Week 5 Lecture 5 - spatial vision

Question 1

What is the tilt after-effect?

Answer 1

• look at titled black and white bars
• look at straight bars
• bars appear titled in the opposite direction

Question 2

What are the 3 components that explain the tilt-after effect?

Answer 2

1. Orientation tuned neurons respond best to preferred orientation but also respond to other similar orientations
2. Perceived orientation determined by distribution of responses across cells
3. Adaptation – cell’s response decreases following prolonged activity

Question 3

In the tilt-after effect, what do the lines look like before, at the start of, during and after adaptation?

Answer 3

• Before adaptation vertical line looks vertical
• Start of adaptation tilted line looks tilted
• During adaptation tilted line continues to look tilted, but cells’ responses decrease
• After adaptation vertical line looks tilted due to asymmetrical response distribution

Question 4

What does the size of the tilt after-effect depend on?

Answer 4

• Size of after-effect depends on difference
  between adapt and test
• biggest effect found at 10-20 degrees
• effect disappears around 50 degrees

Question 5

What does the tilt after effect provide evidence for?

Answer 5

Tilt after-effect provides evidence for orientation tuned cells in human visual system

Question 6

What is the size after-effect?

Answer 6

• see thick bars on top and thing bars underneath
• then look at just thin bars
• see bottom thin bars as thick

Question 7

What is the explanation for size after-effects?

Answer 7

• Before adaptation size perceived veridically
• Adapt to fatter bars
• During adaptation cells’ response decreases
• After adaptation lines look thinner due to
  asymmetrical response distribution

Question 8

What do size after-effects provide evidence for?

Answer 8

Size after-effect provides evidence for size-tuned cells in human visual system

Question 9

Size and orientation are fundamental features of what?

Answer 9

Size and orientation are fundamental features of parts of the visual scene and the brain has cells tuned to these features

Question 10

What is spatial frequency?

Answer 10

• Spatial frequency = number of bars per unit distance (usually cycles per degree)
• Fat bars = low spatial frequency
• Thin bars = high spatial frequency

Question 11

What details are provided in high spatial frequency and low spatial frequency?

Answer 11

• High spatial frequency = fine details
• Low spatial frequency = course information

Question 12

What do we have greater and lower sensitivity to?

Answer 12

• We have greater sensitivity to intermediate spatial frequencies. They can be perceived at low contrasts
• We have lower sensitivity to high and low spatial frequencies. They need higher contrast to be perceived

Question 13

What does spatial frequency tell us about?

Answer 13

size on the retina – it does NOT indicate real size in the world since the projected size depends on distance

Question 14

What is size constancy?

Answer 14

we perceive an object’s real size in the
world regardless of distance

Question 15

What is orientation constancy?

Answer 15

we perceive an object’s orientation in the world regardless of the orientation on the retina