Much of the published literature on the immobility reflex (IR) of birds has been interpreted to indicate that the IR is an innate fear response that results from aversive environmental conditions. We chose to test the fear hypothesis in rabbits using two usual approaches: testing the effects of chlorpromazine, a presumed fear-reducing drug, and repeated handling. Chlorpromazine, even in large mildly toxic doses did not interfere with IR duration but, rather, enhanced it markedly. The influence of repeated trials was a function of intertrial interval; long intervals produced short-duration trials and short intervals produced long durations. Little habituation occurred with short intervals, even after as many as 774 trials in one of the rabbits. All rabbits in the long intertrial interval group developed shorter durations but not to the point of extinction; these durations could be made somewhat longer by nociceptive electric shock to the ear. Retest of IR after a 4-week lapse revealed that IR durations had returned to the initial level. Analysis of the test contingencies and data lead to the conclusion that, in the rabbit, fear is not the sole or necessary cause of IR and that there are other important variables, including, perhaps, instrumentally conditioned control over the IR duration by the animal. © 1977 Academic Press, Inc.
- Handling (psychology)
- Dose-Response Relationship, Drug
- Habituation, Psychophysiologic