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1 Psychology, University of Western Ontario, London, Ontario, Canada; Graduate Program in Neuroscience, University of Western Ontario, London, Ontario, Canada
2 Physiology and Pharmacology, University of Western Ontario, London, Ontario, Canada
3 Psychology, University of Western Ontario, London, Ontario, Canada; The Laboratory for Magnetic Resonance Research, Robarts Research Institute, London, Ontario, Canada
4 The Laboratory for Magnetic Resonance Research, Robarts Research Institute, London, Ontario, Canada; Graduate Program in Neuroscience, University of Western Ontario, London, Ontario, Canada
5 Psychology, University of Western Ontario, London, Ontario, Canada; Graduate Program in Neuroscience, University of Western Ontario, London, Ontario, Canada; Physiology and Pharmacology, University of Western Ontario, London, Ontario, Canada
6 Psychology, University of Western Ontario, London, Ontario, Canada; Physiology and Pharmacology, University of Western Ontario, London, Ontario, Canada; The Laboratory for Magnetic Resonance Research, Robarts Research Institute, London, Ontario, Canada
* To whom correspondence should be addressed. E-mail: severlin{at}uwo.ca.
Previous functional imaging studies have shown increased haemodynamic signal in several cortical areas when subjects perform memory-guided saccades compared with 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 colour 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 three 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.
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