Stimuli will be 'better' exemplars of a category the more closely related they are to the category's exemplars (Dopkins & Gleason, 1997, p.1). Categories are "mentally represented simply as collections of exemplars; categories themselves have no independent representation" and there is no prototype (Dopkins & Gleason, 1997, p.3). In prototype theory, the prototype of a category does not have to correspond to an actually perceived exemplar but can exist as an "abstract, idealized prototype" (Mobius 2004, p.11). This is a "controversial assumption" given that it assumes a level of abstract, one could higher-order thinking that is arguably only be present in the minds of individuals with certain types of education, past a particular developmental stage, or of particular cultural mindsets (Mobius 2004, p.11).

Exemplar theory has also been called 'cloud' theory in that each category of exemplars is represented in memory by a "cloud of remembered exemplars" (Mobius 2004, p.3). "The exemplar cloud is continuously updated by new speech events" (Mobius 2004, p.3). Random selection of matching, however, "causes increasingly broader, fuzzy-edged categories" that are, in their own way, just as flexible as the typicality of prototype theories (Mobius 2004, p.5). Exemplar theory very explicitly 'maps' the brain, stating that each new stimulus encoded as a location on the organism's cognitive map, and while areas of map are associated with specific functions, stimuli are "categorized on the basis of most similarity to the most exemplars near that location" not according to prototypical types (Pierrehumbert, 2003, p.8)

In contrast, in the cognitive map of prototype models," the mental representation of a category is its prototype," and in this singular model the prototype is the paradigmatic "exemplar with average values on all of the dimensions along which the category's exemplars vary…In most prototype models, a category's prototype is mentally represented as a point in a psychological space. The dimensions of this space correspond to the perceptual dimensions along which the category's exemplars vary" (Dopkins & Gleason,...

But one of the difficulties in testing prototype theory against exemplar theory is the different ways in which a 'prototype' may be defined. Even within its community of its advocates, there is controversy about the nature of the prototypical model. In distance models of prototypes "the crucial quantity is the distance between the points that correspond to the stimulus and the prototype in psychological space" in terms of whether an example is deemed prototypical (Dopkins & Gleason, 1997, p.2). "In similarity models, the crucial quantity is the degree of similarity between the stimulus and the prototype" (Dopkins & Gleason, 1997, p.2). However, both prototype and exemplar theories remain controversial within the psychological field of cognitive learning, given the difficulty of translating observable phenomena of classificatory behavior into quantitative data derived from test subjects.
Works Cited

Dopkins, Stephen & Theresa Gleason. (1997). "Comparing exemplar and prototype models of categorization." Canadian Journal of Experimental Psychology. FindArticles.com.

Retrieved July 12, 2009 at http://findarticles.com/p/articles/mi_qa3690/is_199709/ai_n8781479/

Green, Stuart. (2000, January 19). Prototype theory and the classification of offenses in a revised model penal code: A general approach to the special part. Buffalo Criminal Law Review. 4:301. Retrieved July 12, 2009 at http://wings.buffalo.edu/law/bclc/bclrarticles/4(1)/greenpdf.pdf

Mobius, Bernd. (2004). Exemplar Theory. GK: Phonetics and Phonology. University of Stuttgart. Retrieved July 12, 2009 at http://www.ims.uni-stuttgart.de/lehre/teaching/2004-SS/GK-Phon/exemplartheory4.pdf

Pierrehumbert, Janet. (2003, January 3). Exemplar theory. Probability theory in linguistics 2:

LSA. Northwestern University. Retrieved July 12, 2009 at http://staff.science.uva.nl/~rens/janet.pdf

Zaki, Safa, R., Robert M. Nosofsky, R.D. Stanton, A.L. Cohen. (2003, Nov). Selective attention, and stimulus generalization. Journal of Experimental Psychology, Learning, Memory and Cognition. 29(6):1160-73.