Eye tracking and physiological reactivity to threatening stimuli in posttraumatic stress disorder

Journal of Anxiety Disorders 25 (2011) 668–673

Contents lists available at ScienceDirect

Journal of Anxiety Disorders

Eye tracking and physiological reactivity to threatening stimuli in posttraumatic stress disorder Kim L. Felmingham a,b,∗ , Chris Rennie b,c , Barry Manor b , Richard A. Bryant a,b a b c

School of Psychology, University of New South Wales, Sydney, Australia Brain Dynamics Centre, Westmead Millenium Institute, Westmead Hospital, Sydney, Australia School of Physics, University of Sydney, Sydney, Australia

a r t i c l e

i n f o

Article history: Received 5 August 2010 Received in revised form 23 February 2011 Accepted 24 February 2011 Keywords: PTSD Posttraumatic stress disorder Eye tracking Attention Skin conductance response

a b s t r a c t This study tested the vigilance-avoidance model of anxiety and attention bias in posttraumatic stress disorder (PTSD). This study used eye tracking technology to record initial fixations, pupil dilation, fixation time and concurrent skin conductance response to examine initial orienting towards threat stimuli and subsequent fixations. Twenty-one traumatized participants (11 diagnosed with PTSD and 10 traumaexposed participants without PTSD) viewed 32 stimuli (with four words in each quadrant). Sixteen trials contained a trauma-relevant word in one quadrant and 16 had four neutral words. PTSD patients reported significantly greater number of initial fixations to trauma words, and a greater number of skin conductance responses to initial threat fixations. There were no significant differences in subsequent fixations to trauma words between groups. Although this study provides evidence of attentional bias towards threat that is accompanied by specific autonomic arousal, it does not indicate subsequent avoidance of threat stimuli in PTSD. © 2011 Elsevier Ltd. All rights reserved.

Hypervigilance towards threat is a core feature of PTSD. Network models propose that trauma-related memories are readily activated in PTSD, leading to physiological arousal and an attentional bias towards threat stimuli (Chemtob, Roitblatt, Himada, Carlson, & Twentyman,1989; Litz & Keane, 1989). Extending from these models, the vigilance-avoidance theory of anxiety suggests that initial orienting towards threat results in subsequent avoidance of threat stimuli (Williams, Watts, MacLeod, & Mathews, 1997). Evidence for attentional bias towards threat arises from emotional Stroop paradigms, which reveal longer latencies to color-name trauma-relevant words in PTSD patients compared to traumatized and anxiety-disorder controls (Bryant & Harvey, 1995; McNally, Kaspi, Reimann, & Zietlin,1990). Mechanisms underlying Stroop interference are unclear, however, because longer response latencies may reflect attentional bias, emotional arousal, or cognitive avoidance (De Ruiter & Brosschot, 1994; Fox, 1994). Dot-probe methodologies provide a more precise measure of attentional bias because they examine the speed of allocation of visual attention to emotionally-valenced spatial locations. For example, PTSD patients were shown to name targets faster when spatially adjacent to threat words (Bryant & Harvey, 1997). In a recent review,

∗ Corresponding author at: School of Psychology, University of New South Wales, Anzac Parade, Kensington, Sydney, New South Wales 2052, Australia. Tel.: +61 2 93853245; fax: +61 2 93853641. E-mail address: [email protected] (K.L. Felmingham). 0887-6185/$ – see front matter © 2011 Elsevier Ltd. All rights reserved. doi:10.1016/j.janxdis.2011.02.010

