A study of transcallosal inhibition in schizophrenia using transcranial magnetic stimulation

Interhemispheric Brain Communication and the Evolution of Turn-Taking in Mammals

In the last 20 years, research on turn-taking and duetting has flourished in at least three, historically separate disciplines: animal behavior, language sciences, and music cognition. While different in scope and methods, all three ultimately share one goal—namely the understanding of timed interactions among conspecifics. In this perspective, we aim at connecting turn-taking and duetting across species from a neural perspective. While we are still far from a defined neuroethology of turn-taking, we argue that the human neuroscience of turn-taking and duetting can inform animal bioacoustics. For this, we focus on a particular concept, interhemispheric connectivity, and its main white-matter substrate, the corpus callosum. We provide an overview of the role of corpus callosum in human neuroscience and interactive music and speech. We hypothesize its mechanistic connection to turn-taking and duetting in our species, and a potential translational link to mammalian research. We conclude by illustrating empirical venues for neuroethological research of turn-taking and duetting in mammals.

TMS for the functional evaluation of cannabis effects and for treatment of cannabis addiction: A review

The knowledge about the effects of cannabis on human cortical brain processes is increasing. In this regard, transcranial magnetic stimulation (TMS) enables the evaluation of central nervous system function, including drug effects. Moreover, repetitive TMS (rTMS) has been used therapeutically in several substance use disorders. In this scoping review, we summarize and discuss studies that have employed TMS and rTMS techniques in users of cannabis for recreational purposes. In subjects with a history of persistent cannabis use, TMS studies showed reduced short-interval cortical inhibition (SICI). This observation points more at neurobiological changes of chronic cannabis use than to a direct effect of cannabis on gamma-aminobutyric acid (GABA) A receptors. Moreover, individuals vulnerable to becoming long-term users of cannabis may also have underlying pre-existing abnormalities in SICI. Of note, the use of cannabis is associated with an increased risk of schizophrenia, and the down-regulation of GABAergic function may play a role. Less frequent cannabis use and spontaneous craving were observed following rTMS applied to the dorsolateral prefrontal cortex (DLPFC). There is emerging evidence that the posterior cingulate cortex and the precuneus are potential targets for rTMS intervention in cannabis use disorder. However, larger and randomized trials should corroborate these encouraging findings.

Assessment of Cortical Plasticity in Schizophrenia by Transcranial Magnetic Stimulation

Neural plasticity refers to the capability of the brain to modify its structure and/or function and organization in response to a changing environment. Evidence shows that disruption of neuronal plasticity and altered functional connectivity between distinct brain networks contribute significantly to the pathophysiological mechanisms of schizophrenia. Transcranial magnetic stimulation has emerged as a noninvasive brain stimulation tool that can be utilized to investigate cortical excitability with the aim of probing neural plasticity mechanisms. In particular, in pathological disorders, such as schizophrenia, cortical dysfunction, such as an aberrant excitatory-inhibitory balance in cortical networks, altered cortical connectivity, and impairment of critical period timing are very important to be studied using different TMS paradigms. Studying such neurophysiological characteristics and plastic changes would help in elucidating different aspects of the pathophysiological mechanisms underlying schizophrenia. This review attempts to summarize the findings of available TMS studies with diagnostic and characterization aims, but not with therapeutic purposes, in schizophrenia. Findings provide further evidence of aberrant excitatory-inhibitory balance in cortical networks, mediated by neurotransmitter pathways such as the glutamate and GABA systems. Future studies with combining techniques, for instance, TMS with brain imaging or molecular genetic typing, would shed light on the characteristics and predictors of schizophrenia.

A Review of Studies Leveraging Multimodal TMS-fMRI Applications in the Pathophysiology and Treatment of Schizophrenia

The current review provides an overview of the existing literature on multimodal transcranial magnetic stimulation, and functional magnetic resonance imaging (TMS/fMRI) studies in individuals with schizophrenia and discusses potential future avenues related to the same. Multimodal studies investigating pathophysiology have explored the role of abnormal thalamic reactivity and have provided further evidence supporting the hypothesis of schizophrenia as a disorder of aberrant connectivity and cortical plasticity. Among studies examining treatment, low-frequency rTMS for the management of persistent auditory verbal hallucinations (AVH) was the most studied. While multimodal TMS/fMRI studies have provided evidence of involvement of local speech-related and distal networks on stimulation of the left temporoparietal cortex, current evidence does not suggest the superiority of fMRI based neuronavigation over conventional methods or of active rTMS over sham for treatment of AVH. Apart from these, preliminary findings suggest a role of rTMS in treating deficits in neurocognition, social cognition, and self-agency. However, most of these studies have only examined medication-resistant symptoms and have methodological concerns arising from small sample sizes and short treatment protocols. That being said, combining TMS with fMRI appears to be a promising approach toward elucidating the pathophysiology of schizophrenia and could also open up a possibility toward developing personalized treatment for its persistent and debilitating symptoms.

Morphological alterations of the corpus callosum in antipsychotic-naive first-episode schizophrenia before and 1-year after treatment

OBJECTIVE

The corpus callosum (CC) is known to be altered in patients with schizophrenia. However, its morphologic characteristics are less well studied in treatment-naive first-episode schizophrenia patients, as is the effect of antipsychotic treatment on this structure.

METHODS

T-1 weighted MRI scans were obtained from 160 antipsychotic-naïve first-episode schizophrenia patients (AN-FES) and 155 healthy controls (HCs) before treatment initiation. Among the patients, forty-four were available for follow-up studies after one year of antipsychotic treatment, and were divided into good-outcome (n = 31) and poor-outcome subgroups (n = 13) based on whether there was a 50% reduction in Positive and Negative Symptom Scale (PANSS) total scores from baseline. A computer algorithm was applied to automatically identify the mid-sagittal plane (MSP) and obtain morphological measurement parameters of the CC.

RESULTS

Compared with HCs, AN-FES patients showed a significant reduction of thickness in the posterior midbody of the CC. This deficit was correlated with severity of negative symptoms. After one year of antipsychotic treatment, there was no significant change in CC morphological measurements in schizophrenia patients, nor was there a significant difference of CC morphological measurements between good-outcome and poor-outcome subgroups at baseline or at 1-year follow-up.

CONCLUSION

Thickness of the posterior midbody of the CC is reduced in the early course of schizophrenia before treatment. This alteration was not affected by antipsychotic treatment and was unrelated to treatment outcome at 1-year.

