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A growing number of studies have used the PD component to demonstrate that the sensory processing of irrelevant information can be strongly modulated by top-down attentional control. The primary aim of this thesis was to further our understanding of the PD component, as well as to yield further insight into how the visual system deals with

irrelevant information during visual search. The following subsections outline the motivation for the four electroencephalography studies that comprise this thesis. Recurring goals that will be readdressed throughout the thesis are, i) to better understand what the processing indexed by the PD reflects, ii) to determine the stimulus conditions necessary to elicit distractor suppression, and iii) to explore what factors might affect an individual's ability to suppress.

1.4.1.

Chapter 2: Eliciting signal suppression during visual search

Chapter 2 explores the stimulus conditions necessary to elicit the distractor suppression indexed by the PD. While several studies have utilized the PD component to provide compelling evidence that to-be-ignored distractor singletons can be actively suppressed, the perceptual conditions necessary to elicit the component remain an issue of debate. Some have argued that when distraction is highly likely, the PD could be strategically recruited to suppress highly salient distractors that would otherwise be prioritized for attentional selection (Gaspar & McDonald, 2014; Sawaki & Luck, 2010).

Alternatively, the PD could instead index a mechanism that is generally recruited to deal with all unique distracting objects, regardless of salience; other studies have reported instances where physically inconspicuous distractor singletons have also been shown to elicit the PD (Hickey et al., 2009; Hilimire, Hickey & Corballis, 2012). In this chapter, I examine how differences in distractor salience relate to signal suppression and the impact these differences have on selective attention.

To investigate this, EEG was recorded from 40 participants while they performed a unidimensional variant of the additional singleton search task (Gaspar & McDonald, 2014; Theeuwes, 1992). Participants were instructed to search a multi-item array for a target singleton while attempting to ignore a task-irrelevant distractor singleton that could simultaneously appear within the display. On 33% of trials, a lone yellow target singleton was presented alongside nine uniformly coloured non-targets. On the remainder of trials, one non-target item was replaced with either a red or a blue distractor singleton.

Participants were instructed to report the orientation of the line inside the yellow target singleton while ignoring the presence of any distractor singleton. The color of the non-targets was varied across two experimental conditions (green in one condition and orange

in the other) in order to reverse the salience of the red and blue distractor singletons.

Consequently, the red distractor was the highly salient when presented against green non-targets, whereas it was no more salient than the target when presented against orange non-targets. The reverse was true for distractors presented against orange non-targets.

In this chapter, I provide evidence that distractor suppression is implemented only when a target competes with a more salient distractor. These findings will be discussed further in the context of current models of attentional selection.

1.4.2.

Chapter 3: Individual differences in working memory and visual search

Chapter 3 explores how individual differences in target and distractor processing contribute to variations in visual working memory (vWM) capacity. Studies have shown that, when required to process multiple visual objects, low-capacity individuals have difficulty filtering relevant from irrelevant information (McNab & Klingberg, 2008;

Shipstead, Zach, Lindsey, Marshall & Engle, 2014; Unsworth, Nash & Robison, 2014;

Vogel, McCollough & Machizawa, 2005). Together these findings suggest that an individual’s working memory capacity might be determined by the degree to which relevant information is remembered and irrelevant information is ignored; however, the neural basis for such a filtering mechanism remains unknown. Theoretically, the inefficient filtering observed in low-capacity individuals could be specifically linked to problems enhancing target representations, ignoring distractor representations, or a combination of both. In this chapter, I address these possibilities by examining how individual differences in vWM relate to both target and distractor processing during visual search.

To investigate this, vWM capacities were derived for 48 normal young adults using a delayed visual change detection task (Identical to that used by Luck & Vogel, 1997).

EEG was then recorded while subjects performed a competitive visual search task.

Identical to the visual search task used in Chapter 2, participants searched an array for a pre-specified target singleton while attempting to ignore a simultaneously presented high- or low-salience distractor singleton. ERP components associated with target selection (N2pc) and distractor suppression (PD) were then separately assessed for individuals with

indicating that differences in working memory capacity are specifically related to distractor-suppression (but not target processing) activity in visual cortex. Specifically, high-capacity individuals are able to actively suppress distractors, whereas low-capacity individuals cannot suppress in time to prevent salient distractors from capturing attention.

