Comparison of memory- and visually guided saccades using event-related fMRI

Previous functional imaging studies have shown an increased hemodynamic signal in several cortical areas when subjects perform memory-guided saccades than that when they perform visually guided saccades using blocked trial designs. It is unknown, however, whether this difference results from sensory processes associated with stimulus presentation, from processes occurring during the delay period before saccade generation, or from an increased motor signal for memory-guided saccades. We conducted fMRI using an event-related paradigm that separated stimulus-related, delay-related, and saccade-related activity. Subjects initially fixated a central cross, whose color indicated whether the trial was a memory- or a visually guided trial. A peripheral stimulus was then flashed at one of 4 possible locations. On memory-guided trials, subjects had to remember this location for the subsequent saccade, whereas the stimulus was a distractor on visually guided trials. Fixation cross disappearance after a delay period was the signal either to generate a memory-guided saccade or to look at a visual stimulus that was flashed on visually guided trials. We found slightly greater stimulus-related activation for visually guided trials in 3 right prefrontal regions and right rostral intraparietal sulcus (IPS). Memory-guided trials evoked greater delay-related activity in right posterior inferior frontal gyrus, right medial frontal eye field, bilateral supplementary eye field, right rostral IPS, and right ventral IPS but not in middle frontal gyrus. Right precentral gyrus and right rostral IPS exhibited greater saccade-related activation on memory-guided trials. We conclude that activation differences revealed by previous blocked experiments have different sources in different areas and that cortical saccade regions exhibit delay-related activation differences.