Altered interhemispheric signal propagation in schizophrenia and depression

OBJECTIVE

Altered interhemispheric connectivity is implicated in the pathophysiology of schizophrenia (SCZ) and major depressive disorder (MDD) and may account for deficits in lateralized cognitive processes. We measured transcranial magnetic stimulation evoked interhemispheric signal propagation (ISP), a non-invasive measure of transcallosal connectivity, and hypothesized that the SCZ and MDD groups will have increased ISP compared to healthy controls.

METHODS

We evaluated ISP over the dorsolateral prefrontal cortex in 34 patients with SCZ and 34 patients with MDD compared to 32 age and sex-matched healthy controls.

RESULTS

ISP was significantly increased in patients with SCZ and patients with MDD compared to healthy controls but did not differ between patient groups. There were no effects of antidepressant, antipsychotic, and benzodiazepine medications on ISP and our results remained unchanged after re-analysis with a region of interest method.

CONCLUSION

Altered ISP was found in both SCZ and MDD patient groups. This indicates that disruptions of interhemispheric signaling processes can be indexed with ISP across psychiatric populations.

SIGNIFICANCE

These findings enhance our knowledge of the physiological mechanisms of interhemispheric imbalances in SCZ and MDD, which may serve as potential treatment targets in future patients.

TMS-induced silent periods: A review of methods and call for consistency

Transcranial magnetic stimulation (TMS)-induced silent periods provide an in vivo measure of human motor cortical inhibitory function. Cortical silent periods (cSP, also sometimes referred to as contralateral silent periods) and ipsilateral silent periods (iSP) may change with advancing age and disease and can provide insight into cortical control of the motor system. The majority of past silent period work has implemented largely varying methodology, sometimes including subjective analyses and incomplete methods descriptions. This limits reproducibility of silent period work and hampers comparisons of silent period measures across studies. Here, we discuss methodological differences in past silent period work, highlighting how these choices affect silent period outcome measures. We also outline challenges and possible solutions for measuring silent periods in the unique case of the lower limbs. Finally, we provide comprehensive recommendations for collection, analysis, and reporting of future silent period studies.

Pharmacological mechanisms of interhemispheric signal propagation: a TMS-EEG study

Interhemispheric connections across the corpus callosum have a predominantly inhibitory effect. Previous electrophysiology studies imply that local inhibitory circuits are responsible for inducing transcallosal inhibition, likely through inhibitory GABA_B-mediated neurotransmission. We investigated the neurochemical mechanisms involved in interhemispheric connectivity by measuring transcranial magnetic stimulation (TMS)-induced interhemispheric signal propagation (ISP) in the motor cortex and dorsolateral prefrontal cortex (DLPFC) with electroencephalography (EEG) recordings under the pharmacological effects of baclofen, L-DOPA, dextromethorphan, and rivastigmine. We hypothesized that for both stimulated regions, GABA_B receptor agonist baclofen would decrease ISP when compared against baseline while drugs that target other neurotransmitter systems (dopaminergic, acetylcholinergic, and glutamatergic systems) would have no effect on ISP. Twelve right-handed healthy volunteers completed this study and underwent TMS across five sessions in a randomized order. In the motor cortex, participants showed a significant decrease in ISP under baclofen, but not in the other drug conditions. There were no drug-induced changes in ISP in the DLPFC and baseline ISP did not differ across experimental sessions for both brain regions. Together, our results suggest that the inhibitory effects observed with interhemispheric signal transmission are mediated by a population of interneurons involving GABA_B receptor neurotransmission. Inhibitory mechanisms of ISP may be more salient for motor-related functions in the motor cortex than for cognitive control in the DLPFC. These findings are a fundamental step in advancing our understanding of interhemispheric connectivity and may be used to identify treatments for disorders in which transcallosal transmission is dysfunctional.

Investigational and Therapeutic Applications of Transcranial Magnetic Stimulation in Schizophrenia

PURPOSE OF REVIEW

This current review summarizes the investigational and therapeutic applications of transcranial magnetic stimulation (TMS) in schizophrenia.

RECENT FINDINGS

Fairly consistent findings of an impaired cortical excitation-inhibition balance, cortical plasticity, and motor resonance have been reported in schizophrenia. Cortical connectivity impairments have also been demonstrated in motor and prefrontal brain regions. In terms of treatment, the best support is for 1-Hz TMS to the left temporoparietal cortex for the short-term treatment of persistent auditory hallucinations. High-frequency TMS to the left prefrontal cortex improves negative and cognitive symptoms, but with inconsistent and small effects. TMS combined with diverse brain mapping techniques and clinical evaluation can unravel critical brain-behavior relationships relevant to schizophrenia. These provide critical support to the conceptualization of schizophrenia as a connectopathy with anomalous cortical plasticity. Adaptive modulation of these aberrant brain networks in a neuroscience-informed manner drives short-term therapeutic gains in difficult-to-treat symptoms of schizophrenia.

Towards a neurodynamical understanding of the prodrome in schizophrenia

The identification of biomarkers for the early diagnosis of schizophrenia that could inform novel treatment developments is an important objective of current research. This paper will summarize recent work that has investigated changes in oscillatory activity and event-related potentials with Electro/Magnetoencephalography (EEG/MEG) in participants at high-risk for the development of schizophrenia, highlighting disruptions in sensory and cognitive operations prior to the onset of the syndrome. Changes in EEG/MEG-data are consistent with evidence for alterations in Glutamatergic and GABAergic neurotransmission as disclosed by Magnetic Resonance Spectroscopy and brain stimulation, indicating changes in Excitation/Inhibition balance parameters prior to the onset of psychosis. Together these data emphasize the importance of research into neuronal dynamics as a crucial approach to establish functional relationships between impairments in neural circuits and emerging psychopathology that together could be fundamental for early intervention and the identification of novel treatments for emerging psychosis.

Investigating Cortical Inhibition in First-Degree Relatives and Probands in Schizophrenia

Deficits in GABAergic inhibitory neurotransmission are a reliable finding in schizophrenia (SCZ) patients. Previous studies have reported that unaffected first-degree relatives of patients with SCZ demonstrate neurophysiological abnormalities that are intermediate between probands and healthy controls. In this study, first-degree relatives of patients with SCZ and their related probands were investigated to assess frontal cortical inhibition. Long-interval cortical inhibition (LICI) was measured from the dorsolateral prefrontal cortex (DLPFC) using combined transcranial magnetic stimulation (TMS) and electroencephalography (EEG). The study presents an extended sample of 129 subjects (66 subjects have been previously reported): 19 patients with SCZ or schizoaffective disorder, 30 unaffected first-degree relatives of these SCZ patients, 13 obsessive-compulsive disorder (OCD) patients, 18 unaffected first-degree relatives of these OCD patients and 49 healthy subjects. In the DLPFC, cortical inhibition was significantly decreased in patients with SCZ compared to healthy subjects. First-degree relatives of patients with SCZ showed significantly more cortical inhibition than their SCZ probands. No differences were demonstrated between first-degree relatives of SCZ patients and healthy subjects. Taken together, these findings show that more studies are needed to establish an objective biological marker for potential diagnostic usage in severe psychiatric disorders.

