An ERP investigation of sensory responses preceding first-saccade onsets during visual search.

The timing of eye movements is known to vary as a function of task and stimulus processing difficulty, but there are also differences both within and between individuals when these are controlled. Previously, we reported that the number of saccades needed to find a visual target is influenced by the latency of the first saccade following search display onset. Namely, waiting longer to produce the first saccadic eye movement in visual search results in more accurate first saccades and fewer total eye movements to find the target. One possibility is that longer dwell times may be the result of enhanced perceptual processing prior to the saccadic decision. To understand if this accounts for some variability in saccadic timing, we investigated early sensory responses to search display onsets, before any eye movements were initiated. In particular, we examined how these differed depending on the subsequent saccade produced. Search images were composed of Gabor patches that were oriented to form a contour target, which was embedded among randomly oriented distractor Gabor patches. Targets were placed at four, seven, and ten degrees retinal eccentricity. EEG activity was recorded and saccade latency and direction were determined through bipolar HEOG. Results from posterior electrodes showed that the search display generated greater N1 amplitudes when they preceded the initiation of faster saccades relative to slower saccades. This effect was also independent of saccade direction, suggesting it was not purely an artifact of the eye movement itself. Instead, we posit that results indicate that the N1 effect preceding long saccades reflects enhanced attentional engagement with the target. Generally, these results demonstrate the utility in examining stimulus-locked ERPs as they relate to the characteristics of the subsequent eye movements produced.