Representation of magnitude in rhesus monkeys (Macaca mulatta) Open Access

Diamond, Rachel (2014)

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Humans mentally represent numbers along a number line, and the direction of this representation is culturally influenced (Dehaene et al., 1993). Evidence for spatial organization of magnitude has been demonstrated in humans using the Spatial Numerical Association of Response Codes, or SNARC, task. Evidence from list learning tasks with monkeys has suggested a spatial organization of ordinal information by providing evidence for distance effects, and similar cognitive mechanisms might control both ordinal information and magnitude information. While the distance effect has been demonstrated in monkeys, the SNARC paradigm has not been directly evaluated. To the extent that spatial organization of magnitude information is an ancestral primate trait, monkeys, despite lacking cultural tools such as number lines, should show similar evidence of spatial organization. Because the direction of putative spatial representations cannot be known a priori, in Experiment 1 we trained monkeys on touchscreen computers to associate "small" magnitude items with a rightward response and "large" magnitude items with a leftward response to establish a right-left orientation of representational space. In Experiment 2, we evaluated whether monkeys use a spatial organization of magnitude information when processing numerosity by presenting them with a SNARC task. Monkeys learned to associate magnitude with left-right spatial locations, and demonstrated a SNARC effect congruent with the direction of the spatial training in Experiment 1. However, the SNARC effect attenuated after extended testing. Thus, results indicate that monkeys represent information spatially, but additional testing is required to determine the robustness of spatial representation of magnitude.

Table of Contents

Table of Contents Introduction 1 Experiment 1: Magnitude Training 5 Subjects 6 Apparatus 6 Procedures 7 Size Discrimination 7 Numerosity Discrimination 8 Length Discrimination 9 All Discriminations Intermixed 9 Results and Discussion 9 Experiment 2: SNARC Test 11 Subjects and Apparatus 11 Procedure 11 Data Analyses 12 Results and Discussion 13 General Discussion 16 References 19 Appendix 21

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