Evidence that the neural pathways involved in backward conditioning are different from those involved in forward conditioning

Effects of forward and backward conditioned-unconditioned stimulus (CS-US) intervals on classical conditioning of the flexion reflex were examined in a spinal cat preparation. A less intense conditioned stimulus (CS) was employed (activation of A-alpha cutaneous fibers) compared to that of a previous study (activation of both A-alpha and A-delta cutaneous fibers). Interstimulus intervals (ISIs) ranging from +3.0 to -3.0 sec were examined in 9 experimental groups, and results contrasted to those of an explicitly unpaired control group. The ISI of -0.25 sec produced optimal backward excitatory conditioning, paralleling the previous results. However, in contrast to the previous study, no conditioning was observed in any of the forward ISI groups. These observations support the hypothesis that backward and forward conditioning may be fundamentally different phenomena, induced by CS activation of different spinal reflex pathways: backward conditioning involves spinal reflex pathways activated by A-alpha cutaneous fibers of the CS, while forward conditioning involves spinal reflex pathways activated by A-delta cutaneous fibers of the CS.