Tonic Immobility Does Not Uniquely Predict ...

Psychological Trauma: Theory, Research, Practice, and Policy 2012, Vol. 4, No. 3, 278 –284

© 2011 American Psychological Association 1942-9681/11/$12.00 DOI: 10.1037/a0023272

Tonic Immobility Does Not Uniquely Predict Posttraumatic Stress Symptom Severity Murray P. Abrams, R. Nicholas Carleton, and Gordon J. G. Asmundson University of Regina Tonic immobility (TI) is an involuntary state of temporary motor inhibition believed to occur in response to events that provoke extreme fear and the perception of inescapability. Human TI has been documented in a range of traumatic events and several researchers have reported associations between TI and posttraumatic stress symptoms (PTSS); however, it remains unclear if TI is a unique predictor of PTSS. This study was designed to determine whether TI severity would account for variance in PTSS severity over and above the influence of peritraumatic dissociation and trait anxiety. Participants were community members (n ⫽ 75; 88% women; ages 18 – 65, Mage ⫽ 31.49, SD ⫽ 12.21) who reported TI during a traumatic event. TI, peritraumatic dissociation, and trait anxiety were assessed as part of a larger investigation. Results of hierarchical regression analyses indicated trait anxiety and peritraumatic dissociation, but not TI, were significant and substantive predictors of PTSS scores. In all analyses TI scores failed to account for significant variance in PTSS scores (all ps ⬎ .05). Results suggest the TI construct may add little to understanding PTSS beyond what can be ascertained by assessing peritraumatic dissociation and trait anxiety. Given mixed findings to date, further investigation is required to disentangle what is shared and what is distinct among these constructs. Keywords: tonic immobility, peritraumatic dissociation, trait anxiety, trauma, posttraumatic stress

characterize predatory encounters) may in some individuals provoke TI. Several studies have linked TI with increased PTSS severity. For example, Heidt and colleagues (Heidt et al., 2005) found that psychiatric inpatients and undergraduates who reported TI during CSA also reported more severe PTSS together with increased symptoms of depression, anxiety, and peritraumatic dissociation. Similarly, in a treatment study of persons with PTSD consequent to exposure to urban violence (primarily armed robbery), participants who reported a TI experience also reported increased PTSD symptom severity (r2 ⫽ .18 – a medium effect size; Cohen, 1988), relative to those not reporting TI (Fiszman et al., 2008). Few investigations have specifically examined whether TI is uniquely predictive of increased PTSS severity. In one such study of sexual assault survivors, Bovin and colleagues (Bovin, Jager-Hyman, Gold, Marx, & Sloan, 2008) examined TI as a potential mediator of relationships between perceived inescapability, peritraumatic fear, and PTSS severity. Using guidelines proposed by Baron and Kenny (Baron & Kenny, 1986; Kenny, Kashy, & Bolger, 1998), they found that TI mediated relationships between perceived inescapability and overall PTSS severity, as well as PTSD reexperiencing and avoidance/numbing symptom clusters. They also reported that TI fully mediated relationships between fear and reexperiencing symptoms, and partially mediated relationships between fear and overall PTSS severity. In a recent study of persons with current PTSD diagnoses, Rocha-Rego and colleagues (Rocha-Rego et al., 2009) investigated the relative contributions of TI, peritraumatic dissociation, and panic reactions as predictors of PTSS severity. Results of multiple regression analyses were interpreted as indicating TI may be the core peritraumatic symptom associated with PTSS severity.

