Lecture 4 - When perception goes wrong

Question 1

Fraser Spiral illusion?

Answer 1

• look at diagram
• a series of concentric circles
• misaligned parts (black and white strands within the circles ) distort the perception of the regular pattern (circles)

Question 2

Jastrow illusion?

Answer 2

• look at diagram
• objets with a longer radius appear to be shorter

Question 3

Irradiation illusion?

Answer 3

• look at diagram
• white square appears to be bigger but they are actually the same size
• light areas appear to be larger than dark areas because light from a white region irradiates adjacent dark regions

Question 4

What if the image on the retina is ambiguous?

Answer 4

• the retinal image doesn’t change but we perceive different objects
• the same feature can be different parts of different objects

Question 5

Erroneous perception with visual illusions?

Answer 5

what we perceive is not always what is represented on our retina

Question 6

Equivocal perception with ambiguous figures?

Answer 6

we perceive different objects from the same retinal image

Question 7

What did Gibson say?

Answer 7

such illusions and figures are carefully constructed to mislead us and do not exist in the real world, they have no ecological validity

Question 8

What are some examples of real world visual illusions?

Answer 8

• moon illusion
  -> the moon appears larger when it is close to the horizon as compared to when it is high up in the sky
• waterfall illusion
  -> motion aftereffect: after observation of motion in one direction (waterfall for 30-60 seconds) stationary objects (e.g. trees) appear to move in the opposite direction
• wagon wheel effect:
  -> stroboscopic effect: a moving wheel appears to stand still or move in opposite direction to its true rotation

Question 9

Ambiguous retinal images - the inverse projection problem?

Answer 9

• A 3D object is represented on a 2D (retinal) surface
• The same pattern of light on the retina can be caused by different objects
• The real-world 3D object cannot be derived from the retinal image

Question 10

The fact that we experience perceptual illusions and ambiguity tells us what?

Answer 10

• perception is more than just sensation
• sensory information from the retina is insufficient, perception is a matter of interpretation

Question 11

what do top down accounts of perception say?

Answer 11

perception is an interaction between sensation and cognition

Question 12

Richard Gregory 1970?

Answer 12

• came up with the constructive theory of perception
• goal = explain how we attach meaning to sensory input
• perception = interpretation of sensation
• basic idea:
  -> perception is indirect and a constructive process of hypothesis testing
  -> for ambiguous figures 2 equally plausible hypotheses are established
  -> as perception is based on individual factors, incorrect hypotheses can be formed, which lead to perceptual errors

Question 13

Evaluation of the constructive theory of perception?

Answer 13

(+) Theoretical account (tries to explain how perception and object recognition work)
(+) Is able to explain perceptual failure (visual illusion) and ambiguity
(-) Cannot explain why illusions persist even when they are known
(-) Suggests that perception is not effortless (active and continuous hypothesis testing)
(-) doesn’t often match our subjective experience

Question 14

Gestalt psychologists?

Answer 14

• Two light flashes presented in a rapid alternating fashion create illusory movement
• Modern version of the phi phenomenon: Apparent motion induced by sequentially presented static stimuli
• The sensory experience (two light flashes) is not sufficient to explain the perceptual experience (illusion of movement)
• ‘The whole is more than the sum of its parts’

Question 15

How do we achieve the whole (the object)?

Answer 15

• by perceptual organisation such as:
  -> grouping
  -> segmentation

Question 16

What are Gestalt’s principles (heuristics) of perceptual organisation that lead to whole?

Answer 16

1. proximity - elements that are close together are grouped together
2. similarity - elements that look similar are grouped together, similarity seems to override proximity
3. common fait - elements that appear to move together are grouped together
4. good continuation - points that, when connected, result in straight or smoothly curving lines are seen as belonging together
5. closure - elements that close a figure are grouped together
6. relative size
7. surroundedness
8. orientation
9. symmetry
   - elements that are relatively smaller, in a surronded area, horizontally/ vertically oriented + symmetrical are grouped together

Question 17

What is the law of Prägnanz?

Answer 17

“Of several geometrically possible organisations that one will actually occur which possesses the best, simplest, and most stable shape.”

Question 18

Evaluation of Gestalt psychology?

Answer 18

(+) Provides a set of useful perceptual heuristics (principles of how sensory input is grouped and segmented)
(-) Aims to be a theoretical account but is mainly descriptive (describes what grouping principles are, not how object perception and recognition work)