Neural activity associated with target processing does not predict

The relationship between vWM capacity and target processing was assessed in two ways. First, to visualize the relationship between vWM capacity and target processing, participants were again apportioned into three subgroups contingent on their vWM capacity (Figure 3.4a). In contrast to the differences observed for the PD component, one-way ANOVAs revealed no significant differences for the N2pc across the K subgroups (mean amplitude: Fs < 0.7, Ps > 0.512; negative area: Fs < 0.5, Ps > 0.62; onset latency:

Fs < 0.6; Ps > 0.552). Next, correlations were computed to assess the relationship between target processing and vWM capacity. As shown in Figure 3.4a and 3.4b, neither the mean amplitude nor the signed negative area of the N2pc was found to correlate with vWM capacity in either of the display configurations (rs < 0.08; Ps > 0.59). In addition to this, the jackknifed estimates of N2pc onset latency were also found to not correlate with vWM capacity (rs < 0.13; Ps > 0.38). In contrast to the relationship observed for distractor

suppression, the results here reveal no discernable association between the enhancement of the target singleton and vWM.

Figure 3.4. Neural activity associated with target processing not predictive of visual working memory capacity. (A) Correlation between memory capacity (k) and pure N2pc area for lateral-target displays of interest.

(B) Contralateral-minus-ipsilateral difference waveforms for high-, medium-, and low-capacity groups.

3.4. Discussion

Cognitive-control based theories of vWM propose that individual differences in performance are closely associated with variability in attentional control (e.g., Engle &

Kane, 2004; Kane, Conway, Hambrick, & Engle, 2007). Numerous studies have repeatedly found high-capacity individuals to outperform their low-capacity counterparts across a broad assortment of attention tasks (e.g., Kane et al., 2001; Bleckley et al., 2003;

Sobel et al., 2007). Based on such findings, it has been proposed that individual variability may stem, not from a difference in memory capacity, but rather from differences in attentional filtering (Cowan & Morey, 2006; Vogel et al., 2005). Specifically, low-capacity individuals are thought to be inefficient filterers, which results in an inability to effectively encode behaviourally relevant information in the presence of irrelevant, distracting information (Fukuda & Vogel, 2009; Vogel et al., 2005). However, what has remained unclear is whether inefficient filtering in low-capacity individuals reflects a deficit to enhance relevant representations, suppress irrelevant representations, or both.

The present chapter sought to better understand the relationship between attentional neural filtering and vWM capacity. Dissociable ERP measures of attentional enhancement and suppression were isolated while participants performed a visual search task. The results presented here revealed distractor suppression processing during visual search to underlie optimal vWM performance. Specifically, an vWM capacity was found to positively correlate with both the magnitude and onset latency of the PD component.

Additionally, whereas salient-but-irrelevant distractors were observed to evoke the PD for high- and medium-capacity individuals, the PD was altogether absent for low-capacity individuals. These results demonstrate that individual differences in vWM capacity are associated with the magnitude and timing of a specific attentional control operation that is necessary for suppressing the processing of salient-but-irrelevant visual objects.

Difficulty ignoring distracting information has been associated with general deficits in cognitive performance and is symptomatic of several neurological disorders (e.g., Enright & Beech, 1993; Hahn et al., 2010; Melnick, Harrison, Park, Bennetto & Tadin, 2013; Wang et al, 2016). Electrophysiological studies of attention have found deficits in inhibiting distractor representations to be predictive of lower vWM capacity (e.g.,

Gulbinaite et al., 2014; McNab & Klingberg, 2008; Rutman et al., 2010; Sreenivasan &

Jha, 2007; Zanto & Gazzaley, 2009). For example, individuals with low vWM capacity more readily express deficits modulating the sensory processing of irrelevant information at very early stages of visual processing (Sreenivasan, Katz, & Jha, 2007; Zanto &

Gazzaley, 2009). Furthermore, this impaired modulation in low-capacity individuals appears to relate to a decrease in functional connectivity between the medial frontal and dorsolateral prefrontal cortex, areas critical for attentional control (Gulbinaite et al., 2014).