Consensus paper of the WFSBP Task Force on Biological Markers: Criteria for biomarkers and endophenotypes of schizophrenia part I: Neurophysiology

The neurophysiological components that have been proposed as biomarkers or as endophenotypes for schizophrenia can be measured through electroencephalography (EEG) and magnetoencephalography (MEG), transcranial magnetic stimulation (TMS), polysomnography (PSG), registration of event-related potentials (ERPs), assessment of smooth pursuit eye movements (SPEM) and antisaccade paradigms. Most of them demonstrate deficits in schizophrenia, show at least moderate stability over time and do not depend on clinical status, which means that they fulfil the criteria as valid endophenotypes for genetic studies. Deficits in cortical inhibition and plasticity measured using non-invasive brain stimulation techniques seem promising markers of outcome and prognosis. However the utility of these markers as biomarkers for predicting conversion to psychosis, response to treatments, or for tracking disease progression needs to be further studied.

Evidence for inhibitory deficits in the prefrontal cortex in schizophrenia

Abnormal gamma-aminobutyric acid inhibitory neurotransmission is a key pathophysiological mechanism underlying schizophrenia. Transcranial magnetic stimulation can be combined with electroencephalography to index long-interval cortical inhibition, a measure of GABAergic receptor-mediated inhibitory neurotransmission from the frontal and motor cortex. In previous studies we have reported that schizophrenia is associated with inhibitory deficits in the dorsolateral prefrontal cortex compared to healthy subjects and patients with bipolar disorder. The main objective of the current study was to replicate and extend these initial findings by evaluating long-interval cortical inhibition from the dorsolateral prefrontal cortex in patients with schizophrenia compared to patients with obsessive-compulsive disorder. A total of 111 participants were assessed: 38 patients with schizophrenia (average age: 35.71 years, 25 males, 13 females), 27 patients with obsessive-compulsive disorder (average age: 36.15 years, 11 males, 16 females) and 46 healthy subjects (average age: 33.63 years, 23 females, 23 males). Long-interval cortical inhibition was measured from the dorsolateral prefrontal cortex and motor cortex through combined transcranial magnetic stimulation and electroencephalography. In the dorsolateral prefrontal cortex, long-interval cortical inhibition was significantly reduced in patients with schizophrenia compared to healthy subjects (P = 0.004) and not significantly different between patients with obsessive-compulsive disorder and healthy subjects (P = 0.5445). Long-interval cortical inhibition deficits in the dorsolateral prefrontal cortex were also significantly greater in patients with schizophrenia compared to patients with obsessive-compulsive disorder (P = 0.0465). There were no significant differences in long-interval cortical inhibition across all three groups in the motor cortex. These results demonstrate that long-interval cortical inhibition deficits in the dorsolateral prefrontal cortex are specific to patients with schizophrenia and are not a generalized deficit that is shared by disorders of severe psychopathology.

Prolonged cortical silent period among drug-naive subjects at ultra-high risk of psychosis

BACKGROUND

Deficits in gamma-aminobutyric acid (GABA) inhibitory neurotransmission have been associated with pathophysiological mechanisms underlying schizophrenia. However, little is known about whether these deficits occur before or after the onset of psychosis.

METHOD

We recruited 16 drug-naive subjects at ultra-high risk of psychosis (UHR), 17 schizophrenia patients and 28 healthy controls. Cortical inhibition was determined using transcranial magnetic stimulation (TMS) over the left primary motor cortex. TMS markers such as short-interval cortical inhibition (SICI), cortical silent period (CSP) and intracortical facilitation (ICF) were obtained from each subject. While SICI can reflect GABA type A (GABAA) mediated inhibition, CSP is thought to indicate GABA type B (GABAB) mediated inhibitory circuits.

RESULTS

As compared with healthy controls, UHR subjects showed a prolonged CSP with no change in SICI, whereas schizophrenia patients demonstrated both a prolonged CSP and a reduced SICI. No group differences were found for ICF. CSP in schizophrenia patients also had a positive correlation with positive symptom score of the positive and negative symptom scale (PANSS).

CONCLUSIONS

Cortical inhibitory deficits among UHR subjects were relatively limited compared to those among schizophrenia patients. Alterations might occur in some subgroup of GABA-mediated neurotransmitter systems before the onset of psychosis, while alterations in both GABAA and GABAB networks might contribute to full-blown psychosis.

Disrupted prefrontal interhemispheric structural coupling in schizophrenia related to working memory performance

BACKGROUND

Prominent regional cortical thickness reductions have been shown in schizophrenia. In contrast, little is known regarding alterations of structural coupling between regions in schizophrenia and how these alterations may be related to cognitive impairments in this disorder.

METHODS

T1-weighted magnetic resonance images were acquired in 54 patients with schizophrenia and 68 healthy control subjects aged 18-55 years. Cortical thickness was compared between groups using a vertex-wise approach. To assess structural coupling, seeds were selected within regions of reduced thickness, and brain-wide cortical thickness correlations were compared between groups. The relationships between identified patterns of circuit structure disruption and cognitive task performance were then explored.

RESULTS

Prominent cortical thickness reductions were found in patients compared with controls at a 5% false discovery rate in a predominantly frontal and temporal pattern. Correlations of the left dorsolateral prefrontal cortex (DLPFC) with right prefrontal regions were significantly different in patients and controls. The difference remained significant in a subset of 20 first-episode patients. Participants with stronger frontal interhemispheric thickness correlations had poorer working memory performance.

CONCLUSIONS

We identified structural impairment in a left-right DLPFC circuit in patients with schizophrenia independent of illness stage or medication exposure. The relationship between left-right DLPFC thickness correlations and working memory performance implicates prefrontal interhemispheric circuit impairment as a vulnerability pathway for poor working memory performance. Our findings could guide the development of novel therapeutic interventions aimed at improving working memory performance in patients with schizophrenia.

