object recognition

Question 1

perception

Answer 1

• Perception refers to our ability to extract meaning from sensory input.
  
  • It includes audition, taste, touch and olfaction (smell), but research is dominated by vision.
  • Vision alone accounts for over 50% of all neurons in our cortex.
  • How many senses do we have? - 9-22
• PERCEPTION IS A CONSTRUCTIIVE PROCESS!!!!!

Question 2

visual system

Answer 2

• Brain receives sensory input allowing us to perceive an object.
• Cognitive system “constructs” perception.

Question 3

object recognition - a 3 stage model

Answer 3

• Image
  
  • Stage 1 - local features - edge detection/contrast, etc
  • Stage 2 - shape representation - Gestalt principles/feature integration
• Stage 3 - object representation - stored representations/knowledge.

Question 4

gestalt principles

Answer 4

• The whole visual percept is more than the sum of its parts.
  
  • Our perceptual system constantly tries to impose organisation on its input.
  • Components of an image are grouped together on the basis of certain visual properties.
• Laws of perceptual organisation e.g., “good continuation” and “closure” give rise to “illusory contours”.

Question 5

shape perception

Answer 5

• Primarily “bottom up” processes produce a “primal sketch” (Marr, 1982).
  
  • This sketch contains “primitives” - edges/ orientations/ positions/ lengths/ colours etc.
• “Top down” processes (e.g., Gestalt laws) are used to group collections of primitives together into “lines, curves, larger blobs, groups and small patches” - symbolic primitives.

Question 6

object recognition - 3 models

Answer 6

• Template matching (prototype theory).
  
  • Feature analysis
• Recognition by components (structural theory)

Question 7

template matching

Answer 7

• A template is an internal representation.
  
  • A memory against which the visual input is matched.
  • Computer based object recognition programmes use templates.
  • Intuitively plausible - object recognition must involve some kind of contact with a “comparable internal form”.
  • But what rules determine whether a match is made?
• How many different templates are needed?

Question 8

feature analysis

Answer 8

• Assumes lower level features are analysed first.
  
  • The perceptual system searches for simple characteristic features of an object.
  • Supported by neurological evidence (e.g., orientation selective cells in visual cortex).
  • Most research focusses on letter/ word recognition.
  • Letter A made up a / \ and -
  • But what about V and X? both made of / and ... Spatial relationship importants.
• What about complex natural objects?

Question 9

recognition by components

Answer 9

• Any specific view of an object can be represented as an arrangement of simple 3D shapes - geons (Biederman, 1987)
  
  • Geons are “viewpoint invariant” - easily “recoverable” from a 2D retinal image.
  • “Invariant properties” include cotermination and parallelism.
• Object recognition is impaired when geons are made “non-recoverable” by removing termination points.

Question 10

summary

Answer 10

• Object recognition is rapid and apparently effortless.
  
  • It is a constructive process, not a passive one.
  • It combines “bottom up” processes (e.g., edge detection) with top down processes (e.g., expectations/ memory).
  • A number pf models of object recognition have been put forward.
  • These models are not necessarily mutually, exclusive, and object recognition may well involve elements of template matching/ feature analysis/ recognition by components.
• Different strategies may be used in different situations.