This is a highly subjective
review of the literature concerning the mechanisms of visual categorization
in pigeons. In the first half of the review, it is suggested that the way
pigeons sort complex sets of visual stimuli into experimenter-defined categories
does not require conceptual abilities and is therefore accessible to simpler
lines of analysis such as associative learning theory. A few experiments,
conducted in our laboratory using carefully constructed stimulus sets,
are described in order to illustrate this point. In the second half of
the chapter, it will be shown that pigeon categorization studies suffer
from overly simplistic assumptions concerning the perceptual aspects of
natural categorization. Both the nature of natural visual classes and the
capacity of the pigeon's perceptual system to exploit these classes are
underrepresented in the psychologist's laboratory. In a recent series of
experiments, we found that pigeons were able to detect the diagnostic class
characteristics in the surface domain of natural images.
pigeon pecks rapidly at a small photograph of Harvard Yard containing trees,
buildings, people, sky. After a few seconds, a hopper of grain appears
and the pigeon eats. Now the scene changes to a treeless Manhattan street.
The bird emits a few desultory pecks, then turns away and paces about.
After a minute or so, a picture of a leafy suburban garden appears and
the bird begins pecking again."
The above quote describes a now famous
experiment in which pigeons demonstrated their ability to discriminate
complex classes of stimuli (Herrnstein, 1979). In this and other experiments
of its kind-- commonly called category or concept discriminations--pigeons
are presented with large sets of pictorial stimuli; ranging from simple
figures to more complex visual arrays including color photographs and even
real objects (Cabe, 1976; Lumsden, 1977; Delius, 1992; Watanabe, 1993).
The birds are trained, by means of operant procedures, to respond differentially
to the stimuli according to an experimenter-defined class rule (e.g. "tree"
and "non-tree" in the above example). This type of experiment has received
considerable attention not only because the results suggest that pigeons
possess an ability that transcends the discrimination of simple stimulus
dimensions such as wavelength, intensity, or frequency (Honig & Urcuioli,
1981; Mostofsky, 1965), but also because they imply that the pigeons' classification
behavior is mediated by abstract, or conceptual, rules, and therefore resembles
the cognitive solution accomplished by humans.
Categorization--as it is reviewed
here--lies intermediate between discrimination of elementary stimuli and
the linguistic manipulation of classes of objects, events or ideas by using
symbolic representations and by attaching to them verbal names. Categorization
can be viewed as the ability to treat similar, but not identical, things
as somehow equivalent, by sorting them into their proper categories and
by reacting to them in the same manner (Rosch, 1978; Medin & Smith,
1984; Harnard, 1987; Neisser, 1987). Considering the vast amount of information
arriving at the perceptual systems of mobile organisms and the few behavioral
output patterns possible in non-human animals, categorization may be conceptualized
as an adequate solution to this "informational bottleneck" (Delius, Siemann
& Jitsumori; 2000). The drastic information reduction is a quite fundamental
principle of cognitive economy and therefore widely dispersed among species.
The evolutionary pressures to minimize processing requirements in small
information-processing systems such as non-human animals are self-evident
(Cook, Wright & Kendrick, 1990). Nevertheless, the specific solutions
found by a wide range of species to compress the amount of information
to be retained vary at different levels, from the pure perceptual (selective
attention), to the level of learning or representation and, finally, to
the level of reasoning (see Marler, 1982; Premack, 1976).
At the upper end, language and--more
generally--the ability to form symbolic representations widens the bottleneck
in a countless number of ways. Humans can think about an indefinite variety
of things. Although the basic principles of categorization shade into human
cognition, and many "implicit" solutions found by humans to artificial
categorization problems are best explained in simple associative terms
(Mackintosh, 1995), language clearly involves far more than mere categorization.
This is even true for the language competences of great apes (Ristau &
Robbins, 1982). It would lead to a regrettable underestimation of their
competencies if they were interpreted as simple categorization and,
at the same time, to an overestimation of the distinctiveness of those
processes that give rise to the bottleneck solutions found in many other
species. While language depends on categorization, categorization does
not depend on language (Herrnstein, 1984). We therefore need an adequate
model system to investigate the "middle" range of categorization phenomena--lying
between simple discriminations and the formation of symbolic representations.
(Columba livia) obvious lack of language competencies, but extraordinary perceptual
render it an ideal species for investigating categorization in its more
general dimensions (Lea, 1984; Wasserman, 1991; Mackintosh, 1995).