Cortical inhibition, excitation, and connectivity in schizophrenia: a review of insights from transcranial magnetic stimulation

Schizophrenia (SCZ) is a debilitating mental illness with an elusive pathophysiology. Over the last decade, theories emphasizing cortical dysfunction have received increasing attention to explain the heterogeneous symptoms experienced in SCZ. Transcranial magnetic stimulation (TMS) is a noninvasive form of brain stimulation that is particularly suited to probing the fidelity of specific excitatory and inhibitory neuronal populations in conscious humans. In this study, we review the contribution of TMS in assessing inhibitory and excitatory neuronal populations and their long-range connections in SCZ. In addition, we discuss insights from combined TMS and electroencephalography into the functional consequences of impaired excitation/inhibition on cortical oscillations in SCZ.

Motor cortical excitability assessed by transcranial magnetic stimulation in psychiatric disorders: a systematic review

BACKGROUND

Transcranial magnetic stimulation (TMS) is a popular neurostimulation technique suitable for the investigation of inhibitory and facilitatory networks in the human motor system. In the last 20 years, several studies have used TMS to investigate cortical excitability in various psychiatric disorders, leading to a consequent improvement in pathophysiological understanding. However, little is known about the overlap and specificity of these findings across these conditions.

OBJECTIVE

To provide a systematic review of TMS studies (1985-2013) focusing on motor cortical excitability in dementia, schizophrenia, affective disorders (major depression and bipolar), attention deficit hyperactivity disorder (ADHD), obsessive compulsive disorder (OCD), Tourette Syndrome (TS), substance abuse (alcohol, cocaine, cannabis, nicotine) and other disorders (borderline personality disorder, posttraumatic stress disorder (PTSD)).

METHODS

Systematic literature-based review.

RESULTS

Across disorders, patients displayed a general pattern of cortical disinhibition, while the most consistent results of reduced short-interval intracortical inhibition could be found in schizophrenia, OCD and Tourette Syndrome. In dementia, the most frequently reported finding was reduced short-latency afferent inhibition as a marker of cholinergic dysfunction.

CONCLUSIONS

The results of this systematic review indicate a general alteration in motor cortical inhibition in mental illness, rather than disease-specific changes. Changes in motor cortical excitability provide insight that can advance understanding of the pathophysiology underlying various psychiatric disorders. Further investigations are needed to improve the diagnostic application of these parameters.

Effect of Anodal Versus Cathodal Transcranial Direct Current Stimulation on Stroke Rehabilitation

Objective. We compared the long-term effect of anodal versus cathodal transcranial direct current stimulation (tDCS) on motor recovery in patients after subacute stroke. Methods. Forty patients with ischemic stroke undergoing rehabilitation were randomly assigned to 1 of 3 groups: Anodal, Cathodal (over-affected and unaffected hemisphere, respectively), and Sham. Each group received tDCS at an intensity of 2 mA for 25 minutes daily for 6 consecutive days over of the motor cortex hand area. Patients were assessed with the National Institutes of Health Stroke Scale (NIHSS), Orgogozo’s MCA scale (OMCASS), the Barthel index (BI), and the Medical Research Council (MRC) muscle strength scale at baseline, after the sixth tDCS session and then 1, 2, and 3 months later. Motor cortical excitability was measured with transcranial magnetic stimulation (TMS) at baseline and after the sixth session. Results. By the 3-month follow-up, all groups had improved on all scales with P values ranging from .01 to .0001. Improvement was equal in the Anodal and Cathodal groups. When these treated groups were combined and compared with Sham, significant interactions were seen for the OMCASS and BI scales of functional ability ( P = .002 for each). There was increased cortical excitability of the affected hemisphere in all groups with the changes being greater in the real versus sham groups. There were borderline significant improvements in muscle strength. Conclusion. A brief course of 2 types of tDCS stimulation is superior to sham stimulation in enhancing the effect of rehabilitation training to improve motor recovery after stroke.

Impaired motor cortex responses in non-psychotic first-degree relatives of schizophrenia patients: a cathodal tDCS pilot study

BACKGROUND

Schizophrenia has recently been described as a disorder of impaired plasticity and dysconnectivity. Several lines of evidence suggest that alterations in glutamatergic neurotransmission underlie different symptom domains of schizophrenia. Little is known about the impact of genetic liability on cortical plasticity and connectivity in schizophrenia.

OBJECTIVE

To compare N-methyl-d-aspartate receptor (NMDAR)-dependent cortical plasticity and connectivity in schizophrenia patients and unaffected first-degree relatives to that in healthy subjects.

METHODS

Cortical plasticity can be induced in the motor cortex with cathodal transcranial direct current stimulation (tDCS). Animal and human research indicates that this long-term depression-like plasticity (LTD-like) is NMDAR dependent, and that these plasticity shifts can last for several hours. tDCS-induced plasticity was assessed by measuring motor-evoked potentials (MEPs) generated by applying transcranial magnetic stimulation (TMS) to both hemispheres in healthy controls, chronically ill schizophrenia patients and unaffected first-degree relatives.

RESULTS

Compared to healthy controls, both first-degree relatives and schizophrenia patients showed abolished motor-cortical LTD-like plasticity of the stimulated hemisphere. On the non-stimulated hemisphere, plasticity was again abolished in schizophrenia patients, whereas first-degree relatives had a reversed plasticity.

CONCLUSIONS

Non-psychotic and clinically unaffected first-degree relatives showed an alteration and a reversal of LTD-like cortical plasticity, indicating functional alterations of glutamatergic transmission as a result of a genetic liability for developing schizophrenia. These results provide new evidence for the association between plasticity dysregulation and functional cortical connectivity, and the importance of these networks in the pathophysiology of schizophrenia.

A meta-analysis of cortical inhibition and excitability using transcranial magnetic stimulation in psychiatric disorders

OBJECTIVE

To evaluate transcranial magnetic stimulation (TMS) measures of inhibition and excitation in obsessive-compulsive disorder (OCD), major depressive disorder (MDD) and schizophrenia (SCZ).

METHODS

Paradigms included: short-interval cortical inhibition (SICI), cortical silent period (CSP), resting motor threshold, intracortical facilitation, and motor evoked potential amplitude. A literature search was performed using PubMed, Ovid Medline, Embase Psychiatry and PsycINFO 1990 through April 2012.

RESULTS

A significant Hedge's g was found for decreased SICI (g=0.572, 95% confidence interval [0.179, 0.966], p=0.004), enhanced intracortical facilitation (g=0.446, 95% confidence interval [0.042, 0.849], p=0.030) and decreased CSP (g=-0.466, 95% confidence interval [-0.881, -0.052], p=0.027) within the OCD population. For MDD, significant effect sizes were demonstrated for decreased SICI (g=0.641, 95% confidence interval [0.384, 0.898], p=0.000) and shortened CSP (g=-1.232, 95% confidence interval [-1.530, -0.933], p=0.000). In SCZ, a significant Hedge's g was shown for decreased SICI (g=0.476, 95% confidence interval [0.331, 0.620], p=0.000).

