Differential effect of task relevance on early and late components of cortical and subcortical somatic evoked potentials in man.

The effect of "task relevance" on early and late components of cortical and subcortical somatic evoked potentials (SEPs) was studied in a group of Parkinsonian patients operated on under local anesthesia for treatment of prominent unilateral tremor. 1. SEPs produced by median nerve stimulation were found at contralateral cortical (ss), thalamic (vcpci, vcai), lemniscal (Lm), postilemniscal (PoLm), prelemniscal (Raprl) and reticular (Ttc) regions. No SEPs were found in other contiguous thalamic (M,Pf, ce) and subthalamic (Q) regions. 2. Subcortical early SEP components consisted of two monophasic positive potentials distributed within a circumscribed thalamo-lemniscal region where electrical stimulation elicited consistent sensory responses circumscribed to contralateral hand and face. In contrast, subcortical late SEP components consisted of monophasic or polyphasic, positive or negative potentials distributed in a widespread, thalamic, lemniscal, prelemniscal and reticular region where elecrical stimulation elecited sensory or motor responses of various types. Subcortical early and late SEP components appeared together in lemniscal, thalamic and cortical regions but they wers separated at postlemniscal (only early) and prelemniscal and reticular ones (only late). 3. Significant amplitude changes in cortical and subcortical late SEP componets were found concomitant to variations in "task relevance": they decreased when patients shifted from novelty to habituation, they increased when patients shifted from habituation to attention and they decreased when patients shifted from attention to distraction. In contrast, no significant ampiltude changes in cortical and subcortical early components were found when patients shifted through these various "task relevance" conditions.