Lecture 6: Extrastriate cortex

Question 1

Magnocellular channel

Answer 1

*analysis of object motion

• project 4Ca to 4B
• respond to light bars of certain orientation

Question 2

Parvocellular channel

Answer 2

*analysis of object shape

• project to 4Cb
• which projects to inter blob and blob regions in layers 2 and 3

Question 3

Visual area 2

Answer 3

• complete map of visual space is represented
• cells respond to orientation, spatial frequency (motion) and colour

*receptive fields of V2 cells are 2-3x bigger than in V1

Question 4

Functioning connectivity

Answer 4

• V1 projects mainly to v2
• V2 equal projections to V3/V3A and to V4
• V3/V3A projects mainly to V4
• Largest projection from V4 is to IT
• Area MT receives very small projections from V1, V2 and V3
• It projects the MST as well as several other areas including the frontal eye fields.

Question 5

V1 connectivity with V2

The magno

Answer 5

form the beginning of the motion pathway

Question 6

V1 connectivity with V2:

The Parvo

Answer 6

form the beginning of the colour and form pathways

Question 7

V1 connectivity with V2:

Konicellular

Answer 7

part of the parvo pathway

Question 8

V1 connectivity with V2: Parasol ganglion cells

Answer 8

project to M cells in LGN and then 4C alpha in V1 before terminating on 4B in V1 and pass onto the thick striped in V2

Question 9

What versus where

Answer 9

• ventral ‘what’ stream specialises in processing info about object features and identity
• dorsal ‘where’ stream specialises in visuospatial analysis such as movement and reaching

Question 10

Magno vs parvo

Answer 10

M=motion in dorsal stream
(fast pathway for motion)(where)

P=form and colour in ventral stream
slow pathway for objects
(what)

Question 11

V4

Answer 11

• 3rd area in ventral stream
• receiving strong feedforward input from V2 sending strong connections to the PIT
• receives direct inputs from V1
• also has weaker connections to V5
• Strong attentional modulation
• likes simple geometric shapes

Question 12

Evidence for V4 function

Answer 12

Optical imaging in macaque monkeys (Roe et al, 2012)
Colour/luminance (B and D) and orientation (C and D) maps. Composite map (F) illustrates segregation of color/lum (pixels in B and D coded pink) and orientation (pixels in C and E coded green) preference bands. Scale bar: 1 mm (Tanigawa et al., 2010).

Question 13

Dorsal stream

Answer 13

a processing stream including cortical areas V3, MT, and MST, which is claimed to specialise in the analysis of movement, depth and action

Question 14

V5 perception of motion

Answer 14

• Area MT arranged into direction of motion columns
• cells here have large receptive fields which respond to stimuli in narrow range of directions
• almost all cells in area MT are direction selective , leading to hypothesis of motion perception

Question 15

Identifying ‘motion’ areas in the human brain

Answer 15

• Subjects viewed a pattern of moving squares.
• Regions of high cerebral blood flow are indicated in the PET scan.
• Area MT (or V5) receives increased blood flow during the task, as do areas V1 and V2.

Question 16

MT connectivity

Answer 16

**

Question 17

Further motion processing MST

Answer 17

Cells in the MST (parietal lobe) are selective for:
– (i) Linear motion (as in area MT)
– (ii) Radial motion (either inward or outward);
– (iii) Circular motion (either clockwise or counter clockwise).
Researchers do not fully understand how the complex motion processing properties of these cells lead to visual perception.
However, several roles have been proposed:
– (a) Navigation
– (b) Directing eye movements
– (c) Motion perception
– (e.g. for survival).

Question 18

Relative latency

Answer 18

=the time interval between the presentation of a stimulus on the retina and the initiation of a response in the cell

• % of cells: V4 has more colour sensitive cells and MT(V5) has more direction selective cells.
• Relative receptive field size
V1 = 1, v2 = 2-3, V3 = 4-5, V4 = 5-6, MT = 7-10