Failure to handle more than one internal representation in visual detection tasks.

Perceptual studies make a clear distinction between sensitivity and decision criterion. The former is taken to characterize the processing efficiency of the underlying sensory system and it increases with stimulus strength. The latter is regarded as the manifestation of a subjective operation whereby individuals decide on (as opposed to react reflexively to) the occurrence of an event based on factors such as expectation and payoff, in addition to its strength. To do so, individuals need to have some knowledge of the internal response distributions evoked by this event or its absence. In a natural, behaviorally relevant multistimulus environment, observers must handle many such independent distributions to optimize their decision criteria. Here we show that they cannot do so. Instead, while leaving sensitivity unchanged, lower and higher visibility events tend to be reported respectively less and more frequently than when they are presented in isolation. This behavior is in quantitative agreement with predictions based on the notion that observers represent a multistimulus environment as a unitary internal distribution to which each stimulus contributes proportionally to its probability of occurrence. Perceptual phenomena such as blindsight, hemineglect, and extinction may be, at least in part, accounted for in such a way.