CONCLUSION

Inhibitory deficits are a ubiquitous finding across OCD, MDD, SCZ and enhancement of intracortical facilitation is specific to OCD.

SIGNIFICANCE

Provides a clear platform from which diagnostic procedures can be developed.

Neuroleptic treatments and overflow movements in schizophrenia: are they independent?

Neurological soft signs (NSS) are minor neurological abnormalities that can be revealed by a clinical examination focused on sensory and motor information processing. NSS include overflow movements (OMs), which are defined as involuntary movements that may accompany the production of voluntary movements. OM is generally considered to be a characteristic feature of schizophrenia. White matter abnormalities might be involved in the pathogenesis of OMs. Dopamine receptors play a role in oligodendrocytes development. There is a direct link between antipsychotic agents that bind to dopamine receptors on oligodendrocytes and the development of oligodendrocytes and myelin formation. In this paper, we review the current knowledge of the effects of antipsychotic agents on NSS in schizophrenic patients. As a result of this critical review we hypothesize that the neuroleptic actions described in this paper could explain why antipsychotic agents have no effect on the resolution of NSS in patients with schizophrenia.

Deficient inhibitory cortical networks in antipsychotic-naive subjects at risk of developing first-episode psychosis and first-episode schizophrenia patients: a cross-sectional study

BACKGROUND

Impaired cortical inhibition is a well-established finding in schizophrenia patients and has been linked to dysfunctional gamma-aminobutyric acid (GABA)ergic transmission. However, there have been no previous studies investigating cortical excitability with particular regard to intracortical inhibitory networks in antipsychotic-naive subjects at risk of developing first-episode psychosis.

METHODS

A total of 18 subjects at risk, 18 first-episode schizophrenia patients, and 18 healthy control subjects were included in this study. Transcranial magnetic stimulation over the left primary motor cortex was used to determine short-latency intracortical inhibition, intracortical facilitation, and the contralateral silent period (CSP). Short-latency intracortical inhibition can be considered as a parameter of GABA type A (GABA(A))-mediated inhibition and it has been proposed that CSP can test GABA type B (GABA(B))-mediated inhibitory intracortical networks.

RESULTS

Subjects at risk and first-episode patients showed a reduced short-latency intracortical inhibition compared with healthy control subjects, suggesting reduced GABA(A)-mediated inhibition. First-episode patients had a prolonged CSP duration compared with the other two groups, implying a GABA(B) imbalance only in patients with full-blown psychosis. Analyses did not reveal group differences for intracortical facilitation.

CONCLUSIONS

These results indicate specific alterations in inhibitory cortical networks in subjects at risk and in first-episode patients. It appears that there is already a cortical inhibitory deficit in at-risk individuals. These results suggest a possible GABA(A) dysfunction early in the disease course, whereas alterations in GABA(B) functionality seem to occur later in the disease's progression. Future longitudinal studies will be needed to clarify this inhibitory deficit and its relation to the transition to psychosis.

Inhibition of the cortex using transcranial magnetic stimulation in psychiatric populations: current and future directions

Several lines of evidence suggest that deficits in γ-aminobutyric acid (GABA) inhibitory neurotransmission are implicated in the pathophysiology of schizophrenia, bipolar disorder, major depressive disorder and obsessive-compulsive disorder. Cortical inhibition refers to a neurophysiological process, whereby GABA inhibitory interneurons selectively attenuate pyramidal neurons. Transcranial magnetic stimulation (TMS) represents a noninvasive technique to measure cortical inhibition, excitability and plasticity in the cortex. These measures were traditionally specific to the motor cortex, which is an important limitation when nonmotor neurophysiological processes are of primary interest. Recently, TMS has been combined with electro encephalography (EEG) to derive such measurements directly from the cortex. This review focuses on neurophysiological studies related to inhibitory and excitatory TMS paradigms, linking dysfunctional GABAergic neurotransmission to disease states. We review evidence that suggests cortical inhibition deficits among psychiatric populations and demonstrate how each disorder has a specific neurophysiological response to treatment. We conclude by discussing the future directions of TMS combined with EEG, demonstrating the potential to identify biological markers of neuropsychiatric disorders.

Abnormal gamma and beta MEG activity during finger movements in early-onset psychosis

Patients with psychosis often exhibit abnormalities in basic motor control, but little is known about the neural basis of these deficits. This study examines the neuro-dynamics of movement using magnetoencephalography (MEG) in adolescents with early-onset psychosis and typically developing controls. MEG data were imaged using beamforming then evaluated for task and group effects before, during, and after movement onsets. Primary findings included weaker activation in patients during movement execution in cerebellar cortices. Such aberrations likely contribute to the decreased motor control exhibited by patients with psychosis, and may reflect GABAergic-based inhibitory deficits comparable to those seen in cellular and system-level studies.

Impaired long-term depression in schizophrenia: a cathodal tDCS pilot study

BACKGROUND

Neural plasticity involves the reorganization of synaptic connections and represents the ability of the brain to adjust its function in response to challenge. Disturbed cortical plasticity has been linked to the pathophysiology of schizophrenia, with indirect evidence for disturbed plasticity in the disease state having been provided by postmortem studies and various animal models. However, glutamate-dependent long-term depression (LTD)-like cortical plasticity has not yet been investigated.

OBJECTIVE

To investigate LTD-like cortical plasticity after transcranial direct current stimulation (tDCS) in schizophrenia patients.

METHODS

Using excitability-diminishing cathodal tDCS, we performed the first in vivo assessment of glutamate-dependent LTD-like cortical plasticity in 21 schizophrenia patients and 21 matched healthy control subjects. To reveal the physiologic basis of the hypothesized plasticity deficits, we tested different inhibitory and excitatory neuronal circuits with transcranial magnetic stimulation (TMS).

RESULTS

Cathodal tDCS failed to reduce motor-evoked potential amplitudes in schizophrenia patients, indicating abolished LTD-like plasticity. Furthermore, schizophrenia patients had a prolonged GABA(B)-dependent cortical silent period (CSP) at baseline and tDCS failed to modulate the duration of CSP in the patient group. Finally, schizophrenia patients presented an elevated resting-motor threshold at baseline in comparison to healthy controls.

CONCLUSIONS

The pattern of our results provides evidence for a specific plasticity deficit in schizophrenia patients, which might be associated with a hyperglutamatergic state. These findings may reflect a reduced signal-to-noise ratio and a disturbed filter function in schizophrenia patients. An increase of GABA(B)-activity may be a compensatory mechanism to dysfunctional LTD-like plasticity in schizophrenia.

