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 conditioned 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 and 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 acquisition were high and constant across all tone durations. Likewise, responding during 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 had been conducted with shorter tone durations, responding to tone onset was only moderate but, nonetheless, higher than responding to tone offset, which was negligible. At the longer tone durations, responding to tone onset declined to lower levels, whereas responding to tone offset rose to a high level equal to the total level of responding. These effects were consistent across manipulations of tone intensity and prior delay conditioning, in which only tone onset could function as a CS. Consequently, the trace CS appears to act as a compound stimulus that engages complex associative processes as well as simple associative processes. The implications of the present results for understanding the susceptibility of trace conditioning to hippocampal damage in the rabbit nictitating membrane preparation are discussed.