The mechanisms underlying the fear system serve a normative function, allowing individuals to rapidly detect threat in the environment and appropriately respond to the threat (Bar-Haim, Lamy, Pergamin, Bakermans-Kranenburg, & van IJzendoorn, 2007
; Öhman, 2005
). Hypervigilant processing of threat, a heightened tendency to direct attention preferentially to threatening stimuli is often found in anxious individuals (Mathews & MacLeod, 1985
; MacLeod, Mathews, & Tata, 1986
; Mogg, Mathews, & Eysenck, 1992
; Mathews, Mackintosh, & Fulcher, 1997
). This attention bias to threat has been shown in both clinically and non-clinically anxious individuals, in a wide variety of tasks and may be involved in both the development and maintenance of anxiety disorders (Bar-Haim et al., 2007
However, accumulating evidence suggests that stressful circumstances alter anxious individuals’ typical pattern of attention to threat. In the emotional Stroop task, where participants are asked to name the color ink in which a set of words are printed, anxious individuals typically evidence greater interference, reflected in slower reaction times for emotionally evocative words compared to neutral words, an effect not seen in nonanxious individuals (Williams, Mathews, & MacLeod, 1996
). Several studies have shown that this effect disappears when anxious participants are asked to perform the task while experiencing a larger threat or stressor (Mathews & Sebastian, 1993
; Mogg, Kentish, & Bradley, 1993
; Amir, McNally, Riemann, & Burns, 1996
; Constans, McCloskey, Vasterling, Brailey, & Mathews, 2004
). Mathews and Sebastian (1993)
, for example, showed that the slowed reaction times of snake phobics to both snake-related words and generally threatening words disappeared when told that they would be asked to touch a live boa constrictor after the task. Likewise, the attention bias effect for highly anxious participants disappeared when they were either told they would be taking an IQ test after the task (Mogg et al., 1993
), or, in the case of social phobics, when they were told they would be asked to give a speech after the task (Amir et al., 1996
). Vietnam veterans with PTSD also showed a suppression of an attention bias to threat when told they would be asked to watch combat videos after performing the emotional Stroop task (Constans et al., 2004
Two hypotheses have been proposed for why anxious individuals’ slowed reaction time to threatening words disappears under conditions of high stress. Williams et al. (1996)
have suggested that the anxiety produced by stressful conditions leads participants to increase task effort, thereby compensating for the interference effect. Alternatively, Mathews and Sebastian (1993)
have suggested that the addition of a stressor might lead to a “shift in processing priorities”; a more imminent threat (e.g., giving a speech) inhibits attention to, and detection of, less threatening stimuli in the Stroop task.
Performance on the emotional Stroop cannot discriminate between these two hypotheses, because both increased effort and decreased attention to the words will lead to a decrease in response time for emotional words. An alternative paradigm for measuring attention bias to threat in anxious individuals is the dot-probe task (MacLeod et al., 1986
). In this task, participants simultaneously view two stimuli, one threatening and one neutral. After a set time these stimuli disappear and a probe appears in the same location as one of the two stimuli. The participant is asked to respond to the probe as quickly and accurately as possible. In a large number of studies, anxious participants have been shown to respond more quickly when the probe appeared in the same location as the threatening stimulus compared to when it appeared in the location of the neutral stimulus, suggesting an attention bias toward threat. This finding is not seen in non-anxious participants (reviewed in Bar-Haim et al., 2007
). The dot-probe task, thus, appears to have an advantage in examining the underlying processes involved in the suppression of the attention bias to threat in the presence of an environmental stressor. Specifically, if anxious participants increase effort when under stress, one would expect a general decrease in reaction times to probe detection on all trials (i.e., both when target probes appear in the threat location and in the neutral location), leaving the threat bias unchanged. By contrast, if anxious individuals diminish attention to threatening stimuli due to a change in processing priorities, one would expect to see a decrease in attention bias toward threat, with no overall decrease in reaction time.
