Differences in target processing between high-anxiety and low-

To assess the relationship between selective target processing and anxiety, target N2pc waves were isolated for lateral target, distractor absent display configurations. Trials where the distractor was absent were used to assess target processing here, as the N2pc elicited on these trials would in no way be confounded by any attentional processing associated with the salient distractor (see Chapter 2 for an explanation of how N2pc amplitude can be modulated by distractor processing). Figured 5.5a shows grand averaged ERP waveforms for lateral target, distractor absent trials for both high- and low-anxiety individuals. The N2pc was measured as the difference in mean amplitude between contralateral and ipsilateral activity at electrodes PO7/PO8 from 230 to 290 ms after the presentation of the search array. The mean N2pc amplitudes for these lateral-target display configurations were found to differ significantly from zero for both high- [t(18) = 5.57, p < .001] and low-anxiety [t(18) = 2.87, P = .01] individuals. An internal consistency test found this N2pc to be highly reliable (r = 0.69; P < 0.001).

Figure 5.5b illustrates the difference in target processing ERPs for high- and low-anxiety individuals. In addition to the conventional mean amplitude, the N2pc was isolated from the waveform by computing the signed negative area within a 200 ms time window and subtracted from an equally wide pre-stimulus baseline (noise) interval. Despite appearing to differ in Figure 5.5b, neither the mean of the resultant N2pc area nor mean amplitude were not found to reach statistical significance [t(18) = 1.702; P < .11; t(18) = 1.49; P = .15]². The onset latency was also not found to differ between high- and low-anxiety individuals [244 ms vs. 250 ms; tc = .95, P = .36].

2 It should be noted that the resultant mean amplitudes did not significantly differ due to a single outlier in the high-anxiety group that had a large positivity in the N2pc time range. When the subject with the most extreme positive amplitude was removed from each group, this difference

Figure 5.5 N2pc ERPs elicited by trials with displays containing a lateral target and no distractor. Time 0 reflects the onset of the search display, and negative voltage deflections are plotted above the x-axis, by convention. Waveforms were recorded over the lateral occipital scalp (electrodes PO7 and PO8). (A) ERPs recorded contralateral and ipsilateral to a target for high- and low-anxiety individuals. (B) Contralateral-minus-ipsilateral difference waveforms for high- and low-anxiety individuals.

To determine if response efficiency was associated with a unique sequence of target processing, differences in target selection ERPs were examined separately for fast- and slow-response trials. As can be seen in Figure 5.6, the N2pc component did not differ for the high-anxiety group on fast- versus slow-response trials [t(18) = 0.05, P = .96]. In contrast, the N2pc was observed to be markedly attenuated for the low-anxiety group on slow-response trials [-0.24 vs. -0.87 μV; t(18) = 3.38, P = .003]. A reduction in the amplitude of the N2pc component on slow response trials has been previously reported by Jannati and colleagues (2013). Considered together, the results here indicate that

inefficient search is associated with a reduction in target processing for low-anxiety individuals; however, no such relationship exists for the high-anxiety group.

Figure 5.6 ERPs for displays containing a midline target and a lateral distractor, separately for fast- and slow-response trials. (A) High-anxiety group ERPs recorded contralateral and ipsilateral to a distractor for fastest and slowest trials. (B) High anxiety group contralateral-minus-ipsilateral difference waveforms for high- and low-anxiety individuals.

5.4. Discussion

High levels of trait anxiety are associated with an increased sensitivity to threat-related information, even when that information is known to be behaviourally inconsequential (Bar-Haim et al., 2007). This negative attentional bias has been linked to an impairment in the ability to filter out emotionally salient information (e.g., Ansari &

Derakshan, 2011a; 2011b; Berggren & Derakshan, 2013; Bishop, 2009; Pacheco-Unguetti et al., 2010). Several recent studies have reported that this filtering impairment in individuals with high-trait anxiety can also be observed to stimuli that have no affective significance (e.g., Ansari & Derakshan, 2011a, 2011b; Berggren & Derakshan, 2013;

Bishop, 2009). For example, studies have found trait anxiety to effectively predict deficits in filtering out irrelevant emotionally neutral stimuli, which led to their unnecessary storage in visual working memory (Moriya & Sugiura, 2013; Qi et al., 2014; Stout, Shackman &

Larson, 2013). These findings seem to suggest that individuals with high trait anxiety may have a more general deficit in attentional control. To date, however, the precise filtering mechanism(s) used, and how their operation differs between high- and low-anxiety individuals, remains unclear.

