Visuomotor transformations at the cortical level occur along a network where posterior parietal regions are connected to homologous premotor regions. Grasping-related activity is represented in a diffuse, ventral and dorsal system in the posterior parietal regions, but no systematic causal description of a premotor counterpart of a similar diffuse grasping representation is available. To fill this gap, we measured the kinematics of right finger movements in 17 male and female human participants during grasping of three objects of different sizes. Single-pulse transcranial magnetic stimulation was applied 100 ms after visual presentation of the object over a regular grid of 8 spots covering the left premotor cortex (PMC) and 2 Sham stimulations. Maximum finger aperture during reach was used as the feature to classify object size in different types of classifiers. Classification accuracy was taken as a measure of the efficiency of visuomotor transformations for grasping. Results showed that transcranial magnetic stimulation reduced classification accuracy compared with Sham stimulation when it was applied to 2 spots in the ventral PMC and 1 spot in the medial PMC, corresponding approximately to the ventral PMC and the dorsal portion of the supplementary motor area. Our results indicate a multifocal representation of object geometry for grasping in the PMC that matches the known multifocal parietal maps of grasping representations. Additionally, we confirm that, by applying a uniform spatial sampling procedure, transcranial magnetic stimulation can produce cortical functional maps independent of a priori spatial assumptions.

SIGNIFICANCE STATEMENT Visually guided actions activate a large frontoparietal network. Here, we used a dense grid of transcranial magnetic stimulation spots covering the whole premotor cortex (PMC), to identify with accurate spatial mapping the functional specialization of the human PMC during grasping movement. Results corroborate previous findings about the role of the ventral PMC in preshaping the fingers according to the size of the target. Crucially, we found that the medial part of PMC, putatively covering the supplementary motor area, plays a direct role in object grasping. In concert with findings in nonhuman primates, these results indicate a multifocal representation of object geometry for grasping in the PMC and expand our understanding of how our brain integrates visual and motor information to perform visually guided actions.

皮质水平的视运动转换沿网络发生，该网络的后壁区域连接到同源的前运动区域。与抓握有关的活动在顶叶后部区域的弥散，腹侧和背侧系统中表示，但是尚无类似散开抓取表示的运动前对应物的系统因果描述。为了填补这一空白，我们在抓握三个不同大小的物体的过程中，测量了17位男性和女性参与者中右手指运动的运动学。视觉呈现对象后100 ms，在覆盖左前运动皮层（PMC）和2个Sham刺激的8个点的规则网格上进行单脉冲经颅磁刺激。到达时的最大手指孔径用作在不同类型的分类器中对对象大小进行分类的功能。分类准确度被用作衡量运动性抓握转换效率的量度。结果表明，与假刺激相比，经颅磁刺激将其应用于腹侧PMC的2个点和内侧PMC的1个点时，降低了分类的准确性，大约相当于腹侧PMC和辅助运动区的背部。我们的结果表明，用于在PMC中进行抓取的对象几何形状的多焦点表示形式与已知的抓取表示形式的多焦点顶图匹配。此外，我们确认通过应用统一的空间采样程序，先验的空间假设。

重要性声明视觉引导的动作激活了一个大型的额叶前额网络。在这里，我们使用了覆盖整个运动前皮层（PMC）的密集的经颅磁刺激斑点网格，以精确的空间映射来识别抓握运动过程中人类PMC的功能专长。结果证实了先前有关腹侧PMC在根据目标大小预塑指方面的作用的发现。至关重要的是，我们发现PMC的中间部分假定覆盖了辅助运动区域，在抓紧物体方面起着直接作用。与非人类灵长类动物的发现相一致，这些结果表明了用于PMC抓取的对象几何形状的多焦点表示，并扩展了我们对大脑如何整合视觉和运动信息以执行视觉引导动作的理解。