Single-Pulse TMS to the Temporo-Occipital and Dorsolateral Prefrontal Cortex Evokes Lateralized Long Latency EEG Responses at the Stimulation Site

Mapping the routes of perception: Hemispheric asymmetries in signal propagation dynamics

The visual system has long been considered equivalent across hemispheres. However, an increasing amount of data shows that functional differences may exist in this regard. We therefore tried to characterize the emergence of visual perception and the spatiotemporal dynamics resulting from the stimulation of visual cortices in order to detect possible interhemispheric asymmetries. Eighteen participants were tested. Each of them received 360 transcranial magnetic stimulation (TMS) pulses at phosphene threshold intensity over left and right early visual areas while electroencephalography was being recorded. After each single pulse, participants had to report the presence or absence of a phosphene. Local mean field power analysis of TMS-evoked potentials showed an effect of both site (left vs. right TMS) of stimulation and hemisphere (ipsilateral vs. contralateral to the TMS): while right TMS determined early stronger activations, left TMS determined later stronger activity in contralateral electrodes. The interhemispheric signal propagation index revealed differences in how TMS-evoked activity spreads: left TMS-induced activity diffused contralaterally more than right stimulation. With regard to phosphenes perception, distinct electrophysiological patterns were found to reflect similar perceptual experiences: left TMS-evoked phosphenes are associated with early occipito-parietal and frontal activity followed by late central activity; right TMS-evoked phosphenes determine only late, fronto-central, and parietal activations. Our results show that left and right occipital TMS elicits differential electrophysiological patterns in the brain, both per se and as a function of phosphene perception. These distinct activation patterns may suggest a different role of the two hemispheres in processing visual information and giving rise to perception.

Graph Analysis of TMS-EEG Connectivity Reveals Hemispheric Differences following Occipital Stimulation

(1) Background: Transcranial magnetic stimulation combined with electroencephalography (TMS-EEG) provides a unique opportunity to investigate brain connectivity. However, possible hemispheric asymmetries in signal propagation dynamics following occipital TMS have not been investigated. (2) Methods: Eighteen healthy participants underwent occipital single-pulse TMS at two different EEG sites, corresponding to early visual areas. We used a state-of-the-art Bayesian estimation approach to accurately estimate TMS-evoked potentials (TEPs) from EEG data, which has not been previously used in this context. To capture the rapid dynamics of information flow patterns, we implemented a self-tuning optimized Kalman (STOK) filter in conjunction with the information partial directed coherence (iPDC) measure, enabling us to derive time-varying connectivity matrices. Subsequently, graph analysis was conducted to assess key network properties, providing insight into the overall network organization of the brain network. (3) Results: Our findings revealed distinct lateralized effects on effective brain connectivity and graph networks after TMS stimulation, with left stimulation facilitating enhanced communication between contralateral frontal regions and right stimulation promoting increased intra-hemispheric ipsilateral connectivity, as evidenced by statistical test (p < 0.001). (4) Conclusions: The identified hemispheric differences in terms of connectivity provide novel insights into brain networks involved in visual information processing, revealing the hemispheric specificity of neural responses to occipital stimulation.

The external evocation and movement-related modulation of motor cortex inhibition in children and adolescents with Tourette syndrome - a TMS/EEG study

Objective

This study tested the reactivity of motor cortex inhibition to different intensities of external stimulation by transcranial magnetic stimulation (TMS) and its internal modulation during different motor states in children and adolescents with Tourette syndrome.

Methods

TMS-evoked N100 served as an indirect measure of GABAB receptor function which is related to cortical inhibition. Combined TMS/EEG was used to analyze the TMS-evoked N100 component evoked by different stimulation intensities as well as during resting condition, movement preparation (contingent negative variation task) and movement execution. The study included 18 early adolescents with Tourette syndrome and 15 typically developing control subjects.

Results

TMS-evoked N100 showed a less steep increase with increasing TMS intensity in Tourette syndrome together with less modulation (disinhibition) over the primary motor cortex during the motor states movement preparation and movement execution. Children with Tourette syndrome showed equally high N100 amplitudes at 110% resting motor threshold (RMT) intensity during resting condition and a parallel decline of RMT and N100 amplitude with increasing age as control subjects.

Conclusion

Our study yields preliminary evidence that modulation of motor cortical inhibitory circuits, during external direct stimulation by different TMS intensities and during volitional movement preparation and execution is different in children and adolescents with Tourette syndrome compared to controls. These results suggest that a reduced resting motor cortical inhibitory "reserve" could contribute to the production of unwanted movements. Our findings are compatible with increased regulation of motor cortex excitability by perception-action binding in Tourette syndrome instead of top-down / motor regulation and need to be replicated in further studies.

