Electrically evoked turning: asymmetric and symmetric collision between anteromedial cortex and striatum

Electrical stimulation of the anteromedial cortex (AMC) or striatum of rats evoked contraversive eye, head and body movements. In these experiments we test which neurons and which pathways are responsible for the turning by delivering conditioning (C) pulses to one site and test (T) pulses to the second site, and measuring the frequency of pulse pairs required to evoke a full turn in 10 s. Decreases in the required frequency were usually found at C-T intervals from 0.6 to 1.0 ms, whether the C pulses were delivered to the AMC or to the striatum. This symmetric effect is attributed to collision in fast-conducting axons connecting cortex and striatum. Symmetric collision at C-T intervals of 2-4 ms was observed between cortex and 3 dorsal striatal sites, suggesting slower axons from cortex to these dorsal striatal sites. In several animals, asymmetric changes in required frequency also occurred. When the C pulses were presented via the striatal electrode, the recovery in required frequency occurred at C-T intervals of 1-4 ms, but when the C pulses were presented via the cortical electrode, recovery occurred at C-T intervals of 2-50 ms. This asymmetry is attributed to indirect (i.e., transynaptic) activation of corticostriatal or striatal output axons. These results suggest that in both cortex and striatum there are synapses, transmitting from rostral to caudal, which are important for electrically evoked turning. When C and T pulses were delivered to the same site, decreases in required frequency occurred at C-T intervals from 0.4 to 4 ms, attributable to recovery from refractoriness. In 3 striatal sites, however, large changes were also seen at C-T intervals from 6 to 50 ms. In all 3 sites, asymmetric collision occurred at these same intervals. The recovery at long C-T intervals could be due to transynaptic collision also, resulting from the simultaneous activation of presynaptic and postsynaptic axons by a single striatal electrode.