Within-stimulus competition in trace conditioning of the rabbit's nictitating membrane response

In four experiments, it was shown that the onset and offset of a tone condi tioned stimulus (CS) in trace conditioning interact in much the same way as a sequence of two stimuli from different sensory sources, such as a tone a nd a light. The duration of the tone was manipulated across values ranging from 400 to 30,000 msec, whereas the trace interval between the tone offset and the US was fixed at 400 msec. The rate and level of total CR acquisiti on were high and constant across all tone durations. Likewise, responding d uring the trace interval on paired trials was relatively constant. However, when the tone onset and tone offset were tested in relative isolation from each other on test trials using a 30,000-msec tone, there was a reciprocal relationship in responding to tone onset and tone offset. When training ha d been conducted with shorter tone durations, responding to tone onset was only moderate but, nonetheless, higher than responding to tone offset, whic h was negligible. At the longer tone durations, responding to tone onset de clined to lower levels, whereas responding to tone offset rose to a high le vel equal to the total level of responding. These effects were consistent a cross manipulations of tone intensity and prior delay conditioning, in whic h only tone onset could function as a CS. Consequently, the trace CS appear s to act as a compound stimulus that engages complex associative processes as well as simple associative processes. The implications of the present re sults for understanding the susceptibility of trace conditioning to hippoca mpal damage in the rabbit nictitating membrane preparation are discussed.