20-30

Question 4

what are the first 3 stages in visual processing?

Answer 4

edge detection
edge enhancement

filtering of spatial, wavelength, movement and directional info

Question 5

when does lateral inhibition occur?

and what is it?

what is the result on edges?

Answer 5

neurons in same layer of retina are interconnected (either by axons or interneurons) so each neuron inhibits its neighbours mutually

makes edges stand out more as inhibited photoreceptors report seeing less light than they actually do (looks darker)

Question 6

when do lateral inhibitions cancel each other out?

Answer 6

when light falling on group of retinal neurons is uniform

so reciprocal inhibitions cancel each other out

Question 7

examples of horizontal connection cells?

Answer 7

horizontal cells

amacrine cells

Question 8

examples of vertical connections?

Answer 8

fovea - 1 cone linked to 1 bipolar

periphery - many cones to 1 bipolar and many bipolars to 1 ganglion

Question 9

what forms a receptive field of a bipolar and ganglion cell?

and what actually is a receptive field?

Answer 9

cones or rods converging on a bipolar cell form its receptive field

converging bipolar cells on a ganglion cell forms its receptive field

receptive field - region in which stimulus will modify firing of that cell

Question 10

what do bipolar and ganglion cells have?

Answer 10

centre surrounded (classical) receptive fields

Question 11

how do horizontal cells influence bipolar cells?

Answer 11

either directly or by feeding back info to the cones (indirectly)

Question 12

role of bipolar cells in defining activity of ganglion cells?

Answer 12

bipolar cell integrates inhibitory and excitatory post-synaptic potentials

signals from several of these cells define the activity of the ganglion cell

Question 13

what do retina photoreceptors relase when not sitmulated?

Answer 13

glutamate

decreases when exposure to light

Question 14

what happens when photoreceptors and bipolar cells exposed to light?

Answer 14

photoreceptors - exposure hyperpolarises and decreases rate of glutamate

bipolar cells -invert the receptor signal to the standard:

depolarisation when light intensity increases and hyperpolarisation when intensity decreases

(usually the opposite)

Question 15

describe On-centre bipolar cells?

what is the sombrero shaped response?

Answer 15

inhibitory synapse in presence of glutamate in the centre

when light spot is in the off-surround, more inhibitory potentials than excitatory as more cones stimulated in off-surround than in on-centre

when light spot reaches ON-centre, more excitatory potentials than inhibitory as not inhibited by glutamate and more cones in the ON-centre are stimulated than in the OFF-surround

Question 16

describe OFF-centre bipolar cells?

Answer 16

have excitatory synapse in presence of glutamate in the centre

turning off light in centre of off-centre excites as receives more glutamate which depolarises the bipolar cells

(opposite to on-centre bipolar cells)

Question 17

response of retinal ganglion cells to light exposure?

Answer 17

centre-surrounded receptive fields show sombrero response similar to bipolar cells

Question 18

how does On-centre ganglion cell respond to light?

when is it at maximum and minimum?

Answer 18

increases firing (spike) rate

at maximum when light spot covers whole centre as all cones stimulated

at minimum when all of surround but none of ON-centre covered

Question 19

how does OFF-centre ganglion cell respond to light?

Answer 19

decreases firing (spike) rate

spike reduced if spot illuminates centre

spike increased if spot illuminates surround

(opposite of ON-centre ganglion cells)

Question 20

what happens to the ganglion cell when the whole receptive field is equally stimulated (uniform illumination)?

Answer 20

ON and OFF centre ganglion cell is at rest and fires with a spontaneous frequency so doesn’t respond to uniform illumination

Question 21

characteristics of P-ganglion cells?

Answer 21

small receptive fields
slow conduction field
high acuity
colour sensitive

mainly located in fovea

Question 22

characteristics of M-ganglion cells?

Answer 22

large receptive field
high conduction speed
low acuity
sensitive to motion
no colour discrimination

Question 23

difference between P and M pathways?

Answer 23

P - parvocellular small soma small cell
M- magnocellular large soma

project to different layers in V1 so remain segregated

Question 24

what does LGN stand for?

what does it segregate?

Answer 24

lateral geniculate nucleus

segregates left and right eye projections

Question 25

what is V1 and what projects to it?

Answer 25

V1 = first layer of the visual cortex (primary visual cortex) and is important for perceptual processing

m and p pathway neurons project to different layers in the V1

Question 26

what structure does the V1 have?

Answer 26

columnar structure

Question 27

what do neurons in the orientation column of the V1 share?

how is this shown?

who found this out?

Answer 27

same preference for a particular orietation of a bar stimulus in their receptive field

shown by maximal spike rate at that orientation

Hubel and Wiesel (nobel prize in 1981)

Question 28

what have microelectrode recordings revealed about cells?

examples of this?

Answer 28

differ greatly in their receptive fields

visual cells in LGN have concentric receptive fields (like ganglion and bipolar cells) so respond when light in certain part of visual field

visual cells in cerebral cortex more responsive to bars of light and preference for orientation

Question 29

what did Hubel and Wiesel propose about simple cells in orientation columns of V1?

Answer 29

that they receive input from several neighbouring retinal ganglion cells

Question 30

difference between simple and complex cortical cells?

Answer 30

simple: respond best to edge or bar of particular location and orientation in visual field
complex: respond best anywhere within particular area of the visual field

Question 31

what do the dorsal and ventral stream do?

Answer 31

dorsal - interacting with the world - where system

ventral - making sense of the world - what system

Question 32

what is meant by perceptual shape constancy?

Answer 32

We recognise the same shape from different view points and directions despite the distortions in the retinal projections

Question 33

how do receptive fields differ in the fovea and periphery?

Answer 33

receptive fields are large in the periphery (low acuity)

receptive fields are small in the fovea (high acuity)