Relative numerical middle in rhesus monkeys

Abstract

Animals show vast numerical competence in tasks that require both ordinal and cardinal numerical representations, but few studies have addressed whether animals can identify the numerical middle in a sequence. Two rhesus monkeys (Macaca mulatta) learned to select the middle dot in a horizontal sequence of three dots on a touchscreen. When subsequently presented with longer sequences composed of 5, 7 or 9 items, monkeys transferred the middle rule. Accuracy decreased as the length of the sequence increased. In a second test, we presented monkeys with asymmetrical sequences composed of nine items, where the numerical and spatial middle were distinct and both monkeys selected the numerical middle over the spatial middle. Our results demonstrate that rhesus macaques can extract an abstract numerical rule to bisect a discrete set of items.

Keywords: abstract concepts; centre; comparative cognition; middle; number bisection; rhesus monkeys.

Figure 1.

Schematic illustration of the experimental procedure and stimuli. (a) Training trials. A start response square then a three-dot stimulus appeared. A food reward, a green screen and a positive sound occurred after the selection of the middle dot. A grey screen appeared after the choice of either lateral dot; the screen turned black after 5 s with no choice. (b) Testing trials. A start response square then a stimulus appeared. The selection of all dots, lateral or middle, elicited a positive reward. The screen turned black after no choice within 5 s. (c) Stimuli used in the number transfer test: close 3-dots (A1), far 3-dots (A2), close 5-dots (B1), far 5-dots (B2), close 7-dots (C1), far 7-dots (C2), close 9-dots (D1) and far 9-dots (D2). (d) Stimuli used in the asymmetrical test: asymmetrical left condition (E) and asymmetrical right condition (F) (https://www.nature.com/articles/s41598-020-74533-8).

Figure 2.

Results of the number transfer test. Monkeys primarily selected the middle dot in all sequences ((a) 3 items, (b) 5 items, (c) 7 items, (d) 9 items), and (e) accuracy diminished as the number of items increased. (f) Laterality index. Both monkeys showed a side effect in the function of numerical magnitude: Arrow showed a right bias while Tolman showed a left bias. In all graphs, the black bars represent the standard errors and the dashed line indicates chance level, and the asterisks indicate p < 0.05.