Two important capabilities of the brain in controlling movement are to (1) determine how rapidly the movement is to be performed and (2) control how large the movement will be. For instance, a person may write the letter “a” slowly or rapidly. Also, he or she may write a small “a” on a piece of paper or a large “A” on a chalkboard. Regardless of the choice, the proportional characteristics of the letter remain nearly the same.
In patients with severe lesions of the basal ganglia, these timing and scaling functions are poor—in fact, sometimes they are nonexistent. Here again, the basal ganglia do not function alone; rather, they function in close association with the cerebral cortex. One especially important cortical area is the posterior parietal cortex, which is the locus of the spatial coordinates for motor control of all parts of the body, as well as for the relation of the body and its parts to all its surroundings. Damage to this area does not produce simple deficits of sensory perception, such as loss of tactile sensation, blindness, or deafness. Instead, lesions of the posterior parietal cortex produce an inability to accurately perceive objects through normally functioning sensory mechanisms, a condition called agnosia. Figure 1 shows how a person with a lesion in the right posterior parietal cortex might try to copy drawings. In these cases, the patient’s ability to copy the left side of the drawings is severely impaired. Also, such a person will always try to avoid using his or her left arm, left hand, or other portions of his or her left body for the performance of tasks, or even to wash this side of the body (personal neglect syn drome), almost not knowing that these parts of his or her body exist.
Fig1. Illustration of drawings that might be made by a person who has neglect syndrome caused by severe damage in his or her right posterior parietal cortex compared with the actual drawing the patient was requested to copy. Note that the person’s ability to copy the left side of the drawings is severely impaired.
Because the caudate circuit of the basal ganglial system functions mainly with association areas of the cerebral cortex such as the posterior parietal cortex, presumably the timing and scaling of movements are functions of this caudate cognitive motor control circuit. However, our understanding of function in the basal ganglia is still so imprecise that much of what is conjectured in the last few sections is analytical deduction rather than proven fact.
