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

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Nictitating membrane (NM) and heart rate (HR) responses were investigated in a conditional discrimination (A→X+ vs. B→X−), using feature-target intervals of 0, 5, 15, and 45 sec. Conditional control of NM responses, but not of HR responses, was acquired to the 400-msec X stimulus in all the groups tested. However, differential conditioning of both the NM and the HR responses 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 responding to X, in which the highest level of responding occurred at or near the interval used in training. The results are discussed with respect to the relationship of simple conditioning of the feature cues to their control over responding 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.