Tonic immobility (TI) is an involuntary state of temporary motor inhibition believed to occur in response to events that provoke extreme fear and the perception of inescapability (Marx, Forsyth, Gallup, Fuse, & Lexington, 2008). TI has been conceptualized as the terminal defensive response, occurring after flight and fight behaviors have been exhausted ( Blanchard & Blanchard, 1988; Marks, 1987). The response has been documented for centuries in numerous species (Gallup & Maser, 1977) and is posited to have evolved as an adaptive response to animal predation (Gallup, 1974, 1977; Marks, 1987). TI has been studied extensively in nonhuman animals but there have been relatively few investigations of human TI. In humans, TI has been reported in contexts of adult sexual assault (Burgess & Holmstrom, 1976; Galliano, Noble, Travis, & Puechl, 1993), childhood sexual abuse (CSA; Heidt, Marx, & Forsyth, 2005), armed robbery (Fiszman et al., 2008), motorvehicle accidents (MVAs), trauma involving exposure to death, (Abrams, Carleton, Taylor, & Asmundson, 2009), and air, naval, and other disasters (Leach, 2004). Physical immobilization resembling TI has also been reported in the context of panic attacks among persons with panic disorder (Cortese & Uhde, 2006). Apparently, situations that involve elements of intense fear, perceived inescapability, and perceived threat of death (all believed to

This article was published Online First April 11, 2011. Murray P. Abrams, R. Nicholas Carleton, and Gordon J. G. Asmundson, Department of Psychology, University of Regina. Correspondence concerning this article should be addressed to Gordon J. G. Asmundson, Department of Psychology, University of Regina, 3737 Wascana Parkway, Regina SK, S4S 0A2. E-mail: Gordon.Asmundson@ uregina.ca 278

TONIC IMMOBILITY

Peritraumatic dissociation is a further reaction to trauma posited to be related to TI. Peritraumatic dissociation refers to dissociative reactions during a traumatic event and is characterized by alterations in perceptions of time, place, and person. Peritraumatic dissociative experiences include feelings of unreality, depersonalization, disorientation, altered pain perception (a feature of TI), and tunnel vision (Marmar, Weiss, & Metzler, 1997). It has been identified as a correlate of (Abrams et al., 2009; Fuse´, Forsyth, Marx, Gallup, & Weaver, 2007; Heidt et al., 2005) and possible precondition for TI (Abrams et al., 2009). Several researchers have linked peritraumatic dissociation to the development of PTSD (Birmes et al., 2003; Brewin, Andrews, & Valentine, 2000; Koopman, Classen, & Spiegel, 1994; Ozer, Best, Lipsey, & Weiss, 2003), whereas others report little support for such a relationship (van der Velden et al., 2006; van der Velden & Wittmann, 2008). Despite the aforementioned research linking TI to the development of PTSD, the debate regarding the nature of this relationship is ongoing. Some theorists have posited that reported TI may be a proxy indicator of event severity; that is, events during which TI occurs are likely to be very severe, and worsened posttraumatic symptoms may be attributable to the severity of the event rather than the occurrence of TI (Zoellner, 2008). Other have suggested TI and peritraumatic dissociation are intrinsically related (Heidt et al., 2005; Marx et al., 2008), which may confound the relative effect of these constructs on PTSS. Finally, some researchers have highlighted problems with validity of the TI construct. For example, persons endorsing a TI experience may be confusing the state with volitional acquiescence (Zoellner, 2008). The current study attempted to clarify whether TI uniquely contributes to PTSS severity over and above associations already established for peritraumatic dissociation (e.g., Ozer et al., 2003) and trait anxiety. Given suggestions that TI and peritraumatic dissociation constructs may overlap (Heidt et al., 2005; Marx et al., 2008), or be aspects of the same phenomena (e.g., Scaer, 2001), we hypothesized that TI would not significantly predict variance in PTSS severity over and above that accounted for by trait anxiety and peritraumatic dissociation. We included trait anxiety as a predictor in an effort to control for the possibility generally anxious responding would account for variance in PTSS scores. We also sought to explore whether TI was differentially related to the PTSD symptom clusters of reexperiencing, avoidance/numbing, and hyperarousal delineated in the Diagnostic and Statistical Manual of Mental Disorders-IV (DSM–IV; American Psychiatric Association, 2000).

