Lecture 1 - Theories of Visual Perception

Question 1

What state are the eyes in:

Answer 1

the eyes are constantly moving even when fixated

Question 2

How is an image perceived?

Answer 2

Image on back of the retina is inverted - eyeball has a curved surface and the brain compensates for the inability to perceive straight lines

receptors are unevenly distributed

image is inverted and tiny and curved and flat

Question 2

what is perception according to Roth 1986

Answer 2

the means of which information is acquired from the environment via the sense organs and transformed into experiences of objects, events, sound tastes etc

Question 3

Theories of visual perception:

Answer 3

1. Gestalt approach to perception
2. Gibson’s ecological theory of perception
3. Marr’s information processing theory of perception
4. The constructivist approach to perception

Question 4

Gestalt Approach:

Answer 4

• top-down approach
• whole is greater then the sum of its parts
• segregation and grouping
• perceptual organisation

Question 5

what is perceptual organisation

Answer 5

The unified whole is different from the sum of the parts - we can see a bike as a bike not the sum of its parts

Question 6

Gestalt Laws of Perceptual Organisation (9 laws)

Answer 6

1. similarity (similar things grouped together)
2. good continuation (lines are seen in a way that follows smoothest path)
3. proximity (close things grouped together)
4. connectedness
5. closure (closed figure preferred)
6. common fate (same direction/orientation)
7. familiarity (explanation or description?)
8. invariance (different circumstances/orientations help with identification)
9. figure-group segregation (separating figure from ground - some objects more prominent in scene)

Question 7

problems with Gestalt Approach:

Answer 7

• underplays parallel processing and unconscious processing

- some explanations wrong - description of how things work BUT not explanation

Question 8

Gibson’s Ecological Theory of Perception

Answer 8

• Bottom-up approach
• perception is direct
• complex cognitive processes unnecessary
• no difference between perception and sensation as perception is direct
• importance of movement
• retinal image provides rich info sufficient for interaction
• ecological (natural environments)

Question 9

Invariants of Gibson’s ecological theory

Answer 9

1. horizon ratio relation - proportion of object above horizon is constant with changes in distance but NOT size
2. texture gradients - changes in textures in optic array informs us of diatnce, orientation and curvature of surfaces.

Question 10

What did Gibson and Bridgerman (1987) find:

Answer 10

Ps could identify objects and state colours, lighting conditions, orientations just from B&W photos supporting Gibson’s ecological theory - and the invariants

Question 11

Why is motion important in Gibson’s theory

Answer 11

• motion necessary to perceive the invariant info
• two types: observer movements or object movement
• motion parallax - far away objects move more slowly then nearby things - the speed informs us of the distance
• observer movement causes changes in AMBIENT OPTIC ARRAY

Question 12

Optic Flow:

Answer 12

• expansion
• is a combination of parallax and retinal size changes
• observer movement causes changes in AMBIENT OPTIC ARRAY

Question 13

Examples of practical implications of Gibson’s Optic Flow ideas:

Answer 13

• horizontal lines painted on road when approaching junction - lots of repititive white lines - tells brain you are going faster then you are.

Question 14

Criticisms of Gibson’s Ecological theory

Answer 14

• vague - how is this info picked up
• ignores top down - no mention of experience and memory
• little man controlling us?
• no mention of visual cortex

Question 15

Marr’s Information Processing Approach to Perception (four stages of image anaylsis)

Answer 15

1. grey level description
2. Primary sketch (Raw and full)
3. 2 1/2 D sketch
4. 3D object centred description

Question 16

grey level description:

Answer 16

• the intensity of light is measured at each point on retina

- produced by activation of RETINAL PHOTORECEPTORS

Question 17

Raw Primal Sketch:

Answer 17

• Raw primary sketch: identifies edges of objects
• process:
• 1: gaussian blurring - bigger changes in edge = less sensitive to blurring
  2. identify intensity changes
  3. assign primitives: there are four types of intensity charge

Question 18

in Marrs ifnromation processing approach - raw primal sketch - what are the four types of intensity change

Answer 18

1. edge segment
2. bar
3. termination
4. blob

Question 19

Full primal sketch:

Answer 19

• identifies object outline
  process:
  1. group primitives together - assign place taken
  2. place tokens can be grouped to form higher-order place tokens
  3. grouping based on CLUSTERING and CURVILINEAR aggregation

Question 20

2 1/2 D sketch:

Answer 20

• the primal sketch is combined with depth cues + colour + motion to form the 2 1/2 D sketch
• not 3D as is observer orientated (can only see from one orientation)

Question 21

3D representation:

Answer 21

• 2 1/2 D sketch analysed for 3D volume primitives

- Conscious experience of vision

Question 22

Marr’s Informational Processing approach - summary and contributions

Answer 22

• computational - looks at approach as a whole
• bottom-up
• biologically plausible

Question 23

The constructivist approach to perception

Answer 23

• perception depends upon stored knowledge and experience
• retinal image doesn’t provide sufficient info
• (Helmholtz) - relates to unconscious interference, likelihood principle, most likely outcome seen
• unconscious interference: involuntary, pre-rational and reflex-like mechanisms - to form visual impressions
• bottom-up AND top-down

bit vague