Transcranial magnetic stimulation: a neuroscientific probe of cortical function in schizophrenia

Transcranial magnetic stimulation (TMS) is a neuropsychiatric tool that can serve as a useful method to better understand the neurobiology of cognitive function, behavior, and emotional processing. The purpose of this article is to examine the utility of TMS as a means to measure neocortical function in neuropsychiatric disorders in general, and schizophrenia in particular, for the Cognitive Neuroscience Treatment Research to Improve Cognition in Schizophrenia initiative. When incorporating TMS paradigms in research studies, methodologic considerations include technical aspects of TMS, cohort selection and confounding factors, and subject safety. Available evidence suggests benefits of TMS alone or in combination with neurophysiologic and neuroimaging methods, including positron emission tomography, single photon emission computed tomography, magnetic resonance imaging, functional magnetic resonance imaging, functional near infrared spectroscopy, magnetoencephalography, and electroencephalography, to explore neocortical function. With the multiple TMS techniques including single-pulse, paired-pulse, paired associative stimulation, and repetitive TMS and theta burst stimulation, combined with neurophysiologic and neuroimaging methods, there exists a plethora of TMS experimental paradigms to modulate neocortical physiologic processes. Specifically, TMS can measure cortical excitability, intracortical inhibitory and excitatory mechanisms, and local and network cortical plasticity. Coupled with functional and electrophysiologic modalities, TMS can provide insight into the mechanisms underlying healthy neurodevelopment and aging, as well as neuropsychiatric pathology. Thus, TMS could be a useful tool in the Cognitive Neuroscience Treatment Research to Improve Cognition in Schizophrenia armamentarium of biomarker methods. Future investigations are warranted to optimize TMS methodologies for this purpose.

Effects of low versus high frequencies of repetitive transcranial magnetic stimulation on cognitive function and cortical excitability in Alzheimer's dementia

The aim of the study was to compare the long-term efficacy of high versus low frequency repetitive transcranial magnetic stimulation (rTMS), applied bilaterally over the dorsolateral prefrontal cortex (DLPFC), on cognitive function and cortical excitability of patients with Alzheimer's disease (AD). Forty-five AD patients were randomly classified into three groups. The first two groups received real rTMS over the DLPFC (20 and 1 Hz, respectively) while the third group received sham stimulation. All patients received one session daily for five consecutive days. In each session, rTMS was applied first over the right DLPFC, immediately followed by rTMS over the left DLPFC. Mini Mental State Examination (MMSE), Instrumental Daily Living Activity (IADL) scale and the Geriatric Depression Scale (GDS) were assessed before, after the last (fifth) session, and then followed up at 1 and 3 months. Neurophysiological evaluations included resting and active motor threshold (rMT and aMT), and the duration of transcallosal inhibition (TI) before and after the end of the treatment sessions. At base line assessment there were no significant differences between groups in any of the rating scales. The high frequency rTMS group improved significantly more than the low frequency and sham groups in all rating scales (MMSE, IADL, and GDS) and at all time points after treatment. Measures of cortical excitability immediately after the last treatment session showed that treatment with 20 Hz rTMS reduced TI duration. These results confirm that five daily sessions of high frequency rTMS over the left and then the right DLPFC improves cognitive function in patients with mild to moderate degree of AD. This improvement was maintained for 3 months. High frequency rTMS may be a useful addition to therapy for the treatment of AD.

Impaired inter-hemispheric facilitatory connectivity in schizophrenia

OBJECTIVES

To investigate the inter-hemispheric connections between the dorsal premotor cortex (dPM) and contralateral primary motor cortex (M1) in schizophrenia.

METHODS

Sixteen medicated, nine unmedicated schizophrenia patients and 20 healthy age-matched subjects were studied by twin-coil Transcranial Magnetic Stimulation. To activate distinct facilitatory and inhibitory transcallosal pathways between dPM and the contralateral M1, the intensity of dPM stimulation was adjusted to be either suprathreshold (110% of resting motor threshold) or subthreshold (80% of active motor threshold). Interstimulus intervals between conditioning stimulus and test stimulus were 6, 8 and 15 ms.

RESULTS

Schizophrenia patients had comparable efficacy of the inhibitory pathway. On the other hand, medicated patients showed less facilitation of contralateral M1 following dPM stimulation at 80% of active motor threshold, at interstimulus interval=8 ms. The individual amount of facilitation induced by dPM conditioning at 80% of active motor threshold at interstimulus interval=8 ms correlated negatively with negative symptoms.

CONCLUSIONS

Inter-hemispheric facilitatory dPM-M1 connectivity is selectively altered in schizophrenia.

SIGNIFICANCE

This study produced evidence that dPM-M1 connectivity is dysfunctional and that correlates with negative symptoms. These results converge with previous studies which strongly hypothesize that inter- and intra-hemispheric connectivity disturbances may play a major role in schizophrenia.

Increased cortical inhibition deficits in first-episode schizophrenia with comorbid cannabis abuse

RATIONALE/OBJECTIVES

There is a high prevalence of substance use disorder (SUD) in first-episode schizophrenia (SZ), but its contribution to the underlying SZ pathophysiology remains unclear. Several studies using transcranial magnetic stimulation (TMS) have observed abnormalities in human motor cortex (M1) excitability in SZ. Studies on cortical excitability comparing SZ patients with and without comorbid substance abuse are lacking.

METHODS

A total of 29 first-episode SZ patients participated in this study; 12 had a history of comorbid cannabis abuse (SZ-SUD) and 17 did not (SZ-NSUD). We applied TMS to right and left M1 areas to assess the resting motor threshold (RMT), short-interval cortical inhibition (SICI), intracortical facilitation (ICF), and the contralateral cortical silent period (CSP).

RESULTS

In SICI and ICF conditions, right M1 stimulation led to significantly higher motor evoked potential ratios in SZ-SUD compared to SZ-NSUD. This suggests lower cortical inhibition and increased ICF in first-episode SZ with previous cannabis abuse. There were no group differences in RMT and CSP duration. Neither were there any significant correlations between psychopathology (as indexed by Positive and Negative Syndrome Scale), disease characteristics, the extent of cannabis abuse, and TMS parameters (SICI, ICF, and CSP).

CONCLUSIONS

Comorbid cannabis abuse may potentiate the reduced intracortical inhibition and enhanced ICF observed in first-episode SZ patients in some previous studies. This finding suggests an increased alteration of GABA(A) and NMDA receptor activity in cannabis-abusing first-episode patients as compared to schizophrenia patients with no history of substance abuse. This may constitute a distinct vulnerability factor in this special population.

