During and after 15-min occlusion of the middle cerebral artery (MCA) in cats, local CBF and neuronal activity were measured in cortical areas varying in the degree of CBF reduction. In an area within the ischemic center (primary auditory cortex, middle ectosylvian gyrus), CBF was severely suppressed. Click-induced auditory evoked potentials and evoked as well as spontaneous single-unit activity ceased within 1 min after occlusion. Recirculation resulted in a recovery of the different neurophysiological parameters with a time delay ranging from several minutes to 2 h. In two areas surrounding the ischemic focus (a visual area in the marginal gyrus and the forelimb representation area in the primary somatosensory cortex), CBF was reduced but remained above 30 ml/100 g/min during MCA occlusion. Visual flash-induced evoked potentials and somatosensory evoked potentials induced by median nerve electrical stimulation ceased in the corresponding areas with a somewhat slower time course as compared to the auditory responses and they recovered faster after recirculation. In another somatosensory area (hindlimb projection area in the primary somatosensory cortex), CBF stayed nearly at control levels during occlusion. Evoked potentials and single-unit activity induced by tibial nerve electrical stimulation decreased ∼5 min after occlusion and were abolished ∼5 min later. At that time, single-unit activity had changed to a nonresponsive pattern but persisted. However, potentials evoked transcallosally by electrical stimulation of the contralateral hemisphere were still recorded. After reopening the MCA, the recovery of neuronal functions was usually complete and occurred within ∼5 min. We conclude that attention has to be focused on those areas surrounding an acute ischemic focus that show either no or only slight CBF alterations. The functional impairment found in such areas is caused by the injury of subcortical structures leading to a cortical deafferentation. Considering the apparent lack of a CBF disturbance, such a condition should be distinguished from the so-called cortical ischemic penumbra.
The effect of peripheral high-frequency electrical stimulation on the primary somatosensory cortex in pigs