Cisler and Koster (2010) propose three underlying mechanisms of attention bias: facilitated attention (attention drawn to threat stimulus/orienting of attention), disengagement of attention (degree to which threat stimulus captures attention and impairs switching attention to another stimulus) and attentional avoidance (attention preferentially located to opposite location of threat cue). Dot-probe methodologies are limited as it is problematic differentiating the allocation of threat (facilitated attention) from the disengagement of attention from threat. Modified spatial cueing tasks have been designed to better discriminate components of attention bias. Spatial cueing tasks involve presenting a cue (e.g., a neutral or threat stimulus) for a subsequent target in a spatial location. Attention biases are reflected in faster responses to valid (compared to neutral) cues, and slower responses to invalid cues (Cisler & Koster, 2010). However, spatial cueing tasks have been shown to reflect both threat-related attention cueing and response slowing effects (Mogg, Holmes, Garner, & Bradley, 2008). Finally, both Stroop and dot-probe methodologies rely on verbal or motor responses and involve both early and late information processes. Eye tracking tasks provide a more direct measure of attentional bias that does not require verbal or motor responses, and that can better delineate the proposed mechanisms of attention bias (facilitated attention, disengagement, and attentional avoidance). Eye movements are central components of the orienting response (Sokolov, 1990) and reflect the allocation of processing resources (orienting response: Andreassi, 1989; Fogarty & Stern,

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1989). Several theories propose that the activation of fear networks increase autonomic fear responses, which in turn lead to the preferential processing of threat-related stimuli (Chemtob et al., 1989; Lang, Bradley, Fitzsimmons, Cuthbert, Scott, Mulder, & Nangia, 1998). Although there is robust evidence for physiological reactivity to trauma-related stimuli in PTSD (Pole, 2007), few studies have examined arousal in relation to attentional bias. In one previous study, PTSD participants displayed more initial fixations to trauma-relevant than neutral words compared to controls (Bryant, Harvey, Gordon, & Barry, 1995). In this study, skin conductance (SCR) was recorded concurrently, and PTSD participants displayed increased numbers of skin conductance responses (SCRs) to both trauma and neutral words compared to non-traumaexposed controls (Bryant et al., 1995). It is unclear from this study whether attention biases and increased SCRs are a result of trauma exposure or whether they are specific to PTSD. The present study extends this previous research by indexing eye movements and SCRs concurrently in PTSD and trauma-exposed controls. A recent study examined eye tracking in traumatized samples and employed pupil dilation as an index of sympathetic arousal (Kimble, Fleming, Bandy, Kim, & Zambetti, 2010). This study compared traumatized Iraq war veterans with high and low levels of PTSD symptoms (although only two participants met diagnostic criteria for PTSD) in relation to eye movements and pupil dilation to neutral or negative images from the International Affective Picture Series (IAPS: Lang, Bradley, & Cuthbert, 1997). They reported that participants with high PTSD symptoms had larger pupil dilations to negatively valenced stimuli (trauma-specific and non-trauma specific) and longer viewing times compared to low symptom PTSD participants. Furthermore, they reported a trend for more initial fixations to Iraq images in the high symptom PTSD group. Yet, conclusions from this study are limited as they employed a traumatized, but largely non-PTSD sample (Kimble et al., 2010). The current study will extend this research by examining eye fixations, skin conductance response and pupil dilation in a civilian sample of traumatized controls and PTSD participants. Cognitive methodologies have supported the vigilanceavoidance model in social phobics (Amir, Foa, & Coles, 1998) and spider phobics (Pflugshaupt, Mosimann, Von Wartburg, Schmitt, Nyffeler, & Muri, 2005), and generalized anxiety disorder (Weinberg & Hajcak, in press) using dot probe, event-related potential or visual search tasks (see Cisler & Koster, 2010 for review). The vigilance-avoidance theory has also received some support in non-clinical, high trait anxiety samples, predominantly with longer stimulus durations (Cisler & Koster, 2010; Koster, Crombez, Vershuere, Van Damme, & Wiersma, 2006; Mogg, Bradley, Miles, & Diron, 2004). As noted previously, eye tracking methodologies provide a more precise methodology for delineating component mechanisms of attention bias (facilitated attention, disengagement, and avoidance). Eye tracking studies provide mixed evidence for the vigilanceavoidance model. Pflugshaupt et al. (2005) revealed that spider phobics displayed initial eye movement fixations towards spider stimuli, followed by subsequent fixations away from these stimuli. However, Kimble et al. (2010) tested the vigilance-avoidance theory of anxiety by examining time spent on second fixations, but they failed to find evidence for subsequent avoidance following initial fixation in PTSD (Kimble et al., 2010). Such inconsistency may relate to differences in measurements employed, therefore, the current study will examine the direction (rather than time) of subsequent fixations following initial fixation to test the vigilance-avoidance model in a PTSD sample. The present study examined attentional bias, autonomic reactivity and avoidance of threatening stimuli in PTSD and traumatized controls by recording eye movements, SCRs and pupil area to trauma-related and neutral words. It was predicted that the PTSD

