The main objective of this study was to identify a possible neuroanatomical signature of real-time fMRI-based emotional brain regulation by detecting structural features predictive of AI-guided emotion regulation. To this aim, we used a KRR model to estimate the predictive value of GM and WM regional volume for individual differences in self-regulation of AI activity. Our multivariate regression analysis revealed that both GM and WM volumetric characteristics of several cortical and subcortical areas successfully predicted regulation of AI activity, considering either single runs or averaged activity of all runs. Overall, the individual predictive value of both GM and WM regions was rather small suggesting that successful prediction mainly relied on a combined effect of specific anatomical networks. Accordingly, GM network-based analysis indicated significant predictive values, in particular of the DMN and CEN, and to a lesser extent of the SN.
Altogether, these findings represent an indication that volumetric differences of specific neuroanatomical structures can impact the ability to attain voluntary regulation of emotional brain activity. However, the nature of neuroanatomical differences affecting self-regulation of brain activity is likely heterogenous and cannot be simplistically considered unidirectional. As multivariate regression analysis is sensitive to any systematic volumetric difference contributing to prediction of behavioral data, independently of directionality, our results should be interpreted as the effect of both increased and reduced GM and WM volumetric characteristics that altogether influence AI regulation performance.
Nevertheless, our results revealed that several GM regions including areas in the prefrontal cortex, medial temporal cortex, lateral occipital cortex, globus pallidus, hippocampus, parahippocampal gyrus and the cerebellum, as well as WM regions including the fronto-occipital fasciculus, tapetum and fornix, were successful predictors of overall self-regulation of AI activity.
We have not observed any GM and WM region predictive of learning-related indices. This result might be related to the heterogeneity of learning strategies among participants as well as to non-linear learning curves across runs. On the other hand, average AI activity might partially capture individual differences and more complex learning processes associated to emotional brain regulation.
In addition, analysis of single runs appears to reflect well-known learning effects. For instance, in accordance with previous functional studies (Lee et al. 2011) our results showed that the predictive anatomical network is more circumscribed in the last run, a phase when regulation is usually more consolidated, whereas in the initial phase the structural pattern is more distributed. Notably, common frontal, prefrontal, occipital, medio-temporal and cerebellar regions are observable in both first and last run, whereas the caudate and pallidum contribute only to the first run. Moreover, the cerebellum, although it participates to prediction of AI activity during both first and last run, has the largest weight in the last run when participants attained the highest increase of AI activity.
Our findings are generally in line with previous real-time fMRI studies as well as recent theoretical models on the general neural mechanisms underlying brain regulation, both indicating increased activity of the dorsolateral prefrontal cortex and lateral occipital cortex associated with attentional processes in relation to feedback control (Paret et al. 2018; Sitaram et al. 2017; Shibata et al. 2019; Caria 2020), and also pointing to the basal ganglia as critical regions for core learning-related processes such as salience-based strategy selection and reinforcement assessment (Skottnik et al. 2019). In our analysis, the striatum contributes only to the first run and not to the last, it is then conceivable that the observed attenuation might be related to a decreased relevance of strategies’ individuation process when regulation is more easily attainable.
The ventral striatum has been also proposed to enable monitoring the implicit reward value of fMRI feedback (Paret et al. 2018; Sitaram et al. 2017), whereas the orbitofrontal cortex would process feedback failure-associated signals (Paret et al. 2019b). In addition, other regions such as the insula and anterior cingulate cortex (Emmert et al. 2016; Paret et al. 2019a; Shibata et al. 2019) would participate to regulation processes, likely by supporting continuous error monitoring and evaluation (Gaume et al. 2016). Our results, unlike these previous evidences, suggest that structural differences of the SAL network, including anterior cingulate gyrus, amygdala, ventral striatum and SN/VTA, might be less relevant for predicting AI regulation.