An endocrine perspective on the role of steroid hormones in the antidepressant treatment efficacy of transcranial magnetic stimulation

Evidence from recent meta-analyses indicates that transcranial magnetic stimulation (TMS) is moderately effective in the treatment of major depressive disorder (MDD). Individual differences in the susceptibility to TMS are suggested to underlie a significant portion of the variability in antidepressant efficacy observed in TMS trials. Interestingly, recent findings suggest a moderating role for steroid hormones in the antidepressant efficacy of TMS in women. Steroid hormones are known to have strong activational and organizational influences on the brain and may upregulate the efficacy of TMS by way of modulating cortical excitability in a sex-dependent manner. Here we propose that the measurement and manipulation of steroid hormones could be crucial steps in the development of successful individually based TMS protocols for the treatment of MDD.

Modulation of interhemispheric inhibition by volitional motor activity: an ipsilateral silent period study

Brief interruption of voluntary EMG in a hand muscle by focal transcranial magnetic stimulation (TMS) of the ipsilateral primary motor cortex (M1), the so-called ipsilateral silent period (ISP), is a measure of interhemispheric motor inhibition. However, little is known about how volitional motor activity would modulate the ISP. Here we tested in 30 healthy adults to what extent and under what conditions voluntary activation of the stimulated right M1 by moving the left hand strengthens interhemispheric inhibition as indexed by an enhancement of the ISP area in the maximally contracting right first dorsal interosseous (FDI). Left index finger abduction, already at low levels of contraction, significantly enhanced the ISP compared to left hand at rest. Even imagination of left index finger movement enhanced the ISP compared to rest or mental calculation. This enhancement occurred in the absence of motor-evoked potential amplitude modulation in the left FDI, thus excluding a non-specific contribution from an increase in right M1 corticospinal excitability. Contraction of the left extensor indicis, but not contraction of more proximal left upper limb or left or right lower limb muscles also enhanced the ISP. A reaction time experiment showed that the ISP enhancement developed at a late stage of movement preparation just before or at movement onset. Interhemispheric inhibition of the motor-evoked potential as tested by a bifocal paired-pulse TMS protocol and thought to be mediated via a neuronal circuit different to the ISP was not enhanced when tested under identical motor task conditions. Finally, ISP enhancement by contraction of the left FDI correlated inversely with EMG mirror activity in the right FDI during phasic abductions of the left index finger. Our findings strongly suggest that voluntary M1 activation by real or imagined movement of the contralateral hand increases interhemispheric motor inhibition of the opposite M1. This phenomenon shows substantial topographical, temporal and neuronal circuit specificity, and has functional significance as it probably plays a pivotal role in suppressing mirror activity.

Aberrant high-frequency desynchronization of cerebellar cortices in early-onset psychosis

Sensorimotor integration deficits are routinely observed in both schizophreniform and mood-disordered psychoses. Neurobiological theories of schizophrenia and related psychoses have proposed that aberrations in large-scale cortico-thalamic-cerebellar-thalamic-cortical loops may underlie integration abnormalities, and that such dysfunctional connectivity may be central to the pathophysiology. In this study, we utilized a basic mechanoreception task to probe cortical-cerebellar circuitry in early-onset psychosis. Ten adolescents with psychosis and 10 controls completed unilateral tactile stimulation of the right and left index finger, as whole-head magnetoencephalography (MEG) data were acquired. MEG data were imaged in the frequency domain, using spatial filtering, and the resulting event-related synchronizations and desynchronizations (ERS/ERD) were subjected to voxel-wise analyses of group and task effects using statistical parametric mapping. Our results indicated bilateral ERD activation of cerebellar regions and postcentral gyri in both groups during stimulation of either hand. Interestingly, during left finger stimulations, adolescents with psychosis exhibited greater alpha and gamma ERD activity in right cerebellar cortices relative to controls. Subjects with psychosis also showed greater ERD in bilateral cerebellum and the right postcentral gyrus during right finger stimulation, and these differences were statistically stronger for higher frequency bins. Lastly, controls exhibited greater alpha ERS of the right postcentral gyrus during right finger stimulation. These findings provide new data on the neurodevelopmental trajectory of basic mechanoreception in adolescents, and also indicate aberrant cerebellar functioning in early-onset psychoses, especially in the right cerebellum, which may be the crucial dysfunctional node in cortico-thalamic-cerebellar-thalamic-cortical circuits.

Morphology of the corpus callosum in treatment-resistant schizophrenia and major depression

OBJECTIVE

To identify possible differences in the mean midsagittal corpus callosum (CC) total and subdivision areas in treatment-resistant schizophrenia and depression (TRS and TRD) patients.

METHOD

Areas of the total CC and its five equidistant subregions (from CC1 to CC5) obtained by parallel grid partitioning schemes were manually segmented from brain MRI of 42 TRS, 45 TRD patients and 30 healthy controls. The intracranial volume (ICV) normalized areas were calculated and compared between groups.

RESULTS

When compared with controls, patients with TRS had reduced ICV and a larger CC5, and TRD patients had a smaller CC4 while no significant difference in CC total area in patients with TRS or TRD was found. Multiple individual segments and total CC areas were significantly larger in TRS than TRD patients after normalization.

CONCLUSION

Patients with TRS and TRD have different CC morphological characteristics, and therefore there may be aberrant interhemispheric connectivity in schizophrenia and major depressive disorder patients.

Connectivity between posterior parietal cortex and ipsilateral motor cortex is altered in schizophrenia

BACKGROUND

Recent advances have highlighted the hypothesis of schizophrenia as a disorder causing defective connectivity among distinct cortical regions. Neurophysiological evidence supporting this hypothesis, however, is still lacking.

METHODS

In the present study, we used a novel twin-coil transcranial magnetic stimulation (tcTMS) approach to investigate ipsilateral parieto-motor connectivity in 20 schizophrenic patients (14 medicated, 6 unmedicated) and in 15 healthy age-matched volunteers.

RESULTS

In healthy subjects, a conditioning TMS pulse applied over the ipsilateral posterior parietal cortex (PPC) at 90% of resting motor threshold (RMT) intensity was able to increase the excitability of the hand area of the right primary motor cortex, with peaks at interstimulus intervals (ISIs) of 4 and 15 msec. This paradigm of stimulation failed to reveal any facilitatory parieto-motor interaction in medicated and unmedicated schizophrenic patients. The between-group difference in paired-pulse facilitation was not ISI-specific. In following analyses, we found that the effects across ISIs induced by PPC conditioning at 90% RMT correlated with the Global Assessment Functioning score and with the negative subscale of the Positive and Negative Syndrome Scale, showing that patients with a better global functioning and lower negative symptoms had less impaired connectivity. Moreover the same parameter correlated with illness duration.

