The role of neuronal elements in regional cerebral impedance changes in alerting, orienting and discriminative responses

Electrical impedance was measured in the hippocampus, amygdala and midbrain reticular formation during alerting, orienting and discriminative performances in the cat. Measurements were made in focal volumes of approximately 1 mm 3 at 1000 cycle/sec with coaxial electrodes. In the fully trained animal, computed averages of hippocampal impedance decreased by as much as 8 per cent of baseline during visual discrimination, whereas alerting and orienting responses immediately preceding were not accompanied by comparable impedance changes. Similar measurements in the rostral midbrain reticular formation showed small responses during orientation and discrimination, and less constantly during alerting responses. The amygdala exhibited consistent responses only in the alerting epoch. The magnitude of the responses in hippocampus and midbrain increased with the level of behavioral performance. When behavioral cues were reversed, the hippocampal impedance response sharply increased on the first post-reversal day, but rapidly declined thereafter and disappeared. Further retraining was associated with gradual reappearance of the response. Similar relationships to levels of performance, and to cue reversal with retraining, were noted in midbrain responses, but without an enhancement immediately after cue reversal. Variability of impedance was calculated in early, mid and late training, and after cue reversal and during retraining, for the consecutive epochs of alerting, orienting and discriminative behavior. In hippocampus and midbrain, variability declined progressively for the whole test epoch at increasing performance levels, but increased sharply immediately after cue reversal, declining again with retraining. Amygdaloid variability was lowest immediately after cue reversal. Unilateral visual cortical resection, leading to retrograde loss of about 80 per cent of lateral geniculate neurons, was followed by perturbations in geniculate impedance baseline from 10 to 30 days postoperatively. Subsequently, responses to a cyclohexamine drug were reduced in the degenerated nucleus to about 20 per cent of those in the intact nucleus. In modeling these impedance phenomena of regional specificity and apparent dependence on intact neuronal populations, it is proposed that significant current pathways may involve both neuroglial elements and an intercellular substance containing appreciable quantities of mucoproteins and mucopolysaccharides. Possible modulation of conductivity i