Abstract
BackgroundElectrophysiological studies in monkeys showed that the intention to perform a saccade and the covert change in motor plan are reflected in the neural activity of the posterior parietal cortex (PPC).MethodsTo investigate whether such covert intentional processes are demonstrable in humans as well we used event related functional MRI. Subjects were instructed to fixate a central target, which changed its color in order to indicate the direction of a subsequent saccade. Unexpectedly for the subjects, the color changed again in half of the trials to instruct a spatially opposite saccade.ResultsThe double-cue induced synergistic and prolonged signals in early visual areas, the motion specific visual area V5, PPC, and the supplementary and frontal eye field. At the single subject level it became evident that the PPC split up in two separate subareas. In the posterior region, the signal change correlated with the change in motor plan: activation strongly decreased when the cue instructed an ipsiversive saccade while it strongly increased when it instructed a contraversive saccade. In the anterior region, the signal change was motor related irrespective of the spatial direction of the upcoming saccade.ConclusionThus, the human PPC holds at least two different areas for planning and executing saccadic eye movements.
Highlights
Electrophysiological studies in monkeys showed that the intention to perform a saccade and the covert change in motor plan are reflected in the neural activity of the posterior parietal cortex (PPC)
The signal change correlated with the change in motor plan: activation strongly decreased when the cue instructed an ipsiversive saccade while it strongly increased when it instructed a contraversive saccade
In this study we have shown that the PPC together with visual area V5, the frontal eye field (FEF) and supplementary eye field (SEF) are involved in covertly changing the plan for saccadic eye movements
Summary
Electrophysiological studies in monkeys showed that the intention to perform a saccade and the covert change in motor plan are reflected in the neural activity of the posterior parietal cortex (PPC). Our eyes are continuously on the move, since we perform about three eye movements per second This operation requires continuous transformation of visual input to motor output and continuous shifts of visual attention since we attend to where we look. Electrophysiological studies in non-human primates showed the involvement of a specific region within PPC, i.e. the lateral intraparietal area (LIP) in either visual attention [6,7,8] or motor intention [9,10,11]. In view of the functional heterogeneity of the PPC Bracewell used the 'change of motor plan' paradigm to isolate the intention to perform a saccade from attentional processes [19].
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