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group would display increased initial fixations to trauma-relevant words. Second, it was predicted this attentional bias would be associated with heightened arousal, reflected in increased SCRs to trauma-words. Finally, if the vigilance-avoidance model is correct, it was predicted there would be significantly fewer subsequent fixations to a trauma word following an initial traumatic fixation in the PTSD group. 1. Materials and methods 1.1. Participants Eleven participants (6 male, 5 female) with a diagnosis of PTSD of mean age 34.2 years (SD = 9.6) and 10 traumatized control participants (5 male, 5 female) of mean age 37.8 years (SD = 15.1) participated in the study. PTSD participants satisfied DSM-IV criteria for PTSD following non-sexual physical assault, and trauma-exposed controls had experienced a physical assault but did not develop PTSD symptoms and did not have a current diagnosis of PTSD. The average time since trauma was 28.0 months (SD = 13.7) for PTSD patients and 24.2 months (SD = 15.6) for controls. No participants had a history of head injury or other neurological disorder, or current or recent (within the past year) psychotic disorder or substance use. Participants with a history of childhood trauma were excluded from the study to reduce sample heterogeneity. Two trauma controls, and 1 PTSD participant were taking antidepressant medication (SSRI). Participants using lithium, benzodiazepines, methadone, or barbiturates were excluded because of the influence of these substances on the oculomotor system (Griffiths, Marshall, & Richens, 1984). Five PTSD patients had comorbid depression (45%) and 1 had panic disorder (9%), and 1 trauma-exposed control had comorbid depression (10%). 1.2. Measures 1.2.1. PTSD diagnosis Diagnoses were established by clinical psychologists using the Clinician Administered PTSD Scale (CAPS-1; Blake, Weathers, Nagy, Kaloupek, Klauminzer, Charney, & Keane,1990). The CAPS indexes the 17 symptoms described by DSM-IV (APA, 2000) PTSD criteria. Each symptom is rated on severity and frequency in a semistructured interview, which provides both diagnostic information and an ordinal measure of the severity of PTSD symptoms. The CAPS is widely used and has excellent validity and reliability (Blake et al., 1990). 1.2.2. Axis 1 diagnoses To diagnose comorbid disorders such as current major depression, generalized anxiety disorder, obsessive compulsive disorder, panic disorder and social anxiety disorder, and to exclude psychotic disorders and current substance abuse, we administered the structured clinical interview for DSM IV Disorders (SCID IV: Spitzer, Williams, Gibbons, & First, 1996). 1.2.3. Impact of event scale (IES: Horowitz, Wilner & Alvarez, 1979) The IES is a 22 item self-report measure that evaluates the frequency of intrusions and avoidance symptoms relating to a traumatic event. It is widely used, with excellent validity and reliability. 1.2.4. State trait anxiety inventory (STAI: Spielberger, Gorsuch, Lushene, Vagg, & Jacobs, 1983) The STAI contains two twenty item self-report scales to measure state anxiety and trait anxiety. It is used in this study to examine anticipatory anxiety during the testing.

