Reduced mu suppression and altered motor resonance in euthymic bipolar disorder: Evidence for a dysfunctional mirror system?

Neurophysiological profiles of patients with bipolar disorders as probed with transcranial magnetic stimulation: A systematic review

AIM

Bipolar disorder (BD) has a significant impact on global health, yet its neurophysiological basis remains poorly understood. Conventional treatments have limitations, highlighting the need for a better understanding of the neurophysiology of BD for early diagnosis and novel therapeutic strategies.

DESIGN

Employing a systematic review approach of the PRISMA guidelines, this study assessed the usefulness and validity of transcranial magnetic stimulation (TMS) neurophysiology in patients with BD.

METHODS

Databases searched included PubMed, MEDLINE, Embase, and PsycINFO, covering studies from January 1985 to January 2024.

RESULTS

Out of 6597 articles screened, nine studies met the inclusion criteria, providing neurophysiological insights into the pathophysiological basis of BD using TMS-electromyography and TMS-electroencephalography methods. Findings revealed significant neurophysiological impairments in patients with BD compared to healthy controls, specifically in cortical inhibition and excitability. In particular, short-interval cortical inhibition (SICI) was consistently diminished in BD across the studies, which suggests a fundamental impairment of cortical inhibitory function in BD. This systematic review corroborates the potential utility of TMS neurophysiology in elucidating the pathophysiological basis of BD. Specifically, the reduced cortical inhibition in the SICI paradigm observed in patients with BD suggests gamma-aminobutyric acid (GABA)-A receptor-mediated dysfunction, but results from other TMS paradigms have been inconsistent. Thus, complex neurophysiological processes may be involved in the pathological basis underlying BD. This study demonstrated that BD has a neural basis involving impaired GABAergic function, and it is highly expected that further research on TMS neurophysiology will further elucidate the pathophysiological basis of BD.

Investigational applications of transcranial magnetic stimulation (TMS) in Mood Disorders: Studies from a tertiary care center in India

The investigational potential of TMS in psychiatry is largely underutilized. In the current article, we present the results of five studies with similar TMS protocols that looked at the investigative applications of TMS via measuring cortical reactivity as potential biomarkers in mood disorders. The first two studies, evaluate potential of TMS parameters and Motor neuron system (MNS) as state or trait markers of BD. Third and fourth studies evaluate these as endophenotypic markers of BD. The fifth study which is an RCT evaluating add-on yoga in UD, evaluates if markers of CI can index the therapeutic response of yoga. In study one MT1 was significantly greater in the SM (symptomatic-mania) group compared to HC (healthy-control) (P=0.032). The cortical inhibition measures SICI was reduced in SM(P=0.021) and BD (remitted Bipolar) (P=0.023) groups compared to HC. LICI was increased in the SM(0.021) and BD(P=0.06) groups compared to HC. In study two, a significant group x time interaction effect was observed indicating higher putative MNS-activity mediation in patients compared to HC on SlCl(P=0.024), LlCl(P=0.033). There were no significant group differences noted in the endophenotype studies. The fifth study showed a significant time X group interaction for CSP, favoring improvement in YG (yoga-group) (p<0.01).No significant change was observed for LICI(p=0.2), SICI(p=0.5). Limitations of these studies notwithstanding, we conclude that cortical reactivity measured using TMS is a potential biomarker across the course of mood disorders, starting from state and trait markers to understanding the therapeutic mechanism of a particular treatment modality in these disorders.

Predominant polarity as a neurobiological specifier in bipolar disorder: Evidence from a multimodal neuroimaging study

BACKGROUND

While predominant (PP) and onset polarity (OP) have considerable clinical and treatment implications in bipolar disorder (BD), the neurobiological underpinnings of PP and OP from a radiological perspective remain largely unknown. The main objective of this study is to investigate the neuroanatomical profile of polarity subphenotypes (PP and OP) in euthymic BD patients, using a standardized multimodal neuroimaging protocol to evaluate regional gray matter (GM) volumes, cortical thickness, as well as white matter (WM) integrity of major projection, commissural and association tracts.

METHODS

Forty-two euthymic BD patients stratified for PP and OP and 42 healthy controls (HC) were included in this computational neuroimaging study to comprehensively characterize gray and white matter alterations. Univariate analyses of covariance (ANCOVAs) were conducted with Bonferroni corrections for each MRI modality and Cohen's d effect sizes were calculated for group comparisons.

