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

We previously 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.