Contraversive circling elicited from the internal capsule and substantia nigra: evidence for a continuous axon bundle mediating circling.

Electrical stimulation of many brain sites (e.g., anteromedial cortex, internal capsule, substantia nigra, superior colliculus, rostro-medial tegmentum, and medial pons) evokes circling. The collision method of Shizgal et al. (J. Comp. Physiol. Psychol., 94 (1980) 227-237) was used to determine whether these sites are functionally connected for the production of circling in rats. If connectivity was evidenced, then refractory period and conduction velocity distributions were determined for axons passing through the connected stimulation sites. Collision of up to 90% was found between electrodes placed in internal capsule and substantia nigra, suggesting that these sites are connected by continuous axons that mediate circling. The refractory periods of these axons ranged from 0.5 to 4.5 ms, and the conduction velocities of these axons ranged from 0.9 to 4.4 ms. These velocities are similar to those of striatonigral axons. No collision was found between anteromedial cortex and any other sites tested, nor between pontine sites and internal capsule or substantia nigra.