Overcoming response bias using symbolic representations of number by chimpanzees (Pan troglodytes)

Abstract

Wepreviously reported that chimpanzees were unable to optimally select the smaller of two candy arrays in order to receive a larger reward. When Arabic numerals were substituted for the candy arrays, animals who had had prior training with numerical symbols showed an immediate and significant improvement in performance and were able to select reliably the smaller numeric representation in order to obtain a larger reward. Poor performance with candy arrays was interpreted as reflecting a response bias toward the intrinsic incentive and/or perceptual features of the larger array. In contrast, the Arabic numerals represent numerosity symbolically and appear to promote response choice on the basis of abstract processing of numerosity, with minimal interference from the inherent properties of the choice stimuli. The present study tested the hypothesis that, for mixed symbol-candy choice pairs, the requisite processing of the abstract numeral may foster a mode of numerical judgment that diminishes the interfering incentive/perceptual effects of the candy stimuli. The results were consistent with this hypothesis. Whereas performance on candy-candy arrays was significantly below chance levels, performance on numeral-candy choice pairs was significantly above chance and comparable with performance on numeral-numeral pairs.Chimpanzees have been shown to demonstrate remarkable cognitive capabilities that are often strikingly reminiscent ofhuman reasoning abilities and conceptual skills (see, e.g., Boysen, Berntson, Shreyer, & Hannan, 1995;Fujita & Matsuzawa, 1986;Gillan, Premack, & Woodruff, 1981; Matsuzawa, 1985a Matsuzawa, , 1985bPremack, 1986; Savage-Rumbaugh, 1986). For example, they have been shown to have impressive capabilities in numerical processing, symbolic representation ofnumber, and even numerical reasoning that does not require the support of language, as typically defined (see, e.g., Boysen, 1997;, Boysen & Berntson, 1989 Matsuzawa, 1985b;Murofushi, 1997). Behavioral and cognitive differences nonetheless exist between chimpanzees and humans, and those differences can sometimes be more informative than similarities for the comparative understanding of cognitive function (Boysen, Berntson, Hannan, & Cacioppo, 1996). Several years ago, we encountered a rather striking training failure with two chimpanzees (Boysen & Berntson, 1995). The task required the seemingly simple choice between two different-sized candy arrays. A reversed reinforcement contingency was imposed, so that the candies in the selected array were removed and the animal received the remaining, nonchosen array as a reward. In order to maximize payoff, it was in the best interest of the chimpanzee subject to choose the dish with the smaller number of candies, in order to reap the larger remainder. Both of the animals in the preliminary study failed to do so, and even after hundreds of training trials, they persisted in selecting the larger candy array and, thus, consistently received the smaller amount of reward.To further explore their seeming...

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