Available online at www.sciencedirect.com Symbolic representation of number in chimpanzees Tetsuro Matsuzawa This paper aims to summarize the existing evidence for the chimpanzees possess for numerals. Fourth, it introduces symbolic representation of number in chimpanzees. common characteristics of the difficulties experienced by Chimpanzees can represent, to some extent, both the cardinal chimpanzees in various kinds of cognitive tasks. Finally, and the ordinal aspect of number. Through the medium of it discusses the number concept within the larger context Arabic numerals we compared working memory in humans and of cognition, illuminating cognitive development in chimpanzees using the same apparatus and following the same chimpanzees and the unique features of human cognition. procedure. Three young chimpanzees outperformed human adults in memorizing briefly presented numerals. However, we The Ai project: psychophysical measurement found that chimpanzees were less proficient at a variety of and the underlying logic other cognitive tasks including imitation, cross-modal The Ai project – encompassing a series of studies whose matching, symmetry of symbols and referents, and one-to-one principal subject has been a female chimpanzee named Ai correspondence. In sum, chimpanzees do not possess human- – began when Ai, at the age of 1.5 years, first touched an like capabilities for representation at an abstract level. The experimental keyboard on April 15th 1978 [3–5]. The last present paper will discuss the constraints of the number in a succession of ape-language projects launched in the concept in chimpanzees, and illuminate some unique features second half of the 20th century, the Ai-project was also of human cognition. the forerunner of a new research paradigm called Com- parative Cognitive Science (CCS) [6]. CCS is the com- Address bined study of psychophysics and ape-language, using Primate Research Institute, Kyoto University, 41 Kanrin, Inuyama, Aichi computer-controlled apparatus. In its original form, CCS 484-8506, Japan aimed to compare perception, memory, and cognition in Corresponding author: Matsuzawa, Tetsuro humans with those of closely related species such as ([email protected]) chimpanzees. Crucially, such inter-species comparisons rely on identical methods: subjects, irrespective of species, use the same apparatus and follow the same Current Opinion in Neurobiology 2009, 19:92–98 procedure during testing. This review comes from a themed issue on Cognitive neuroscience Psychophysics is the classic psychological study of Edited by Michael Platt and Elizabeth Spelke measuring human sensation, perception, memory, and cognition in general. Psychophysics tries to unravel the Available online 14th May 2009 relationship between physical events and psychological 0959-4388/$ – see front matter events. Psychophysical researchers have established # 2009 Elsevier Ltd. All rights reserved. many laws, including, for example, Weber–Fechner’s DOI 10.1016/j.conb.2009.04.007 law and Steven’s power law. There exist well-established methods for measuring the relationship between the stimuli presented and the corresponding internal psycho- logical states. Thus, psychophysics can be extended to Introduction issues related to concept formation, such as the under- Using the Arabic numerals 0 through 9, the chimpanzee standing of number. Ai can represent both the cardinal and the ordinal aspect of number to some extent [1]. Similar skills have now The Ai project has two important features: the research been partially confirmed in six other chimpanzees [2,3]. presents humans and chimpanzees with exactly the same A recent study has used numerical stimuli to demonstrate tasks so that their performance can be compared in detail, an extraordinary memory capability in young chimpan- and that the research undertakes that comparison by zees [2]. In contrast with the memory test, chimpanzees using the methods of psychophysics. There is the syner- show poor performance in many other cognitive tasks that gistic connection between these two features for the require some form of abstraction. The present paper will enterprise of CCS. CCS aims to compare different species address the following five topics. First, it describes the at the level of cognitive mechanisms, and it is the great logic and framework for the study of the number concept virtue of psychophysical method that serves not only to in nonhuman animals. Second, it reviews the long-run- describe the performance of humans and nonhuman ning Ai project, focusing in particular on studies dealing animals but also to analyze that performance into its with the symbolic representation of number. Third, it component mechanisms. By adopting psychophysical describes the extraordinary memory