Method Participants and Procedure Participants were from a larger sample (N ⫽ 249) of community members (n ⫽ 169) and undergraduate students (n ⫽ 80) who reported experiencing a traumatic event. Individuals who reported TI during trauma comprised the sample used in this study (n ⫽ 75: 18 students/57 community members, 88% women; ages 18 – 65, Mage ⫽ 31.49, SD ⫽ 12.21). Participants resided primarily in the city of Regina or surrounding area in the Canadian province of Saskatchewan. The majority identified their ethnicity as Caucasian (84%) with Aboriginal being the next largest reported ethnic background (6.7%). A small proportion of the sample self-

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identified as Asian (1.3%), Hispanic (1.3%), or other (1.3%). Several participants (5.3%) elected not to report their ethnic background. Recruitment was conducted via web-based advertising (e.g., advertisements on a local radio station’s homepage), posters placed in busy community locations (e.g., laundromats, supermarket notice boards), and a university classifieds email list. Potential participants were asked if they had ever experienced a traumatic event. Community participants were not financially compensated; however, eligible undergraduate students received class marks for participation. Participation involved the completion of a webbased questionnaire battery to assess TI, peritraumatic dissociation, trait anxiety, and symptoms of posttraumatic stress. Data collection was conducted using Survey Monkey and took place over several weeks in the winter of 2007. This study was approved by the University of Regina Research Ethics Board.

Measures Tonic Immobility Questionnaire – Revised [TIQ; Taylor, Stapleton, & Asmundson, 2007). The TIQ is a 12-item self-report measure developed to assess TI across a range of traumatic events. Items are rated on a five point Likert scale ranging from 0 (not at all) to 4 (very much). Development of the TIQ was based, in part, on the item content of the Tonic Immobility Scale – Adult Form (TIS-A; Forsyth, Marx, Fuse´, Heidt, & Gallup, 2000); however, items were designed to generalize across a range of traumatic events rather than being specific to sexual assault. Exploratory factor analysis supports a three-factor solution for the TIQ that is consistent both with reported features of TI in nonhuman animals and with phenomenological and observational reports of humans (Abrams et al., 2009). The three factors were characterized as physical immobility, fear, and dissociation. Prior to responding to TIQ items participants were presented with the following: “When people experience extremely stressful or traumatic events, they sometimes feel “frozen” or “paralyzed with fear.” Individuals may be unable to move even though they remain conscious and are not physically restrained. For example, during an assault a person may feel frozen or paralyzed with fear and be unable to resist their attacker.” The current study focused on the core symptoms of TI based on a precedent analytic approach (Rocha-Rego et al., 2009); accordingly, only the TI physical immobility and dissociation scales were included in analyses in order to distinguish the influence of tonic immobility from peritraumatic fear. Sample items from the immobility scale included: “My body felt frozen,” “My legs felt paralyzed,” and from the dissociation scale: “My ability to feel pain was diminished,” “I had trouble keeping my eyes open,” and, “I felt faint or light-headed.” The TIQ fear subscale scores were not included in analyses. For the current sample, internal consistency was acceptable for the TIQ total scale (␣ ⫽ .83), for the immobility subscale (␣ ⫽ .84), and for the composite TI variable (␣ ⫽ .82, i.e., physical immobility ⫹ dissociation subscales) used in regression analyses herein. Internal consistency was low for the dissociation (␣ ⫽ .62) subscale. Average interitem correlations were as follows: TIQ total scale ⫽ .33; immobility subscale ⫽ .52; dissociation subscale ⫽ .29; and composite TI variable ⫽ .35. Item data were evaluated for

