Circling elicited from the anteromedial cortex and medial pons: refractory periods and summation.

Contraversive circling is evoked by stimulating the anteromedial cortex (AMC) of rats, and ipsiversive circling is evoked by stimulating the medial pons (PONS). During AMC circling, lateral and vertical head movements and vibrissae movements were exhibited. During PONS circling, although lateral head movements were exhibited, vertical head movements and vibrissae movements were not exhibited. Refractory periods were estimated by delivering trains of paired pulses and measuring the frequency thresholds for circling at various intrapair intervals. Refractory periods at AMC circling sites were much longer (range 1.4-3.3 ms) than at PONS circling sites (range 0.5-1.0 ms). To determine the degree of summation between the AMC and contralateral PONS, the two sites were stimulated concurrently. Summation of 95-100% was observed for AMC and PONS circling. No collision was observed at short intrapair intervals of paired pulses. Thus, the AMC and PONS are not connected axonally but are related, perhaps serially, for the production of circling.