Effector-specific fields for motor preparation in the human frontal cortex.
We investigated the neural correlates of advance motor preparation in two experiments that required a movement in response to a peripheral visual stimulus. In one experiment (the memory delay paradigm), subjects knew the target location during a preparatory 'memory delay' interval; in the other experiment they did not know the target location during a 'gap period' (the gap paradigm). In both experiments we further varied the effector that was instructed, either the eye or the forelimb. An area that codes motor preparation should exhibit increases during the memory delay and gap period and such increases should predict some attribute of performance (planning to use the eye or the forelimb). We first identified the frontoparietal visuomotor areas using standard fMRI block designs. Subjects were then scanned using event-related fMRI. With the exception of primary motor cortex (M1), all areas (putative lateral intraparietal area (putLIP), dorsal premotor cortex (PMd), frontal eye field (FEF), ventral frontal eye field (FEFv), supplementary motor area (SMA)) showed gap and memory delay activation for both saccades and pointing. Gap activity in the frontal areas was higher than in the parietal area(s) investigated. The observation that 'memory delay' activity was equivalent or less than gap activity in all areas suggests that what is commonly considered to be memory-related responses largely represents advance motor preparation. Certain areas showed increased activation during the gap or memory delay intervals for pointing (PMd, FEF, FEFv) or saccades (SMA, putLIP). These observations suggest an important role of the frontal cortex in advance motor preparation.