This study investigated the question whether spatial working memory related to movement plans (motor working memory) and spatial working memory related to spatial attention and perceptual processes (perceptual spatial working memory) share the same neurophysiological substrate or there is evidence for separate motor and perceptual working memory streams of processing. Towards this aim, ten healthy human subjects performed delayed responses to visual targets presented at different spatial locations. Two tasks were attained, one in which the spatial location of the target was the goal for a pointing movement and one in which the spatial location of the target was used for a perceptual (yes or no) change detection. Each task involved two conditions: a memory condition in which the target remained visible only for the first 250 ms of the delay period and a delay condition in which the target location remained visible throughout the delay period. The amplitude spectrum analysis of the EEG revealed that the alpha (8-12 Hz) band signal was smaller, while the beta (13-30 Hz) and gamma (30-45 Hz) band signals were larger in the memory compared to the non-memory condition. The alpha band signal difference was confined to the frontal midline area; the beta band signal difference extended over the right hemisphere and midline central area, and the gamma band signal difference was confined to the right occipitoparietal area. Importantly, both in beta and gamma bands, we observed a significant increase in the movement-related compared to the perceptual-related memory-specific amplitude spectrum signal in the central midline area. This result provides clear evidence for the dissociation of motor and perceptual spatial working memory.
In the present study, reaction time (RT) was measured in 12 healthy subjects in a saccade and antisaccade task while recording electroencephalographic activity (EEG) from 62 electrodes on the scalp. Event-related potentials averaged both on target appearance and on saccade onset were larger in amplitude (increased negativity) for the antisaccade task compared to the saccade task. The relation of RT variability to EEG amplitude was studied by averaging stimulus-aligned and movement-aligned individual trials for each subject into four RT quartile groups. The analysis showed a relation of EEG amplitude to RT for both saccades and antisaccades. More specifically, the ERP negativity at 100-120 ms after stimulus onset in the saccade task and at 160-200 ms after stimulus onset in the antisaccade task for stimulus-aligned ERPs decreased monotonically with increasing RT as would be expected if this signal would be related to the eye movement preparation processes. This was much more pronounced and wide spread for the antisaccades than for visually triggered saccades. The peak negativity before movement onset for movement-aligned ERPs also covaried with RT suggesting no relation of this activity to movement preparation processes. This study then confirmed that only a particular ERP signal variation was related to the saccadic eye movement preparatory processes while other components of the ERP have no specific relation to the movement preparation. This particular signal was more prominent for antisaccades compared to visually triggered saccades supporting previous evidence for the cortical involvement in the preparation of these voluntary eye movements. In conclusion, this study validates the use of ERPs in the study of the planning and execution of saccadic eye movements.
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