Temporal order judgment in mice.
A temporal order judgment task was developed for mice. After training male mice (C57BL6NCrj, n=15) to poke their noses into a hole, two stimuli (brief puffs of air) were delivered to the whiskers with a fixed interval of 750 ms in one of four orders: right (R)-left (L), L-R, L-L, and R-R. The mice were rewarded when they oriented their heads toward the first (n=5) or second (n=10) stimulus after a visual go signal. The mice were trained for up to 50 days. All mice met the criterion for task achievement (daily correct response rate >70% on 3 consecutive days) in response to unilateral stimuli (L-L and R-R), and 9 of the 15 mice met the criterion for task achievement in response to bilateral stimuli (L-R and R-L). The median periods for task achievement were 15 and 34 days for unilateral and bilateral stimuli, respectively. The correct response rate dropped to approximately the chance level after all whiskers had been removed. The nine successful mice were trained further and tested with smaller interstimulus intervals. The probability of right-first judgment plotted against the stimulation interval was fitted with a sigmoid function (r2=0.92) with asymptotes of 0.29 and 0.73 and a temporal resolution of 160 ms. The sigmoid curve was biased horizontally by 133 ms, reflecting the fact that stimuli delivered simultaneously were judged as left-first rather than right-first. The results show that mice can be trained to judge the temporal order of tactile stimuli delivered to whiskers and that such judgment might be lateralized to the right hemisphere.