Brain, Vol. 125, No. 1, 123-139,
January 1, 2002
© 2002 Oxford University Press
The effect of spatial and temporal information on saccades and neural activity in oculomotor structures
1Departments of Psychology and Psychiatry, McGill University, 2Montreal Neurological Institute, 3McConnell Brain Imaging Centre, Montreal Neurological Institute, Montreal and 4Douglas Hospital Research Centre, Verdun, QC, Canada Correspondence to: Dr Gillian ODriscoll, Department of Psychology, McGill University, 1205 Dr Penfield Ave., Montreal, QC, Canada H3A 1B1 E-mail: gillian{at}hebb.psych.mcgill.ca
It has been argued that saccade generation is supported by two systems, a where system that decides the direction and extent of an impending saccade, and a when system that is involved in the timing of the release of fixation. We evaluated the contributions of these systems to saccade latencies, and used functional MRI to identify the neural substrates of these systems. We found that advance knowledge of the direction and the timing of an impending target movement had both overlapping and discrete effects on saccade latencies and on neural activation. Knowledge of either factor decreased regular saccade latencies. However, knowledge of target direction increased the number of predictive and express saccades while knowledge of target timing did not. The brain activation data showed that advance knowledge of the direction or the timing of the target movement activated primarily overlapping structures. The precentral gyrus, in the region of the frontal eye fields, was more active in conditions in which some aspect of the target movement was predictable than in saccade control and fixation conditions. In the basal ganglia, activation discriminated between advance knowledge of target timing and target direction. The lenticular nuclei were more active when only target timing was known in advance, while the caudate was more active when only target direction was known in advance. These data suggest that the neural structures supporting the where and when systems are highly overlapping, although there is some dissociation subcortically. Knowledge of target timing and target direction converge in precentral gyrus, a region where there is strong evidence of context-dependent modulation of neural activity.
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