Sex differences in electrophysiological indices of conflict monitoring
Highlights
► Sex differences in cognitive control were investigated using the N2 component of the event-related potential. ► Males demonstrated larger N2 amplitudes than females. ► No sex differences were demonstrated for N2-related conflict adaptation effects. ► Results indicate potential sex differences in conflict-monitoring brain activation.
Introduction
The ability to recruit, engage, and modulate cognitive resources to monitor and regulate task-related behavior is an important part of healthy cognitive and behavioral functioning (Botvinick et al., 2001, Folstein and Van Petten, 2008, van Veen and Carter, 2002a, Yeung et al., 2004, Yeung and Cohen, 2006). Central to these processes is the ability to monitor and adapt to conflict. Conflict occurs when information from multiple stimuli or responses overlap or recruit identical neural pathways, disrupting parallel information processing. Such conflict is common during dual-task performance or when task-irrelevant and task-relevant information compete for attentional control (Botvinick et al., 2001). Due to limited capacity of the information processing system, the increased attentional demand associated with conflict necessitates the activation of compensatory mechanisms to enhance cognitive efficiency. Conflict monitoring is thought to increase attentional control by evaluating contextually relevant information for conflict and subsequently signaling for increased cognitive resources to reduce the effects of the conflict (Botvinick et al., 2001, Botvinick et al., 2004, Folstein and Van Petten, 2008, McLoughlin et al., 2009, Yeung and Cohen, 2006).
A growing body of literature shows sex differences in behavioral manifestations of conflict monitoring, specifically in the processing of task-irrelevant information (Bayliss et al., 2005, Garcia-Garcia et al., 2008, Li et al., 2006, Li et al., 2009, Stoet, 2010). For example, females attended to irrelevant visual cues more than males in a Flanker-type task (Bayliss et al., 2005), and females were more distracted by novel auditory-visual stimuli in a negative emotional context than males (Garcia-Garcia et al., 2008). Building on these studies, Stoet (2010) examined sex differences in the processing of irrelevant information in a sample of 80 individuals (40 males, 40 females) using a novel GO/NOGO and Flanker task combination. During the task, a red, green, or blue flanker circle appeared in a grid followed by a central target circle. Participants were instructed to respond if the target stimulus was green or withhold a response if the target stimulus was red. The irrelevant flanker circles created conflict, as blue flankers were response-neutral and red or green flankers were either response-compatible if they matched the target or response-incompatible if they differed from the target. Females had slower response times (RTs) for incongruent trials and made more errors on go and no-go trials than males, possibly indicating that females were more distracted by high-conflict incompatible flankers than males, significantly slowing their performance (Stoet, 2010). Altogether, these studies suggest that females attend to irrelevant stimuli more than males, potentially demonstrating differential conflict monitoring.
One way to examine the neural underpinnings of conflict-related processing is through the conflict N2, a stimulus-locked fronto-central negative deflection of the scalp-recorded event-related potential (ERP) that is thought to reflect conflict monitoring processes (Botvinick et al., 2001). N2 amplitude is larger following high-conflict than low-conflict stimuli and is attenuated after increased attentional control (Albrecht et al., 2008, Dickter and Bartholow, 2010, van Veen and Carter, 2002b, Yeung and Cohen, 2006). Indeed, N2 latencies are closely related to RTs, suggesting that the N2 may index the duration of the response selection process (Gajewski et al., 2008). Source localization studies indicate the anterior cingulate cortex (ACC) is the neural generator of the N2 (Ladouceur et al., 2007, Ridderinkhof et al., 2004, van Veen and Carter, 2002a, van Veen and Carter, 2002b, Yeung et al., 2004). Several studies also indicate that negative affect influences conflict N2 amplitude (Durston et al., 2003, Righi et al., 2009, Sehlmeyer et al., 2010). For example, larger conflict N2 amplitudes correlate with high levels of trait anxiety (Righi et al., 2009, Sehlmeyer et al., 2010); however, both larger and smaller conflict N2 amplitudes have been associated with high levels of depression (Bruder et al., 1998, Durston et al., 2003, lv et al., 2010, Ogura et al., 1993, Ruchsow et al., 2008).
