We present the results of two experiments that probe the ability of rhesus macaques to match visual arrays based on number. Three monkeys were first trained on a delayed match-to-sample paradigm (DMTS) to match stimuli on the basis of number and ignore continuous dimensions such as element size, cumulative surface area, and density. Monkeys were then tested in a numerical bisection experiment that required them to indicate whether a sample numerosity was closer to a small or large anchor value. Results indicated that, for two sets of anchor values with the same ratio, the probability of choosing the larger anchor value systematically increased with the sample number and the psychometric functions superimposed. A second experiment employed a numerical DMTS task in which the choice values contained an exact numerical match to the sample and a distracter that varied in number. Both accuracy and reaction time were modulated by the ratio between the correct numerical match and the distracter, as predicted by Weber's Law.
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Two out of three monkeys exhibited PSEs nearer to the geometric mean than the arithmetic mean (Monkey M, 3.51 and Monkey F, 3.04). Monkey S had a PSE of 4.86. We follow the convention of previous bisection studies in animals that have focused on group data/PSEs and have therefore not highlighted individual differences (Emmerton et al. 1997; Meck et al. 1985; Roberts and Mitchell 1994; Roberts 2005; Siegel 1986). Future studies with larger sample sizes should explore individual differences.
As in Experiment 1A, two out of the three monkeys had PSEs near the geometric mean (Monkey M, 5.05; Monkey F, 5.13; Monkey S, 7.96).
Latencies to touch the sample numerosity were not recorded in Experiment 1.
Monkey M developed motivation problems during Experiment 1, resulting in entire sessions during which she would not participate. These difficulties prevented her from serving as a subject in Experiment 2.
Average Weber fractions for each sample numerosity were as follows: 1, 0.229; 2, 0.395; 3, 0.498; 4, 0.560; 5, 0.591; 6, 0.597; 7, 0.582; 8, 0.549; 9, 0.500.
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We thank Jessica Cantlon, Patrick Cacchio, Evan Maclean, Pierre Rojas, and Jessica Ward for help with running subjects; Dr. Jamie Roitman for help with analyses; and Dr. Warren Meck for comments on an earlier draft of this manuscript. This work was supported by NICHD RO1 HD49912, an NSF CAREER award, and a Merck Scholars award to EMB and an NSF Graduate Fellowship to KEJ. The experiments described in this paper comply with IACUC standards for the care and use of laboratory animals at Duke.
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Jordan, K.E., Brannon, E.M. Weber's Law influences numerical representations in rhesus macaques (Macaca mulatta). Anim Cogn 9, 159–172 (2006). https://doi.org/10.1007/s10071-006-0017-8
- Numerical competence
- Nonhuman primate
- Weber's Law