Parametric manipulation of the conflict signal and control-state adaptation

Abstract

Mechanisms by which the brain monitors and modulates performance are an important focus of recent research. The conflict-monitoring hypothesis posits that the ACC detects conflict between competing response pathways which, in turn, signals for enhanced control. The N2, an ERP component that has been localized to ACC, has been observed after high conflict stimuli. As a candidate index of the conflict signal, the N2 would be expected to be sensitive to the degree of response conflict present, a factor that depends on both the features of external stimuli and the internal control state. In the present study, we sought to explore the relationship between N2 amplitude and these variables through use of a modified Eriksen flankers task in which target-distracter compatibility was parametrically varied. We hypothesized that greater target-distracter incompatibility would result in higher levels of response conflict, as indexed by both behavior and the N2 component. Consistent with this prediction, there were parametric degradations in behavioral performance and increases in N2 amplitudes with increasing incompatibility. Further, increasingly incompatible stimuli led to the predicted parametric increases in control on subsequent incompatible trials as evidenced by enhanced performance and reduced N2 amplitudes. These findings suggest that the N2 component and associated behavioral performance are finely sensitive to the degree of response conflict present and to the control adjustments that result from modulations in conflict.

Figure 1

Schematic of trial procedure for the modified Eriksen flanker task. The example trial shown is for HHSSSHH. The stimuli to the right are examples for each of the congruency levels.

Figure 2

Mean RTs (A) and ERs (B) by stimulus congruity condition. Mean RT was found to increase with stimulus incongruity (CON < INC_LO < INC_MED < INC_HI; p < .001 for all comparisons). Mean ER also increased with stimulus incongruity (CON < INC_LO < INC_MED < INC_HI) with CON versus INC_LO and INC_MED versus INC_HI comparisons reaching significance (p < .001) and INC_LO versus INC_MED approaching significance at p = .113.

Figure 3

Behavioral adaptation as indexed by the mean difference between incongruent and congruent trials, given the incongruity of the previous stimulus for RT (A) and ER (B). After the INC_HI condition behavioral adjustment, effects were greater than those after both the INC_LO condition (for both RT, p = .004 and ER, p < .001) and the INC_MED condition (for both RT, p = .021 and ER, p = .002). These findings are in line with our hypothesis that control recruitment, after conflict, depends on the level of conflict previously detected.

Figure 4

Stimulus-locked ERPs at FCz and Cz by stimulus congruity condition. N2 amplitude was found to increase with stimulus incongruity (CON < INC_LO < INC_MED < INC_HI) at both FCz and Cz. Although there was no significant difference between N2 amplitudes after INC_LO and INC_MED stimuli at channel FCz, all other relevant comparisons were significant. The differentiation in N2 observed across stimulus conditions suggests that the amplitude of this component reflects the level of response conflict present.

Figure 5

Stimulus-locked ERPs at FCz and Cz by congruency (congruent vs. incongruent) of the previous and present trial. The main effect of previous stimulus congruity was nonsignificant at both FCz and Cz, and there was no significant previous–present congruency interaction. However, the mean voltage of conINC –incINC difference waves was significantly less than zero at the latency of the N2 for both FCz and Cz, suggesting that the congruity of the previous stimulus does affect subsequent N2 amplitude.

Figure 6

Mean N2 amplitude by stimulus congruity condition (CON, INC_LO, INC_MED, INC_HI) of the previous trial and congruency (CON vs. INC) of the present trial for a subset of 22 subjects. No significant difference in N2 amplitude by previous stimulus congruity was found for current congruent trials, as predicted. For current incongruent trials, however, N2 amplitude was found to decrease with previous stimulus incongruity at both FCz and Cz. N2 amplitudes were significantly less negative after INC_HI stimuli than any other condition at both FCz and Cz were significantly less negative after INC_MED than con stimuli at FCz. These findings suggest that the level of conflict signaled on the previous trial predicts control recruitment that, in turn, determines subsequent N2 amplitude.

Figure 7

Across-subject correlations between the difference in RT observed for incongruent trials preceded by incongruent trials and those preceded by congruent trials (incINC – conINC) and corresponding differences in N2 amplitude for each incongruity condition. Control recruitment should result in decreased incINC RT, increased conINC RT, increased (less negative) incINC N2 amplitude, and decreased (more negative) conINC N2 amplitudes, so a negative correlation was expected. Negative correlations were observed for each level of previous stimulus incongruity and were significant for measures after the INC_HI condition.