Form Perception Flashcards
What is Marr’s approach to visual perception concerned with?
concerned with the representation of edges, contours and other areas of contrast change
What is Gestalt’s approach to visual perception concerned with?
Rules of perceptual organisation
Give features of Marr’s approach
- ‘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
What is a computational model?
- 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
What are Marr’s stages in his model?
- 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)
How is the 2 1/2 2D sketch turned into a 3D representation?
- 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
Why is the computational approach important?
- 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
What is Gestalt Psychology?
the whole is greater (different) than the sum of its parts (Max Wertheimer, 1912)
What is the Gestalt school?
- 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
What is the Gestalt approach?
- The whole is different to the sum of its parts
- Don’t see lines and figures, but forms and shapes
- Top-down approach
What is perceptual organisation (Gestalt school)?
- 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
What are the 8 Gestalt laws of perceptual organisation?
- Similarity
- Good continuation
- Proximity
- Connectedness
- Closure
- Common fate
- Familiarity
- Invariance
What is similarity?
- Similar things appear to be grouped together
- Grouping can occur due to shape, lightness, hue, orientation, size
What is good continuation?
- 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
What is Reification?
When we perceive more spatial information than is present
What is proximity?
Things that are near to one another appear to be grouped together
What is connectedness?
Things that are physically connected are perceived as a unit
What is closure?
- 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
What is Common fate?
- Things that are moving in the same direction are grouped together
- Objects with same orientation are grouped together
What is familiarity?
Things are more likely to form groups if the groups appear familiar or meaningful
What is invariance?
- 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
What is Pragnanz?
- 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’
What is figure-ground segregation and why is Gestalt interested in this?
- 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
What are the properties that affect whether an area is seen as figure or ground?
- 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
What are the problems with the Gestalt approach?
- 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?
What are some positive points about the Gestalt approach?
- 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
What is bottom-up versus top-down processing?
- 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
During perception what do we form and test hypotheses regarding percept’s based on?
- What we sense (sensory data)
- What we know (knowledge stored in memory)
- What we can infer (using thinking)
- What we expect
