Modulations of the executive control network by stimulus onset asynchrony in a Stroop task

Abstract

Background: Manipulating task difficulty is a useful way of elucidating the functional recruitment of the brain's executive control network. In a Stroop task, pre-exposing the irrelevant word using varying stimulus onset asynchronies ('negative' SOAs) modulates the amount of behavioural interference and facilitation, suggesting disparate mechanisms of cognitive processing in each SOA. The current study employed a Stroop task with three SOAs (-400, -200, 0 ms), using functional magnetic resonance imaging to investigate for the first time the neural effects of SOA manipulation. Of specific interest were 1) how SOA affects the neural representation of interference and facilitation; 2) response priming effects in negative SOAs; and 3) attentional effects of blocked SOA presentation.

Results: The results revealed three regions of the executive control network that were sensitive to SOA during Stroop interference; the 0 ms SOA elicited the greatest activation of these areas but experienced relatively smaller behavioural interference, suggesting that the enhanced recruitment led to more efficient conflict processing. Response priming effects were localized to the right inferior frontal gyrus, which is consistent with the idea that this region performed response inhibition in incongruent conditions to overcome the incorrectly-primed response, as well as more general action updating and response preparation. Finally, the right superior parietal lobe was sensitive to blocked SOA presentation and was most active for the 0 ms SOA, suggesting that this region is involved in attentional control.

Conclusions: SOA exerted both trial-specific and block-wide effects on executive processing, providing a unique paradigm for functional investigations of the cognitive control network.

Figure 1

Illustration of paradigm. Examples of a) a −400 ms SOA incongruent condition; b) a −200 ms SOA congruent condition; and c) a 0 ms SOA control condition. Duration (ms) of each stimulus is indicated to the right.

Figure 2

Behavioural data. a) Mean RTs for each congruency and SOA (standard error in parentheses). b) Stroop; c) interference; and d) facilitation effects, with significant differences between SOAs, as determined by paired-sample t-tests (two-tailed), indicated (§ = trend, p < 0.1; * = p < 0.05; ** = p < 0.01; *** = p < 0.001).

Figure 3

Stroop contrast in the fMRI data. Overlaid contrasts for the Stroop comparison (incongruent > congruent) for all SOAs, with clusters of interest labelled (LIPL = left inferior parietal lobe; LSFG = left superior frontal gyrus). Axial slices are shown from z = −25 to z = 70.

Figure 4

Interference contrast in the fMRI data. Overlaid contrasts for the interference comparison (incongruent > control) for all SOAs, with clusters of interest labelled (LSPL = left superior parietal lobe; RSPL = right superior parietal lobe).

Figure 5

Facilitation contrast in the fMRI data. Overlaid contrasts for the interference comparison (control > congruent) for all SOAs, with clusters of interest labelled.

Figure 6

Interaction of interference and SOA in the fMRI data. Results of the 3-way ANOVA identifying significant interactions of a) interference and c) facilitation magnitude with SOA, with ROIs indicated. Panels to the right show the percent signal change effect in b) interference (incongruent signal change minus control signal change) and d) facilitation (control signal change minus congruent signal change) for each ROI and SOA.

Figure 7

Global SOA effects in the fMRI data. Results of the 3-way ANOVA collapsing across congruency to investigate global (i.e. block-wide) SOA effects. a) Axial slices presented at three z-coordinates to illustrate two distinct clusters in the right inferior frontal gyrus, as well as a cluster in the superior parietal lobe. b) The percent signal change for each ROI and SOA, collapsed over congruency (the RIFG percent signal change was extracted from the larger cluster at z = 0).