This exploratory study is an investigation of temporally dynamic regional brain activation patterns underlying cue-reactivity, response-inhibition, and their interaction in individuals with MUD. While similar sliding window techniques are relatively common in dynamic functional connectivity analyses 54,55 and despite decades of evidence for temporal variation in regional sensitization and habituation in cognitive/affective neuroscience 31–33, dynamic analyses of regional activation in addiction remain rare and this is the first study which explored this dynamic interaction in response-inhibition in the context of cue-reactivity.
Dynamic cue-reactivity
Dynamic cue-reactivity was observed in the bilateral STG, the right amygdala, and rostral hippocampus, and the left Pcun and ITG. Many of these regions have previously been indicated in methamphetamine cue-reactivity 16,17,19, and drug cue-reactivity more widely 56,57. Notably, dynamic amygdala activity with a similar downward slope over time has been observed in two recent cue-reactivity studies in individuals with MUD and opioid use disorder 37,38. A study on individuals with heroin use disorder estimating dynamic causal modeling parameters in overlapping windows has also demonstrated craving inputs to the amygdala increase during a cue-reactivity task, and that the DlPFC’s (dorsolateral prefrontal cortex) modulatory impact on the connection between the VMPFC (ventromedial prefrontal cortex) and the amygdala decreases over time 58. Ekhtiari et al. also similarly reported bilateral dynamic cue-reactivities in the STG, but they observed an initially escalating and subsequently decreasing activation whereas we observed a consistent habituation response 37. Broadly, our results suggest generalized habituation to drug cues across the task duration.
The dynamic cue-reactivity LME showed no significant condition-by-time interactions in the VMPFC and the VSTR (ventral striatum), indicating a lack of dynamic activity, unlike another dynamic study using similar analytical procedures 37. Unexpectedly, these regions also showed no static activity, potentially showing that they were not recruited by our task components. Methamphetamine use duration had a significant uncorrected correlation with dynamic cue-reactivity in the right Pcun, similar to a previous study in which addiction severity was found to be correlated with Pcun activation during cue-reactivity tasks 59.
Dynamic response-inhibition
More than a hundred regions in our LME model showed dynamic response-inhibitory activity. This may not be surprising, as response-inhibition is associated with large-scale neural activity 53 and dynamic brain network reconfiguration 41. Also, notable is that dynamic prefrontal activations were also observed in the response-inhibition model, whereas only FDR-uncorrected prefrontal activations were observed in the other two models (cue-reactivity and cue-reactivity/inhibition interaction). There have been reports of prefrontal sensitization to salient cues 36, and it has been observed that the prefrontal cortex is implicated in the dysfunctional behavioral regulation seen in the MUD during response control tasks 22. The observation of dynamic activity in prefrontal regions was expected, given their involvement in inhibitory control networks51 and response-inhibition in substance use disorders 60,61.
Most of the regions involved in response-inhibition in a recent meta-analysis of Go-NoGo tasks 53 had dynamic activation patterns in this study. Notably, while dynamic cue-reactivity was associated with a generalized habituation effect, these regions showed two broad temporal activation patterns. The MTG, the left INS, the right CG, the right MTG, and supramarginal gyrus, showed falling inhibitory activations while the PrG, the left SPL, and IFG (operculum) showed increasing activations (sensitization). This might reflect differences in response-inhibitory processes that these regions contribute to, such as error monitoring and attentional control 62,63, or the involvement of these regions in other networks that interact with the response-inhibition network in individuals with substance use disorders, such as the INS in the salience network or the MFG in self-directed processing 14.
Commission error rates have been used as a measure of response-inhibitory success in Go-NoGo tasks, and have been correlated with activities in the right SPL and DLPFC in individuals with addictive disorders 64. In our study, commission error rates had significant uncorrected correlations with dynamic response-inhibition slopes commission error rates in the ITG, IPL, Pcun, and dorsal and ventral anterior CG, potentially implicating these regions in response-inhibition dysfunctions in the MUD. These regions can play an important role in the development of response control-related biomarkers in addictive disorders 65.
Dynamic response-inhibition during cue exposure
The bilateral PhG and the right Pcun were the only regions with a dynamic interaction of response-inhibition and cue-reactivity. Several meta-analyses have demonstrated that drug cue-reactivity is associated with heightened precuneal activation 66,67, and based on the response-inhibition literature, dopaminergic inhibition and network decoupling of precuneal activity may be important for successful response-inhibition 68–70. Precuneal involvement in cue-reactivity in substance use disorders might be related to its role in the default mode network and self-referential processing in general 14, and, interestingly, it has been argued that the Pcun might be an important node for the integration of contradictory executive control and cue-reactivity processes 71. Considering the above, the decreasing activation associated with drug-related inhibition in the right Pcun may reflect a lessening effect of drug cues in hampering response-inhibition across the task duration. Since it appears that the response-inhibition contrast in the Pcun is mostly stable across time while cue-reactivity and interaction contrasts decline, habituation to drug cues or top-down suppression of precuneal cue-reactivity, rather than the role of the Pcun in response-inhibition per se, maybe the responsible mechanisms.
The PhG have also been implicated in substance use disorders. Addictive disorders are associated with parahippocampal gray matter changes 72 and increases in its connectivity within the default mode network 73, both the right and the left parahippocampus generally show higher activations in response to drug-related cues compared to neutral cues 66,74,75, and response-inhibition-associated parahippocampal dysfunction has been observed in individuals with substance use disorders compared to healthy controls 20. As part of the default mode network and given its association with drug cue-reactivity, it was expected that similar to the Pcun, the cue-reactivity contrast in the PhG would decrease, reflecting both habituation processes and task-engagement-related suppression. Some evidence also exists for parahippocampal habituation during exposure to emotionally salient stimuli 76,77 and for the role of the parahippocampus in the extinguishing of drug cue associations 78. However, the parahippocampus is also involved in neural networks involved in associational memory and learning 79,80 and might be activated to support learning during response-inhibition tasks 14. Indeed, increasing parahippocampal recruitment during a learning task has been observed before 81. These dual roles of the parahippocampus in cue habituation and learning could explain why the cue-reactivity contrast decreased while the response-inhibition contrast increased in the PhG during the task, and is supported by the observation that drug-related inhibition remained mostly stable, while inhibition during neutral cue exposure was associated with increasing parahippocampal activity.
An interesting observation in this study was the right-lateralization of dynamically active regions across the three contrasts. Some evidence exists that the right hemisphere may be more important in response-inhibitory and attentional control processes 82,83, and right lateralization of dynamic response to salient stimuli has been observed in the right amygdala, IPL, and hippocampus 31,84. It has been argued that while the left amygdala is involved in sustained stimulus evaluation, the right amygdala might be more specialized for dynamic stimulus processing 34.
Limitations
While the results of this exploratory investigation are promising, several limitations are important to point out. Firstly, we included no healthy control group, and so the specificity of observed patterns to individuals with MUD is unclear. Also, all participants were men, treatment seeking individuals MUD, limiting the generalizability of our observations. Regarding the task design, an inherent limitation introduced by our use of a mixed drug cue and negative emotional Go-NoGo task is the potential carry-over effects of salient cues on brain activity during subsequent blocks 85,86. While such issues may be ameliorated by the choice of a blocked presentation of different cue types, the results are likely confounded by these effects. Lastly, we used no measure of craving across the task duration, which would have allowed the analysis of temporal correlations between craving and neural activity, as in one recent study by Murphy et al. 38.