Apart from the raw visual input, people’s perception of temporally varying ambiguous visual stimuli is strongly influenced by earlier and recent summary statistics of the sequence, by its repetition/alternation structures, and by the subject’s earlier decisions and internal biases. Surprisingly, neither a thorough exploration of these effects nor a framework relating those effects exist in the literature. To separate the main underlying factors, we ran a series of nine 2-AFC visual decision making experiments. Subjects identified serially appearing abstract shapes in varying level of Gaussian noise (uncertainty), appearance probabilities and repetition-alternation ratio. We found a) an orderly relationship between appearance probabilities on different time-scales, the ambiguity of stimuli and perceptual decisions; b) an independent repetition/alternation effect, and c) a separation of bias effects on RT and decision, suggesting that only the latter is appropriate for measuring cognitive effects. We confirmed our main human results with behaving rats making choices based on luminance between stimuli appearing at different locations. These findings are compatible with a probabilistic model of human and animal perceptual decision making, in which not only decisions are taken so that short-term summary statistics resemble long-term probabilities, but higher order salient structures of the stimulus sequence are also encoded.

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