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Skew and Kurtosis and found to be within recommended limits (Tabachnick & Fidell, 2001). Peritraumatic Dissociative Experiences Questionnaire (PDEQ; Marmar et al., 1997). The PDEQ is a 10-item questionnaire that asks respondents to recall dissociative experiences (e.g., derealization, depersonalization, amnesia, altered time perception) that may have occurred during the traumatic event. Items on the PDEQ are responded to on a Likert scale ranging from 1 (not at all true) to 5 (extremely true). Sample items include: “My sense of time changed—things seemed to be happening in slow motion”; “I felt as though things that were actually happening to others were happening to me—like I was being trapped when I really wasn’t”; and, “What was happening seemed unreal to me, like I was in a dream or watching a movie or play.” The PDEQ has demonstrated good internal consistency and convergent validity with other measures of traumatic responding (Marmar et al., 1994). Peritraumatic dissociation as measured by the PDEQ is generally believed to be a unitary construct (Marmar et al., 1994; 1997); however, a recent investigation of the psychometric properties of the PDEQ instead suggested an eight-item two-factor structure consisting of altered awareness and derealization (Brooks et al., 2009). For the purposes of this investigation, the original 10-item PDEQ was used in all analyses. Internal consistency for the current sample was acceptable (␣ ⫽ .91) and item data were within recommended limits for Skew and Kurtosis (Tabachnick & Fidell, 2001). The average interitem correlation was .50. State–Trait Anxiety Inventory – Trait scale (STAI-T; Spielberger, Gorsuch, & Luschene, 1970). The STAI-T is a 20-item self-report measure designed to assess trait anxiety (e.g., “I get in a state of tension or turmoil as I think over my recent concerns and interests”). Items are endorsed on a four point Likert scale ranging from 1 (almost never) to 4 (almost always). The STAI-T has been shown to have good internal consistency, good stability, as well as adequate validity (Spielberger et al., 1970). Internal consistency for the current sample were acceptable with a scale alpha of .92. Item data were within recommended limits for Skew and Kurtosis (Tabachnick & Fidell, 2001). Average scale interitem correlation was .38. PTSD Checklist Civilian Version (PCL-C; Weathers, Litz, Huska, & Keane, 1994). The PCL-C is a 17 item self-report measure designed to assess symptoms of PTSD as described in the DSM-IV (American Psychiatric Association, 2000). The PCL-C has demonstrated high diagnostic efficiency of .90 (Blanchard, Jones-Alexander, Buckley, & Forneris, 1996) and strong convergent validity with other measures of response to trauma (Weathers, Litz, Herman, Huska, & Keane, 1993). Respondents are asked to indicate the degree to which they have been disturbed by each trauma-related symptom during the past month on a 5-point Likert scale ranging from 1 (not at all) to 5 (extremely). For this study, reexperiencing, avoidance, numbing, and hyperarousal were each scored separately to be consistent with evidence suggesting avoidance and numbing are better conceptualized as distinct (Asmundson, Stapleton, & Taylor, 2004). Consistencies for the current sample were acceptable for the PCL-C total scale (␣ ⫽ .91), reexperiencing (␣ ⫽ .84), numbing (␣ ⫽ .80), and hyperarousal subscales (␣ ⫽ .79), but low for the avoidance subscale (␣ ⫽ .63). All PCL-C item data were within recommended limits for Skew and Kurtosis (Tabachnick & Fidell, 2001). The average interitem correlations were as follows: PCL-C total scale ⫽ .39; reexperi-

encing ⫽ .51; avoidance ⫽ .46; numbing ⫽ .52; and hyperarousal ⫽ .44. For the current sample, average PCL-C scores were in the clinical range (M ⫽ 43.79, SD ⫽ 14.91) with almost half (48%) meeting or surpassing the cut-off score of 44 shown to be predictive of DSM–IV PTSD diagnosis (Blanchard, JonesAlexander, Buckley, & Forneris, 1996b).

Results Initial analyses assessed for and found no significant sexdifferences (all ps ⬎ .05) on all predictor and dependent variables (i.e., TI composite variable, PDEQ, STAI-T, PCL-C subscales). The traumatic events during which TI was reported included sexual assaults, (n ⫽ 19, 89.5% women), physical assaults (n ⫽ 9, 88.9% women), motor vehicle accidents (n ⫽ 5, 80% women), other accidents (n ⫽ 1, 0% women), trauma involving exposure to death (primarily, learning of the unexpected death of a loved one; n ⫽ 17, 88.2% women), and other events that did not fall under the previous categories (e.g., armed robbery, dog attack, witnessing violent assault, confrontations in social contexts; n ⫽ 24, 91.7% women). Bivariate correlations among all predictor and dependent variables were calculated (see Table 1). TI was strongly correlated with peritraumatic dissociation, moderately correlated with all PTSD symptom cluster scores, and not significantly correlated with trait anxiety. Peritraumatic dissociation was moderately correlated with PTSD symptom cluster scores but not significantly correlated with trait anxiety. Trait anxiety was not significantly correlated with PTSD avoidance symptoms, moderately correlated with reexperiencing and numbing scores, and strongly correlated to hyperarousal scores. A series of four hierarchical regression analyses were conducted to examine whether the effect of TI on PTSS would remain after controlling for trait anxiety (STAI-T) and peritraumatic dissociation (PDEQ). STAI-T scores were entered on the first step, PDEQ scores were entered on the second step, and TI composite variable scores were entered on the third and final step. The entry order was consistent with recommended practice for hierarchical regression that predictors be entered into the model in temporal order (Petrocelli, 2003); specifically, trait anxiety first— by definition a

