Vision: Primary visual pathway

Question 1

Where is the primary visual cortex?

Answer 1

Striate cortex, V1 in the occipital lobe.

Question 2

What are the three main information stages?

Answer 2

Retinal, lateral geniculate body and visual cortex.

Question 3

Where does information pass from, through and to?

Answer 3

Photoreceptors, bipolar cells, retinal ganglion cells, then optic nerve, through lateral geniculate body to the visual cortex.

Question 4

What is the experimental strategy to reveal mechanisms of visual perception?

Answer 4

Studying the different neuronal responses at different stages of the visual pathway provides an understanding of how the visual perception system works.

Question 5

Which two people made seminal contributions to our understanding of visual information processing, winning a Nobel Prize in 1981?

Answer 5

David Hubel and Thorsten Wiesel.

Question 6

What are the two types of photoreceptors?

Answer 6

Rods and cones.

Question 7

Describe rods.

Answer 7

• Abundant (120 million in human retina)
• No colour (wavelength) discrimination
• Sensitive in low light levels
• Higher density in periphery
• Track high-rate changes

Question 8

Describe cones.

Answer 8

• Less abundant (6 million in human retina)
• 3 types (S=blue M=green L=red) discriminate wavelengths
• Less sensitive to low light
• Higher concentration in fovea
• Can’t follow rapid changes.

Question 9

Which cells use an analogue signal?

Answer 9

Photoreceptors and bipolar cells - they vary their voltage as they’re stimulated.

Question 10

Which cells use a digital signal?

Answer 10

Everything subsequent to photoreceptors and bipolar cells - vary spike-rate.

Question 11

What is photoreceptor detection of light translated into and via what?

Answer 11

Translated into excitation or inhibition of retinal ganglion cells via bipolar cells.

Question 12

Without cones, our vision would be…?

Answer 12

Black and white.

Question 13

Without rods, our vision would be…?

Answer 13

In colour, but we’d find it difficult to see in low light.

Question 14

What are the receptive fields of visual neurons?

Answer 14

The portion of the retina in which visual stimulation will evoke a change in the firing rate of a given visual neuron.

Question 15

What is the substructure of a receptive field?

Answer 15

The way in which visual stimuli need to be presented in the receptive field of a visual neuron in order to evoke firing-rate changes.

Question 16

From where do retinal ganglion cells receive their input?

Answer 16

Photoreceptors via bipolar cells.

Question 17

How do retinal ganglion cells respond to visual stimuli?

Answer 17

They have on-off centre-surround receptive fields.

Question 18

In an on-off centre-surround receptive field, light presented in the ‘on’ region…?

Answer 18

Excites the cell, and light presented in the ‘off’ region inhibits the cell.

Question 19

What is the response rate of a retinal ganglion cell based on?

Answer 19

The sum of stimulation in the on region minus stimulation in the off region. This leads to enhancement of contrast and boundaries.

Question 20

What neurons respond to visual stimuli in a similar way to retinal ganglion cells?

Answer 20

Neurons in the lateral geniculate body.

Question 21

Why are centre-surround fields important?

Answer 21

• They’re efficient - changes and boundaries are important (constants need no response)
• The luminance of features is represented relative to their surround, meaning that object appearance is preserved regardless of light levels.
• However, this relative luminance can result in illusions!

Question 22

Are retinal ganglion and LGN neurons sensitive to colour?

Answer 22

Yes, they receive input from cones and have receptive fields that show centre-surround colour opponency.

Question 23

What is the functional significance of colour opponency?

Answer 23

It’s unclear.

Question 24

How can negative afterimages be explained?

Answer 24

A combination of colour opponency and firing-rate adaptation (rebound effects) in retinal ganglion cells.

Question 25

What are orientation-selective cells in V1?

Answer 25

Most V1 neurons respond to elongated stimuli with specific orientation.

Question 26

What are the two main types of orientation-sensitive V1 neurons?

Answer 26

Simple and complex cells.

Question 27

Describe simple cells.

Answer 27

• Fields have inhibitory and excitatory regions.

- Can be thought of as combining inputs from on and off cells.

Question 28

Describe complex cells.

Answer 28

• Fields have no discrete on and off regions
• They respond best to moving stimuli (reflecting response adaptation)
• Can be thought of as combining inputs from simple cells.

Question 29

What is the retino-topic map?

Answer 29

An orderly mapping of the retina/visual field onto the visual cortex.

Question 30

V1 is divided into small columnal modules. What do they do?

Answer 30

They combine neurons sensitive to different aspects of stimuli presented in a small part of the visual field.

Question 31

What is blindsight?

Answer 31

The phenomenon whereby subjects with primary visual cortex lesions and apparent blindness can show appropriate reactions to visual stimuli of which they are not conscious.

Question 32

Once visual information is in the modules in V1, what happens to it to result in perception and memory of the holistic properties of whole objects and scenes?

Answer 32

It is combined and further processed in the visual association cortices (V2-V5, inferior temporal cortex, posterior parietal cortex) and other regions.

Question 33

Give examples of blindsight.

Answer 33

‘Looking’ or pointing toward visual stimuli, detection of movement.

Question 34

What does blindsight highlight?

Answer 34

That although the primary visual pathway is critical for conscious vision, there are additional pathways involved.
Also that the brain can perform visual information processing which can guise subjects’ behaviour without their conscious awareness.

Question 35

What does recent research by Schmid et al (2010) suggest about blindsight?

Answer 35

That direct LGN projections to the extrastriate cortex are critical.