| It is important for people to control thoughts and behaviors. However, individuals vary greatly in their control ability. For example, some people can control the impulse to smoke, whereas others cannot suppress their craving, despite being aware of the hazards of cigarettes. From the view point of cognitive psychology, such variation represents individual differences in cognitive control. The main function of cognitive control is to accomplish goal-directed behaviors by monitoring interference or response conflict, and dynamically adjusting performance(Botvinick et al., 2001; Egner, 2011). Such dynamic adjustments can be studied via the ―conflict adaptation effect‖(Gratton et al., 1992), which is frequently observed in congruency tasks, such as the color-work Stroop task.In the Stroop task, participants are required to respond to the ink color of a printed color name while ignoring the word’s meaning that may either be congruent with the ink color(congruent trial, C) or incongruent with the ink color(incongruent trial, I). Subjects typically respond more slowly to incongruent than to congruent trials, and the performance difference in terms of incongruent minus congruent trial reaction times(RTs) is a classic interference effect. The conflict adaptation effect refers to the tendency in which the interference effect following an incongruent trial(II–IC) is smaller than that following a congruent trial(CI–CC)(Gratton et al., 1992; Mansouri et al., 2009). Task-based functional magnetic resonance imaging(fMRI) has been used to study the underlying neural basis of conflict adaptation. This has revealed strong links to the prefrontal cortex, especially the anterior cingulate cortex(ACC) and dorsolateral prefrontal cortex(DLPFC). The ACC appears to be associated with the function of conflict monitoring(Botvinick et al., 2001; Botvinick et al., 2004; MacDonald et al., 2000), while the DLPFC is involved in attempts to resolve conflict by transiently amplifying neural representations of task-relevant information(Biswal et al., 1995; Egner & Hirsch, 2005a) and/or suppressing the processing of irrelevant information(Zhang et al., 2013).However, there are large individual differences in conflict adaptation. These differences have been used to explore specific cognitive and neural dysfunction in psychiatric patients(Volter et al., 2012). For example, a negative correlation has been found between the conflict adaptation effect and the schizotypal trait(Volter et al., 2012). Examination of individual differences in conflict adaptation in normal subjects is helpful for understanding dysfunction, particularly for investigations of the neural markers of the schizotypal trait(Egner, 2011). However, only one study has examined individual variability in conflict adaptation and linked it with the functional activation of the ventrolateral prefrontal cortex(VLPFC)(Egner, 2011). To our knowledge, no study to date has used resting-state fMRI(RS-f MRI) to examine the intrinsic neural basis of inter-individual differences in conflict adaptation. Therefore, the present study attempted to explore the conflict adaptation effect using RS-fMRI.Study 1 evaluated the regional homogeneity(Re Ho) of RS-fMRI signals in order to explore the neural basis of individual differences in conflict adaptation across two independent samples comprising a total of 67 normal subjects. A partial correlation analysis was carried out to examine the relationship between ReHo and behavioral conflict adaptation, while controlling for reaction time, standard deviation and flanker interference effects. This analysis was conducted on 39 subjects’ data(sample 1); the results showed significant positive correlations in the left dorsolateral prefrontal cortex(DLPFC) and left ventrolateral prefrontal cortex. We then conducted a test-validation procedure on the remaining 28 subjects’ data(sample 2) to examine the reliability of the results. Regions of interest were defined based on the correlation results. Regression analysis showed that variability in ReHo values in the DLPFC accounted for 48% of the individual differences in the conflict adaptation effect in sample 2. The present findings provide further support for the importance of the DLPFC in the conflict adaptation process. More importantly, we demonstrated that ReHo of RS-fMRI signals in the DLPFC can predict behavioral performance in conflict adaptation, which provides potential biomarkers for the early detection of cognitive control deterioration.Study 2 used the color-naming Stroop task to explore the brain regions subserving conflict adaptation, and combining the PPI analysis method to investigate the neural network of conflict adaptation. We first examined the regional activation during the processing of conflict adaptation for all participants. Data showed that activated brain regions including ACC, VLPFC, PPC, etc. Then we using the voxel-vise psychophysiological interaction analysis(PPI), investigate functional connectivity during conflict adaptation. Results showed that the ACC and PPC have different functional connectivity modes. Brain regions, which showed positive connection with ACC, contained bilateral insula, right anterior frontal cortex and cerebellum. These areas are important nodes of SN network; regions showing positive