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?

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)

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

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.

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

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...

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

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.

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

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.

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

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

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”

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:

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?

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

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?

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

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??]

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)

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

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

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

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

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

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