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1.2.5. Alcohol use disorders inventory (AUDIT: Babor, De La Fuente, Saunders, & Grant, 1989) The AUDIT is a 10 item self-report measure which screens for alcohol abuse and dependence. It has good reliability and validity, and provides a measure of the extent of current alcohol use in our participants. 1.2.6. Beck depression inventory (BDI: Beck, Ward, Mendelsohn, Mock, & Erbaugh, 1961) The BDI is a self-report measure of the severity of current depression symptoms. It was included in the study to provide a measure of severity of current depressed mood. 1.3. Eye movement task To examine attentional bias, word stimuli were used in the present study to allow the simultaneous presentation of discrete, simple visual stimuli in different spatial locations. On each trial, four different words were presented in each of four quadrants of the monitor within parafoveal range (2–7◦ ).

blood

steps

ahead

drapes

radio

land

track

term

Trauma Trial Example

Neutral Trial Example

On the 16 trauma trials, a trauma-relevant word (relating to physical assault: e.g., blood, attack, agony, terror and dead) and three neutral words (e.g., drapes, land, ahead, steps, radio) were presented in each of the four quadrants. On the 16 neutral trials, four neutral words were presented in each of the four quadrants. The trauma-relevant and neutral words were selected after ratings were made by 30 assault victims in a pilot study. These participants rated the emotional valence of each word on a 7-point Likert scale (0 = “not at all distressing”, 7 = “extremely distressing). The words were matched for word length and frequency of usage (Francis & Kucera, 1982). Horizontal dimensions of each word were 4◦ and vertical dimensions 2◦ visual angle. Each stimulus word was presented once; the orders of neutral and traumatic trials, and quadrant orders were randomized. The words remained on the screen for 1 s, with an inter-stimulus interval of 5 s. To ensure that all participants commenced each trial from the same fixation point, words were presented only after participants had maintained a fixation on a central dot (±0.75◦ ) for 1 s.

oratory. Eye movements were recorded monocularly from the right eye using a computerized video-oculographic infrared eye monitoring system (CEDRIC Mark II, Santech Pty. Ltd., South Australia). A near infrared (700–1100 nm) light source and a low light television camera were directed at the eye, and retinal and corneal reflections produced by the infrared light were recorded by the camera every 20 ms (50 Hz) to obtain eye gaze positions. The error of resolution for determining the point of gaze was 0.75◦ . Eye position, fixation duration, and pupil area data were obtained at each fixation. A fixation was defined as a set of consecutive gaze coordinates within an area of 1◦ for at least 200 ms (Kojima, Matsushima, Ando, Sakurada, & Ohta, 1992). Initial fixations on stimulus words were defined as the first 200 ms-period of continuous gaze within one degree of a word. Blinks (identified by loss of corneal reflection) and “off screen” gazes (identified by analyzing vertical and horizontal coordinates for each eye position) were excluded from data analysis. Electrodermal activity was recorded from the distal phalanx of the second and third digits of the non-dominant hand using Ag/Ag Cl electrodes (of approximately 0.8 cm2 contact area) linked with a constant voltage. Electrodes were filled with 0.05 M sodium chloride gel. Skin conductance signals were recorded continuously for 6.5 min using a 32-channel PC based system. SCRs were defined as an increase in electrodermal activity from baseline within 1–3 s of the stimulus, with a minimum amplitude of .05 ␮S (Barry, 1990). Time-locked orienting responses to the traumatic and neutral trials were calculated on this basis. Pre-stimulus skin conductance levels (SCL) were indexed on the basis of the initial 1 s period preceding stimulus presentation. Skin conductance data was selected as an index of physiological arousal rather than other indices (such as heart rate), as SC is innervated specifically by the sympathetic nervous system which is thought dysregulated in PTSD (Boucsein, 1992; Pole, 2007). 2.1. Data analysis Analyses focused on number of initial fixations, the pupil area to initial fixations, the fixation duration to initial fixations, and SCR responses associated with initial fixations. The number of initial eye fixations and SCRs were compared with Fisher Exact chi-squared analyses. Mean pupil area (baseline and poststimulus) and stimulus duration time were examined with 2 (group: PTSD/Trauma control) × 2 (stimulus: trauma/neutral) analyses of variance (ANOVAs). Mean pupil area for baseline and post-stimulus data were calculated from the relevant fixations in the 1 s prior to the stimulus, and following the stimulus. Tukey post hoc tests were used for post hoc analyses. 3. Results

