Humans’ reliance on expectations based on past experience to evaluate uncertain sensory events of their environment has been interpreted either as local adaptation or probabilistic implicit inference. However the exact interplay between immediate past and longer-term sensory experiences in influencing these expectations has not yet been experimentally explored. In a simple probabilistic visual 2-AFC task, we assessed how human judgments depended on unbalanced base-rate appearance statistics and the immediate history of possible events. Participants detected the appearance of visual shapes in blocked random visual sequences, where base-rate appearances were systematically manipulated throughout the blocks. Expectations were assessed implicitly by reaction times and explicitly by interspersed numerical estimates and confidence judgments. Implicit expectations were reliably influenced by both probabilities of the immediate past (F(3,14)=34.733,p <.000) and the base-rate (F(3,14)=28.674,p<.000) in an additive manner. In addition, implicit and explicit measures were consistent, as participants’ accuracy and confidence judgments in estimating the probability of likely and unlikely events followed the same pattern for the two measures. The results confirm previously untested assumptions about the interaction of base-rate statistics and short-term effects in forming visual expectations, and suggest that behavioral changes are based on internal probabilistic models not just on local adaptation mechanisms.

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