Low-level visual processing

Question 1

What is ‘accommodation’?

Answer 1

The adjustment of the lens to focus light on the retina.

Question 2

What percentage of the brain comprises the visual cortex?

Answer 2

20%

Question 3

How many steps are there before visual stimuli reach the visual cortex?

Answer 3

3

Question 4

Which steps have to occur before visual stimuli reach the visual cortex?

Answer 4

• Reception: absorption of physical energy by receptors in the eye.
• Transduction: the conversion of physical energy to electrochemical patterns in the neurons.
• Coding: One to one mapping between the physical stimulus and neural activity. One ‘block’ of light is dealt with by one neuron.

Question 5

What are cone cells responsible for in the eye?

Answer 5

• colour vision

- visual acuity

Question 6

What are rod cells responsible for in the eye?

Answer 6

• vision in low light

- movement

Question 7

Which types of receptors are more numerous in the eye?

Answer 7

Rod cells.

Question 8

What are the two (important) visual pathways?

Answer 8

Parvocellular and magnocellular.

Question 9

What does the parvocellular pathway involve?

Answer 9

• Input comes from cone cells

- Deals with colour and fine detail

Question 10

What does the magnocellular pathway involve?

Answer 10

• Input comes from rod cells

- Movement/motion

Question 11

Describe the 5 components of the pathway from the eye to the brain.

Answer 11

• Retina
• Optic nerve
• Optic chiasma
• Lateral geniculate nucleus
• Cortical area V1

Question 12

What is the retina?

Answer 12

A layer of light sensitive cells at the back of the eye.

Question 13

What is the optic nerve?

Answer 13

A pair of nerves which transmit impulses from the retina to the brain.

Question 14

What is the optic chiasm?

Answer 14

The location at which the optic nerve crosses over.

Question 15

What is the lateral geniculate nucleus?

Answer 15

Two bodies in the thalamus where the optic nerve terminates.

Question 16

What is the receptive field?

Answer 16

The region of sensory space in which light will trigger the firing of that relevant neuron.

Question 17

What is lateral inhibition?

Answer 17

A reduction in the activity of one neuron caused by activity in a neighbouring neuron.

Question 18

What is the center-surround receptive field?

Answer 18

Specific neurons are linked to specific patterns of light in a receptive field. (e.g dark middle or light outside)

Question 19

What is retinotopic organisation?

Answer 19

The arrangement of neurons being directly linked to the section of the visual field that they respond to.

Question 20

What is V1 responsible for?

Answer 20

Basic information from the visual field, such as edges, orientations and wavelengths of light.

Question 21

Several LGN cells combine to form…

Answer 21

Simple cells

Question 22

Several simple cells combine to form…

Answer 22

Complex cells.

Question 23

What is cortical blindness?

Answer 23

A condition which is diagnosed when patients cannot consciously report objects presented in a region of space. Caused by damage to V1.

Question 24

What is the ‘what’ pathway involved in?

Answer 24

Colour and form processing. Temporal/ventral.

Question 25

What is the 'where' pathway involved in?

Answer 25

Movement processing. Parietal/dorsal

Question 26

Who devised functional specialisation theory?

Answer 26

Zeki (1992, 1993)

Question 27

What does the functional specialisation theory suggest about the visual cortex?

Answer 27

That different parts of the visual cortex are specialised for different functions.

Question 28

According to functional specialisation theory, what are the roles of each visual area?

Answer 28

V1 & V2 - shapes V3 (V3a) - form & moving objects V4 - colour V5 - visual motion

Question 29

What evidence is there for V4 and V5?

Answer 29

Zeki (1991) conducted a PET study and found that: - V4 was more active for coloured images compared to greyscale images. - V5 was more active for moving dots compared with static dots.

Question 30

What evidence is there against V4 being linked to colour?

Answer 30

Patients with cortical achromatopsia, who can't see colours due to V4 damage. However, damage also to V2 and V3.

Question 31

What is the name of the condition that arises when there is damage to visual area V5 (MT)?

Answer 31

Akinetopsia.

Question 32

What evidence is there for V5?

Answer 32

Patient LM had damage to V5, was good a locating stationary objects, but could not detect motion in objects.

Question 33

What is the synchronisation hypothesis?

Answer 33

Form, colour, motion etc is bound together across the different areas.

Question 34

What are the three different aspects of colour?

Answer 34

- Hue - Brightness - Saturation

Question 35

What is hue?

Answer 35

The actual colour category. (red, blue, yellow, etc)

Question 36

What is brightness?

Answer 36

The perceived intensity of light

Question 37

What is saturation?

Answer 37

The paleness of the colour.

Question 38

What is trichromatic colour theory and who devised it?

Answer 38

Hermann von Helmholtz postulated that there must be 3 types of colour receptors in the eye, each responding to different wavelengths of light.

Question 39

What were the three colours and wavelengths that Trichromatic theory proposed?

Answer 39

- Blue (short) - Yellow/green (medium) - Red (long)

Question 40

What does the opponent-process theory suggest?

Answer 40

There are three 'opponent processes' - pairs of colours. Suggests that we cannot perceive both colours in an opponent-process at once.

Question 41

Which two studies have investigated the opponent-process theory?

Answer 41

Hering (1878) DeValois & DeValois (1975)

Question 42

What does the dual-process theory suggest?

Answer 42

One achromatic channel combines activity of medium and long-wavelength cones to measure the intensity of the light. A blue yellow channel measures the difference between medium and long wavelength cone signals A red-green channel represents the difference between these signals.

Question 43

What is colour constancy?

Answer 43

The tendency for a surface or object to appear to have the same colour, despite the fact they have changed slightly.

Question 44

What is chromatic adaptation?

Answer 44

The decrease in the sensitivity to light of any given colour over time.