Fig. 2

Diurnal change of color-taste association task. a A Japanese macaque in a cage with two water bottles. b The method for the color-taste association task. The color-taste association task is done in five consecutive days consisting of three parts, three days of “practice”, one day of “training”, and one day of “testing” at either three-time points, ZT 1.5 (Group 1), ZT 5.5 (Group 2), or ZT 10 (Group 3). In the “practice,” two bottles of bitter water and normal water equipped with nozzles in different colors were presented to the monkeys. The “practice” let animals learn that nozzle color is associated with water taste. Monkeys were allowed to freely drink water from the bottles for 2 h with the location exchange at 1-h after the bottle setting. “Training” and “testing” were carried out at the same time of day as the “practice,” and nozzle color sets are the same in the “training” and “testing” but different from those used in the “practice.” In the “training,” the bottles are presented for 2 h with the location exchange at 1-h after the bottle setting. In the “testing,” the two bottles were both filled with normal water, and the animals were allowed to drink water from the bottles for 30 min, with the location exchanged at 15-min after the bottle setting. An association between specific nozzle color and water taste formed during the "training” was evaluated in the” testing.” c The accuracy rate was examined at ZT1.5, ZT5.5, or ZT10. *p = 0.002 (ZT5.5) by Student’s t-test (versus 0.25 chance level). Error bars, s.e.m. (n = 6 macaques, a total of 16 task trials for ZT1.5, 13 task trials for ZT5.5, 16 task trials for ZT10.0). The dotted line represents performance by chance 0.25. n.s., not statistically significant