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Jamie D. Roitman, Elizabeth M. Brannon, Michael L. Platt; Implicit discrimination of visual arrays by number in rhesus macaques. Journal of Vision 2005;5(8):1044. doi: 10.1167/5.8.1044.
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Research suggests that animals, including humans, internally represent number as an analog magnitude on a subjective ‘number line’. The internal representation of number is thought to be less precise with increasing magnitude, accounting for the size and distance effects on numerical judgments consistent with Weber's law. A critical prediction of Weber's law is that discriminability of two numbers depends on their ratio, regardless of actual magnitude. To test this prediction, we trained a monkey to perform an implicit visual numerical discrimination task which varied the number of elements in a visual array, while holding their ratios constant and controlling for element size and density. Specifically, midway through the delay period of a visually-guided saccade task, an array of n elements flashed briefly (300ms) in the hemifield opposite the saccade target. On the majority of trials the number of elements in the array was ‘standard’ (e.g. 8), and correct saccades were followed by a small reward (100ms juice delivery). On remaining trials, an array with an ‘oddball’ number of elements (e.g. 4 or 16) was presented and a large reward (300ms juice) followed correct saccades. The values of the standard and oddball arrays were varied across blocks of trials. Task execution was thus independent of the number of elements in the array, while differences in reward value encouraged attending to the array. Saccade reaction time differed on oddball trials, suggesting that the number of elements in the array was processed implicitly. This task thus provides a powerful paradigm for investigating implicit visual numerical processing and its neurophysiological correlates in the primate brain.
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