On the other hand, in line with previous functional data (Shibata et al. 2019), we observed a remarkable influence of cerebellar structures for learned regulation. The cerebellum, besides its clear role in adaptive motor learning (Galea et al. 2011), is implicated in higher-level cognitive functions such as encoding of internal models of mental representations (Ito 2008; Sokolov et al. 2017). It also contributes to adaptive predictive mechanisms during error-based and reinforcement learning (Swain et al. 2011; Ito 2008; Sokolov et al. 2017). An increasing number of studies demonstrated that the cerebellum processes reward- and error-related signals (Heffley and Hull 2019; Kostadinov et al. 2019; Larry et al. 2019; Sendhilnathan et al. 2020; Wagner et al. 2017). Existing interconnections of the posterior cerebellum with ventral and dorsal striatum (Bostan et al. 2013) might then support exchanging of reward- and error-related information during real-time fMRI training. The cerebellum has been recently described as part of an integrated network along with basal ganglia, and prefrontal cortex subserving multiple non-motor functional domains (Bostan and Strick 2018) including affective and socio-cognitive behavior (Sokolov et al. 2017; Van Overwalle et al. 2014; Pierce and Peron 2020). The widespread cerebellar interplay with several regions of the limbic network might then support a modulatory function during emotion regulation (Baumann and Mattingley 2012; Schutter and van Honk 2009; Pierce and Peron 2020). Cerebellum-mediated adaptive predictive mechanisms would enable selection and adjustment of appropriate regulatory strategies. During our real-time fMRI training participants explicitly induced self-generated affective states through emotional recalling and autobiographical memory retrieval. The efficacy of different emotional strategies was then continuously estimated and assessed so as to retain and fine-tune those more predictive of successful regulation. A probabilistic representation of the adopted emotional strategies was thus likely implemented and dynamically updated. Additional networks such as the DMN, might also support the instantiation of predictable future events (Suddendorf and Corballis 2007; Spreng and Grady 2010). The DMN is posited to constitute a reinforcement learning agent mediating higher order predictive control of behavior through decision process based on prediction error estimation and reward feedback assessment (Dohmatob et al. 2020). In line with this perspective, the DMN along with the CEN might thus play an important role in real-time fMRI-mediated regulation of brain activity.
Emotional imagery and affective memory recalling have been proved effective in supporting emotional brain regulation (Linhartova et al. 2019). Interindividual differences in the capacity to adopt such strategies, possibly reflected by specific neuroanatomical features, are then expected to significantly influence brain regulation ability. Accordingly, our results showed that volumetric differences of medial temporal regions such as the perirhinal cortex, hippocampus and parahippocampal gyrus, as well as the fornix, all regions implicated in autobiographical memory and emotional memory retrieval (Ritchey et al. 2019; LaBar and Cabeza 2006), were good predictors of AI regulation. Overt self-regulation might then capitalize on the capacity to adopt explicit emotional strategies that rely on structural and functional characteristics of the medial temporal lobe. On the other hand, a number of studies demonstrated that brain activity can be actually regulated implicitly, without specific instructions, and even covertly with participants being not aware of the regulation process (Ramot et al. 2016; Watanabe et al. 2017; Taschereau-Dumouchel et al. 2018).
Finally, our results are also in line with studies investigating the neural correlates of cognitive emotion regulation (Gross 2014). For instance, we also observed involvement of the dorsolateral and medial prefrontal cortex regions, assumed to mediate top-down emotional regulatory processes. However, these regions were typically associated with down-regulation of emotions, whereas in our study participants were mainly instructed to up-regulate AI activity. Disentangling the specific impact of these regions on both regulation conditions separately it was here not possible as up-regulation activity was strictly dependent on the following down-regulation phase, not having a baseline condition. In future studies, it would be interesting to investigate whether morphological characteristics of the frontal and prefrontal circuits differentially affect up and down emotional brain regulation.
In conclusion, our findings corroborate previous functional evidences of brain networks implicated in real-time fMRI-guided brain regulation and extend them by highlighting neuroanatomical topography relevant to self-regulation of emotion-related brain nodes. However, considering the small sample size any conclusion from the observed effects should be cautious. Further studies with larger samples, adopting homogenous experimental protocols, are still required. In particular, future studies should explore brain structural landmarks at both macro- and mesoscale specifically contributing to specific emotional brain regulation strategies in contrast to more general regulatory mechanisms. In the field of cognitive emotion regulation it has been proposed a distinction, with some overlaps, between brain circuits supporting emotion regulation and those contributing to general neurophysiological regulation (Gross 2014). An analogous perspective is plausible in the context of real-time fMRI-based emotional brain regulation. Ultimately, we reason that neuroanatomical predictors, along with other important methodological factors shown to impact real-time fMRI-based regulation (Haugg et al. 2021), might be relevant for designing and optimizing future protocols of patient-tailored interventions for emotional disorders.