Altering the synchrony of stimulus trace processes: tests of a neural-network model.

A previously described neural-network model (Desmond 1991; Desmond and Moore 1988; Moore et al. 1989) predicts that both CS-onset-evoked and CS-offset-evoked stimulus trace processes acquire associative strength during classical conditioning, and that CR waveforms can be altered by manipulating the time at which the processes are activated. In a trace conditioning paradigm, where CS offset precedes US onset, the model predicts that onset and offset traces act in synchrony to generate unimodal CR waveforms. However, if the CS duration is subsequently lengthened on CS-alone probe trials, the model predicts that onset and offset traces will asynchronously contribute to CR output and bimodal CRs will be generated. In a delay conditioning paradigm, in which US onset occurs prior to CS offset, the model predicts that only the onset process will gain associative strength, and hence, only unimodal CRs will occur. Using the rabbit conditioned nictitating membrane response preparation, we found experimental support for these predictions.