Table 1 TI and PTSS—Correlations Among Predictors and PCL-C Subscales Predictors 1. 2. 3. 4. 5. 6. 7.

PCL-C reexp. PCL-C avoid. PCL-C numb. PCL-C hyp. STAI-T PDEQ TI comp.

1

2

3

4

5

6

.66ⴱⴱ .64ⴱⴱ .61ⴱⴱ .33ⴱⴱ .41ⴱⴱ .33ⴱⴱ

.54ⴱⴱ .48ⴱⴱ .20 .35ⴱⴱ .23ⴱ

.60ⴱⴱ .36ⴱⴱ .46ⴱⴱ .35ⴱⴱ

.57ⴱⴱ .40ⴱⴱ .40ⴱⴱ

.12 .20

.50ⴱⴱ

Note. N ⫽ 74. PCL-C ⫽ PTSD Checklist – Civilian Version; reexp. ⫽ reexperiencing; avoid. ⫽ avoidance; numb. ⫽ numbing; hyp. ⫽ hyperarousal; STAI-T ⫽ State Trait Anxiety Inventory - Trait scale; PDEQ ⫽ Peritraumatic Dissociative Experiences Questionnaire; TI comp. ⫽ Tonic Immobility composite variable. ⴱ p ⬍ .05. ⴱⴱ p ⬍ .01.

TONIC IMMOBILITY

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Table 2 TI and PTSS: Multiple Regression PCL-C Reexperiencing Scores Dependent Model

Predictors

M (SD)

1 STAI-T

R2

⌬R2

.33

29.09 (10.87)

.29ⴱⴱ .38ⴱⴱ

.29 .37

8.38 (5.06)

.27ⴱ .32ⴱⴱ .12

.27 .27 .10

⌬F

F ⴱⴱ

.11

.33ⴱⴱ

3 STAI-T PDEQ TI

part r

47.66 (11.12)

2 STAI-T PDEQ

␤

8.95

.25

.14ⴱⴱ

.26

.01

11.81ⴱⴱ

13.17ⴱⴱ

8.19ⴱⴱ

.95

Note. PCL-C reexperiencing scale descriptive statistics: M ⫽ 13.27, SD ⫽ 4.88. ⴱ p ⬍ .05. ⴱⴱ p ⬍ .01.

dispositional variable (Spielberger et al., 1970), followed by peritraumatic dissociation—posited to precede TI (Abrams et al., 2009), and finally TI— conceptualized as the terminal defensive response (Blanchard & Blanchard, 1988; Marks, 1987). Dependent measures were PCL-C symptom cluster scores (i.e., reexperiencing, avoidance, numbing, hyperarousal). All assumptions for regression were evaluated and met (i.e., outliers, normality, linearity, homoscedasticity, independence of residuals). Regression results indicated TI did not account for variance in PTSD symptom cluster scores over and above the influence of trait anxiety and peritraumatic dissociation (Tables 2, 3, 4, & 5). Across all analyses, trait anxiety and, especially, peritraumatic dissociation were stronger and more substantial predictors of PTSS scores.

