Implicit short-term memory and event frequency effects in visual search.

Numerous experiments have shown that animals and humans behave as if guided by an implicit knowledge of the relative frequency of occurrence of events in their environment. A well-known example of such trait is "Hick's Law" for reaction times: responses to more frequent stimuli are faster than to less frequent ones. In the present study, we demonstrate that an important source of the effects produced by Hick's law in the context of a visual search task is to be found in a form of implicit short-term memory, previously described as Priming of Pop-out. We report the results of experiments in which we have disrupted or enhanced the accumulation of implicit short-lived memory traces in the context of visual search tasks where stimulus frequency was varied. With target frequencies greater than 20%, these memory manipulations resulted in the elimination or enhancement of the effect of stimulus frequency on reaction times, thus indicating that an implicit, finite-memory accumulator is an important underlying mechanism for frequency effects in visual search paradigms. We characterize the properties of the accumulator and discuss the kinds of behaviors in which it may be implicated.