Physiological changes during tonic immobility in Gallus gallus var domesticus

Physiological parameters were examined in the hen in response to the induction of tonic immobility (TI). The induction of TI was associated with EEG arousal, low shank temperature and elevated heart rate. However, shortly after the adoption of TI, the EEG showed a predominantly slow-wave deactivated pattern, muscle tone fell and sympathetic nervous activity was reduced, as indicated by declining heart rate and peripheral vasodilation. Core temperature remained constant at 42°C throughout the immobility response. Several instances of EEG-behavioural dissociation were observed. The results are discussed in terms of “cut-off” and waning of the induced fear state.