Avian Visual Cognition

Young & Wasserman
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V. Conclusions 

Recent Related Work with Nonhuman Animals

Our research on abstract categorization in the pigeon has focused on the acquisition and nature of the same-different concept. We have identified variability as an important underlying dimension to this concept, demonstrated the generality of the concept, and are currently conducting further research into the nature of this concept. We are not alone, however, in our endeavors to understand the characteristics of the same-different concept in nonhuman species.

Research into the same-different concept in pigeons began with investigations using matching-to-sample (MTS) and non-matching-to-sample (NMTS) tasks (reviewed by Delius, 1994). In these tasks, the pigeon is first given a sample stimulus (e.g., a red light) and subsequently given a choice of two stimuli (e.g., a red light or a green light). In MTS, the pigeon must choose the stimulus that matches the sample (in this case the red light), whereas in NMTS the pigeon must choose the stimulus that does not match the sample (in this case the green light). Although it has been clearly demonstrated that pigeons can perform well during transfer tests (see Delius, 1994), MTS and NMTS tasks may not involve an understanding of same-different at the conceptual level. In fact, Premack (1976) has argued that strong evidence of the presence of a concept of same-different would require that an observer be able to assign arbitrary responses to items that were either the same as or different from one another. Our work follows a tradition of demonstrating that pigeons are intelligent enough to meet Premack's criterion for abstract conceptual behavior.

Wright, et al. (1983) trained both pigeons and rhesus monkeys to perform a same-different discrimination in which two identical slides required one response and two different slides required a second response. After considerable training, both of the monkeys learned the discrimination and showed near perfect transfer to novel item pairs. In contrast, none of the pigeons performed well on the transfer tests. This observation led Wright et al. (1983) to a rather pessimistic conclusion: "There is the possibility that monkeys can abstract the task and learn the Same/Different concept whereas pigeons cannot" (p. 316). Our research corroborates the difficulty of teaching pigeons to classify two items as same or different; but, we have further demonstrated that they can learn the concept when eight or more items are the same as or different from one another. Thus, the difference between pigeons and monkeys noted by Wright et al. (1983) appears to be a quantitative one and not a qualitative one.

Oddity detection. Cook and colleagues (e.g., Cook, 2001; Cook, Cavoto, & Cavoto, 1995; Cook, Katz, & Cavoto, 1997) have used a different paradigm to investigate same-different processing in the pigeon. They typically present pigeons with arrays containing a large collection of elements (e.g., 384); the pigeons' task is to choose one key ("Same") when the array is uniform (containing identical elements) and a second key ("Different") when a region of different elements (all identical to one another) is present on a uniform background. Their pigeons have little difficulty acquiring this discrimination, despite the fact that the entropy differences (0.0 for Same displays, somewhat less than 1.0 for Different displays) were smaller than that between our 2-item Same and Different displays (0.0 and 1.0, respectively), displays which our pigeons had great difficulty discriminating (Young, Wasserman & Garner, 1997).

These results and others (see Cook et al., 1997) reveal that pigeons learning our variability discrimination task and those learning Cook and colleagues' oddity detection task may have learned quite different rules. Whereas our task prompts discrimination based on variability, their task may prompt a more oddity-based evaluation of the same-different relations (see Cook et al., 1997, for a similar conclusion).

Relative numerosity. Honig and colleagues (1991; Honig & Matheson, 1995; Honig & Stewart, 1989) have reported evidence which suggests that pigeons are sensitive to the relative numerosity of two items arranged in multi-element arrays. One of their experimental methods represents a simpler version of the simultaneous same-different task that we have used. Honig and Matheson (1995, Experiment 2) used a uniform-mixture discrimination task that required the pigeons to peck at one side of the screen for uniform arrays, consisting of identically-colored squares, and the other side for mixture arrays, consisting of various proportions of two colors of squares. Honig (1991) and Honig and Matheson (1995) found that pigeons could learn to discriminate between uniform arrays and all of the various mixture arrays in the uniform-mixture task. In a reanalysis of their data (Young & Wasserman, 1997) we discovered that entropy, rather than relative numerosity, might be producing the pigeons' discriminative performance in the uniform-mixture task.

