The intracortical excitability cycle following stimulation of the optic pathway of the cat

Like many neurones of the central nervous system, those of the optic cortex of the cat exhibit a brief phase of facilitation, followed by a more prolonged phase of depression, as tested by a shock to the optic radiation following a conditioning shock. Recovery toward normal occurs gradually, and is complete in about 200 msec. The negative phase of the specific response returns only much later than the preceding portions of the response. This cortical depression is not complete, and varies in intensity in different preparations treated similarly. A weak first shock depresses the response to a second to some degree, and as the first shock is made stronger this effect is greater, in some cases to complete occlusion of the second response. The depression is accompanied by a surface-negative swing of the baseline. A more severe depression occurs in the dorsal nucleus of the lateral geniculate body, as previously described by Marshall. Its recovery of responsiveness is slower than is that of the cortex. Thus to optic nerve stimulation the cortical activity is affected by two conditions of depression operating serially. The depressive effect at the geniculate can be recognized in responses from the cortex in terms of the amplitude of the first spike representing responses of radiation axons. Two shocks, the second within the facilitatory period of the first, can break through the depression left by a previous response, when neither can do so alone. Similarly two such shocks may cause a larger single response than can any one stimulus. From these superimposed effects of facilitation and depression exhibited by arbitrary volley responses, certain speculations are offered as to the normal manner of cortical functioning.