ACTIVITY OF ANTERIOR SUPRASYLVIAN GYRUS IN RESPONSE TO TRANSCALLOSAL AFFERENT VOLLEYS

Abstract

Cortico-cortical transcallosal responses were initiated by bipolar cortical stimulation at various parameters. An attempt was made to record the afferent volley at the surface and to accumulate more data to aid in the analysis of this response. Results can be summarized as follows: (1) A spike sequence can be recorded at the surface. The 1st spike is considered to be afferent volley, the subsequent ones postsynaptic in origin. (2) Differences in the response to short compared to long pulses are suggested to be due to greater current flux in the depths as a result of high intensity short-pulse shocks which result in direct fiber stimulation and greater antidromic activity. (3) The presence of a stationary dipole at a depth consistent with the region of axon terminals leads to the conclusion that much of the slow activity of the cortical response is afferent fiber in origin brought about by discontinuities in membrane characteristics of the afferent fiber, its terminal, and the postsynaptic element. Additional slow activity as a result of synaptic transmission cannot be clearly differentiated from that resulting from orthodromic and antidromic activity in the afferent fibers and their endings. (4) Strychnine topically applied to cortex appears to set up a characteristic kind of activity consisting of high-voltage spikes which can be triggered at latencies dependent on the strength of the afferent volley. This activity, when it occurs at very short latencies with respect to the afferent volley, can sum with it to'' give the appearance of a potentiated evoked cortical response.