Dissociating the Role of Ventral and Dorsal Premotor Cortex in Precision Grasping

Location of the TMS coil positions to induce virtual lesion of PMv (blue) and PMd (red). To stimulate PMv, the coil was positioned over the caudal portion of the pars opercularis of the inferior frontal gyrus (BA44), corresponding to the following normalized MNI coordinates: −60 ± 2, 16 ± 3, 23 ± 9 mm (x, y, z, mean ± SD; n = 10) and 56 ± 6, 16 ± 4, 26 ± 9 mm (n = 6) for the left and right PMv, respectively. To target PMd, the coil was positioned over the superior portion of the precentral gyrus, as delimited by the superior frontal sulcus. The mean MNI coordinates of stimulation sites for the left and right PMd were, respectively, −22 ± 3, −4 ± 4, 71 ± 4 mm (x, y, z, mean ± SD; n = 10) and 24 ± 4, −5 ± 6, 72 ± 3 mm (n = 6). Each ellipse was centered on the mean MNI coordinates of PMv and PMd stimulation points, and their surface shows the 95% confidence interval of the normalized coordinates calculated for each subject.

Distinctive effects of the left or right PMv or PMd inactivation on the grip–lift movement. Virtual lesions of PMv specifically altered the grasping phase of grip–lift movement, whereas lesions of the left PMd only affected the lifting phase. Histograms showing the dissociation between the effects of virtual lesions of the left and right PMv (light gray) and PMd (dark gray) when compared with the control condition (white) for four movement parameters: A, the delay between the intrinsic hand muscle recruitment, i.e., the APB and 1DI; B, the index–thumb horizontal distance (Ind-Th H dist); C, the BrR muscle onset; and D, the preloading phase duration. *p < 0.05

Effects of the virtual lesions of the left or right PMv on the fingertip positioning. Side view of the manipulandum showing the distribution of the fingertip positions for the thumb (black circles) and the index finger (white circles). The two graspable surfaces of the manipulandum were superimposed to represent the thumb and index fingertip positions on the same graph. The subject’s hand came from the right side of the figure. The ellipses represent the area in which 95% of the fingertip positions were found. A, Distribution of the thumb and index fingertip positions for 30 control trials in one subject. Note that the index finger contact points were always slightly farther than those of the thumb. B, A virtual lesion of the left PMv increased significantly both the horizontal distance between the thumb and index finger contact points and their variability. C, Similar effects, although less pronounced, of a virtual lesion of the right PMv on fingertip positions and distributions.