Form Perception

Question 1

What is Marr’s approach to visual perception concerned with?

Answer 1

concerned with the representation of edges, contours and other areas of contrast change

Question 2

What is Gestalt’s approach to visual perception concerned with?

Answer 2

Rules of perceptual organisation

Question 3

Give features of Marr’s approach

Answer 3

• ‘Bottom up’ approach
• Starts with input to perceptual system in form of retinal image and describes the stages in processing of this image
• Each stage takes as its input the information from the previous stage and transforms it into a more complex description or representation
• Computational model

Question 4

What is a computational model?

Answer 4

• Computational theory: what is the model trying to do? What are the processes for? What is the goal?
• Algorithmic level: what algorithm is needed? What process?
• Mechanism level: What mechanism is needed to implement the algorithm – biological, neural mechanism

Question 5

What are Marr’s stages in his model?

Answer 5

• Retinal image
• Grey level description – measuring intensity of light at each point in the image
• Primal sketch – representation of abrupt contrast change (blobs, edges, bars etc) over range of spatial frequencies
• 21/2D sketch – representation of orientation, depth, colour relative to the observer
• 3D representation – Representation of objects independent of observer (only stage we are aware of)

Question 6

How is the 2 1/2 2D sketch turned into a 3D representation?

Answer 6

• 2 ½ sketch analysed for 3D volume primitives (cylinders, cones, etc.)
• Produced 3D representation that is independent of observer
• Uses memory experience of world
• E.g. if looking at a nose you will know that it sticks out
• Conscious experience of vision

Question 7

Why is the computational approach important?

Answer 7

• An algorithm/ rule/ system is more likely to be understood by understanding the problem that has to be solved, rather than examining the mechanism (and hardware) in which it is embodied
• To understand perception (purely) by studying neurons is like trying to understand bird flight by studying only feathers: function not form (AI argument)
• Neurons may not only be the thing that can carry out human processes

Question 8

What is Gestalt Psychology?

Answer 8

the whole is greater (different) than the sum of its parts (Max Wertheimer, 1912)

Question 9

What is the Gestalt school?

Answer 9

• Max Wertheimer
• Kurt Koffka
• Wolfgang Kohler
• Series of experiment on Kohler and Koffka by Wertheimer
• Together they developed the Gestalt school
• Thought you needed to see problems as a whole

Question 10

What is the Gestalt approach?

Answer 10

• The whole is different to the sum of its parts
• Don’t see lines and figures, but forms and shapes
• Top-down approach

Question 11

What is perceptual organisation (Gestalt school)?

Answer 11

• Ambiguity generally does not arise in the real world. We usually see a stable and organised world
• For example, most people see a set of overlapping circles, rather than one circle touching two adjoining shapes that have ‘bites’ taken out of them. Why?
• Argue that we see objects according to their elements taken as a whole
• Sought to isolate principles of perception: seemingly innate ‘laws’ which determine ways in which objects are perceived
• The unified whole is different from the sum of the parts, e.g. a bike

Question 12

What are the 8 Gestalt laws of perceptual organisation?

Answer 12

• Similarity
• Good continuation
• Proximity
• Connectedness
• Closure
• Common fate
• Familiarity
• Invariance

Question 13

What is similarity?

Answer 13

• Similar things appear to be grouped together

- Grouping can occur due to shape, lightness, hue, orientation, size

Question 14

What is good continuation?

Answer 14

• Points that, when connected, result in straight or smoothly curving lines, are seen as belonging together, and the lines tend to follow the smoothest path. (we tend to continue lines)
• Reification – more spatial info than is present

Question 15

What is Reification?

Answer 15

When we perceive more spatial information than is present

Question 16

What is proximity?

Answer 16

Things that are near to one another appear to be grouped together

Question 17

What is connectedness?

Answer 17

Things that are physically connected are perceived as a unit

Question 18

What is closure?

Answer 18

• Of several geometrically possible perceptual organisations, a closed figure will be preferred to an open figure
• We tend to ‘complete’ a broken figure because of strong closure cue for what we see
• Reification

Question 19

What is Common fate?

Answer 19

• Things that are moving in the same direction are grouped together
• Objects with same orientation are grouped together

Question 20

What is familiarity?

Answer 20

Things are more likely to form groups if the groups appear familiar or meaningful

Question 21

What is invariance?

Answer 21

• Recognise an object even if they are presented in different orientations
• Major problem in computer vision – CAPTCHA Test – completely automated public turing test to tell computer and humans apart

Question 22

What is Pragnanz?

Answer 22

• The central law of Gestalt Psychology
• Many of the laws are manifestations of Pragnaz
• What is the most likely explanation of sensory input
• ‘Of several geometrically possible organisations that one will occur which posses the best, simplest and most stable shape’

Question 23

What is figure-ground segregation and why is Gestalt interested in this?

Answer 23

• Interested in how we separate figure from ground
• Usually no doubt – but some reversible figure-ground patterns.
• Normally in a visual scene some objects (figures) seem prominent and other aspects of field recede into the background (ground)
• Lecturer (figure), other objects (background)
• Gestalt interested in this because it infers top-down processes

Question 24

What are the properties that affect whether an area is seen as figure or ground?

Answer 24

• Symmetry: symmetrical areas usually figure
• Convexity: convex shapes usually figure
• Area: stimuli with comparatively smaller area usually figure
• Orientation: vertical and horizontal orientations usually figure
• Meaning/Importance: meaningful objects are more likely to be seen as figure. Implies attention – top down

Question 25

What are the problems with the Gestalt approach?

Answer 25

- Underplay the parallel processing and unconscious processing that the brain does - Explanation of how some of their laws worked was wrong - Their laws provide a description of how things work rather than an explanation - Their laws are ill defined - Stating the obvious?

Question 26

What are some positive points about the Gestalt approach?

Answer 26

- Laws appear to generally be correct - Percept’s can be analysed into basic elements - The whole is greater than the sum of its parts - Context and experience effect perception

Question 27

What is bottom-up versus top-down processing?

Answer 27

- Bottom up: start from the bottom, considering physical stimuli being perceived and then work their way up to higher cognitive processes - Top down: perceiver constructs a cognitive understanding of a stimulus

Question 28

During perception what do we form and test hypotheses regarding percept's based on?

Answer 28

- What we sense (sensory data) - What we know (knowledge stored in memory) - What we can infer (using thinking) - What we expect