The effect of experimental ischaemia on the direct cortical response of the motor cortex in primates.

We studied the changes in amplitude of the first short latency positive potential (2.3 +/- 0.3 msec, mean +/- S.D.) of the direct cortical response (DCR) elicited by surface electrical stimulation of the motor cortex in anaesthetised baboons. Local cortical blood flow, measured by the hydrogen clearance method, was progressively reduced by acute middle cerebral artery occlusion and subsequent hypotension and was related to the amplitude of this potential. With flow levels greater than 25 ml/100 g/min the DCR was essentially unaffected, but it was lost with flows below 20 ml/100 g/min. These results indicate a threshold relationship between the generation of the electrical activity evoked in the cortical elements and local cortical flow, similar to that previously demonstrated for cortical somatosensory evoked potentials.