The Dynamics of Proactive and Reactive Cognitive Control Processes in the Human Brain

Abstract

In this study we leveraged the high-temporal resolution of EEG to examine the neural mechanisms underlying the flexible regulation of cognitive control that unfolds over different timescales. We measured behavioral and neural effects of color-word incongruency as different groups of human participants performed three different versions of color-word ‘Stroop’ tasks in which the relative timing of the color and word features varied trial-to-trial. For this purpose we used a standard ‘Stroop’ color-identification task with equal congruent-to-incongruent proportions (50/50%), along with two versions of the ‘Reverse Stroop’ word-identification tasks, for which we manipulated the incongruency proportion (50/50% and 80/20%). Two canonical ERP markers of neural processing of stimulus incongruency, the fronto-central negative-polarity incongruency wave (NINC) and the late positive component (LPC), were evoked across the various conditions. Results indicated that color-word incongruency interacted with the relative feature timing, producing greater neural and behavioral effects when the task-irrelevant stimulus preceded the target, but still significant effects when it followed. Additionally, both behavioral and neural incongruency effects were reduced by nearly half in the word-identification task (ReverseStroop-50/50) relative to the color-identification task (Stroop-50/50), with these effects essentially fully recovering when incongruent trials appeared only infrequently (ReverseStroop-80/20). Across the conditions, NINC amplitudes closely paralleled reaction times, indicating this component is sensitive to the overall level of stimulus conflict. In contrast, LPC amplitudes were largest with infrequent incongruent trials, suggesting a possible readjustment role when proactive control is reduced. These findings thus unveil distinct control mechanisms that unfold over time in response to conflicting stimulus input under different contexts.

The neural dynamics of stimulus and response conflict processing as a function of response complexity and task demands

“…As has been previously observed in conflict-related tasks, two main conflict-related processing components of interest emerged (see Appelbaum et al, 2014; Liotti et al, 2000). The first component was the intermediate-latency, centrally-distributed, incongruency-related negativity, the “N inc ,” peaking between 300 and 500 ms, and the second component was the longer-latency, posteriorally-distributed late positive complex, the “LPC.” For each of these components, the data were collapsed across the two response conditions (i.e., the two and four response-choice conditions) for each task to obtain an ROI for the N inc and for the LPC (see Supplementary Figure 1).…”

“…The Stroop stimuli consisted of colored rectangles that subtended 5 by 16 degrees of visual angle (see Appelbaum et al, 2009; 2012; 2014; for previous studies with similar stimulus configurations), presented 3.75 degrees below fixation (see Figure 1). Each colored rectangle contained a color word written in white ink.…”

“…The stimulus structure used in this task, therefore, differed somewhat from those often used in most Stroop tasks (e.g., Liotti et al, 2000), in which the word is written in the font color that either matched or did not match its semantic meaning. We chose to use the bar/word stimuli here, in order to be able to compare our ERP activation patterns with that which we had previously observed in a version of the Stroop task where we had used such a stimulus structure in order to give us the ability to vary the relative temporal onsets of the color/word information (Appelbaum et al, 2009; 2012; 2014). This previous work had confirmed that such Stroop stimuli do elicit robust incongruency effects, both behaviorally and neurally, in line with that which has been observed with the more traditional Stroop stimuli (Liotti et al, 2000).…”

“…In some instances, this negativity has been termed the N450 in the Stroop task (e.g., West & Alain, 2000), as it tends to peak at approximately 450 ms post-stimulus onset). In cases where the timing of this effect might vary, others have referred to this effect as the N inc , standing for the negative-polarity incongruency wave without regard to a specific latency (e.g., Appelbaum et al, 2009; Appelbaum, Boehler, Davis, Won, & Woldorff, 2014; Coderre et al, 2011). …”

“…In EEG studies employing the Stroop task, the number of response options is typically greater than that in the Flanker task. Specifically, Stroop tasks used for EEG typically have had four response options (Appelbaum et al, 2014; Atkinson, Drysdale, & Fulham, 2003; Badzakova-Trajkov et al, 2009; Coderre et al, 2011; Hanslmayr et al, 2008; Huster et al, 2009; Liotti et al, 2000; Markela-Lerenc et al, 2004; Silton et al, 2010; Tillman & Wiens, 2011; West & Alain, 1999; 2000 but see Caldas, Machado-Pinheiro, Souza, Motta-Ribeiro, & David, 2012; Killikelly & Szűcs, 2013), whereas EEG studies of the Flanker task have tended to only use two response options (Appelbaum et al, 2011; Bartholow et al, 2005; Beste et al, 2008; Brydges et al, 2012; Danielmeier et al, 2009; Freitas, Banai, & Clark, 2009; Frühholz et al, 2011; Tillman & Wiens, 2011; Wendt, Heldmann, Münte, & Kluwe, 2007). Having a limited set of response-options, as in the Flanker task, could be the cause of an earlier onset of this conflict-related activity, which could explain the differential timing of the incongruency-related negativity that has observed in these two tasks.…”