Stimulus control of visual fixation duration in a single saccade paradigm.

Subjects performed a task in which they successively inspected two stimuli presented at an equal distance of the left (SL) and the right (SR) of the visual meridian and subtending a 45 degrees visual angle. This was followed by a joint response on the basis of the status of both stimuli. The instruction was to fixate the position of SL at the start of a trial, followed by a single saccade to SR. In earlier studies on this paradigm it was suggested that the fixation time of SL (TL) may serve as a modern version of the (d)-reaction in which the time for achieving perceptual identification is measured uncontaminated by decision. This suggestion has the implicit assumption that, at least in this single saccade paradigm, the saccade from SL to SR is triggered when perceptual identification has been completed. There is the potential alternative that the duration of TL is programmed in advance, depending on the general processing demands of the stimuli in a block of trials. These two options were tested in two experiments. In the first, presentation of SL was delayed with a fixed or with a variable interval (200-400 ms) during a block of trials. In the second, the stimulus quality of SL was varied between and within blocks of trials. The results of either manipulation argue against preprogramming TL, and they are consistent with the hypothesis that the saccade is triggered upon completion of perceptual processing. Thus, TL in the single saccade paradigm appears a serious candidate for the (d)-reaction.