Conditioning of cortical neurons in cats with antidromic activation as the unconditioned stimulus.

Single-cell activity was recorded from the postcruciate cortex of acutely prepared cats during a differential classical conditioning procedure. The conditioned stimuli (CS) were hind paw stimuli, and the unconditioned stimulus (US) was pyramidal tract stimulation that produced an antidromic response in the recorded cortical neuron. A control group was also examined in which the pyramidal stimulus was set below the threshold to produce an antidromic response. Clear differential conditioning was found for the experimental group, with antidromic activation of the neuron as the US. There was no evidence of differential conditioning in the control group without antidromic activation. Any activation of orthodromic pathways should have been the same in the control and experimental groups. The absence of conditioning in the control group demonstrated that orthodromic pathways were not contributing to the differential conditioning observed in the experimental group. This indicates that it was activation of the neuron produced by antidromic firing which was important for conditioning. All the evidence suggests that the site of learning was in the cortex. It is concluded that the the role of the US in conditioning may be simply to activate the neuron at an appropriate interval following the CS.