Handedness-dependent and -independent cerebral asymmetries in the anterior intraparietal sulcus and ventral premotor cortex during grasp planning.

When planning grasping actions, right-handers show left-lateralized responses in the anterior intraparietal sulcus (aIPS) and ventral premotor cortex (vPMC), two areas that are also implicated in sensorimotor control of grasp. We investigated whether a similar cerebral asymmetry is evident in strongly left-handed individuals. Fourteen participants were trained to grasp an object appearing in a variety of orientations with their left and right hands and with a novel mechanical tool (operated with either hand). BOLD fMRI data were then acquired while they decided prospectively whether an over- or under-hand grip would be most comfortable for grasping the same stimulus set while remaining still. Behavioral performances were equivalent to those recorded previously in right-handers and indicated reliance on effector-specific internal representations. In left-handers, however, grip selection decisions for both sides (left, right) and effectors (hand, tool) were associated with bilateral increases in activity within aIPS and vPMC. A direct comparison between left- and right-handers did reveal equivalent increases in left vPMC regardless of hand dominance. By contrast, aIPS and right vPMC activity were dependent on handedness, showing greater activity in the motor-dominant hemisphere. Though showing bilateral increases in both left- and right-handers, greater increases in the motor dominant hemisphere were also detected in the caudal IPS (cIPS), superior parietal lobule (SPL) and dorsal premotor cortex (dPMC). These findings provide further evidence that regions involved in the sensorimotor control of grasp also participate in grasp planning, and that for certain areas hand dominance is a predictor of the cerebral organization of motor cognitive functions.