Discussion The current study was designed to clarify whether TI severity was uniquely predictive of symptoms of posttraumatic stress after controlling for the influence of trait anxiety and peritraumatic dissociation. Hierarchical regression analyses were used to assess the relative contributions of TI, trait anxiety, and peritraumatic dissociation to PTSS severity. The most robust and substantive predictor of PTSS severity was peritraumatic dissociation. Similarly, trait anxiety was found to be a significant predictor of all PTSS scores, except avoidance. By contrast, TI severity failed to predict PTSS scores over and above the influence of trait anxiety and peritraumatic dissociation. These results are contrary to find-

ings that suggest TI may be uniquely linked to PTSS severity (e.g., Bovin et al., 2008; Fiszman et al., 2008; Heidt et al., 2005; Rocha-Rego et al., 2009). There are several possible reasons the current data did not replicate precedent findings of a unique relationship between TI and PTSD. First, traumatic events severe enough to provoke a TI response are likely to occur at the extreme end of the severity continuum and, accordingly, are inherently more likely to result in more severe PTSS (Zoellner, 2008). If this is the case, reported TI may function as a proxy indicator for trauma severity. The current study did not include a measure of event severity; however, the nature of the events reported by many participants appear to fall into what would be considered by most as severe (e.g., sexual assault, physical assault, armed robbery). Moreover, attempting to measure event severity may not be feasible. Consistent with current PTSD conceptualizations (American Psychiatric Association, 2000), event severity is a largely a matter of subjective experience and efforts to operationalize it are likely to be viewed as arbitrary. A second reason for contradictory results may stem from the possibility TI and peritraumatic dissociation are interrelated constructs (Heidt et al., 2005; Marx et al., 2008). Evidence to date suggests an overlapping but distinct relationship between TI and peritraumatic dissociation; on the one hand, dissociation (by definition) interferes with higher cognitive functioning (American Psychiatric Association, 2000), whereas evidence from animal research suggests intact central processing during TI (Gallup,

Table 3 TI and PTSS: Multiple Regression PCL-C Avoidance Scores Dependent Model

␤

Predictors

M (SD)

part r

STAI-T

47.66 (11.12)

.20

.20

29.09 (10.87)

.16 .33ⴱⴱ

.16 .33

8.38 (5.06)

.15 .31ⴱ .05

.15 .27 .04

1

R2

⌬R2

.04

2 STAI-T PDEQ 3 STAI-T PDEQ TI

Note. PCL-C avoidance scale descriptive statistics: M ⫽ 5.78, SD ⫽ 2.45. ⴱ p ⬍ .05. ⴱⴱ p ⬍ .01.

F

⌬F

2.87

.15

.11ⴱⴱ

6.14ⴱⴱ

9.08ⴱⴱ

.15

.00

4.08ⴱ

.12

ABRAMS, CARLETON, AND ASMUNDSON

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Table 4 TI and PTSS: Multiple Regression PCL-C Numbing Scores Dependent Model

Predictors

M (SD)

1 STAI-T

R2

⌬R2

.33

29.09 (10.87)

.31ⴱⴱ .43ⴱⴱ

.31 .42

8.38 (5.06)

.30ⴱⴱ .38ⴱⴱ .11

.29 .33 .09

⌬F

F ⴱⴱ

.13

.36ⴱⴱ

3 STAI-T PDEQ TI

part r

47.66 (11.12)

2 STAI-T PDEQ

␤

10.95

.31

.18ⴱⴱ

16.09ⴱⴱ

18.56ⴱⴱ

.32

.01

10.99ⴱⴱ

.85

Note. PCL-C numbing scale descriptive statistics: M ⫽ 11.82, SD ⫽ 5.19. ⴱ p ⬍ .05. ⴱⴱ p ⬍ .01.

Boren, Suarez, Wallnau, & Gagliardi, 1980). This latter finding is consistent with reports that sexual assault survivors who report TI are able to recall the sequence of events during their attack (Suarez & Gallup, 1979). Given that reexperiencing is a hallmark symptom of PTSD, further study of the cognitive states that manifest during TI may yield further understanding of these symptoms and provide new avenues for treatment. A third and related reason for the lack of replication involves the question of whether TI is better conceptualized as a dimensional or categorical construct (Marx et al., 2008). Much of our knowledge of TI derives from comparative research in which the response appears to have a distinctly categorical nature (i.e., it is present or not). Nonetheless, it is plausible that nonhuman animals may experience attenuated TI (i.e., partial or less pronounced immobility) and experiential states analogous to those reported by humans. Some evidence suggests support for a dimensional conceptualization of TI. Case studies have described persons for whom the inability to walk was a prominent symptom of panic disorder and severe social phobia (George & Lydiard, 1989). In these individuals immobility did not present stereotypically (i.e., complete immobility), as is characteristic of TI documented in laboratory investigations of nonhuman animals. It is plausible that TI severity is a function of the nature of the traumatic event, with partial immobility responses more probable in contexts perceived as frightening but not immediately life threatening. If correct, then TI may differ dimensionally across trauma type, with more severe symptoms of TI occurring in traumatic events that impose the

