The visual pathway - visual cortex - L5&L6

Question 1

Learning objectives

Answer 1

Describe the structure of the visual cortex.

Explain how visual space is organized in V1.

Explain how orientation sensitivity works.

Describe some important types of cortical cells,
and explain what they are sensitive to.

Question 2

What is the V1?

Answer 2

The LGN projects to the primary visual cortex (V1) via optic radiations. V1 aka striate cortex.

V1 has about 100m cells per hemisphere.

V1 is organised in layers.

Question 3

Where does LGN input come into V1?

Answer 3

LGN input comes into V1 at layer 4:

Magnocellular in upper layer 4.

Parvocellular with lower layer 4.

Then they connect to upper and lower layers

Question 4

Where do K cells go straight to from the LGN?

Answer 4

K cells go straight to layers 1-3.

Question 5

What are cells in layer 4 driven by?

Answer 5

Cells in layer 4 are driven by the input from one eye only.

Question 6

How are ocular dominance columns created?

Answer 6

If a particular block of cells receives input from the right eye, the cells above and below it will also receive input from the right eye.

But adjacent blocks of cells either side will receive input from the opposite eye.

Question 7

What are ocular dominance columns

Answer 7

ocular dominance columns penetrate perpendicular to the surface. They are organised in patterns visible in staining.

Question 8

How are adjacent regions in the retina mapped?

Answer 8

Like the LGN, adjacent regions of the retina are mapped onto adjacent regions of the cortex – the retinotopic map is maintained.

Question 9

How are the cells distributed in the retinotopic map?

Answer 9

the distribution of cells associated with each retinal region is distorted: 80% of cortical cells are devoted to the central 10deg of the visual field (remember no convergence in the fovea).

Question 10

What is the disproportionate weighting of cortical power called?

Answer 10

referred to as cortical magnification.

Question 11

What does the cortica magnification mirror?

Answer 11

how the vast majority of RGCs are devoted to the fovea.

Question 12

How are Foveal RFs different from Peripheral RFs?

Answer 12

Since foveal RFs are small relative to peripheral RFs, many more are required to cover the same area of visual space.

Question 13

What are the cortical cells differences when compared with RGCs and LGN cells from which they get their input?

Answer 13

Selectivity to orientation.

They are sensitive to size in a different way.

They can be binocular.

They are more sensitive to colour.

They are sensitive to direction of motion.

Question 14

What are the cortical cells similarities when compared with RGCs and LGN cells from which they get their input?

Answer 14

They maintain the retinotopic map.

They aren’t particularly sensitive to the illumination level.

They respond best to abrupt changes in luminance (lines, bars).

Question 15

Why dont all cells have the same features?

Answer 15

V1 circuitry is extremely complex

Question 16

What do most cortical cells have a marked preference for?

Answer 16

Unlike RGC and LGN, most cortical cells have marked preference for particular orientations.

Question 17

Why do cortical RFs have a preference for particular orientations?

Answer 17

Cortical RFs are organised and shaped differently so that they obtain a maximum response to a line of a specific orientation

Question 18

What happens when a cell is in its preferential location?

Answer 18

Each cell will have a preferential orientation. In this case, the response is maximal when the bar is vertical. Firing rates highest in each cells preferred orientation

Question 19

What does staining on cortical cells do?

Answer 19

staining can tell us about the orientation preference of an array of cortical cells.

Question 20

What is a map of orientation preference called?

Answer 20

By doing a stain for all orientations, we can create a map of orientation preference, called pinwheels.

Question 21

Can cortical cells come in different types?

Answer 21

Cortical cells come in different types, some of which are sensitive to location, and some to size.

Question 22

What is a simple cell (type of cortical cell)?

Answer 22

Optimum response to an appropriately oriented stimulus and a certain position within the RF. Phase sensitive.

Question 23

What is a complex cell (type of cortical cell)?

Answer 23

Optimum response to an appropriately oriented stimulus placed anywhere within the RF. Phase insensitive.

Question 24

What is a hypercomplex cell (type of cortical cell)?

Answer 24

Optimum response depends not only on orientation but also on contour length. Maximum response occurs when the bar length matches the width of the receptive field.
This is “end stopping” or “length-width inhibition”.

Question 25

What are cells in the V1 layer?

Answer 25

Monocular

Question 26

What is binocularity?

Answer 26

other layers to which cells in V1 send signals are binocular – they can be driven by either eye.

If a visual stimulus is delivered to each eye in turn, the cell will respond.

If the same stimulus is then delivered to both eyes, the response is more vigorous.

Question 27

How many RFs do Binocular cells have?

Answer 27

Binocular cells have two RFs –

LE & RE

Question 28

What are binocular cells like?

Answer 28

They are matched in type (e.g. simple) and respond to similar preferred orientations, locations, and directions of motion.

Question 29

When will binocular cells response be maximal?

Answer 29

their response will be maximal when corresponding regions in each eye are stimulated by stimuli of similar size and orientation.

Question 30

Where are colour sensitive cells concentrated?

Answer 30

Colour sensitive cells are concentrated in the cortical blobs. Each blob is centred on an ocular dominance column.

Question 31

How are cortical blobs seperated?

Answer 31

Within a blob, cells will either have red/green opponency or blue/yellow opponency – these are not mixed in a single blob.

Question 32

Where do blobs receive their input from?

Answer 32

Blobs receive their input from lower layer 4, which gets its input from the parvocellular LGN layers.

Question 33

What is the difference between colour sensitive cells and most other V1 cells?

Answer 33

These cells show no preference for a particular orientation, unlike most other V1 cells.

Question 34

What do most cortical cells have a preference for?

Answer 34

A large proportion of cortical cells display preferences for stimuli moving in a particular direction

Question 35

What do motion sensitive cells usually respond to?

Answer 35

Motion-sensitive cells usually respond only to one direction, i.e., they do not respond to motion in the opposite direction. This is direction selectivity.

Question 36

What motions do simple and complex cells respond to?

Answer 36

Simple cells respond to slow motion.

Complex cells respond to faster motion.

Question 37

How is the visual cortex composed?

Answer 37

The visual cortex is composed of columns of cells.

Each column consists of cells with the same orientation preference and the same ocular dominance preference.

Question 38

What is a hypercolumn?

Answer 38

A set of 18-20 columns (~1mm) traverse a complete range of orientations and ocular dominance. This collection of adjacent columns is referred to as a hypercolumn (HC).

Each left and right eye is alternated in ocular dominance slab

Question 39

What is the structural arrangement of hypercolumns called?

Answer 39

ice cube model.

Question 40

What does each HC contain?

Answer 40

Each HC contains the neural machinery required to simultaneously analyse multiple attributes of an image (orientation, size, colour, direction of motion) falling on a localized region of the retina.

Question 41

How much does the Foveal HC cover the visual field?

Answer 41

0.05deg of the visual field;

At 10deg eccentricity, each one covers ~0.7deg.

Question 42

What do neighbours HCs look at?

Answer 42

Neighbouring HCs look at neighbouring regions of the retina

Question 43

V1 Summary

Answer 43

Visual information enters V1 at layer 4 (of 6).

The majority of V1 is dedicated to central vision due to cortical magnification.

V1 cells can be simple, complex, or hypercomplex.

V1 cells share some attributes with LGN and retinal cells, but others are unique to the cortex.

V1 cells are organised into columns and hypercolumns based on ocular dominance and orientation.