HUMAN TONIC IMMOBILITY: MEASUREMENT AND CORRELATES

Background: Tonic immobility (TI) is a temporary state of motor inhibition believed to be a response to situations involving extreme fear. Limited attention has been directed to studying TI in humans; however, the phenomenon has been well documented in the animal literature. In humans, TI is believed to occur during sexual assault, and there have been reports of fear-induced freezing in the contexts of air, naval, and other disasters. Methods: This study had three main purposes: (1) to assess the factor structure of a new self-report measure-the Tonic Immobility Questionnaire([1]) -designed to assess human TI in a range of traumatic events; (2) to explore associations among discovered TIQ factors and a measure of posttraumatic symptoms in the context of trauma type; and (3) to determine whether TI is related to suspected and empirically supported predictors of posttraumatic stress disorder. Participants were a subset of undergraduate students (n = 78) who reported a TI experience in the context of a traumatic event. Results: No differences were found in frequency or severity of TI reported across trauma types. Exploratory factor analysis of Tonic Immobility Questionnaire item responses resulted in a three-factor solution (i.e., physical immobility, fear, and dissociation). Significant positive correlations were found between the Tonic Immobility Questionnaire and measures of posttraumatic symptoms, dissociation, anxiety sensitivity, and absorption. Regression analysis revealed that peritraumatic dissociation scores alone accounted for 51% of the variance in TI scores. Conclusions: TI may represent an extreme behavioral expression of trauma-induced peritraumatic dissociation. Implications and directions for future research are discussed. Depression and Anxiety 26:550-556, 2009. (c) 2009 Wiley-Liss, Inc.