Habituation of the cold shock response: novel drivers of the habituation process in an integrated model of cold shock

Habituation is defined as a diminished response to a stimulus of the same magnitude but is transient and may not result in adaptation. Habituation of the cold shock response (CSR), a life-threatening response if accidentally immersed, can be achieved by as few as four separate short immersions and is retained for 11 to 14-months. Hence, inducing an habituation of the CSR may be practical for those at daily risk of accidental cold water immersion (CWI) to reduce the risk of drowning. Yet the CSR remains variable both before and after habituation suggesting novel drivers of the habituation process require consideration. We have undertaken four studies where the primary variable was the role that acute anxiety prior to and during CWI plays in the extent of the CSR in unhabituated and habituated participants. Forty-eight participants (34 male, 14 female; mean [SD] age 20.3 [1.7] years, height 1.75 [0.1] m, mass 76.2 [16.7] kg) were recruited across these ethically approved studies in order to test the hypotheses that: study 1) acute anxiety could magnify the CSR in unhabituated participants; study 2) acute anxiety after habituation could reverse the habituation of the CSR; study 3) habituation includes a significant perceptual component and study 4) repeated anxiety would prevent CSR habituation. For CWIs, participants were immersed for 7-minutes using a standardised technique to the same depth in15 °C water. When required, anxiety levels were manipulated by deception about water temperature (they were instructed it would be 1°C colder than previous; studies 1,2 & 4) and a punitive maths task where incorrect responses extended immersion to a maximum of 7-minutes (study 4 only). Where CWI habituation was studied (i.e. studies 2 & 4) a total of seven CWIs were completed. In study 3, five thermoneutral water (35 °C) immersions were undertaken between two CWIs to establish the resultant effect of familiarisation (i.e. a change in threat perception) on the CSR. Acute anxiety (20 cm visual analogue scale) was measured prior and after 1,3,5 and 7 minutes of immersion. The cardiorespiratory responses (cardiac frequency [f_c], respiratory frequency [f_R], tidal volume [V_T], minute ventilation [V̇_E ]) were measured by ECG and spirometry. We found: study 1) acute anxiety magnifies the CSR in unhabituated participants; study 2) acute anxiety reverses habituation of the cardiac component of the CSR after habituation has occurred; study 3) habituation includes a significant perceptual component; study 4) habituation is delayed or prevented by repeatedly high(er) levels of anxiety when present throughout habituation immersions. We offer a new integrated model to explain the role anxiety plays in habituation of the CSR considering the neuroanatomical, perceptual and attentional components contributing to the CSR from the perspective of magnifying compared to reducing acute anxiety levels.