Representational format determines numerical competence in monkeys

Abstract

A range of animal species possess an evolutionarily ancient system for representing number, which provides the foundation for simple arithmetical operations such as addition and numerical comparisons. Surprisingly, non-human primates tested in ecologically, highly valid quantity discrimination tasks using edible items often show a relatively low performance, suggesting that stimulus salience interferes with rational decision making. Here we show that quantity discrimination was indeed significantly enhanced when monkeys were tested with inedible items compared with food items (84 versus 69% correct). More importantly, when monkeys were tested with food, but rewarded with other food items, the accuracy was equally high (86%). The results indicate that the internal representation of the stimuli, not their physical quality, determined performance. Reward replacement apparently facilitated representation of the food items as signifiers for other foods, which in turn supported a higher acuity in decision making.

Figure 1. Percent of trials in which the larger quantity was chosen in the different test conditions (means and standard error of means).

Performance in the 'Food' condition was significantly worse than in the other two conditions (GLMM, N=16 subjects; P<0.001).

Figure 2. Effects of relative and absolute difference between choice stimuli on performance.

Percent of trials in which the larger amount was chosen (means and standard error of means) in relation to the ratio between the quantities presented for the three different conditions (a) Food, (b) Pebbles, (c) Food replaced; and in relation to the absolute difference between quantities (d) Food, (e) Pebbles, (f) Food replaced. There was a combined effect of relative and absolute difference on performance (GLMM, N=16 subjects, Effect of absolute difference P=0.0001, Effect of ratio P=0.0024).