Habituation and sensitization of protective reflexes: dissociation between cardiac defense and eye-blink startle.

We examined the habituation and recovery of two protective reflexes, cardiac defense and eye-blink startle, simultaneously elicited by a white noise of 500ms as a function of the time interval between stimulus presentations. Participants were 90 volunteers (54 women) randomly distributed into 6 inter-trial interval (ITI) conditions. They all received three presentations of the stimulus with a time interval of 30min between the first and third noise. The timing of the second noise was manipulated in six steps, using a between-group design, in order to increase the ITI between Trials 1 and 2 and symmetrically decrease the ITI between Trials 2 and 3. Cardiac defense showed fast habituation at the shortest ITI (2.5min), but reduced habituation and increased recovery at the longest ITI (27.5min). In contrast, eye-blink startle showed sensitization irrespective of the ITI. This pattern of findings highlights dissociations between protective reflexes when simultaneously examined. The results are discussed in the context of the cascade model of defense reactions.