capacity that young approach, you can go beyond questions of what trained Current Opinion in Neurobiology 2009, 19:92–98 www.sciencedirect.com Symbolic representation of number in chimpanzees Matsuzawa 93 chimpanzees can do and ask how chimpanzees and enumeration, quantity discrimination, and sequential humans do what they do: What cognitive capacities are responding. Today, much attention continues to be paid shared by the two species and what capacities distinguish to the concept of number in nonhuman animals [16,17]. them. Various researchers have published studies that they described as tests of numerical competence in macaque Ai was the first chimpanzee who learned to discriminate monkeys [18–20], cotton-top tamarin [21], elephant [22], the 26 letters of the alphabet during tests of shape parrot [23], pigeons [24], even salamanders [25] and bees perception and visual acuity [7]. She also mastered [26]. However, none of these compared the performance additional visual symbols such as specially devised geo- of human and nonhuman subjects directly, using the metric shapes (called lexigrams) and Japanese Kanji same methodology. Almost all the literature on nonhu- characters signifying different colors, in the course of man animals has focused on rudimentary forms of the experiments designed to examine her classification of number concept. By contrast, the Ai-project’s series of various colors [8]. number concept studies stands out clearly for two reasons. First, it focuses on the symbolic representation of number Ever since the beginning of the Ai-project, my colleagues using Arabic numerals. This symbolic representation and I have been focusing on the mathematical skills of allowed us to conduct direct comparisons between chimpanzees rather than on any form of bilateral com- humans and chimpanzees. Second, we applied psycho- munication between humans and chimpanzees—the latter physical measurements using a fully automated compu- having been the central theme of previous ape-language ter-controlled apparatus that completely excluded any studies. There were four reasons why we decided to kind of social cueing. concentrate on issues related to number concepts. The fourth reason why we decided to concentrate on First, the world of logico-mathematical skills was small. issues related to number concepts is the connection to the Mathematical cognition is a part of human cognition in psychophysical approach. Most domains of human cogni- general, but the number system is narrower and more tion are not yet amenable to psychophysical analysis, clearly and precisely defined in comparison to the lin- because the proper mathematical characterization of guistic system. The number system includes the levels of the cognitive domain is not clear. Number, however, is Integer that is a part of Real number, and that is a part of highly amenable to psychophysical analysis. Indeed, the Complex number. The scales of the number system Weber–Fechner’s law of psychophysics holds in the advance from Nominal to Ordinal, Interval, and Ratio numerical domain. scales. Therefore, focusing on the Integer, for example, we can examine both cardinal aspects (‘one, two, three, Cardinal and ordinal aspects of number and four...’) and ordinal aspects (‘first, second, third, introducing zero fourth...’) of the number concept in chimpanzees. From Ai is the first chimpanzee who mastered the use of Arabic the perspective of future work, we can examine the numerals to represent numbers [4]. She learned both combination of numerical elements to create new mean- cardinal and ordinal aspects of the number system. She ing: such as putting 1 and 0 together to create ‘10’ in the used the numerals to label real-life items, shown to her in decimal number system. Moreover, further questions a display window, in terms of their numerosity. She also open up regarding numerical operations such as addition became proficient at responding to Arabic numerals in and subtraction. These kinds of numerical manipulations ascending order, selecting them from a touch-screen in can be viewed as corresponding to the syntactical struc- the correct sequence. When the numeral ‘0’ was intro- ture of the language system. duced, Ai mastered its use in both the cardinal and the ordinal domains; however, the acquisition process and the Our second reason for focusing on number concepts was generalization tests in this case clearly highlighted the the accumulation of knowledge about mathematical constraints of the symbolic representation of number thinking in human subjects. Several important papers acquired by the chimpanzee [1]. Further detail on Ai’s had just been published about the child’s understanding experience with numerals follows below. of number, including Gelman and Gallistel’s