The main objective of the present study was to investigate whether the filtering inefficiency in highly anxious individuals is related to the ability to actively suppress salient-but-irrelevant distractor stimuli during a competitive visual search task. Specifically, the question asked here was: does the suppressive processing indexed by the PD differ between high- and low-anxiety individuals? To investigate this, distractor processing was isolated in the ERPs by segregating trials where the distractor was the only lateralized singleton in the search array (see previous chapters for further details regarding this methodology). On these trials, distractor suppression was observed for all individuals: the lateralized high-salience distractor stimulus was found to elicit a contralateral distractor positivity in both low- and high-trait anxiety groups. Furthermore, the observed PD was not found to differ in either latency or amplitude across the two groups. However, among individuals with high trait anxiety, the PD was preceded by an initial early deflection in the ERPs of the opposite polarity. This negativity—which began at approximately 170 ms after the onset of the search display and continued until the onset of the PD—likely reflects an early N2pc component, indicating that the distractor singleton initially captured attention.

Furthermore, the presence of the subsequent PD suggests that this initial capture of attention was rapidly followed by the active suppression of the distractor location. Notably, similar patterns of suppression (in which an N2pc is immediately followed by a PD

component) have been previously reported in several other studies (Jannati et al., 2013;

Sawaki et al., 2012; Sawaki & Luck, 2010). This sequence of events is thought to reflect a reactive suppression mechanism. By this account, in situations in which a distracting

stimulus erroneously captures attention, a corrective mechanism is invoked to suppress the location of the distractor, which in turn facilitates the selection of the target at another location (see Geng, 2014).

Recently, models of anxiety have begun to distinguish between proactive and reactive mechanisms of attentional control (Braver et al., 2007; Braver, 2012; Aron, 2011).

Whereas low-anxiety individuals are thought to engage top-down attentional control in a sustained and proactive manner, high-anxiety individuals have been shown to rely more on a reactive recruitment of attentional control (Braver, Gray & Burgess, 2007; Fales et al., 2008). This notion is consistent with the findings reported here. Among the high-anxiety group, a preceding shift of attention was observed to the distractor prior to it being suppressed. This suggests that the inefficient filtering in high-anxiety individuals may not stem from an inability to suppress distractor representations but rather from an inability to proactively ignore them.

On first blush, the finding that the PD did not differ across groups appears to conflict with ACT’s prediction that inhibitory processing is impaired in high-anxiety individuals; this link between anxiety and inhibition is central to ACT and has been reported in a number of studies (e.g., Derakshan et al., 2009; Eysenck and Byrne, 1992; Fox, 1993a; 1993b;

Wieser et al., 2009; Wood, Mathews, Dalgleish, 2001). However, one intriguing explanation for this contradiction may relate to the different functional networks that govern varying aspects of inhibitory processing and attentional control. Whereas the ventral attention network is involved in stimulus-driven attentional orienting, the fronto-parietal network is responsible for implementing increased levels of top-down cognitive control.

While anxiety is associated with various patterns of deficits for both networks (see Sylvester et al., 2012 for a review), it may be the case that either i) the deficit is more pronounced for the ventral network or ii) the compensatory effort observed in high-anxiety individuals may primarily activate the fronto-parietal network. This suggests that activation of fronto-parietal network regions may be necessary to implement the top-down active suppression associated with the PD component. This would be consistent with the finding that distractor suppression is closely correlated with activation of the dorsolateral prefrontal cortex, a key brain region in the fronto-parietal network (Suzuki & Gottlieb, 2013)

that has been previously implicated in proactive attentional control (Braver & Barch 2006;

Braver et al. 2007).

In several the behavioral studies examining trait anxiety, reaction time measures have been regarded as a principal index of processing efficiency (Eysenck et al., 2007).