connections with PPC included the left anterior frontal cortex, the left middle frontal gyrus and the left parietal lobe. These brain regions are important node of CEN network. Then, in order to explore the individual difference, we split subjects into two groups(high score group and low score group) according to the behavior performance of conflict adaptation. Results showed that high score group showed more PPC(CEN) network connection mode(L. Wang et al., 2010), and low score group showed more ACC(SN) network connection mode(Seeley et al., 2007). To test the reliability of this result, we conducted a correlation analysis between PPI connection strength and conflict adaptation. Results found that: the connection strength of CEN network was positive correlated with conflict adaptation; and the connectivity of SN network was negatively related to the conflict adaptation. SN network is mainly responsible for detecting salience stimuli, support the bottom-up processing, and CEN network is mainly responsible for the high level of control processing. Thus, we speculate that the high score group mainly rely on the CEN network to resolve conflict in a top-down manner; while the low score group mainly rely on the SN network to resolve conflict in a down-up manner.Individual differences in brain function may originate from differences in brain structure(Kanai & Rees, 2011). For example, regional functional connectivity has significant association with the degree of expansion of the cortex evolution, has positive correlation with the groove depth of brain cortex. Study 3 used VBM and DTI to explore the brain structure basis of conflict adaptation. Results showed that, the volume of superior parietal lobule were positively related to conflict adaptation, which suggests that the PPC is important area for dynamic adjustment. The result is consistent with the results of the research one and research two, fully illustrates the executive control network to adapt to the important role of the conflict, hints at the same time, to adapt to the effects of low conflict prominent individuals facing conflict, which make use of network and perform network synergy to resolve conflicts.According to the views of dual control model(Braver, 2012; Braver et al., 2009), the top-down control is similar to the initiative control, and bottom-up control similar to reactive control. From the current evidence, initiative control is mainly completed by DLPFC and parietal lobe(Boulinguez et al., 2009; Braver et al., 2009); the reactive control is related to the ACC and the insula(Menon & Uddin, 2010). We speculated that the individual with high conflict adaptation effect might use the active control strategy, can effectively use the previous conflict signals to solve the current conflict, thus showed a greater conflict adaptation effect in behavior. While, the individual with high conflict adaptation effect might use reactivity control strategy, which could hardly use the previous conflict signals to influence the current conflict resolution; they need adopt the SN network to detect and signal the conflict to the DLPFC, which subsequent resolve conflict.One important goal of psychology is to understand and predict the behavior of people. However, studies of the neural basis of cognitive tasks in the laboratory can only help us to understand behavior. In order to make the research have more ecological validity, and to determine whether the conflict adaptation process included in the self-control ability of daily life, we define ROI based on the results of experiment 1, examining the predictive power of conflict adaptation on self-control. Results found that ReHo values of DLPFC can predict individual self-control in two dimensions: work and behavior(dimension 2) and health habits(dimension 5), these two dimensions related to adjust self and plan; For other three dimensions(dimension 1: impulse control; dimension 3: resist the temptation and dimension 4: moderation entertainment), there is no predictive power, the three dimension mainly reflects the ability of impulse control. The results show that the individual self-control ability is associated with cognitive control ability is in the laboratory, the use of cognitive control related brain regions can predict individual cognitive control ability, we can be in laboratory study used in the performance prediction of daily life, make it more ecological validity. In addition, the result also shows that the indirect DLPFC adapt to effect the conflict may not only inhibit the action of the control, it may play an important role in the plan and adjust.In conclusion, this study explored the neural mechanisms of conflict adaptation from multiple levels, found that DLPFC and parietal lobe play important roles in the conflict adaptation effect; the results of brain networks analysis implied that, high scores of conflict adaptation adopted a top-down control strategy to complete tasks; while the low score individuals take a reactive control strategy. Finally, in order to make the results more ecological validity, we expand the researches in laboratory to the self-control ability in everyday life; found that ReHo values in DLPFC can predict two dimensions of self-control ability. |
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