Effects of temporal gaps between successive fixation targets on discrimination performance and evoked brain activity.

Planning and executing of action in real-world conditions require continuous sensory input from many modalities. At the same time, sensory functions depend on reafferent and efference-copy information flow as imposed by motor actions. We studied how a specific oculomotor task influences afferent visual processing. Twenty healthy adults performed visually guided saccades. Between the offset of a fixation light and the onset of a new visual target a temporal gap of a duration of about 200 ms was introduced. This time structure is known from previous studies to elicit saccades at express latencies. In a control condition, 'no gap' was used. During eye movements one of four visual patterns with different orientations was presented, triggered by the horizontal electro-oculogram. We analyzed discrimination performance and the simultaneously recorded multichannel EEG activity. In the gap condition, shorter saccadic latencies were accompanied by significant more correct perceptual judgments. However, brain activity, as quantified by global field power, evoked component latency and topographical descriptors (centers of gravity or centroids) were not affected by the gap. This contrasts the notion that parieto-occipital areas are the most important sites of sensorimotor integration. Furthermore, the presence of a visual masking stimulus did not degrade discrimination performance, demonstrating that local retinal afterimages were not used for perceptual decisions. We conclude that intra-saccadic visual processing is influenced by pre-saccadic events. Under the short-time constraints prevalent in the saccadic task, fixation target cues are not only used for motor planning but also influence the visibility of visual patterns presented during the eye movement.