Pattern Recognition

1. Pattern Recognition Pattern - complex composition of sensory stimuli that the human observer may recognize as being a member of a class of objects Issue - what cognitive mechanisms need to be inferred to describe this process of recognition?

2. Bridge with Signal Detection • Detection of sensory stimuli - data driven • Perception of Patterns - conceptually driven • work from the bottom (identifying stuff in the world) to the top (thinking)

3. Necessary Terms and Concepts of Pattern Recognition • Serial and Parallel Processing • Serial or sequential processing means we process information one step at a time, where one process must be finished before the next can be started. • Parallel processing means we can process several tasks at one time

4. Necessary Terms and Concepts of Pattern Recognition • Bottom-up and Top-down processing • Bottom-up processing is similar to inductive reasoning. Basic data are combined into more complex forms. • Top-down processing is similar to deductive reasoning. Higher levels of processing affect lower level tasks. • The following gives examples of how we perceive visual patterns and how positioning or additional information affects our perception.

7. Theories of Perception 1. Gestalt (Canonic Processing)2. Bottom-Up vs. Top Down 3. Template matching 4. Feature Analysis Prototype Theory Form Perception

8. Gestalt Theory • Gestalt theorists are among the earliest to look at the problem of pattern recognition. • They postulate that we perceive stimuli as a whole pattern. That is, that individual parts have no meaning independent of the whole but combine to revel an identifiable pattern. • Gestalt theorists developed 5 rules of perception to explain their ideas...

9. Gestalt Laws 1. Law of Proximity: • Elements that are closer together will be perceived as a coherent object. • On the top, there appears to be three horizontal rows, while on the bottom, the grouping appears to be columns

10. Gestalt Laws • Law of Similarity: • Elements that look similar will be perceived as part of the same form. • There seems to be a triangle in the square.

11. Gestalt Laws • Law of Closure: • Humans tend to enclose a space by completing a contour and ignoring gaps in the figure

12. Gestalt Laws • Law of Prananz: • A stimulus will be organized into as good a figure as possible (symmetrical, simple, and regular) • The figure appears to the eye as a square overlapping triangle, not a combination of several complicated shapes.

13. Summary of Gestalt • Modern conclusion is that some level of “natural organization” of patterns is tied to the perceptual history of the subject • a function of the perceiver rather than the stimulus

14. Canonic Processing • Extension of Gestalt • the first images of an object that comes to mind when thinking of that particular form. • perspectives fluctuate with culture and time. • person from Los Angeles asked to think of a house might recall a one story, 3 bedroom stucco structure; a person living in a poverty-stricken Third world country might imagine a small hut made of tree branches held together with mud

19. Canonic Processing • Through common experience with objects, we develop memories of the most representational view (and gives most amount of info) • Studying this helps to understand form perception, prototype formation, economy of thinking, “visual shorthand”

20. Top-down vs. bottom up processing • Bottom-up processing consists of mental operations influenced by the physical properties of the stimulus. • Top-down processing consists of mental operations influenced by the results of processes already completed. • Reading the following requires both kinds of processing:

22. Reminder... • Problem - perception requires that information in the environment must be matched to internal information about the environment; however, the environmental information is subject to substantial variation. How do we recognize things in the face of this variability?

23. Template Matching • Template - internal constrict that, when matched by sensory stimuli, leads to the recognition of an object • Assumption: a retinal image of an object is faithfully transmitted to the brain and that an attempt is made to compare it directly to various stored patterns • details are vague

25. Template Matching • compare stimulus to large number of literal copies (templates) that are stored in memory to find match against all templates • works well with computers (check-sorting machines) • does not work well with humans -- too inflexible • does not account for similarities among objects • what is the effect of context?

26. Prototype Model • more flexible version of template model - the match does not have to be exact • match against “prototypical A” • advantages • manageable number of representations in memory can account for how people classify similar objects into a common category • disadvantages • lack of explicit information about how stimuli are compared to prototypes

27. Feature Analysis Model • Assumption: stimuli consist of combinations of elementary features; (e.g for the alphabet, features may include horizontal lines, vertical lines, diagonals, and curves) • make discriminations based on a small number of characteristics of stimuli • distinctive feature components stored in memory [a mini-template model??]

28. Feature Analysis Model • What is a feature? • A feature is a distinctive attribute or characteristic of a stimulus. e.g., 'T' has 2 features: ' - ' & '|' (E. Gibson, 1969)

31. Feature Analysis Model • Psychological Evidence: Gibson (1969) • decide whether or not two letters are different • takes longer to respond to P & R versus G & M • P & R share many critical features

32. Feature Analysis Model • Neurological Evidence: Hubel & Wiesel (1962) • microelectrodes in cats’ brains (visual cortex) • some neurons respond only to horizontal lines, others to diagonals... • similar evidence in monkeys (Maunsell & Newsome, 1987) • certain feature detectors are “wired” and help us identify features and simple patterns

33. Neisser example • - Look for the “X” O O P O P O P O P P O P P O P P P O O O P P O X P O P O O P O P O P O P P O P P O P P P O

34. Neisser example • Look for the “X” N N Z N Z N Z N Z Z N Z Z N Z Z N N N N N Z N X N Z N N N Z N Z N Z N Z Z N Z Z N Z Z N N

35. Feature Analysis • advantages • economical to store features in memory • experimental evidence consistent with features • disadvantages • lack of applicability to a wide range of stimuli • analysis of stimuli does not always begin with features • treats all features as equivalent

36. Back to Gestalt...

37. Back to Gestalt...

38. Back to Gestalt...