2. Procedure

3.1. Participant characteristics

Participants started the study by providing written informed consent approved by the Sydney West Area Health Service Human Ethics Board. All participants were then tested with a Snellen chart and had normal or corrected-to-normal vision. Participants reported not taking alcohol for 24 h prior to the study and had refrained from using caffeine or nicotine for 4 h prior to data acquisition. Participants were then interviewed by clinical psychologists to diagnose PTSD and identify any comorbid psychiatric conditions. After completing this assessment, the participant was seated in a dental chair with a soft head restraint maintaining a standard distance of 60 cm from the participants’ eyes to the stimulus display screen. After a calibration procedure, participants were instructed to stare at a fixation dot on the display screen until some words appeared, and then look at the words in any way that they desired. Eye movements were recorded in a sound-attenuated, dimly lit lab-

Mean values for participant characteristics are presented in Table 1. There were no differences between groups for age, education, or time elapsed since trauma. Chi-square analyses revealed no significant differences between proportion of males and females, or medicated and unmedicated individuals. There were more participants with comorbid axis I disorders in the PTSD group [2 = 9.5, p < .005]. The PTSD group had significantly higher scores on state anxiety [F(1,19) = 27.2, p < 001], trait anxiety [F1,19 = 53.3, p < 001], total CAPS [F(1,19) = 21.8, p < 001], IES [F1,19 = 58.5, p < 001], and BDI [F(1,19) = 64.1, p < 001] scores. 3.2. Eye movement data Initial fixation, pupil dilation, fixation time, and subsequent fixation data are presented in Table 2. Fisher’s Exact Chi-square tests

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Table 1 Descriptive statistics for participant characteristics. Variable

Trauma controls

PTSD

Sig.

Age (years) Education (years) Gender Medication Comorbidity Time post-trauma (months) CAPS total IES total State anxiety (STAIS) Trait anxiety (STAIT) Depression (BDI)

37.8 (15.1) 10.5 (0.68) 5 m/5 f 20% 10% 24.2 (15.6) 20.1 (15.4) 3.6 (2.2) 26.1 (2.6) 33.4 (5.9) 3.9 (2.6)

34.2 (9.6) 11.5 (2.3) 6 m/5 f 9% 54% 28 (13.7) 63.6 (25.5) 43.7 (16.4) 44.2 (10.6) 54.7 (7.3) 22.5 (6.9)

n.s. n.s. n.s. n.s.

Effect size

*

n.s. ** ** ** ** **

−2.1 −3.4 −2.3 −3.2 −3.6

Note: Standard deviations appear in parentheses. PTSD, posttraumatic stress disorder group, Sig, statistical significance, m, male, f, female, CAPS, clinician administered PTSD scale, IES, impact of event scale, STAIS, state trait anxiety inventory state version, STAIT, state trait anxiety inventory-trait version, BDI, Beck depression inventory. * p < 05. ** p < 01. Table 2 Eye fixation data. Variable

TC

PTSD

Effect size

Initial fixations to trauma Initial fixations to neutral Mean pupil area to trauma Mean pupil area to neutral Fixation duration to trauma Fixation duration to neutral Subsequent fixations to trauma quadrants

21% (3.5/16) 25% (4/16) 17.4 (3.3) 14.8 (4.3) 282 (42.4) 334.2 (33.1) 14.3%

38% (6/16)*,† 21% (3.5/16) 22.7 (6.3)* 22.5 (4.7)** 300 (34.4) 329.5 (26.3) (2/16)†

−1.1 −1.7 −.47 .16 21% (3.5/16)

Note: Standard deviations appear in parentheses. Between group comparisons: * p < .05. ** p < 01. † = Significantly different from chance (p < .05). Mean pupil area measured in pixels, and fixation duration in milliseconds.

