The time course of stimulus expectation in a saccadic decision task.

Because the time to respond to a stimulus depend markedly on expectation, measurements of reaction time can, conversely, provide information about the brain's estimate of the probability of a stimulus. Previous studies have shown that the quantitative relationship between reaction time and static, long-term expectation or prior probability can be explained economically by the LATER model of decision reaction time. What is not known, however, is how the neural representation of expectation changes in the short term, as a result of immediate cues. Here, we manipulate the foreperiod-the delay between the start of a trial and the appearance of the stimulus-to see how saccadic latency, and thus expectation, varies with different delays. It appears that LATER can provide a quantitative explanation of this relationship, in terms both of average latencies and of their statistical distribution. We also show that expectancy appears to be subject to a process of low-pass filtering, analogous to the spatial blur that degrades visual acuity.