Categorization is essential for survival, and it is a widely studied cognitive adaptation in humans and animals. An influential neuroscience perspective differentiates in humans an explicit, rule-based categorization system from an implicit system that slowly associates response outputs to different regions of perceptual space. This perspective is being extended to study categorization in other vertebrate species, using category tasks that have a one-dimensional, rule-based solution or a two-dimensional, information-integration solution. Humans, macaques, and capuchin monkeys strongly dimensionalize perceptual stimuli and learn rule-based tasks more quickly. In sharp contrast, pigeons learn these two tasks equally quickly. Pigeons represent a cognitive system in which the commitment to dimensional analysis and category rules was not strongly made. Their results may reveal the character of the ancestral vertebrate categorization system from which that of primates emerged. The primate results establish continuity with human cognition, suggesting that nonhuman primates share aspects of humans' capacity for explicit cognition. The emergence of dimensional analysis and rule learning could have been an important step in primates' cognitive evolution.

The controversy over multiple category-learning systems is reminiscent of the controversy over multiple memory systems. Researchers continue to seek paradigms to sharply dissociate explicit category-learning processes (featuring verbalizeable category rules) from implicit category-learning processes (featuring learned stimulus-response associations that lie outside of declarative cognition). We contribute a new dissociative paradigm, adapting from comparative psychology the technique of deferred-rearranged reinforcement. Participants learned matched category tasks that had either a one-dimensional, rule-based solution or a multidimensional, information-integration solution. They received feedback only after each block of trials, with their positive outcomes grouped and their negative outcomes grouped. Deferred-rearranged reinforcement qualitatively eliminated implicit, information-integration category learning. It left intact explicit, rule-based category learning. Moreover, implicit category learners—facing deferred-rearranged reinforcement—turned by default and information-processing necessity to rule-based strategies that poorly suited their nominal category task. The results represent one of the strongest explicit-implicit dissociations yet seen in the categorization literature.

Some studies of nonhuman animals’ metacognitive capacity encourage competing low-level, behavioral descriptions of trial-decline responses by animals in uncertainty-monitoring tasks. To evaluate the force of these behavioral descriptions, six capuchin monkeys were presented with two density-discrimination tasks between sparse and dense stimuli. In one task, difficult trials with stimuli near the middle of the density continuum could be declined through an “uncertainty” response. In the other task, making a “middle” response to the same stimuli was rewarded. In Experiment 1, capuchins essentially did not use the uncertainty response, but they did use the middle response. In Experiment 2, we replicated this result with 5 of 6 monkeys while equating the overall pace and reinforcement structure of the two tasks, although one monkey also showed appropriate use of the uncertainty response. These results challenge a purely associative interpretation of some uncertainty-monitoring performances by animals, while sharpening the theoretical question concerning the nature of the psychological signal that occasions uncertainty responses.

An influential theoretical perspective differentiates in humans an explicit, rule-based system of category learning from an implicit system that slowly associates different regions of perceptual space with different response outputs. This perspective was extended for the first time to the category learning of nonhuman primates. Humans and macaques learned categories composed of sine-wave gratings that varied across trials in bar width and bar orientation. The categories had either a singledimensional, rule-based solution or a two-dimensional, information-integration solution. Humans strongly dimensionalized the stimuli and learned the rule-based task far more quickly. Six macaques showed the same performance advantage in the rule-based task. In humans, rule-based category learning is linked to explicit cognition, consciousness, and to declarative reports about the contents of cognition. The present results demonstrate an empirical continuity between human and nonhuman primate cognition, suggesting that nonhuman primates may have some structural components of humans' capacity for explicit cognition.Keywords category learning; implicit/explicit cognition; primate cognition; comparative cognition; rhesus monkeys Learning and using categories is a basic cognitive function for humans and animals. Consequently, categorization is a focus of research involving humans (Ashby & Maddox, 2005;Brooks, 1978;Murphy, 2003; Nosofsky, 1987;Rosch & Mervis, 1975; Correspondence concerning this article should be addressed to J. David Smith, 346 Park Hall, SUNY Buffalo, Buffalo, NY 14260 psysmith@buffalo.edu. Publisher's Disclaimer: The following manuscript is the final accepted manuscript. It has not been subjected to the final copyediting, fact-checking, and proofreading required for formal publication. It is not the definitive, publisher-authenticated version. The American Psychological Association and its Council of Editors disclaim any responsibility or liabilities for errors or omissions of this manuscript version, any version derived from this manuscript by NIH, or other third parties. The published version is available at www.apa.org/pubs/journals/xan. NIH Public AccessAuthor Manuscript J Exp Psychol Anim Behav Process. Author manuscript; available in PMC 2011 January 1. NIH-PA Author ManuscriptNIH-PA Author Manuscript NIH-PA Author Manuscript 1998) and animals (Cerella, 1979;Herrnstein, Loveland, & Cable, 1976;Jitsumori, 1994;Lea & Ryan, 1990;Smith, Redford, & Haas, 2008;Vauclair, 2002;Wasserman, Kiedinger, & Bhatt, 1988).Early categorization theories assumed that organisms apply a single category-learning system to all category problems. Different descriptions were offered for this system (e.g., Medin & Schaffer, 1978;Reed, 1972). In hindsight, it was predictable that categorization would not be so simple and unitary. Categorization is an important enough capacity that it might deserve (and receive) distributed and varied expression in cognition. Fortunately, many researchers transcended the "single system" claim a...

Researchers have begun to evaluate whether nonhuman animals share humans' capacity for metacognitive monitoring and self-regulation. Using perception, memory, numerical, and foraging paradigms, they have tested apes, capuchins, a dolphin, macaques, pigeons, and rats. However, recent theoretical and formal-modeling work has confirmed that some paradigms allow the criticism that low-level associative mechanisms could create the appearance of uncertainty monitoring in animals. This possibility has become a central issue as researchers reflect on existing phenomena and pause to evaluate the area's current status. The present authors discuss the associative question and offer our evaluation of the field. Associative mechanisms explain poorly some of the area's important results. The next phase of research in this area should consolidate the gains achieved by those results and work toward a theoretical understanding of the cognitive and decisional (not associative) capacities that animals show in some of the referent experiments.Humans feel uncertain. They know when they do not know or remember, and they respond well to uncertainty by deferring response and seeking help or information. Their responses to doubt and uncertainty ground research on metacognition, or thinking about thinking (Benjamin et al.