Avian Visual Cognition

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IV. Similarity and Categorization

A note on similarity and categorization

A word is in order about the relation of similarity to several terms that have appeared fairly often in the avian literature including "categorization", "categorical perception" and "concept".   Much has been written about all of these; a particularly lucid treatment is given by Smith and Medin (1981).  Other chapters in this book introduce the reader to some of this material in the avian context; see especially chapters by Huber (2001), Urcuioli, (2001) and by Young & Wasserman (2001)

Categorization is necessary because the world offers a limited number of  behavioral options.  Perhaps each seed on the ground is unique, but a bird has to decide what to do with it.  When a bird eats it we can say that the item has been categorized as "food".   From the present point of view, similarity underlies this categorization;  the bird computes the similarity of the item to previously experienced (or innate?) representations that have the "food" tag;  if the computed value exceeds a threshold or criterion, the bird eats. 
Items that are categorized in this way are apt to be similar to each other by the measures treated in earlier sections, for example falling into the same cluster.  However, the presence of such groups does not alone justify our saying that an animal categorizes the grouped items together.  After all, computer programs will produce clusters from purely random data.  Only if an animal does something different in the presence of members of one cluster than it does in the presence of members of other clusters does it seem appropriate to use the summary term “categorization”.  For example,  the calls of  Dooling’s budgerigars fell into “alarm”,  “contact” and other similarity clusters, but in addition the birds responded in the same way to members of a given cluster, and this response differed between clusters.   Even if these conditions are met,  one need not assume an internal process that represents a category as such,  as opposed, for example, to a set of  stored exemplars that participate in the criterion/decision process suggested above.  (See Smith & Medin (1981) for a discussion of different representations of categories.)

The case is somewhat different for “categorical perception”, where the human observer has the distinct impression of perceiving an object in one way or another, as opposed to behaving  in one way or another to objects that may function similarly but are perceptually distinct.  Examples are provided by reversible figures and phonemic differences (e.g. the switch from “ba” to “pa”), where at some point during a gradual change in the physical stimulus (or an attentional shift by the observer) there is a sudden flip from one appearance to another.  There should be high similarity between items on the same side of the "flip" discontinuity,  and  much lower similarity between items on different sides of it.  It is difficult to establish categorical perception in humans; I know of no established instance in non-human subjects.

According to this analysis, categories are defined by a common behavioral response, and similarity plays a role in determining the occurrence of this response.  Just what additional meaning, if any,  "concept" adds to "category" for non-verbal species has been controversial.  For example, just because a bird categorizes some items as food, there seems no reason to say that it  "has a concept of food", and it is difficult to say just what one would have to observe to justify  this statement.    To add to the confusion, some writers appear to use "category" and "concept"  interchangeably;  more often, "concept" is used when one is referring to abstract categories, which cannot be defined in terms of perceptual similarity alone (see, for example, the discussion by Young & Wasserman, this book, of the "same-different concept").  The resolution of this matter appears to lie elsewhere than in a discussion of perceptual similarity, and the reader is invited to consult the references given.  For a specific discussion of criteria for pigeon concepts, see Lea (1984).
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