Studies on the mechanism of the action of visual afferents on motor cortex excitability

Abstract 1. 1. The growing interest in the effect of sensory stimulation and especially of light stimulation on epileptic and myoclonic phenomena raises two questions of particular importance. The first is whether light stimulation produces its effect on the motor system by affecting the motor cortex or whether the effect is injected into the motor system at lower levels. The second question is by what pathways the visual afferent impulses reach the motor system in order to affect its threshold. A study of these two questions has been carried out on 30 cats under various types of anaesthesia including chloralose and semicarbazide. In order to test the excitability of the motor cortex, bipolar stimulating electrodes were placed on it and recording electrodes were placed in the pyramidal tract at the medullary pontine junction. The height of the response recorded from the pyramidal tract was used as an index of motor cortex excitability. 2. 2. The response recorded from the pyramidal tract after a single stimulus to the motor cortex is a complex one of prolonged duration. Our analysis of the pyramidal discharge suggests that it contains two sharply differentiated components. The fast component is probably due to direct stimulation of cells and perhaps fibres which contribute directly to the pyramidal tract (D response). The second slow component (I response) is probably due to stimulation of internuncial cortical cells which fire onto cells whose fibres contribute to the pyramidal tract. 3. 3. The effect of a single light flash in the eye or of lateral geniculate electric stimulation is to produce a prolonged period of facilitation of the motor cortex. The time course of this excitability increase is shown in figure 2 and the nature of the effect on the components of the pyramidal discharge is shown in figure 1. In some animals the light flash alone produces a descending volley in the P.T. It is concluded that the light flash affects both the cortical cells of origin of the pyramidal tract and the cortical interneurons. Some preliminary evidence suggests that other sensory systems may produce a similar phenomenon. 4. 4. It is shown that the effect of visual afferents on the excitability of the motor cortex is unaffected by complete removal of the occipital lobe cortex. It is concluded that neither the primary visual receiving area nor the visual association area need be present for the faciliting effect of visual stimulation to occur in the motor cortex. 5. 5. Complete removal of the cortex of the superior colliculus did not affect the time course or size of the facilitating effect of a visual afferent volley on the excitability of the motor cortex. 6. 6. Lesions placed in the pretectal region abolished the facilitatory effect of visual stimulation. 7. 7. It is therefore concluded that neither of the two main visual projection systems are responsible for the facilitory effect of visual afferent volleys on the motor cortex. The possible role of other visual pathways especially the ventral division of the lateral geniculate (the pregeniculate system) is discussed.