Differential effects of cortical lesions in humans

The best-known example of motor deficits after cortical lesions is contralateral paresis and spasticity after damage to the precentral motor strip. After recovery the residual motor functions can be used in a purposive and skillful manner. In patients with lesions of the supplementary motor area (SMA) and cingulate gyrus transient akinesia and mutism have been described. Lesions restricted to more lateral parts of the premotor field interfere with proximal muscle function and interlimb coordination, whereas distal motor activity and bimanual coordination are unimpaired. In contrast, hand function in patients with parietal lesions is severely disturbed. This dysfunction includes deficits such as ataxia, dysmetria and postural instability that are typically observed in deafferented patients. Severe disturbances of the purposive behaviour of the hand during exploratory finger movements and manipulation of objects are seen in patients with posterior parietal lesions. Observations in human patients are compatible with the hypothesis that lesions of the frontal agranular motor fields interfere with the control of postural and force control whereas parietal lesions are associated with motor programme disorders affecting the use of the hand or the eye as a sense organ or affecting more complex motor behaviour.