Temporal expectations play an important role in optimizing future perception and behavior, but the nature of the attentional processes affected by our expectations is still widely debated. To investigate effects of expectations on the speed of visual processing, we employed a cued single-letter recognition task with stimuli of varied durations terminated by pattern masks. To obtain an estimate of processing speed unconfounded by motor components, we used accuracy-based measures rather than reaction times.

Eight healthy young subjects completed a total of 4,000 trials each. A symbolic cue at the beginning of each trial induced an expectation about the time when the stimulus letter would appear. Waiting times were not fixed but distributed exponentially. The cue indicated which of two distributions with different hazard rates the waiting time would be drawn from. The hazard rate could be either high (1.33 s^-1) or low (0.22 s^-1) corresponding to mean waiting times of 750 ms and 4,500 ms, respectively.

The probability (p) of correct report was approximately an exponential function of the stimulus duration (t): p = 1 - exp[-v (t - t₀)], where t₀ measured the threshold of conscious perception, and v measured the processing speed at times t [[gt]] t₀. Cue type did not affect the threshold of conscious perception in any of the subjects, but all subjects showed a highly significant reduction (on average 30%) in processing speed on trials with low as compared with high hazard rate. This effect was found even though no general decrease in processing speed over time occurred; thus, the observed pattern was independent of the actual duration of the waiting time, but depended entirely on expectation. This seems to be the first time speed of vision - unconfounded by motor components - has been shown to depend on temporal expectancy.