However, others have criticized the validity of such measures, arguing reaction time to be an indirect measure of the outcome of processing rather than a measure of the processing itself (Basten, Stelzel & Fiebach, 2012; Eysenck & Derakshan, 2011). In the present study, individuals with high trait anxiety were characterized by their inefficient processing of the distractor at a neural level; however, this difference did not result in any decrement in behavioural performance. Although reaction time and distractor interference costs were observed, no differences were found for the high-anxiety group relative to the low-anxiety group. This dissociation between neural and behavioural efficiency is not entirely unexpected, as several previous studies have shown no effects of trait anxiety on behaviour, yet significant effects of trait anxiety on neural processing as measured by EEG and fMRI (e.g., Ansari & Derakshan, 2011a; Basten et al., 2012; Eysenck & Derakshan, 2011; Fales et al., 2008 Osinsky, Alexander, Gebhardt & Hennig, 2010). ACT accounts for these findings, arguing that under some circumstances high-anxiety individuals may show no behavioural evidence of disrupted attentional control; high-anxiety individuals may be able to compensate for behavioural deficiencies by recruiting additional resources and investing greater effort, allowing them to maintain a level of task performance on par with their low-anxiety counterparts. Support for this idea comes from a study by Hayes and colleagues (2009) that found anxiety to affect performance on a learning task when learning is effortless; however, this behavioural deficit was eliminated when learning was effortful and required higher motivation. In the additional singleton paradigm employed here, where the target and distractor singleton remained fixed throughout, the cognitive demands required to perform the search task would be considerably low. Thus, highly anxious individuals may have been able to directly compensate for their potential inefficiency in processing by recruiting additional cognitive resources to resolve the target.

Neuroimaging studies have substantiated the prediction that reduced efficiency in high-anxiety individuals is associated with increased levels of processing (e.g., Basten, Stelzel & Fiebach, 2011; Fales et al., 2008). For example, Santos, Wall, and Eysenck

(2006) found anxiety to be associated with greater activation in the right lateral prefrontal cortex, an area of the brain implicated in shifting attention. To test the idea that high-anxiety individuals recruit more attentional resources processing than low-high-anxiety individuals, differences in the target N2pc amplitude were assessed here. Recently, it has been proposed that the amplitude of the N2pc can be modulated by the allocation of top-down attentional resources: during target selection, if more attentional resources are directed towards the target, then a higher N2pc amplitude is observed (Liu et al., 2016).

The prediction here was that high-anxiety individuals would have an increased levels of target processing relative to low-anxiety individuals. The mean area of the N2pc differed across the two groups in the direction expected—high-anxiety individuals had a larger and more sustained N2pc than did low-anxiety individuals—however, this difference did not reach statistical significance (but likely would have with a larger sample; see Section 5.3.4). In addition, there was no reduction in N2pc amplitude on fast- relative to slow-response trials for high-anxiety individuals, indicating that high-anxiety individuals devote considerable attentional resources irrespective of their response efficiency. This may suggest that among high-anxiety individuals, relative to their low-anxiety counterparts, response efficiency is more readily associated with a deficit in post-selection processing than that of selection itself. Further research on the link between anxiety and processing efficiency is necessary given the results of previous studies are inconsistent and controversial (e.g., Bishop, 2009; Wood et al., 2001).

General Discussion

Attention is a complex cognitive phenomenon that fills the divide between perception and conscious experience. It is thought to be what “turns looking into seeing”

(pg. 1484, Carrasco, 2011). An ongoing goal of experimental psychology and neuroscience has been to explore the many processes that comprise attention to gain a better understanding of the role they play in our behaviour and cognition. The central theme of this thesis has focused, perhaps counter-intuitively, on the processes that mediate our ability to not pay attention. The ability to ignore irrelevant—but highly salient—

information is essential in allowing our limited cognitive resources to process only the information that is relevant to our current goals. This thesis has focused primarily on an event related potential component, the PD, which is thought to index this ability to ignore distracting information. In the four chapters of experimental data presented herein, I have attempted to expand our nascent understanding of the functional significance of the PD

and its role in attentional processing. In this final chapter I will briefly recapitulate these findings in the context of the broader theoretical questions they address and propose future directions for subsequent research.