Reacquisition after extinction often appears faster than original acquisition. However, data from conditioned suppression studies indicate that this effect may arise from spontaneous recovery and reinstatement of unextinguished contextual stimuli related to the unconditioned stimulus (US). In the present experiments using the rabbit nictitating membrane preparation, spontaneous recovery was eradicated before reaquisition training. US contextual stimuli were controlled by retaining the US during extinction through explicit unpairings of the conditioned stimulus (CS) and US. Attempts were also made to drive the associative strength of the CS into the inhibitory region by differential conditioning and conditioned inhibition procedures. In all cases, reacquisition was very rapid in comparison with a rest control. The results are discussed with respect to their implications for CS and US processing models of conditioning.
Conditioning theories and recent real-time models commonly postulate that a reinforcer is signaled by aseries of stimuli. In both Pavlovian and operant procedures, serial stimuli have heen shown to control the likelihood and timing of responses over intervals of seconds and minutes. The present experiments were conducted to determine whether serial stimuli exercise similar effects over stimulus-reinforcer intervals in the order of hundreds of milliseconds. Such intervals typify those used in conditioning of the rabbit nictitating membrane response. A sequence of four tone pulses (50-100 msec) was used as the CS to assess the effectiveness of serial stimuli. After training with this CS, tests were conducted in which one or more of the pulses were removed. These perturbations of the sequence of stimuli over a 400-msec interval produced large deficits in CR likelihood and smaller alterations in CR timing. The results are discussed with respect to their implications for current real-time models of conditioning, and particularly with respect to their assumptions about the source of internal stimuli, rules for learning, and rules for generating CRs.
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