The effect of feature-target intervals in conditional discriminations on acquisition and expression of conditioned nictitating membrane and heart rate responses in the rabbit

Nictitating membrane (NM) and heart rate (HR) responses were investigated i n a conditional discrimination (A-->X+ vs. B-->X-), using feature-target in tervals of 0, 5, 15, and 45 sec. Conditional control of NM responses, but n ot of HR responses, was acquired to the 400-msec X stimulus in all the grou ps tested. However, differential conditioning of both the NM and the HR res ponses to A versus B feature cues appeared for the three shorter intervals. Following acquisition, all the rabbits were tested with the four different feature-target intervals. All the groups showed a gradient of NM respondin g to X, in which the highest level of responding occurred at or near the in terval used in training. The results are discussed with respect to the rela tionship of simple conditioning of the feature cues to their control over r esponding during presentation of the target stimulus, the putative role of HR as an index of preparatory processes during presentation of feature cues , and mechanisms of temporal specificity in conditional discriminations.