Two recent studies have used modified dot probe tasks to examine aspects of anxious individuals’ performance under conditions of social threat. In Mansell, Clark, Ehlers, and Chen (1999)
, half the participants were told they would be asked to give a speech after performing the task, while the other half of the participants received no such instruction. On each trial, participants were presented with a picture of a household object paired with a picture of a face, with either a neutral, positive, or a negative expression. They found that when participants were not under conditions of stress, there was no reaction time difference in probe detection between high socially-anxious and low socially-anxious participants. However, when participants were told they would be giving a speech after the task, high socially-anxious participants showed bias scores away
from both positive and negative faces, a pattern that was not found in non-anxious participants.
Garner, Mogg, and Bradley (2006)
, using a dot-probe paradigm, presented participants with neutral face–object pairs, neutral face–angry face pairs, and neutral face–happy face pairs. These authors used eye tracking to examine participants gaze during stimulus presentation. Similar to Mansell, et al. (1999)
, they found that under ordinary circumstances, high socially-anxious individuals were more likely to make an initial gaze shift towards the neutral face in a neutral face-object pairing. However, when participants were told they would be asked to give a speech after the task, their initial bias to orient to faces was reduced compared to the low socially-anxious group. Taken together, these two studies show that when a sustained threat is introduced to the task, anxious participants no longer show the typical attention bias to threat
All of the studies discussed above used a sustained stressor—the threat of performing a stress inducing activity after the attention-testing task—to examine changes in patterns of attention allocation to threat under stress. It remains unknown whether transient stressors can produce similar effects on anxiety-related attention bias. In the present study, we used a modified version of the dot-probe task. Each trial began with the presentation of either a neutral or a socially threatening word. It was expected that the socially threatening words would briefly increase stress levels, affecting attention allocation patterns on the immediately subsequent trial of a typical dot-probe task (see for sequence of events in this task). Thus, each participant’s performance could be compared under conditions of greater or lesser social threat.
Sequence of events in the modified dot-probe paradigm.
In addition to behavioral reaction time data, Event-Related Potentials (ERPs) can provide detailed information about the temporal structure of attention allocation. Many studies have used this approach to investigate attentional allocation during manual reaction time tasks (reviewed in Awh & Jonides, 1998
). In addition, several studies have used ERP methodologies to study threat-related processing in anxious vs. non-anxious individuals (e.g., Rossignol, Anselme, Vermeulen, Philippot, & Campanella, 2007
; Bar-Haim, Lamy, & Glickman, 2005
; Holmes, Nielsen, & Green, 2008
). Three ERP components are particularly relevant to the current study, the P1, N1, and P2. The amplitude of the first positivity (P1) and the first negativity (N1) appearing after presentation of a visual stimulus is influenced by the degree of visual attention the stimulus receives (Hillyard, Vogel, & Luck, 1998
). Thus, in the dot-probe task, an increase in amplitude of the P1/N1 complex to the faces display is indicative of an increase in attention directed towards the face images, and would be expected to vary in response to presentation of the face stimuli in a dot-probe task as a function of participant attention. Thus, the ERP results could reveal whether anxiety-related modulations occur in the early attention processes indexed by the P1/N1 complex. Such findings would confirm an initial effect of the prime on attention.
Although relatively little is known about the posterior P2 component (Luck, 2005
), it has been shown to vary in amplitude between anxious and non-anxious individuals while viewing angry faces in an attention disengagement task (Bar-Haim, Lamy, & Glickman, 2005
). Hence, we expected that this component might differ across group and prime conditions in the present study, as well.
The current study presented participants either high or low in self-reported social anxiety with either a threat or neutral prime word prior to a typical dot-probe trial. ERPs in response to presentation of the prime were used to confirm the effectiveness of the threat prime manipulation independently of behavioral responses. Bernat, Bunce, and Shevrin (2001)
have shown that participants display different patterns of ERP activation in frontal and parietal sites to negative vs. neutral words. Therefore, we expected similar findings in the current study.
To summarize, the objectives of the current study were three-fold. First, we hoped to determine if the changes in attention allocation seen under sustained threat occur under transient activation of the fear system. Second, by using a dot-probe paradigm, we hoped to discern whether this shift in attention allocation was due to an increase in task effort, or a shift in processing priorities. Finally, we complemented our behavioral reaction time data with ERP data to examine the chronometry of attention allocation during activation of the fear system.