Since the pioneering experiments
of Herrnstein and Loveland in 1964, psychologists have repeatedly demonstrated
that simple creatures like the pigeon can categorize stimulus classes containing
instances so variable that we cannot physically describe even the class
rule. These animals seem readily capable of categorizing images from natural
scenes; among the reported stimulus classes are aerial photographs (Skinner,
1960; Lubow, 1974), images from people (Herrnstein & Loveland, 1964),
pigeons (Poole & Lander, 1971; Watanabe, 1991), trees and bodies of
water (Herrnstein, Loveland & Cable, 1976), oak leaves (Cerella, 1979),
chairs, cars, humans, and flowers (Bhatt, Wasserman, Reynolds & Knauss,
here to see examples of their stimuli), birds and other animals (Roberts
& Mazmanian, 1988), and pictures of a geographic location (Wilkie,
Willson & Kardal, 1989). But the rather strange flexibility of pigeons
has been demonstrated by using defective pharmaceutical capsules or diodes
(Cumming, 1966; Verhave, 1966), letters of the alphabet (Morgan et al.,
1976; Lea & Ryan, 1983, 1990), line drawings of cartoon characters
(Cerella, 1980), squiggles (Vaughan & Greene, 1984), dot patterns (Watanabe,
1988), schematic faces (Huber & Lenz, 1993, 1996) and real faces (Jitsumori
& Yoshihara, 1997; Troje, Huber, Loidolt, Aust & Fieder, 1999),
color slides of paintings by Monet and Picasso (Watanabe, Sakamoto &
Wakita, 1995) and excerpts from famous pieces of classical music (Porter
& Neuringer, 1984).
In sum, the quite impressive list
of readily solved categorization problems by pigeons indicates a robust
perceptual/cognitive ability. This conclusion is justified even if we take
into account the surprising failures of pigeons to achieve a satisfactory
solution in seemingly similarly difficult tasks. For instance, there are
three unpublished experiments by Herrnstein (conducted 1970-1972) in which
pigeons failed to discriminate photographs of food cups, bottles, and wheeled
vehicles. Such failures only prove that pigeons are not limitlessly malleable
by reinforcement on the stimulus side (Lea & Ryan, 1990; Lea, Lohmann
& Ryan, 1993). Obviously more irritating in the above list of pigeon
categorization experiments is the inability of the experimenter to give
a straightforward answer to the most interesting question of how the pigeons
were able to succeed in all these complex experiments. Although the birds
readily learned to discriminate between the pictures shown during classification
training and also were able to generalize to other instances of the categories,
this tells us little or nothing about the perceptual or cognitive mechanisms
Consider, for instance, Herrnstein
and Loveland's (1964) groundbreaking demonstration of pigeons being able
to quickly sort color slides showing similar natural scenes according to
the "higher-order" concept "human being." This first experiment of
category discrimination remained typical in two respects, its procedure
and implication. Half of the slides contained human forms and half did
not, but otherwise the slides looked comparable (click
here to see some actual stimulus examples from their experiment). In
the presence of a human slide, pecking was intermittently reinforced with
brief access to food. In the presence of a non-human slide, pecking earned
no reinforcement. The pecking behavior in the presence of the two types
of stimuli became increasingly different as the pigeons learned the discrimination;
they frequently pecked in the presence of human slides and completely
withheld pecking in the presence of non-human slides.
Figure 1. Human or Non-Human?
The most interesting aspect
of this experiment was that the stimuli were not collected by any very
exact criteria but according to the natural language concept "human being
present"--in the experimenter's best judgement. In fact, positive and negative
instances varied in a large number of visual dimensions. Even the positive
feature (human being) varied in position, in number (from
a single person to groups of various sizes), in appearance (clothed, semi-nude,
or nude; adults or children; men or women; sitting, standing, or lying;
black, white, or yellow), in lighting and coloration, and so on. Each experimental
session consisted of the successive presentation of 40 positive and 40
negative instances in semi-arbitrary sequence. Over 1200 slides were shown
to the pigeons. In sum, the birds were required to detect human beings
in photographs constituting "a class of visual stimuli so diverse that
it precludes simple characterization" (Herrnstein & Loveland, 1964,
Unsurprisingly, the anthropocentric
approach led to an interpretation of the experiment--and hence a title
of the paper--in terms of "complex visual concept in the pigeon".
The outcome of the experiment had been contrasted with the well-known ability
of pigeons to discriminate stimuli differing in size, shape, or color.