The Eriksen Flanker Task (Eriksen and Eriksen, 1974) is frequently used to elicit a conflict N2 due to the activation of competing response options (Dickter and Bartholow, 2010). In this task, participants respond to the direction of a central stimulus arrow. During congruent trials flanker stimuli cue a response similar to the target arrow (> > > > >). During incongruent trials, flanker stimuli activate competing response options by prompting a response opposite the target arrow (> > < > >). N2 amplitude is larger during high-conflict trials when participants attend more to flanker stimuli vs. low-conflict trials when participants may be less distracted by flanker stimuli, indicating greater awareness of conflict between task-irrelevant (e.g., flankers) and task-relevant (e.g., target arrow) information (Danielmeier et al., 2009, Dickter and Bartholow, 2010, Folstein and Van Petten, 2008, Ullsperger et al., 2005, van Veen and Carter, 2002a, van Veen and Carter, 2002b, Yeung et al., 2004, Yeung and Cohen, 2006, Yeung et al., 2007).
N2 amplitude may also reflect conflict adaptation processes, or increases in cognitive and attentional control associated with previous-trial congruency. Conflict adaptation is based on the finding that performance differences may be sensitive to variations in control states associated with increased allocation of cognitive resources following high conflict trials (Botvinick et al., 2001, Forster et al., 2011, Gratton et al., 1992, Kerns, 2006, Kerns et al., 2004, Larson et al., 2009a). In the context of the conflict monitoring theory, detection of high conflict on incongruent trials should lead to the recruitment of more attentional control and cognitive resources to enhance performance (i.e., faster RTs, improved error rates, and decreased susceptibility to irrelevant stimulus information) on the subsequent trial (Botvinick et al., 2001, Botvinick et al., 2004, Carter and van Veen, 2007). Following a congruent trial, cognitive control is reduced, resulting in longer RTs, decreased error rates, and more processing of irrelevant stimulus information. Recently, Forster et al. (2011) demonstrated that N2 difference waves were larger for incongruent trials preceded by congruent trials (resulting in slower RTs) than incongruent trials preceded by incongruent trials (resulting in faster RTs). Thus, N2 amplitude may reflect neural activity associated with increased attentional control in response to conflict.
Due to the hypothesized role of the ACC in conflict N2 generation (Ridderinkhof et al., 2004) and recent literature revealing sex differences in putative markers of ACC activation (Larson et al., 2011, Li et al., 2006, Li et al., 2009), the purpose of this study was to investigate sex differences in cognitive control as measured by the conflict N2. The findings of Stoet (2010) point toward sex differences in selective attention, suggesting that females may attend more to incompatible flankers. Based on this information, we hypothesized that females would exhibit larger conflict N2 amplitudes than males during a modified Eriksen Flanker Task. Furthermore, we sought to clarify potential sex differences in conflict monitoring by investigating strategic adjustments in cognitive control associated with conflict adaptation.
Section snippets
Participants
All participants provided written informed consent as approved by the local Institutional Review Board. Participants were recruited from undergraduate psychology courses. Exclusion criteria, assessed via participant self-report, included current or previous diagnosis of a psychiatric or neurologic disorder, head injury, psychoactive medication use, substance use or dependence, left-handedness, or uncorrected visual impairment. Initial study enrollment included 121 individuals. Seven
Response times and error rates
Response time and error rate data as a function of congruency are presented in Table 1; RT and error rate data for conflict adaptation effects are presented in Table 2. A Sex × Previous-trial Congruency × Current-trial Congruency ANOVA on mean RTs revealed the expected main effects of previous-trial congruency and current-trial congruency with longer RTs to incongruent than congruent trials for both, F(1, 112) = 239.36, p < .001, η2 = .68; F(1, 112) = 1,866.82, p < .001, η2 = .94, respectively. The main effect
Discussion
The main objective of this study was to investigate sex differences in the neural reflections of selective attention and conflict monitoring. We hypothesized that females would be more distracted by irrelevant flanker information compared to males resulting in slower RTs. Although females showed longer RTs and committed more errors compared to males, females exhibited smaller incongruent N2 amplitudes compared to males. Females also demonstrated shorter N2 latencies than males. No sex
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