The present study complements these findings by showing here that, not only were low-capacity individuals unable to actively suppress salient-but-irrelevant distractors, but these distractors captured their attention. This latter finding is consistent with a number of EEG studies that have shown low-capacity individuals to have an increased propensity for attentional capture by irrelevant distractors (Fukuda & Vogel, 2009). By attending to—

rather than suppressing—distractors, irrelevant items can inadvertently gain entry into working memory and consume the finite resources of the system (Vogel & Machizawa, 2004; Vogel, McCollough, & Machizawa, 2005).

In this study, attentional enhancement was found to be unrelated to vWM capacity, suggesting that vWM does not predict differences in processing a target processing during visual search. The was confirmed twice: once using trials where the salient distractor was present and once using trials where the distractor singleton was absent. The data presented here is consistent with a recent study by Zanto and Gazzaley (2009), which showed an individual’s vWM capacity to be independent of the attentional enhancement of relevant information during the early stages of visual processing. The fact that target processing was unrelated to vWM capacity does not infer that attentional enhancement is unimportant for working memory—attentional enhancement is critical for the encoding of information into the vWM system—but rather that the focusing of attention may represent a more robust attentional mechanism, less susceptible to individual variability.

The finding here—that distractor suppression processing but not target enhancement processing underlies individual differences in vWM—is consistent with previous studies that have demonstrated a similar dissociable relationship in both typical (Fukuda & Vogel, 2009; Zanto & Gazzaley, 2009) and aging populations (Gazzaley et al., 2005; Gazzaley et al., 2008). Since the ability to focus on goal-relevant stimuli is

hypothesized to require working memory (Lavie, 2005), one possible explanation for this finding is that low-capacity individuals may not have the available cognitive resources necessary to sustain top-down control throughout the duration of a visual search task. As a result, when a target and salient distractor compete, the initial selection may rely more on bottom-up selection processes. This resource availability hypothesis is consistent with studies that have shown that, as the availability of cognitive resources is depleted (by increasing cognitive load), observers are more susceptible to attentional capture (Lavie &

de Fockert, 2005a, 2005b). If these resources are not available to begin with, low-capacity individuals may not be able to recruit a distractor suppression mechanism, resulting in the salient distractor capturing attention. Future research will be necessary to elucidate the precise relationship between vWM, cognitive load, and the active suppression indexed by the PD.

Signal suppression during a transient loss of attentional control

4.1. Introduction

The visual environment is replete with information; however, at any given moment only a subset of this information is relevant to our behavioural goals. In order to effectively navigate this environment, we must be able to locate the relevant information amid cluttered and distracting visual conditions. It has been well established that selective attention facilitates this process by directing our cognitive resources toward locations in the environment contingent with our top-down volitional goals. By biasing processing resources toward behaviourally relevant locations in the visual field, both the detectability and discriminability of the information there is markedly improved (e.g., Liu, Pestilli &

Carrasco, 2005; Liu, Abrams & Carrasco, 2009; Luck, Hillyard, Mouloua & Hawkins, 1996;

Spitzer, Desimone & Moran, 1988; Yeshurun & Carrasco, 1998). Over the years, the neural bases of the mechanisms controlling selective attention have been investigated using electrophysiological recordings in humans (see Carrasco, 2011 and Luck, 2014 for comprehensive reviews). Many of these electrophysiological studies have tracked attention using the N2pc, an ERP component known to index spatially selective processing during visual search (Luck & HIllyard, 1994a, 1994b; Hickey et al., 2009;

Mazza et al., 2009; Eimer, 1996). When a laterally presented item is attended, the N2pc component typically presents as a greater negativity over posterior electrode sites contralateral (versus ipsilateral) to the item. This processing is thought to reflect the enhanced processing of an attended target when presented in competition among other irrelevant items (Hickey et al., 2009; Luck, 2012, Mazza et al., 2009a, 2009b).

