the visual system - detecting shape

Question 1

what does it mean for a ganglion cell to have an ON centre receptive field? as opposed to an OFF centre?

Answer 1

on centre means it is excited by light in the centre and inhibited by light in the surround.
off centre means it is excited by light in the surround but inhibited by light in the centre.

Question 2

can ganglion cells respond to objects that are larger than their receptive fields?

Answer 2

NO. - they only give information about small changes eg an edge.

Question 3

global changes in illumination will not change the response of a ganglion cell - why?

Answer 3

because the ratio of excitation/inhibition will remain unaffected.

Question 4

how do bipolar receptive fields affect ganglion cells?

Answer 4

an on centre bipolar cell will depolarise - increases the firing rate
an off centre cell will hyperpolarise - decreses the firing rate.

Question 5

what is lateral inhibition?

Answer 5

the activity of horizontal cells which modulates photoreceptors synapsing with ON centre bipolar cells.
they can depolarise the cells in the centre of the receptive field.

Question 6

receptive fields of ganglion cells overlap, meaning…

Answer 6

adjacent fields will look almost like the same scene.

Question 7

what are ganglion and LGN receptive fields characterised as being?

Answer 7

concentric (circular)

Question 8

receptive fields of V1 are organised how?

Answer 8

in columns - down one column, all cells will respond strongly with the same stimuli.

Question 9

what did Hubel and Wiesel find about V1 receptive fields? (preference)

Answer 9

each cell has an oriental preference.

Question 10

there are 2 types of v1 cell - simple and complex. what do each do?

Answer 10

simple - respond best to lines or edges with a specific orientation and location
- each cell has a specific preference.
complex - also have an orientation preference BUT have a preferred direction of movement too.
- majority of v1 cells are complex.

Question 11

v1 can determine..

Answer 11

movement

Question 12

v1 receptive fields contain ..

Answer 12

the receptive fields of many LGN cells

Question 13

what is spatial frequency in v1?

Answer 13

the level of detail per degree of visual angle. we need this to be able to see contrast.
high spatial frequency allows us to see small details and sharp contrasts.
low frequency allows us to see large uniform areas and gradual transitions.

Question 14

the squareness of a spatial frequency wave determines…

Answer 14

sharpness

Question 15

the amplitude (height) of a spatial frequency wave determines…

Answer 15

contrast

Question 16

magnocellular and parvocellular cells can be independently associated with spatial frequencies..

Answer 16

magnocellular = low frequencies
parvocellular = high frequencies (fine detail and high quality)

Question 17

v1 colums have _ layers. LGN cells send their axons to layer _.

Answer 17

6 layers.

cells go to level 4.

Question 18

layer 4 of V1 is subdivided into 4 sublayers. what are these called

Answer 18

4A
4B
4Ca
4C(B) - weird B

Question 19

from layer 4 of v1, what determines where axons go?

Answer 19

their function

Question 20

what is the flow of information for magnocellular cells in v1?

Answer 20

4Ca - 4b - v2.

Question 21

what is the flow of information for parvocellular cells in v1?

Answer 21

4cb - layers 2+3 - v2.

Question 22

what processing occurs at V2?

Answer 22

shape processing
simple properties of shape like orientation and spatial frequency as well as more complex properties like:
- illusory contours
- retinal disparity
- rigure/ground segregation

Question 23

how is V2 organised?

Answer 23

in stripes. there are 3 types:

• thin
• thick
• inter

Question 24

M-layer neurons send axons to ____ stripes in V2. this serves what function

Answer 24

thick

shows spatial location and depth information

Question 25

P-B neurons send their aoxns to ____ stripes and ____ stripes in V2. what functions do these serve?

Answer 25

thin and inter stripes

give information on form and colour

Question 26

whata does the inferotemporal cortex do?

how is it represented?

Answer 26

it completes object recognition and is located in the temporal lobe.
represented in columns - cells in a column respond to similar but slightly different stimuli.

Question 27

what are the two broad identification pathways in the visual system?

Answer 27

1. the object identification pathway
   - parvo/ventral
   - processes inf related to the identity of the object
   - COLOUR AND SHAPE
2. motion pathway
   - magno/dorsal
   - information about position and movement

Question 28

how do we sum up hte two streams theory?

Answer 28

one tells us WHAT an object is and the other tells us WHERE it is.

Question 29

what will damage to the ventral pathway result in?

Answer 29

a deficit in object recognition

• vision and intelligence remain intact
• eg visual agnosia

Question 30

what will damage to the dorsal pathway result in?

Answer 30

a deficit in locating objects in space

- eg visual ataxia.