Stimulation of site one (click STIMULATE 1) causes the monkey to bring its arm in front of its eyes, regardless of the starting location of the arm, as if the monkey were producing a defensive posture. Stimulation of site two (click STIMULATE 2) causes the monkey to bring its arm to its mouth and open the mouth, regardless of the starting location of the arm, as if it were bringing a piece of food to its mouth (Graziano et al., 2002). Like the somatosensory cortex of the postcentral gyrus, the primary motor cortex is somatotopically organized (Figure 3.3). Stimulation of the anterior paracentral lobule elicits movements of the contralateral leg. As the stimulating electrode is moved across the precentral gyrus from dorsomedial to ventrolateral, movements are elicited progressively from the torso, arm, hand, and face (most laterally). The representations of body parts that perform precise, delicate movements, such as the hands and face, are disproportionately large compared to the representations of body parts that perform only coarse, unrefined movements, such as the trunk or legs. The premotor cortex and supplementary motor area also contain somatotopic maps. Somatotopic representation of motor outputs in motor cortex. One might predict that the motor cortex “ homunculus” arises because neurons that control individual muscles are clustered together in the cortex. That is, all of the neurons that control the biceps muscle may be located together, and all of the neurons that control the triceps may be clustered nearby, and the neurons that control the soleus muscle may be clustered in a region further removed. Electrophysiological recordings have shown that this is not the case, however. Movements of individual muscles are correlated with activity from widespread parts of the primary motor cortex. Similarly, stimulation of small regions of primary motor cortex elicits movements that require the activity of numerous muscles.

Activate Neuro Programmer 2 Review