In addition to the enhanced processing of a relevant target item, the visual system can also act to suppress the processing of irrelevant distractor items. In humans, studies of visual attention have reported that task-irrelevant distractors can be inhibited in a top-down manner when they are anticipated. This inhibitory mechanism serves to actively

suppress distractor representations and prevent these irrelevant objects from erroneously capturing attention, even when they are especially salient (e.g., Gaspar & McDonald, 2014; Gaspelin et al., 2015; Hickey et al., 2009; Janatti, Gaspar & McDonald, 2013). The electrophysiological correlate of this process is a contralateral positive-going voltage recorded from electrodes over posterior-occipital scalp—an ERP component termed the PD. The distractor suppression indexed by the PD is thought to reflect an active mechanism that inhibits certain stimuli based on the parameters of an observer’s top-down attentional set (e.g., Gaspar & McDonald, 2014; Hickey et al., 2009; Hilimire et al., 2012; Jannati et al., 2013; Sawaki & Luck, 2010; Sawaki, Geng & Luck, 2011); however, direct evidence for this active suppression is limited. Alternatively, it is possible that the PD may reflect processing associated with the bottom-up physical properties of an object rather than the object’s top-down status (Fortier-Gauthier et al., 2013).

If the PD is strongly related to endogenous attentional factors, it should be impaired in situations where top-down control would be severely restricted. The primary purpose of the present chapter was to investigate this possibility by disrupting attentional control during visual search and examining how distractor processing is affected. One manner of producing a disruption of attentional control is by presenting a critical target in rapid succession of another. Although participants are easily able to identify the first target (T1), their ability to identify the second target (T2) depends on the amount of time separating the two items. Specifically, if the second target is presented within 200-500 ms of the first, accuracy for the second target is heavily impaired. This impairment for processing the second target is termed the attentional blink (AB; Broadbent & Broadbent, 1987;

Raymond, Shapiro, & Arnell, 1992). Although there is no consensus regarding the precise nature of the mechanism(s) underlying the AB, most theories agree that the deficit in processing T2 is associated with attentional resources being transiently disrupted by the processing of T1 (Chun & Potter, 1995; Raymond, Shapiro, & Arnell, 1995; however, see Olivers & Meeter, 2008; and Taatgen, Juvina, Schipper, Borst & Martens, 2009 for alternative explanations).

In the present study, both the N2pc and PD, as well as behavioural performance, were measured while participants performed a modified rapid serial visual presentation (RSVP)/visual search task. The task here combined attentional-blink methodology with

those of the visual additional singleton search paradigm. The first target (T1) was a number within an RSVP stream of letters and the second target (T2) was a colour singleton that appeared within a visual search array that also contained a salient distractor singleton (Figure 4.1). 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). Target and distractor processing ERPs to the T2 search array at short (within the attentional blink) and at long (outside the attentional blink) separations from T1 were then separately examined. Based on previous electrophysiological studies, it was anticipated that target processing during visual search would be delayed during the blink (Lagroix, Grubert, Spalek, Di Lollo & Eimer, 2015; Pomerleau, et al., 2014). Alternatively, it was less clear was how distractor processing would be affected. If the distractor suppression indexed by the PD reflects an active mechanism contingent on the observer maintaining a high level of attention control, then it is expected that the AB would disrupt this mechanism. However, if the PD reflects an exogenous process that is instead sensitive to the physical properties of a stimulus, the component should remain unimpaired during the AB.

4.2. Methods

4.2.1. Materials and Methods

The Research Ethics Board at Simon Fraser University approved the research protocol used in this study.

4.2.2. Participants

Twenty students from Simon Fraser University participated after giving informed consent. These students were given course credit for their participation as part of a departmental research participation program. Eighteen subjects were included in the EEG analysis (8 women, age 19.61, SD = 1.97; 3 left-handed), as two were excluded due to excessive noise in the ocular channels. All subjects reported normal or

corrected-to-normal visual acuity and were tested for typical color vision, using Ishihara color test plates.