Conditioned stimulus intensity: role of cutaneous fiber size in classical conditioning of the flexion reflex in the spinal cat.

Three intensities of the conditioned stimulus (CS) were used to investigate the role of cutaneous fiber size on classical conditioning of the flexion reflex in the spinal cat. Conditioning occurred when the CS activated A alpha and A delta cutaneous fibers; CS activation of only the largest myelinated (A alpha) fibers or CS recruitment of unmyelinated (C) fibers in addition to A alpha and A delta fibers did not lead to significant conditioning. Comparison with two unpaired CS-US control treatment groups showed that the facilitation of reflex output during conditioning was specific to pairing of the CS and US and was not a consequence of sensitization or differential habituation rates between paired (conditioning) and unpaired (control) groups. These results were comparable to those from intact animal conditioning studies and thus lend further support to the spinal conditioning preparation as an appropriate simplified system for investigations of neural mechanisms of learning.