RESULTS

Phenotype-associated cortical thickness abnormalities and volumetric alterations were identified, but no WM changes ascertained. Specifically, we found a main effect of OP on GM volume of left middle frontal gyrus and of OP and PP (either or both) on cortical thickness of various regions previously implicated in BD, i.e. inferior frontal gyrus-pars opercularis (left) and pars orbitalis (bilateral), left lateral orbitofrontal gyrus, bilateral medial segment of the superior frontal gyrus, left planum polare, right anterior cingulate gyrus, left anterior and posterior insula, bilateral frontal operculum (both OP and PP); left anterior and posterior orbitofrontal gyrus, left transverse temporal gyrus, right posterior insula (only OP); and right medial frontal cortex (only PP). Based on the magnitude of differences on pairwise comparisons, we found a large effect of OP on cortical thickness in a single region (left anterior orbitofrontal gyrus) (OP-M > OP-D), while PP subgroups showed large or medium effect size differences in cortical thickness (PP-M > PP-D) in a wider array of regions (right medial frontal cortex, left frontal operculum, left inferior frontal gyrus-pars opercularis, bilateral medial segment of the superior frontal gyrus). For most regions, PP-D patients showed the greatest decreases in cortical thickness compared to HC while PP-M showed the smallest, with PP-U showing an "unspecified" pattern mostly lying in-between PP-D and PP-M.

CONCLUSIONS

Our multimodal imaging findings suggest specific polarity BD subgroups with compromised cortical thickness; we recorded a greater impact of PP on brain structure compared to OP, which provides additional evidence that PP can be considered as a neurobiological specifier in BD.

Characterization and interrelationships of theory of mind, socially competitive emotions and affective empathy in bipolar disorder

OBJECTIVE

Evidence shows impaired theory of mind (ToM) in patients with bipolar disorder (BD), yet research examining its cognitive and affective components simultaneously is sparse. Moreover, recognition of socially competitive 'fortune of others' emotions (e.g. envy/gloat) may be related to ToM, but has not been assessed in BD. Finally, if and how ToM and 'fortune of others' emotions relate to affective empathy in BD is currently unclear. This study aimed to address these points.

METHODS

64 BD patients and 34 healthy controls completed the Yoni task, a visual task assessing first- and second-order cognitive and affective ToM as well as 'fortune of others' emotions. The Toronto Empathy Questionnaire was used to assess self-reported affective empathy.

RESULTS

Patients with BD showed no deficits in cognitive and affective ToM or recognition of 'fortune of others' emotions. The ability to infer 'fortune of others' emotions correlated with several ToM measures, indicating that these functions are part of the same system. Patients with BD reported similar levels of affective empathy to healthy controls, and this was not related to ToM or 'fortune of others' emotions, suggesting that affective empathy represents a separate social domain.

CONCLUSIONS

These findings highlight areas of spared social functioning in BD, which may be utilized in therapeutic strategies.

PRACTITIONER POINTS

Our results suggest theory of mind and empathy may represent areas of potentially spared cognitive functioning in BD. As many BD patients have experienced adversity during developmental periods in which theory of mind and empathy develop, our findings suggest that these abilities may be markers of resilience in the disorder. Our findings are important for the formulation of therapeutic interventions for BD, which may include considering practical ways that a patients' knowledge of intact ToM and empathy could be utilized to reduce self-stigma and promote self-efficacy, improved well-being and functioning.

Separable and Replicable Neural Strategies During Social Brain Function in People With and Without Severe Mental Illness

OBJECTIVE

Case-control study design and disease heterogeneity may impede biomarker discovery in brain disorders, including serious mental illnesses. To identify biologically and/or behaviorally driven as opposed to diagnostically driven subgroups of individuals, the authors used hierarchical clustering to identify individuals with similar patterns of brain activity during a facial imitate/observe functional MRI task.

METHODS

Participants in the Social Processes Initiative in Neurobiology of the Schizophrenia(s) study (N=179; 109 with a schizophrenia spectrum disorder and 70 healthy control participants) underwent MRI scanning at three sites. Hierarchical clustering was used to identify new data-driven groups of participants; differences on social and neurocognitive tests completed outside the scanner were compared among the new groups.

RESULTS

Three clusters with distinct patterns of neural activity were found. Cluster membership was not related to diagnosis or scan site. The largest cluster consisted of "typical activators," with activity in the canonical "simulation" circuit. The other clusters represented a "hyperactivating" group and a "deactivating" group. Between-participants Euclidean distances were smaller within clusters than within site or diagnostics groups. The deactivating group had the highest social cognitive and neurocognitive test scores. The hierarchical clustering analysis was repeated on a replication sample (N=108; 32 schizophrenia spectrum disorder, 37 euthymic bipolar disorder, and 39 healthy control participants), which exhibited the same three cluster patterns.