Fig. 1. Percentage of initial fixations to traumatic and neutral words for traumaexposed controls (TC: n = 10) and PTSD patients (n = 11).

the PTSD and trauma control groups made initial fixations towards neutral words at chance level [PTSD: 2 = 0.45, p > 05: controls: 2 = 0.002, p > 05]. Repeated measures ANOVAs revealed that the PTSD group had larger pupil area to both trauma and neutral initial fixations [F(1,19) = 16.0, p < 01]. There were no significant differences in baseline pupil area between groups. There were no group differences in mean fixation duration to trauma or neutral words, but fixation duration was shorter to trauma than neutral words across groups [F(1,19) = 9.6, p < 01]. There were no significant differences between groups on the number of subsequent fixations to a trauma word. However, the trauma controls looked less at traumatic quadrants in their subsequent fixations than chance [2 = 3.62, p < 05], whereas the PTSD group did not differ from chance [2 = 0.45, p > 05]. A summary of key findings is presented in Fig. 1. 3.3. Autonomic data

showed that the PTSD group made more initial fixations to trauma words than trauma controls [2 = 5.37, p < .05]; there were no differences in response between groups to neutral words (see Fig. 1). In the PTSD group, initial fixations to trauma words were significantly greater than chance level [2 = 4.9, p < 05], but the trauma controls did not differ from chance level [2 = 1.2, p > 05]. In contrast, both

SCR data are presented in Table 3. Three SCR non-responders in the PTSD group and two non-responders in the control group were excluded from SCR analyses. There were no significant differences in baseline SCL between groups. The PTSD group had more SCRs (see Table 2) associated with initial fixations to trauma words

Table 3 Skin conductance data. Variable

Trauma controls

PTSD

Sig

Baseline SCL % SCRs to trauma fixation % SCRs to neutral fixation SCR amp first trauma word SCR amp first neutral word Mean SCL trauma cue Mean SCL neutral cue

4.9 (2.3) 4% (0.6/16) 4% (0.6/16) 0.10 (.12) 0.02 (.01) 4.8 (2.2) 5.1 (2.4)

7 (2) 32% (5/16) 6% (1/16) 0.36 (.3) 0.06 (.04) 7 (1.9) 7 (2.2)

n.s.

Effect Size

**

n.s. *

n.s. n.s. n.s.

−1.2 −.13 −1.1 −.82

Note: Standard deviations appear in parentheses. Between group comparisons: * p < .05. ** p < 01. † = Significantly different from chance (p < .05). SCL, skin conductance level, SCR, skin conductance response, PTSD, posttraumatic stress disorder group, n.s., non-significant.

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Fig. 2. Percentage of skin conductance responses (SCRs) to traumatic and neutral words for trauma-exposed controls (TC: n = 10) and PTSD patients (n = 11).

than controls [2 = 8.4, p < 01]. This between-group effect was not evident in initial fixations to the neutral words. To examine initial orienting responses, SCR amplitude to the first traumatic and first neutral stimulus was compared between the groups. Significant group [F(1,19) = 7.9, p < 05] and condition effects [F(1,19) = 17.6, p < 001] were qualified by a significant group × condition interaction [F(1,19) = 5.6, p < 05]. Tukey post hoc tests revealed the PTSD group had greater SCR amplitude to the first trauma word than controls, but there were no significant differences between groups to the first neutral word (see Fig. 2). 4. Discussion The key finding of increased number of initial fixations to trauma-related words in PTSD is consistent with previous eye movement studies in PTSD (Bryant et al., 1995; Kimble et al., 2010). We extended earlier findings by showing this initial fixation effect is specific to PTSD compared to trauma-exposed controls. Increased initial fixations reflect greater initial processing and attention (Andreassi, 1989), and in this sense our findings are consistent with findings from dot-probe and Stroop experiments of an attentional bias towards threat in PTSD (Bryant & Harvey, 1995, 1997; McNally et al., 1990). In terms of specific mechanisms underlying attention bias (Cisler & Koster, 2010), our findings provide evidence for facilitated attention to threat in PTSD, but the lack of significant group differences in initial fixation time and in direction of subsequent fixation (to threat stimuli) suggests there is no difficulty disengaging from threat stimuli or any subsequent avoidance of threat stimuli in PTSD. Network models posit that attentional bias to threat is associated with autonomic arousal (Chemtob et al., 1989; Lang et al., 1998). Consistent with this prediction, the present study found increased numbers of SCRs to initially-fixated threat words in the PTSD group. This suggests autonomic reactivity occurs concurrently with attentional bias in PTSD. This effect was not influenced by baseline arousal because although the PTSD group showed higher SCL, these differences did not reach significance. This finding is in line with recent reports of increased pupil dilation, which has been associated with increased sympathetic arousal (Andreassi, 1989) and amygdala activation (Demos, Kelley, Ryan, Davis, & Whalen, 2008), in PTSD to negatively valenced images (Kimble et al., 2010). Although the PTSD group also displayed increased pupil dilation, there was, however, no evidence of a specific increase in pupil dilation to traumatic stimuli in the PTSD group. This difference may relate to the varying nature of the stimuli (visual IAPS in the preceding study, versus word stimuli in the current study), as visual stimuli have been argued to promote greater levels of arousal relative to word stimuli (Felmingham, Bryant, & Gordon, 2003).

