Depth-duration profile of the immobility reflex: theoretical implications for its triggering, sustaining, and terminating mechanisms.

Immobility reflex (IR) durations of rabbits were remarkably consistent (weighted average range of 0.36-0.70 minutes/trial), under the special conditions when rabbits were "habituated" by approximately 25 preliminary trials and when termination of IR was operationally defined as the point at which any attempt, even abortive, at righting occurred. When the mean duration for each rabbit was normalized (into quartile duration points) and then rabbits were tested at these points to determine arousal threshold, each rabbit revealed a clear progressive decrease in the "depth" of IR as the duration of a given episode progressed. Both duration and depth were decreased when body hair was removed. The hippocampal EEg revealed persistent theta activity throughout a given trial; but the specific frequency of theta increased briefly just prior to induction of IR and decreased immediately afterwards. In short-duration trials, theta frequency increased progressively until the IR terminated spontaneously. In long-duration trials frequency changes oscillated. Each of these observations is interpreted to support the theory that, during IR, reverberating neural circuits (in the brainstem reticular formation) are activated and their output inhibits spinal motor neurons.