Trace conditioning as an inhibitory procedure

Virtually no evidence of excitatory CR acquisition is observed in rabbit eyelid conditioning with a trace-conditioning procedure if the CS-UCS trace interval is greater than a few seconds. The lack of overt CRs following training with a trace-conditioning procedure has traditionally been interpreted as due to the passive decay of the CS stimulus trace over time. However, it is conceivable that a trace interval sufficiently long to preclude CR acquisition may be sufficiently long to teach the subject that the CS signals a UCS-free period, thereby producing an inhibitory CS. To test this proposal, eyelid conditioning in rabbits was conducted in two experiments with a 10-sec trace interval (there being no evidence of excitatory conditioning with this procedure). The trace-CS was then tested for evidence of inhibition with retardation-of-acquisition and summation tests. The trace-CS was shown to have conditional inhibitory properties. These results indicate that the interstimulus interval function in trace conditioning may be determined, in part, by inhibitory associations.