There was no hint to suggest that the quick mastery of the concept task
arose from some trivial and unsuspected visual clue in the slides, or from
some non-visual property of the procedure. The authors concluded that the
subjects entered the experiment with some already existing general
Current evidence suggests that this
interpretation was premature because we do not know whether pigeons have
concepts and use them to solve category problems (Watanabe, Lea & Dittrich,
1993; Monen, Brenner & Reynaerts, 1998). During the early period of
pigeon categorization, testing the subjects with novel stimuli--as soon
as they have arrived at a satisfactory level of training performance--was
considered as the critical operation in order to test conceptual categorization.
Experimenters relied upon such transfer tests because successful generalization
of the novel stimuli indicates that the subjects have not memorized the
training pictures together with their psychological consequence. However,
evidence for anything more interesting than pure picture memorization is
not per se indicative of the acquisition or use of a "complex visual concept".
Indeed, Greene (1983)--in replicating the people/non-people experiment--provided
clear evidence that the pigeons had not only attended to the concept-relevant
features of the people instances; their classification behavior had also
been controlled by irrelevant features included in the "background".
In fact, perceptual mechanisms alone
may explain the results of these experiments. Although it is difficult
to prove that conceptual behavior is not involved in classification tasks, I agree with Chater and Heyes (1994) that the idea of a concept has not
yet been sufficiently decoupled from natural language to make this possible.
Furthermore, there is at present no coherent account of what animal concepts
might involve (clusters of features or anything more abstract or knowledge-based).
I do not want to continue a fruitless debate here (but see also Huber,
1995, 1998, and my recent account to this in Huber, 1999). I only want
to emphasize that the early study of visual categorization in pigeons has
a past that is strongly rooted in the experimental analysis of human conceptualization.
In contrast, the understanding of the "stimulus problem in nature" (Herrnstein,
1990; Fetterman, 1996) and the analysis of the actual performance of pigeons
in categorization experiments has been poorly developed in the early period
of animal categorization.
In the late seventies the focus of
pigeon categorization studies shifted towards the classification of much
simpler, albeit more carefully specified, sets of stimuli (Cerella, 1979;
Lea & Harrison, 1978; Morgan, Fitch, Holman & Lea, 1976). Taking
into account the explanations offered by classic theories of discrimination
and generalization, the consensus grew that pigeons were seldom doing anything
more complex than associating a large number of pictures, and/or the features
that they contained, with reward (Mackintosh, 2000). The main question
concerned the type of representation that was acquired during classification
learning. Categories may be temporary clusters of simpler mental categories
(usually called features) that recur together in the encounters of the
sensorium with the external world and so, by association, are stored together.
Or they may be the perceived pictures of all or many instances that maintain
their configural structure in the pigeons' vast memory stores. Finally,
categories may be prototypes that reflect the central tendency of any encountered
class of stimuli, and involve abstract class rules such as those that underlie
In this review, I will focus on the
research that my colleagues and I have conducted with pigeons since 1991.
Using carefully constructed sets of stimuli we started to test the three
main models of human categorization (that we adapted to the pigeon in the
Skinner box) and went forward to prove whether our main findings were applicable
to more natural stimulus classes (without losing control over their feature
content and distribution). I hope to show that today we have very good
reason to doubt that pigeon categorization is "a secret" (Herrnstein,
1985, p. 129), "utterly mysterious" (Marler, 1982, p. 87), or "shrouded
in mystery" (Premack, 1983, p. 357).
Despite the common tenet that categorization
is not a logical device, but a matter of assessing similarity, there is
little agreement concerning the level of abstraction from which the descriptors
of open-ended categories are obtained. After several decades of experimentation,
simulation, and theorizing, essentially three views have taken their place
in human psychological research: the exemplar, the feature,
and the prototype view. (Click
here to learn more about a fourth classical view of categorization).
These three theories can be applied
to perceptual categorization in animals and have, in fact, tacitly guided
its analysis. For example, Herrnstein's (1990) functional approach extended
the model troika only by adding pure discriminations to the "lower" end,
and relational concepts to the "upper" end. In between these two points,
learning by rote corresponds to the exemplar view, open-ended categorization
to the feature view, and the abstraction of concepts to the prototype view.
If the match between human models of categorization and Herrnstein's (1990)
functional approach is not perfect, then differential emphasis can be laid
on the details of processing, representation and learning. However, as
a conceptual framework for guiding discussion of our own, as well as other
closely related studies, the match is sufficiently precise. The next three
sections examine the research related to each of these different views
of categorical representation.