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

Back to Gestalt...

Back to Gestalt...

Back to Gestalt...