The aim of this study was to develop a temporal order judgment (TOJ) task for monkeys and to investigate monkeys’ performance. Two trained monkeys (Macaca Fascicularis) performed a two-alternative forced choice TOJ task in which the subject made a saccadic eye movement toward one of two Gabor stimuli. Two stimuli were presented in the left and right side of fixation with a temporal asynchrony of 0, 10, or 30ms. A few drops of fluid as reward was delivered when the subject made a saccadic eye movement to the earlier one of the two. For the asynchrony of zero, reward was delivered in randomly-chosen half of trials, regardless of the subject’s choice. Analyses of behavioral data revealed following results. First, a significant difference in the mean reaction time between correct and incorrect trials was observed, but the pattern of difference across temporal asynchrony conditions was depended on subjects. Second, response bias and correct rate were correlated with each other, but the pattern and the strength of the relationship differed between two subjects. Third, strong correlation of biases was found between conditions when stimuli were presented asynchronously (i.e. correct answer exists.) or presented simultaneously (i.e. correct answer doesn’t exists.). Fourth, when targets were presented simultaneously, animal’s choice was predictable with a logistic regression model, using reward history and previous choices as indicators. But the models differ between subjects. These results suggest that two monkeys used different strategies while performing the same TOJ task.
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