Priming and backward influences in the human brain: processing interactions during the stroop interference effect

Abstract

This study investigated neural processing interactions during Stroop interference by varying the temporal separation of relevant and irrelevant features of congruent, neutral, and incongruent colored-bar/color-word stimulus components. High-density event-related potentials (ERPs) and behavioral performance were measured as participants reported the bar color as quickly as possible, while ignoring the color words. The task-irrelevant color words could appear at 1 of 5 stimulus onset asynchronies (SOAs) relative to the task-relevant bar-color occurrence: -200 or -100 ms before, +100 or +200 ms after, or simultaneously. Incongruent relative to congruent presentations elicited slower reaction times and higher error rates (with neutral in between), and ERP difference waves containing both an early, negative-polarity, central-parietal deflection, and a later, more left-sided, positive-polarity component. These congruency-related differences interacted with SOA, showing the greatest behavioral and electrophysiological effects when irrelevant stimulus information preceded the task-relevant target and reduced effects when the irrelevant information followed the relevant target. We interpret these data as reflecting 2 separate processes: 1) a 'priming influence' that enhances the magnitude of conflict-related facilitation and conflict-related interference when a task-relevant target is preceded by an irrelevant distractor; and 2) a reduced 'backward influence' of stimulus conflict when the irrelevant distractor information follows the task-relevant target.

Figure 1.

(A) Schematic illustration of the experimental design for a congruent (red–red) Stroop stimulus presented at each of the 5 SOAs. Each temporal separation (−200, −100, 0, +100, and +200 ms) is shown in a separate row with vertical dotted lines indicating times at which stimuli components were presented. Once both stimulus components were presented, they remained on the screen for an additional 1000 ms for all SOA conditions. The participant's task was always to report the color of the bar, which is defined as 0 ms in this schematic. (B) Trials proportions and exemplar stimuli for the main experiment and behavioral control variants of the task. See the Supplementary Materials for movies of the stimuli used in the 2 tasks.

Figure 2.

Behavioral performance for data collected during the main experiment (A) and the behavioral control experiment (B) are shown as a function of SOA. Data in the top panels show reaction times for Incongruent (red), Neutral (green), and Congruent (blue) trials. Congruency differences are shown as gray bars for the Incongruent minus Congruent reaction times in the main experiment, and separated into RT Facilitation (white: Congruent minus Neutral) and Interference (black: Incongruent minus Neutral) in the behavioral control experiment. Mean error rates are shown in the bottom panels for each condition using the same color convention as in above.

Figure 3.

Grand average ERPs (left) and incongruent minus congruent difference waves (right) are shown for the no-delay condition of the Stroop color-discrimination task. ERPs are shown for congruent (blue) and incongruent (red) trials at 4 midline channels (FCz, Cz, CPz, and Pz). The location of these channels are indicated in green in the key at the bottom. Channels marked in orange around the peak of the effect were subjected to repeated-measures ANOVAs.

Figure 4.

Spatial distribution of the negative-polarity (top) and later positive-polarity (bottom) incongruency ERP effects for the no-delay SOA condition. Note that these differences maps are also indicated with asterisks “*” in Figure 6.

Figure 5.

Group average difference waves (incongruent minus congruent) are shown separately for irrelevant-first (left) and relevant-first (right) conditions, with the no-delay difference wave (black traces) present in both sets of plots. Irrelevant-first difference waves shifted monotonically as a function of SOA, with the −200 ms condition showing the largest and temporally sharpest amplitude difference (purple traces). Relevant-first difference waves did not show a strictly monotonic shift, with the +100 and +200 SOA difference waves initiating at nearly the same increased latency relative to the no-delay condition effect, but with the +200 condition effect offsetting later. Horizontal bars below the difference waves correspond to the time points that showed a main effect of congruency according to ANOVAs performed on the 4 channels surrounding Pz (highlighted in orange in Figure 3 inset). These bars are color coded with the same condition convention as for the difference waves.

Figure 6.

Spatiotemporal distributions of the congruency-related difference potentials as a function of SOA. The temporal shift due to SOA is evident in these distributions for both the earlier negative-polarity effect (blue/purple) and the later positive-polarity effect (yellow/orange). As seen in the difference waves in Figure 5, these maps also show that the negative-polarity waves for the +100 and +200 ms SOA conditions onset at roughly the same latency.