The mean fixation duration was significantly shorter to the initially attended threat words than neutral words across both groups. Fixation duration reflects viewing, and presumably, processing time. There is evidence of reduced viewing time in phobics when viewing fear-related pictures (Hamm, Cuthbert, Globisch, & Vaitl, 1997). The finding that all participants tended to view threat stimuli more briefly challenges the proposal that the attention bias in PTSD is related to difficulty in disengaging from threat stimuli, rather than a facilitated attention effect. The vigilance avoidance model suggests that highly anxious individuals initially orient towards threat, and subsequently avoid further processing to contain anxiety (Williams et al., 1997). Lack of group differences in subsequent fixations to threat suggests there is not a vigilance-avoidance effect in PTSD, as the PTSD group did not appear to avoid fixation on subsequent threat stimuli (in fact, they looked at subsequent threat stimuli at chance level). This failure to confirm the vigilance-avoidance model in PTSD accords with a recent eye tracking study (Kimble et al., 2010) who examined the duration of subsequent fixations to threat. Although there were no significant between-group differences, the PTSD participants made subsequent fixations to the trauma word at chance level, whereas the control participants made fewer subsequent fixations to the threat word compared to chance. This finding is consistent with dot-probe evidence that individuals with relatively low anxiety orient away from mild or moderately threatening stimuli (Bradley, Mogg, White, Groom, & De Bono, 1999; MacLeod & Mathews, 1988; Mogg, Bradley, Dixon, Fisher, Twelftree, & McWilliams, 2000). We recognize that the study had small sample sizes, and that we did not index attentional bias to general negative or positive stimuli to rule out a general emotionality effect. Moreover, presenting stimuli subliminally would clarify the stage of processing at which the attentional bias occurs; there is tentative evidence that people with PTSD do have a preconscious bias to threat (Harvey, Bryant & Rapee, 1996). This cross-sectional design also precludes the distinction between the attentional bias being an acquired result of developing PTSD or is a risk factor for PTSD development. Recent work highlights this possibility, with some evidence suggesting that biases in appraising stressful events prior to trauma exposure in fire fighters predicts subsequent PTSD (Bryant & Guthrie, 2007). Future study could assess eye movements to threat in people prior to being exposed to trauma to assess the extent that preferential processing of threat predisposes people to PTSD. 5. Conclusion In conclusion, this study provided further evidence of attentional biases towards threat in PTSD and extended previous eye tracking findings to show that this attentional bias is specific to PTSD, occurs concurrently with elevated autonomic arousal, and specifically involves facilitated attention rather than difficulty disengaging from threat or subsequent avoidance of threat. Our finding accords with network models that predict that attentional bias towards threat is associated with intense states of arousal in PTSD. Importantly, this study found no evidence of avoidance of threatening stimuli in PTSD subsequent to their initial fixation. This contradicts predictions of the vigilance-avoidance model, but accords with predictions that ongoing arousal states lead to facilitated processing of threat, and therefore reduced inhibitory and extinction learning. Acknowledgements We would like to acknowledge the support of Dr. Evian Gordon, Kaye Horley and Kerri Brown who assisted us in completion

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