Matching same-different relations. Thompson, Oden, and Boysen (1997) have recently demonstrated that language-naive chimpanzees can judge relations between relations in a conceptual MTS task. This application of MTS differs from that previously used in studies of the same-different concept. The samples in Thompson et al.'s (1997) conceptual MTS problems consisted of either two identical items (e.g., AA) or two different items (e.g., AB). The chimpanzee was then required to choose from two alternatives: one of which consisted of two identical items (e.g., EE) and the other of which consisted of two different items (e.g., FG); none of the items appearing in the sample were members of the alternatives. This task thus required the chimpanzee to match a relation - which of the alternatives involved the same relation between its constituent items as that between the constituent items of the sample - and is thus decidedly more conceptual than the conventional MTS task. Thompson et al. (1997) found that chimpanzees can learn this task and, furthermore, that prior language training was not a prerequisite, although experience with arbitrary tokens that were consistently associated with abstract relations may be necessary.

Other species. In work involving the more traditional simultaneous MTS or NMTS tasks, ravens and gulls (Benjamini, 1983), and jackdaws, jays, and rooks (Wilson, Mackintosh, & Boakes, 1985) have all been documented to learn to choose the stimulus that matches the sample and to generalize this learning to novel stimuli. Furthermore, Pepperberg (1987) has trained a language-trained grey parrot (Alex) to identify the specific difference between objects (matter, shape, or color). Alex learned to properly identify the dimension on which the objects differed at well above chance levels and to generalize this knowledge to novel objects.

The bird's and the nonhuman primate's demonstrated abilities to learn and use an abstract relational concept make one thing clear: nonhuman animals are capable of conceptual feats heretofore thought to be solely within the ken of the human species.

Related Work with Humans 

In research conducted in our human laboratory (Young & Wasserman, 2001, in press), we were interested in quantifying people's sensitivity to visual display variability and in determining whether their discrimination of these displays would parallel that of the pigeons used in our earlier studies. In our first experiment (Experiment 1, Young & Wasserman, 2001), we used the same simultaneous training procedure previously used with pigeons. During a training phase, people were required to discriminate Same from Different arrays. In tests involving Mixture arrays of intermediate variability, we found that people's predominant inclination was to treat same vs. different displays categorically, not continuously. The majority of our participants (80%) overwhelmingly chose the "different" key on Different trials and Mixture trials (regardless of whether the Mixture involved low variability or high variability); participants chose the "same" key almost exclusively for the Same arrays. Thus, similar tasks prompted two quite different patterns of behavior in pigeons and in the majority of our human participants. There were, nevertheless, some people (20% of our participants) who did respond continuously to manipulations of display entropy.

In subsequent programmatic experiments, we sought to preclude the participants' use of a categorical same-different distinction so that the available contrast was in the degree of variability present. Participants in these experiments were required to discriminate among the Mixture arrays (each involving different levels of variability). After learning to discriminate between low (Entropy 1.0) and high (Entropy 3.0) variability displays (Experiment 2, Young & Wasserman, 2001), people responded to displays of intermediate or more extreme variability in close accord with their use of entropy as the discriminative dimension. After learning to discriminate between low (entropy less than 2.0) and high (entropy greater than 2.0) variability displays (Young & Wasserman, in press), people again responded in close accord with their use of entropy. Although people's use of entropy was only revealed when a categorical distinction between no variability and some variability was precluded, we did document that people and pigeons both rely on entropy to discriminate visual display variability.

The two species differed, however, in two fundamental ways. First, most people were inclined to treat the discrimination of intermediate degrees of variability as a categorical one (all same vs. some different), whereas all pigeons treated the discrimination as a continuous one. Second, although some training methods produced human variability discrimination that was based on entropy, a significant number of college student participants (and no pigeons) relied on relative entropy rather than on absolute entropy.

