Visual Adaptation and After Effects

Question 1

What are the three after effects we experience?

Answer 1

• Tilt After-Effect
• Motion After-Effect / Waterfall Effect.
• Colour After Effect

Question 2

What is visual adaptation?

Answer 2

The process by which the visual system alters its operating properties (i.e. sensitivities to a particular parameter, whether that be light, direction , colour etc) in response to changes in the environment.

Question 3

Why do after effects occur?

Answer 3

• When a particular parameter is strongly stimulated, the visual system becomes less sensitive to that particular parameter. This affects the way in which subsequent stimuli appear until the adaptation effect has worn off.
• For example, sensitivity to dim light stimuli decreases when we are exposed to bright light - (light adaptation).
• Takes time for light sensitivity to return when we enter a dark environment (dark adaptation).

Question 4

True or False- Intraocular transfer of effect is proof of an after effect being monocular

Answer 4

False - the fact that there is intraocular transfer i.e. the effect of what is seen by one eye- is percieved by the other, means that detection of this stimulus must have occured in cortical cells as part of binocular vision

Question 5

Describe how the tilt after effect occurs

Answer 5

Question 6

Why does the tilt after effect occur?

Answer 6

Extended exposure to pattern causes adaptation, reducing neurone firing rate.

Recovery time is needed.

Thus tilt after effect occurs as change in orientation is not correctly processed in cortex during recovery time.

Question 7

When may we make use of the tilt after effect?

Answer 7

The extent of interocular transfer of the after effect is used to assess binocularity of visual cortex e.g. we may choose to do this following squint opperation.

[Normally interocular transfer of after-image is 60-70% of if we had viewed the initial stimulus and after effect in the same eye]

Question 8

Do we get intraocular transfer of the colour after effect - and what does this tell us?

Answer 8

No we do NOT get intraocular transfer of the colour after effect. This tells us the effect is pre-cortical.

Question 9

Why does the colour after effect occur?

Answer 9

Due to Hering’s red-green and blue-yellow colour opponent pathways.

Red-green pathways are stimulated by red and inhibited by green or vice versa.

Decrease in sensitivity to red means increase in sensitivity to green.

Question 10

Summarise After effects

Answer 10

• When we focus on a visual stimulus with a specific orientation or direction of movement, those neurones which respond most strongly to that stimulus adapt.
• The adaptation results in a reduction in sensitivity.
• When the stimulus is removed neurones take time to recover.
• During this time our perception will be distorted by the reduced sensitivity of one group of neurones.
• If we adapt one eye and the after-effect is seen in the other eye, effect must occur in the visual cortex or beyond.
• If adaptation is in one eye only, suggests pre-cortical.

Question 11

True or False- We have cells sensitive to every direction of movement representing every area of the retina, and every orientation of stimulus.

Answer 11

True

Question 12

Why does the motion after effect occur?

Answer 12