Behavioral and neuropsychological research suggests that delayed actions rely on different neural substrates than immediate actions; however, the specific brain areas implicated in the two types of actions remain unknown. We used functional magnetic resonance imaging (fMRI) to measure human brain activation during delayed grasping and reaching. Specifically, we examined activation during visual stimulation and action execution separated by a 18-s delay interval in which subjects had to remember an intended action toward the remembered object. The long delay interval enabled us to unambiguously distinguish visual, memory-related, and action responses. Most strikingly, we observed reactivation of the lateral occipital complex (LOC), a ventral-stream area implicated in visual object recognition, and early visual cortex (EVC) at the time of action. Importantly this reactivation was observed even though participants remained in complete darkness with no visual stimulation at the time of the action. Moreover, within EVC, higher activation was observed for grasping than reaching during both vision and action execution. Areas in the dorsal visual stream were activated during action execution as expected and, for some, also during vision. Several areas, including the anterior intraparietal sulcus (aIPS), dorsal premotor cortex (PMd), primary motor cortex (M1) and the supplementary motor area (SMA), showed sustained activation during the delay phase. We propose that during delayed actions, dorsal-stream areas plan and maintain coarse action goals; however, at the time of execution, motor programming requires re-recruitment of detailed visual information about the object through reactivation of (1) ventral-stream areas involved in object perception and (2) early visual areas that contain richly detailed visual representations, particularly for grasping.
Hippocampal theta oscillations (3-12 Hz) may reflect a mechanism for sensorimotor integration in rats (Bland BH. Prog Neurobiol 26: 1-54, 1986); however, it is unknown whether cortical theta activity underlies sensorimotor integration in humans. Rather, the mu rhythm (8-12 Hz) is typically found to desynchronize during movement. We measured oscillatory EEG activity for two conditions of an instructed delayed reaching paradigm. Conditions 1 and 2 were designed to differentially manipulate the contribution of the ventral visuomotor stream during the response initiation phase. We tested the hypothesis that theta activity would reflect changes in the relevant sensorimotor network: condition 2 engaged ventral stream mechanisms to a greater extent than condition 1. Theta oscillations were more prevalent during movement initiation and execution than during periods of stillness, consistent with a sensorimotor relevance for theta activity. Furthermore, theta activity was more prevalent at temporal sites in condition 2 than condition 1 during response initiation, suggesting that theta activity is present within the necessary sensorimotor network. Mu activity desynchronized more during condition 2 than condition 1, suggesting mu desynchronization is also specific to the sensorimotor network. In summary, cortical theta synchronization and mu desynchronization may represent broadly applicable rhythmic mechanisms for sensorimotor integration in the human brain.
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