greatest perceived threat to victims. Indeed, a dimensional conceptualization of TI may be consistent with suggestions that training aimed at establishing a response repertoire for high stress situations (e.g., among military personnel) likely mitigates the onset of the dissociative reactions that may precede TI (Leach, 2005). This study had limitations that suggest future research directions. First, there are concerns regarding the validity of the TI construct. Specifically, it is possible that respondents who report being frozen or paralyzed with fear may be confounding dissociative reactions or acquiescence with the physical immobility characteristic of TI (Zoellner, 2008). This issue is challenging to resolve using self-report methodologies but may be addressed with in-person interviews to better evaluate the features of TI. A second limitation centers on the probable overlap between TI and peritraumatic dissociation. Continued research is required to determine what is shared and what is distinct between these constructs. Third, the reliability of participant memory for past events is a potential problem in retrospective studies of traumatic responding. Some researchers suggest current psychological status may inflate the reporting of event severity and associated states (Roemer, Litz, Orsillo, Ehlich, & Friedman, 1998; Southwick, Morgan, Nicolaou, & Charney, 1997), whereas others suggest memory is sufficient, especially for traumatic events (Harvey & Bryant, 2000). Continued research, particularly of a prospective nature, is required to address this limitation. Finally, there were limitations associated with our sample. Women comprised the majority of participants

Table 5 TI and PTSS: Multiple Regression PCL-C Hyperarousal Scores as Dependent Variable Model

Predictors

M (SD)

1 STAI-T

.57

29.09 (10.87)

.53ⴱⴱ .34ⴱⴱ

.52 .34

8.38 (5.06)

.50ⴱⴱ .26ⴱ .18

.49 .22 .15

Note. PCL-C hyperarousal scale descriptive statistics: M ⫽ 13.24, SD ⫽ 5.02. ⴱ p ⬍ .05. ⴱⴱ p ⬍ .01.

R2

⌬R2

⌬F

F ⴱⴱ

.32

.57ⴱⴱ

3 STAI-T PDEQ TI

part r

47.66 (11.12)

2 STAI-T PDEQ

␤

34.35

.44

.12ⴱⴱ

27.62ⴱⴱ

14.47ⴱⴱ

.46

.02

19.91ⴱⴱ

2.97

TONIC IMMOBILITY

and it is unknown whether results will apply similarly to men. It is also possible those who elected to participate self-selected in some systematic manner (e.g., perhaps motivated by current distress), thereby limiting the generalizability of findings. Future research should endeavor to employ sex-balanced samples as well as assess current levels of functioning. The findings of this study contradict previous research linking TI with PTSS severity. Controlling for peritraumatic dissociation and trait anxiety removed the unique relationship for TI, though perhaps not surprisingly. Precedent research has linked peritraumatic dissociation to the development of PTSD (Ozer et al., 2003) and to TI (Abrams et al., 2009). However, some researchers have raised questions about the robustness of both peritraumatic dissociation (van der Velden et al., 2006; van der Velden & Wittmann, 2008) and TI (Zoellner, 2008) as predictors of PTSS severity. It remains plausible that both peritraumatic dissociation and TI may reflect the severity of an event rather than denote unique mechanisms contributing to the development of PTSS. Given the heterogeneous nature of trauma and the variable potential for TI responding, further research exploring interrelationships among these constructs seems imperative for improving our understanding of TI with regard to PTSS.

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Received April 19, 2010 Revision received December 23, 2010 Accepted February 7, 2011 䡲