SOMATOSENSORY-EVOKED POTENTIALS IN THE BA BOON - MORPHOLOGY AND CORTICAL INTERAREAL SI-MI RELATIONSHIPS

Primary somatosensory potentials (SEPs) were elicited by electrical st imulation of the medial sciatic (proprioceptive) or sural (cutaneous) nerves. They were detected by contacts on S1 and MI dura on both corti cal sides against a cephalic reference. SEPs were averaged (n = 20). P rimary SI SEP consisted of positive, then negative, P16/N30 waves. N30 war absent from MI records. Local electrocoagulation of the SI cortic e on one side has entailed some reduction, but not suppression of toge ther the homolateral MI and contralateral SI and MI SEP. The residual SEPs have increased in latencies by a few milliseconds. Additional coa gulation of the MI area on the same side has resulted in loss of the S I and MI SEP on the opposite hemisphere when evoked by a stimulation i psilateral to this intact cortex. Normal SEPs were elicitable from the intact cortex by any of the used stimulation. No evoked signal could be evidenced from the lesioned areas; It was concluded that negligible passive electrical diffusion from any SEP area was present onto any o f the other SEP reception sites. From close comparison between the dif ferent records, we came to the following propositions: each of the SI and MI areas harbours a neural mass generator for SEPs elicitable by c ontralateral nerve stimulation. SI and MI SEPs cannot be directly elic ited by ipsilateral stimulus. SI and MI SEP ipsilateral to the nerve s timulation are due To some cortico-cortical trans-sagittal excitatory message arising from the contralateral SI/MI areas. Data stand for ext eroceptive or proprioceptive stimulation as well. The absence of ipsil ateral direct spinocortical projection for SEP evidenced under barbitu rate does also exist in the baboon after total recovery of surgery. A scheme is given which summarizes these active relationships between so mesthetic areas.