Relative entropy is the absolute entropy of a display divided by the maximum possible absolute entropy for a display comprising the same number of items. Thus, any display containing items that are all different from one another would have its maximum possible absolute entropy (producing a relative entropy of 1.0). The distinction between absolute and relative entropy is made evident by examining the effect of the number of items in a display on discriminative behavior. As previously reported (Young, Wasserman, & Garner, 1997), pigeons were increasingly likely to choose the "same" key as the number of items in a Different display was reduced. The absolute entropy of these arrays also decreases with the number of items. In contrast, relative entropy is 1.0 for all of the Different arrays regardless of the number of items comprising those displays. When tested with arrays involving between 2 and 16 items, approximately half of our human participants produced a response pattern consistent with a sensitivity to absolute entropy and half produced a response pattern consistent with a sensitivity to relative entropy.

Given the documented difficulty that pigeons have with discriminating displays involving two identical items from those involving two different items (a distinction between minimal and maximal relative entropy), it is likely that pigeons find it very difficult to compute entropy relative to some reference point (e.g., the maximal entropy possible given the number of items present, as is required to compute relative entropy).

Current and Future Directions

We have continued our investigations of same-different discrimination on multiple fronts. In Wasserman, Young, and Nolan (2000) we extended the notion of temporal organization (used in successive same-different discrimination, Young et al., 1999) into the domain of spatial organization in simultaneous same-different discrimination. Although the spatial organization of the items did have a small, but reliable effect on our pigeons' behavior, we demonstrated that this effect can be accounted for by assuming that the pigeons attended to a contiguous subset (approximately 6) of the 16 items.

In Young and Wasserman (2000), we have explored the possibility that the pigeon's simultaneous same-different discrimination might be driven by differences in perceptual regularities. Analyses of our Same and Different displays did in fact reveal a divergence in their perceptual regularity (as indexed by their spatial frequency profile). In order to control for these differences in a new experiment, we disrupted the regularities in Same arrays by jittering each of the icons; each icon was offset both vertically and horizontally by a small, random number of pixels. The jittering process successfully produced nearly identical spatial frequency spectra for the Same and Different arrays, but this manipulation had absolutely no effect on our pigeons' discriminative performance. A second experiment revealed that mixing orientations of the icons disrupted spatial regularity but not discriminative performance. These results, in conjunction with the pigeon's ability to discriminate variability in lists of icons (Young et al., 1999), strongly indicate that perceptual regularities are not the basis of the pigeon's same-different discrimination.

Finally, we have begun a collaborative research effort with Joel Fagot at the Center for Research in Cognitive Neuroscience in Marseille, France in which baboons are being trained and tested with a broad variety of simultaneous same-different discrimination tasks. Baboons show rapid learning (less than a week to reach criterion), strong transfer to novel items (only a 10% difference between training and testing displays), and continuous responding to same/different mixtures nearly identical to that observed in pigeons (Wasserman, Fagot, & Young, in press). Given the observed differences between the behavior of humans and pigeons in our tasks, the examination of another primate species is helping to determine the uniqueness of the human response profile.


Since its beginning a century ago, the science of comparative cognition has sought to discover commonalities of mind between humans and animals; it has also tried to provide a natural science account of human and animal cognition (Wasserman, 1993; Wasserman, 1997). This movement now enters its second century equipped with new and powerful methodological and technological tools for carrying on its inquiry.

Our research represents one part of the current effort to advance the science of comparative cognition. We combined a behavioral analysis of concepts (Wasserman & Astley, 1994) with information theory (Shannon & Weaver, 1949); used advanced computing and video technology; and, accepted the challenge of cognitivists to explore the most complex realms of memory and cognition instead of complacently concentrating on the most elementary domains of associative learning (Wasserman & Miller, 1997).

We are optimistic that this investigative strategy will continue to disclose new and important parallels between human and animal cognition, and that explanations for even the most advanced forms of memory and cognition can be made exceedingly precise and rigorous. A natural science of mind is not only possible, but necessary if we are to move beyond the stale conventions that humans are qualitatively different from animals and that mind is an inherently inscrutable entity. Finally, showing that even pigeons are capable of abstraction and that deterministic computational principles may lie at the root of this complex cognitive process should help stimulate neuroscientists to uncover the biological mechanisms of this amazing ability.

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