1.4.3.

Chapter 4: Signal suppression during a transient loss of attentional control

The distractor suppression indexed by the PD is thought to reflect an active process contingent on an observer’s top-down attentional set (Hickey et al., 2009; Hilimire, Hickey

& Corballis, 2012). However, others have proposed that the PD may instead characterize a bottom-up process with stimulus characteristics, such as certain colours, given special status in the context of visual search (Fortier-Gauthier, Dell'Acqua & Jolicœur, 2013).

Chapter 4 tests this assertion and asks whether the PD can be elicited during a disruption of top-down attentional control. One manner of producing such a disruption would be to exploit a behavioural phenomenon known as the attentional blink (AB). The attentional blink, characterized as an impairment in identifying the second of two targets when they are presented in close temporal succession, has been shown to affect attentional selection by producing a transient loss of endogenous control over the visual system (Di Lollo, Kawahara, Shahab Ghorashi & Enns, 2005). In this chapter, I examine how target selection and distractor suppression processing operate within the window of the attentional blink during visual search.

To investigate how visual search is affected during a disruption to the availability of attentional control, EEG was recorded from 18 participants while they performed a modified rapid serial visual presentation (RSVP)/visual search task. The first target (T1) was a number within an RSVP stream of letters. The second target (T2) was a colour singleton that appeared within a visual search array that also contained a salient distractor singleton. Subjects were instructed to first make a speeded response to T2 (by identifying the orientation of a line inside the target singleton) and were then subsequently probed to respond to T1 (by identifying whether the number in the RSVP stream had been even or odd). ERPs elicited by the T2 search array at lag 2 (within the attentional blink) and at lag

that during the attentional blink i) the processing of T2 is put on hold until after processing of T1 is complete and ii) distractor suppression is not possible. These findings will be presented in line with contemporary models of distractor suppression and the attentional blink.

1.4.4.

Chapter 5: Individuals with high levels of trait anxiety show differences in selective attentional processing

High levels of anxiety have been associated with deficits in attentional control. The attentional control theory of anxiety (ACT; Derakshan et al., 2009; Eysenck, Derakshan, Santos & Calvo, 2007) posits that these deficits could potentially relate to an inability of high-anxiety individuals to suppress salient-but-irrelevant information. Behaviourally, this has been shown in anti-saccade tasks where high-anxiety individuals are slower to initiate eye-movements away from counter-predictive abrupt-onset cues presented in the periphery (Derakshan et al., 2009). This observed increase in anti-saccade latency is thought to reflect an inability to inhibit the automatic, reflexive shift of attention to the salient counter-predictive cue (Eysenck & Derakshan, 2011). While trait anxiety appears to disrupt the ability to ignore distracting information, the neural correlates of this effect are not well understood. Differences in attentional biases between high- and low-anxiety individuals may be due to an impaired ability to shift attention toward relevant stimuli or a reduced ability to apply active suppression to irrelevant stimuli. Chapter 4 will examine how high- versus low-trait anxiety relate to both target and distractor processing.

To investigate this, 219 young adults were initially screened using the State-Trait Anxiety Inventory (STAI; Spielberger, Gorsuch, Lushene, Vagg & Jacobs, 1983), a 40-item self-evaluation questionnaire pertaining to anxiety affect. To maximize power to detect potential differences in brain responses, an extreme-groups design was used (Yarkoni, Tal & Braver, 2010). Individuals whose trait anxiety scores were among the highest and lowest were selected to participate in the ERP experiment (n = 36; 18 per group). To measure the neural correlates of attention, EEG was recorded while subjects performed an additional singleton search task identical to that previously used by Gaspar and McDonald (2014). In this task participants searched for a color-singleton target and on 50% of trials attempted to ignore a more salient color-singleton distractor. Participants

were instructed to indicate whether the orientation of the line inside the target was either horizontal or vertical. The relationship between anxiety and i) target processing ERPs and ii) distractor suppression processing ERPs were then separately assessed. This chapter will relate these specific aspects of visual-search processing to the attentional control theory of anxiety and show that distractor suppression is intact in high-anxiety individuals;

however, it is deployed reactively after the distractor is initially attended.

Eliciting signal suppression during visual search