CONCLUSIONS

The study findings demonstrate replicable differing patterns of neural activity among individuals during a socio-emotional task, independent of DSM diagnosis or scan site. The findings may provide objective neuroimaging endpoints (biomarkers) for subgroups of individuals in target engagement research aimed at enhancing cognitive performance independent of diagnostic category.

Elevated mirror neuron system activity in bipolar mania: Evidence from a transcranial magnetic stimulation study

OBJECTIVES

The role of the "mirror neuron system" (MNS) in the pathophysiology of mood disorders is not well studied. Given its posited role in the often-impaired socio-emotional processes like intention detection, empathy, and imitation, we compared putative MNS-activity in patients with bipolar mania and healthy comparison subjects. We also examined the association between putative MNS-activity and hyper-imitative behaviors in patients.

METHODS

We studied 39 medication-free individuals diagnosed with mania and 45 healthy comparison subjects. TMS-evoked motor cortical reactivity was measured via single- and paired-pulse stimuli (assessing SICI-short and LICI-long interval intracortical inhibition) while subjects viewed a static image and goal-directed actions. Manic symptom severity and imitative behaviors were quantified using the Young's Mania Rating Scale and a modification of the Echolalia Questionnaire.

RESULTS

Two-way repeated measures analysis of variance demonstrated a significant group ×time interaction effect indicating greater facilitation of cortical reactivity during action-observation (putative MNS-activity) in the patient group as compared to the healthy group. While LICI-mediated MNS-activity had a significant association with manic symptom severity (r = 0.35, P = 0.038), SICI-mediated MNS-activity was significantly associated with incidental echolalia scores in a subgroup of 17 patients with incidental echolalia (r = 0.75, P < 0.001).

CONCLUSIONS

Our findings demonstrate that putative MNS-activity is heightened in mania, possibly because of disinhibition, and associated with behavioral consequences (incidental echolalia).

Investigating the neurobiology of schizophrenia and other major psychiatric disorders with Transcranial Magnetic Stimulation

Characterizing the neurobiology of schizophrenia and other major psychiatric disorders is one of the main challenges of the current research in psychiatry. The availability of Transcranial Magnetic Stimulation (TMS) allows to directly probe virtually any cortical areas, thus providing a unique way to assess the neurophysiological properties of cortical neurons. This article presents a review of studies employing TMS in combination with Motor Evoked Potentials (TMS/MEPs) and high density Electroencephalogram (TMS/hd-EEG) in schizophrenia and other major psychiatric disorders. Studies were identified by conducting a PubMed search using the following search item: "transcranial magnetic stimulation and (Schizophrenia or OCD or MDD or ADHD)". Studies that utilized TMS/MEP and/or TMS/hd-EEG measures to characterize cortical excitability, inhibition, oscillatory activity, and/or connectivity in psychiatric patients were selected. Across disorders, patients displayed a pattern of reduced cortical inhibition, and to a lesser extent increased excitability, in the motor cortex, which was most consistently established in Schizophrenia. Furthermore, psychiatric patients showed abnormalities in a number of TMS-evoked EEG oscillations, which was most prominent in the prefrontal cortex of Schizophrenia relative to healthy comparison subjects. Overall, results from this review point to significant impairments in cortical excitability, inhibition, and oscillatory activity, especially in frontal areas, in several major psychiatric disorders. Building on these findings, future studies employing TMS-based experimental paradigms may help elucidating the neurobiology of these psychiatric disorders, and may assess the contribution of TMS-related measures in monitoring and possibly maximizing the effectiveness of treatment interventions in psychiatric populations.

Sources of mu activity and their functional connectivity in perceiving complexities in reciprocal social interactive motion: An exploratory study using the 'Namaste' task

Cognitive processes underlying reciprocal social interactions are understood by the mechanism of embodiment, which is closely related to the mirror neuron system. Electroencephalographic (EEG) mu activity is a neural marker of the mirror neuron system. This study investigated the mu activity, localization of its sources and functional connectivity, which was induced while watching reciprocal social interactive motion across various degrees of complexity. Eighteen healthy participants underwent high-resolution EEG recording using 256-channels while they watched a specifically designed, culture specific, video task that showed two persons interacting socially using body gestures. Task complexity was determined by (1) whether there was an identical gestural response or a non-identical one; (2) whether the participant watched two persons interacting or was virtually involved in the interaction. Source localization and functional connectivity analysis was conducted for mu activity across various tasks. We also correlated mu activity and functional connectivity measures with serum BDNF. We found that spectral densities in various brain sources of mu activity and their increased functional connectivity distinguished identical and non-identical reciprocal expression observations, while mu suppression alone did not discriminate various degrees of complexities. These findings might have important implications in the understanding of mechanisms underlying mirror neuron dysfunction in various psychiatric disorders.