Differentiation between cortical and subcortical lesions following focal ischemia in cats by multimodality evoked potentials.

Regional ischemia was induced in cats by occluding the middle cerebral artery. Evoked and spontaneous electrical activity as well as regional cerebral blood flow (rCBF) were recorded with platinum depth macroelectrodes in three primary cortical areas: the auditory cortex (A, middle ectosylvian gyrus) and the front and hind limb somatosensory cortex (SF and SH, lateral and medial posterior sigmoid gyrus). To distinguish among the various evoked potentials after click, median or tibial nerve stimulation, electrical field interactions had to be eliminated using a multiplex stimulation and analysis system. Spontaneous electrocortical activity was evaluated by power spectral analysis. In all areas, evoked potentials were abolished 10 min after arterial occlusion. However, rCBF behaved differently in these regions: it was severely reduced in A, decreased moderately in SF and remained unchanged in SH. The graded reduction of rCBF in the three cortical areas was related to changes in electrophysiological activity during the first minutes of ischemia. In A, auditory potentials were abolished within 3 min after occlusion, whereas in SH, the decrease of somatosensory responses started after about 5 min. In SF, two components of the EP changes were found: an early decrease immediately and a later decrease about 5 min after occlusion. The different rates of EP impairment possibly correspond to two types of ischemia. The fast EP abolishment seems to be caused by local cortical damage whereas the delayed EP decrease probably reflects impairment of subcortical white matter structures. Thus, this method may be useful for distinguishing between gray and white matter ischemia.