Current evidence, clinical applications, and future directions of transcranial magnetic stimulation as a treatment for ischemic stroke

Transcranial magnetic stimulation (TMS) is a non-invasive brain neurostimulation technique that can be used as one of the adjunctive treatment techniques for neurological recovery after stroke. Animal studies have shown that TMS treatment of rats with middle cerebral artery occlusion (MCAO) model reduced cerebral infarct volume and improved neurological dysfunction in model rats. In addition, clinical case reports have also shown that TMS treatment has positive neuroprotective effects in stroke patients, improving a variety of post-stroke neurological deficits such as motor function, swallowing, cognitive function, speech function, central post-stroke pain, spasticity, and other post-stroke sequelae. However, even though numerous studies have shown a neuroprotective effect of TMS in stroke patients, its possible neuroprotective mechanism is not clear. Therefore, in this review, we describe the potential mechanisms of TMS to improve neurological function in terms of neurogenesis, angiogenesis, anti-inflammation, antioxidant, and anti-apoptosis, and provide insight into the current clinical application of TMS in multiple neurological dysfunctions in stroke. Finally, some of the current challenges faced by TMS are summarized and some suggestions for its future research directions are made.

Changes in the TMS-evoked potential N100 in the dorsolateral prefrontal cortex as a function of depression severity in adolescents

Studies using transcranial magnetic stimulation with simultaneous electroencephalography (TMS-EEG) revealed an imbalance between cortical excitation and inhibition (E/I) in the dorsolateral prefrontal cortex (DLPFC) in depression. As adolescence is a developmental period with an increase in depression prevalence and profound neural changes, it is crucial to study the relationship between depression and cortical excitability in adolescence. We aimed to investigate the cortical excitability of the DLPFC in adolescents with depression and a dependency of the TMS-evoked potential N100 on the depression severity. 36 clinical patients (12–18 years of age; 21 females) with a major depressive episode were assessed twice in a longitudinal design: shortly after admission (T0) and after six weeks of intervention (T1). GABA-B-mediated cortical inhibition in the left and right DLPFC, as assessed by the N100, was recorded with EEG. Significantly higher depression scores were reported at T0 compared to T1 (p < 0.001). N100 amplitudes were significantly increased (i.e., more negative) at T0 compared to T1 (p = 0.03). No significant hemispheric difference was found in the N100 component. The correlation between the difference in depression severity and the difference in N100 amplitudes (T0–T1) obtained during stimulation of the left DLPFC did not remain significant after correction for testing in both hemispheres. Higher N100 amplitudes during a state of greater depression severity are suggestive of an E/I imbalance in the DLPFC in adolescents with an acute depressive episode. The N100 reduction potentially reflects a normalization of DLPFC over inhibition in association with decreased depressive symptomatology, indicating severity dependency.

Fearful facial expressions reduce inhibition levels in the dorsolateral prefrontal cortex in subjects with specific phobia

BACKGROUND

Specific phobias have the highest prevalence among anxiety disorders. Cognitive control involving the dorsolateral prefrontal cortex (DLPFC) is crucial for coping abilities in anxiety disorders. However, there is little research on the DLPFC in specific phobia.

METHODS

Using transcranial magnetic stimulation (TMS), we investigated the TMS-evoked potential component N100 in the DLPFC at rest and while watching emotional expressions. The TMS-evoked N100 provides a parameter for gamma-aminobutyric acid (GABA)-B-mediated cortical inhibition. Twenty-two drug-free subjects with specific phobia (21 females and 1 male) were compared with 26 control subjects (23 females and 3 males) regarding N100 in the DLPFC at rest and during an emotional 1-back task with fearful, angry, and neutral facial expressions.

RESULTS

At rest, we found reduced N100 amplitudes in the specific phobia compared with the control group. Furthermore, the specific phobia group showed a further reduction in N100 amplitude when memorizing fearful compared with neutral facial expressions.

CONCLUSION

There appears to be a decrease in GABA-B-mediated inhibition in the DLPFC in subjects with a specific phobia at rest. This decrease was more pronounced under emotional activation by exposure to fearful facial expressions, pointing towards additional state effects of emotional processing on inhibitory function in the DLPFC.

Topography and lateralization of long-latency trigeminal somatosensory evoked potentials

OBJECTIVE

Long-latency trigeminal somatosensory evoked potentials (SSEPs) have not been sufficiently studied regarding their topography and lateralization. SSEPs are hypothesized to contribute to the evoked potentials after transcranial magnetic stimulation (TMS). This study focused on trigeminal SSEPs with latencies > 100 ms, potentially overlapping with TMS-evoked N100.

METHODS

In 14 healthy subjects, the trigeminus was electrically stimulated on the left and right forehead, and time-course, topography, and lateralization of trigeminal SSEPs were examined in 64-channel electroencephalogram (EEG). SSEPs were then compared to TMS-evoked potentials when TMS was applied to the left and right dorsolateral prefrontal cortex.

RESULTS

Trigeminal stimulation produced a somatosensory N140 with topographic maximum over centroparietal electrodes with larger amplitudes contra- than ipsilaterally to the stimulation. Contralateral potentials after TMS were partly comparable in their topography but differed in latencies.

CONCLUSIONS

SSEPs generated by electrical stimulation of the trigeminus occurred over somatosensory areas with a contralateral lateralization. Therefore, contralateral potentials after TMS should be interpreted with caution, as they may include somatosensory components.

SIGNIFICANCE

The topography and lateralization of long-latency trigeminal SSEPs should be considered in future TMS-EEG designs.