CONCLUSIONS

Parieto-motor connectivity is impaired in schizophrenia. Cortico-cortical disconnection might be a core feature of schizophrenia.

Reduced cortical inhibition in first-episode schizophrenia

Disturbances in cortico-cortical and cortico-subcortical circuits in schizophrenia have been described by previous neuroimaging and electrophysiological studies. Transcranial magnetic stimulation (TMS) provides a neurophysiological technique for the measurement of cortical excitability, especially of the motoneural system. Previous studies using paired-pulse TMS to investigate short-interval cortical inhibition (SICI) and intracortical facilitation (ICF), mainly involving chronic schizophrenia patients, have been inconsistent and only one study in first-episode patients has been conducted so far. We assessed SICI (interstimulus interval, ISI, 3 milliseconds, ms) and ICF (ISI 7 ms) in 29 first-episode schizophrenia patients (FE-SZ) with limited exposure to antipsychotic treatment against measures of 28 healthy controls (HC). Amplitudes of motor evoked potentials (MEPs) were measured from the left and right first dorsal interosseus muscle (FDI). The conditioning stimulus was set at 80% intensity of resting motor threshold (RMT) and the test stimulus (TS) was set at an intensity that produced an MEP amplitude of about 1 mV. For SICI conditions, FE-SZ demonstrated significantly higher MEP amplitudes from left motor cortex (right FDI) compared to HC, and for MEPs from right motor cortex (left FDI) a similar trend was observable (FE-SZ 41% vs. HC 21% of TS, p=0.017 for left motor cortex, and FE-SZ 59% vs. HC 31% of TS, p=0.059 for right motor cortex; Mann-Whitney U-test). No significant difference in MEPs could be detected for ICF on either hemisphere. In addition, there was no difference in left and right RMT comparing patients and control subjects. Our result of a reduced SICI in a large sample of well characterized first-episode schizophrenia patients suggests that a GABAergic deficit may be involved in schizophrenic pathophysiology, already early in the disease course, supporting the intracortical dysconnectivity hypothesis.

The role of the cerebellum in the pathophysiology and treatment of neuropsychiatric disorders: a review

The cerebellum has traditionally been looked upon as a brain area primarily involved in motor behaviour. The last decade has however heralded the cerebellum as a brain region of renewed interest for neuropsychiatric disorders. This renewed interest is fuelled by new insights obtained from neuroanatomical research, modern functional neuroimaging and transcranial magnetic stimulation studies. In this review, evidence in support of cerebellar involvement in neuropsychiatric disorders will be presented. In addition, transcranial magnetic stimulation will be introduced as a novel way to study cerebellar contributions to the pathophysiology of psychiatric disorders. In conclusion, a new functional concept of the cerebellum as more than simply a brain area regulating motor control appears mandatory and the involvement of the cerebellum should be considered when studying the neurological basis of neuropsychiatric disorders.

Interhemispheric transfer deficit in alexithymia: a transcranial magnetic stimulation study

BACKGROUND

A deficit in interhemispheric transfer was hypothesized in alexithymia more than 30 years ago, following the observation that split-brain patients manifest certain alexithymic characteristics. However, direct evidence of interhemispheric transfer deficit has never been provided. This study investigated the hypothesis of a transcallosal interhemispheric transfer deficit in alexithymia by means of paired-pulse transcranial magnetic stimulation.

METHODS

A random sample of 300 students was screened for alexithymia using the Italian version of the 20-item Toronto Alexithymia Scale. Eight right-handed males and eight females with high alexithymic scores and an age- and gender-matched group with low alexithymic scores were selected. A first (conditioning) magnetic stimulus was delivered to one motor cortex followed by a second (test) stimulus to the opposite hemisphere at different interstimulus intervals for both motor cortices. Motor evoked responses were recorded from the abductor digit minimi muscles.

RESULTS

High alexithymic subjects showed reduced transcallosal inhibition as compared to low alexithymic subjects at interstimulus intervals of 10, 12 and 14 ms in the left-to-right and right-to-left interhemispheric transfer directions.

CONCLUSIONS

Results point to functional differences in transcallosal interactions in high alexithymic as compared to low alexithymic subjects, supporting the hypothesis of an interhemispheric transfer deficit in alexithymia.

[Cortical excitability in schizophrenia. Studies using transcranial magnetic stimulation]

Abnormalities in brain plasticity, including abnormal information processing by disturbed cortical inhibition and facilitation in schizophrenia, have been described several times in the past. Transcranial magnetic stimulation (TMS) provides a neurophysiological technique for the measurement of cortical excitability, especially of the motoneural system. With TMS it is possible to explore important aspects of the pathophysiological mechanisms underlying schizophrenia using recently developed paradigms (paired pulse method, cortical silent period). This review summarizes the results of available diagnostic TMS studies in schizophrenia. Studies investigating the efficacy of therapeutic repetitive magnetic stimulation in patients with treatment resistant hallucinations or predominantly negative symptoms were not considered. The reviewed studies support the assumption of reduced cortical inhibition, probably caused by GABAergic deficit, in schizophrenia. Factors influencing the study results, the limitations of this promising technique in schizophrenia, and further research options are discussed.

Smaller corpus callosum subregions containing motor fibers in schizophrenia

Neuropsychological and neurophysiological studies provide evidence for abnormal interhemispheric communication in schizophrenia. These abnormalities may have a substrate in structural irregularities of the corpus callosum. This study investigated schizophrenia patients (n=27) and healthy comparison subjects (n=31). Global and regional measurements of the corpus callosum were acquired from one midsagittal SPGR slice. Eight subregions were approximately matched to fiber pathways from cortical regions. Overall effects of diagnosis [Wilks' Lambda F(8,46)=2.45, p=0.03] and diagnosis by age interaction [Wilks' Lambda F(8,46)=2.58, p=0.02] were found in a MANCOVA of the eight functionally specific subregions. Specifically, chronic schizophrenia was associated with a smaller rostral body [lower by 6.9%, F(1,53)=9.70, p=0.003] and anterior midbody [lower by 9.7%, F(1,53)=4.89, p=0.03] subregions. The rostral body and anterior midbody subregions of the corpus callosum primarily have premotor, supplementary motor, and motor cortical fibers transversing through them. Functional abnormalities of the associated cortical regions are reported in schizophrenia. These novel findings suggest that structural abnormalities of the corpus callosum exist in schizophrenia, with perhaps the motor-specific subregions affected more than others. Structural differences in the corpus callosum may be a substrate for interhemispheric functional dysconnectivity in schizophrenia.