Altered recovery from inhibitory repetitive transcranial magnetic stimulation (rTMS) in subjects with photosensitive epilepsy

Non-invasive brain stimulation for patients and healthy subjects: Current challenges and future perspectives

Non-invasive brain stimulation (NIBS) techniques have a rich historical background, yet their utilization has witnessed significant growth only recently. These techniques encompass transcranial electrical stimulation and transcranial magnetic stimulation, which were initially employed in neuroscience to explore the intricate relationship between the brain and behaviour. However, they are increasingly finding application in research contexts as a means to address various neurological, psychiatric, and neurodegenerative disorders. This article aims to fulfill two primary objectives. Firstly, it seeks to showcase the current state of the art in the clinical application of NIBS, highlighting how it can improve and complement existing treatments. Secondly, it provides a comprehensive overview of the utilization of NIBS in augmenting the brain function of healthy individuals, thereby enhancing their performance. Furthermore, the article delves into the points of convergence and divergence between these two techniques. It also addresses the existing challenges and future prospects associated with NIBS from ethical and research standpoints.

Disruption of layer-specific visual processing in a model of focal neocortical epilepsy

The epileptic brain is the result of a sequence of events transforming normal neuronal populations into hyperexcitable networks supporting recurrent seizure generation. These modifications are known to induce fundamental alterations of circuit function and, ultimately, of behavior. However, how hyperexcitability affects information processing in cortical sensory circuits is not yet fully understood. Here, we investigated interlaminar alterations in sensory processing of the visual cortex in a mouse model of focal epilepsy. We found three main circuit dynamics alterations in epileptic mice: (i) a spreading of visual contrast-driven gamma modulation across layers, (ii) an increase in firing rate that is layer-unspecific for excitatory units and localized in infragranular layers for inhibitory neurons, and (iii) a strong and contrast-dependent locking of firing units to network activity. Altogether, our data show that epileptic circuits display a functional disruption of layer-specific organization of visual sensory processing, which could account for visual dysfunction observed in epileptic subjects. Understanding these mechanisms paves the way to circuital therapeutic interventions for epilepsy.

Visually sensitive seizures: An updated review by the Epilepsy Foundation

Light flashes, patterns, or color changes can provoke seizures in up to 1 in 4000 persons. Prevalence may be higher because of selection bias. The Epilepsy Foundation reviewed light-induced seizures in 2005. Since then, images on social media, virtual reality, three-dimensional (3D) movies, and the Internet have proliferated. Hundreds of studies have explored the mechanisms and presentations of photosensitive seizures, justifying an updated review. This literature summary derives from a nonsystematic literature review via PubMed using the terms "photosensitive" and "epilepsy." The photoparoxysmal response (PPR) is an electroencephalography (EEG) phenomenon, and photosensitive seizures (PS) are seizures provoked by visual stimulation. Photosensitivity is more common in the young and in specific forms of generalized epilepsy. PS can coexist with spontaneous seizures. PS are hereditable and linked to recently identified genes. Brain imaging usually is normal, but special studies imaging white matter tracts demonstrate abnormal connectivity. Occipital cortex and connected regions are hyperexcitable in subjects with light-provoked seizures. Mechanisms remain unclear. Video games, social media clips, occasional movies, and natural stimuli can provoke PS. Virtual reality and 3D images so far appear benign unless they contain specific provocative content, for example, flashes. Images with flashes brighter than 20 candelas/m² at 3-60 (particularly 15-20) Hz occupying at least 10 to 25% of the visual field are a risk, as are red color flashes or oscillating stripes. Equipment to assay for these characteristics is probably underutilized. Prevention of seizures includes avoiding provocative stimuli, covering one eye, wearing dark glasses, sitting at least two meters from screens, reducing contrast, and taking certain antiseizure drugs. Measurement of PPR suppression in a photosensitivity model can screen putative antiseizure drugs. Some countries regulate media to reduce risk. Visually-induced seizures remain significant public health hazards so they warrant ongoing scientific and regulatory efforts and public education.

Altered Visual Cortical Excitability Is Associated With Psychopathological Symptoms in Major Depressive Disorder

Previous studies suggest that in people with major depressive disorder (MDD), there exists a perturbation of the normal balance between the excitatory and inhibitory neurotransmitter systems in the visual cortex, indicating the possibility of altered visual cortical excitability. However, investigations into the neural activities of the visual cortex in MDD patients yielded inconsistent findings. The present study aimed to evaluate the visual cortical excitability utilizing a paired-pulse stimulation paradigm in patients with MDD and to access the paired-pulse behavior of recording visual evoked potentials (VEPs) as a marker of MDD. We analyzed the amplitudes of VEPs and paired-pulse suppression (PPS) at four different stimulus onset asynchronies (SOAs) spanning 93 ms to 133 ms. Further, the relationship between PPS and the symptom severity of depression was investigated using Spearman's correlation. We found that, whereas the first VEP amplitude remained unchanged, the second VEP amplitude was significantly higher in the MDD group compared to the healthy controls. As a result, the amplitude ratio (second VEP amplitude/first VEP amplitude) increased, indicating reduced PPS and thus increased excitability in the visual cortex. Moreover, we found the amplitude ratios had a significantly positive correlation with the symptom severity of depression in MDD, indicating a clinically useful biomarker for MDD. Our findings provide new insights into the changes in the excitation-inhibition balance of visual cortex in MDD, which could pave the way for specific clinical interventions.

Morphological changes of the dorsal contour of the corpus callosum during the first two years of life

BACKGROUND

In the medicolegal literature, focal concavities or notching of the corpus callosum has been thought to be associated with fetal alcohol spectrum disorders. Recent work suggests corpus callosum notching is a dynamic and normal anatomical feature, although it has not yet been defined in early life or infancy.

OBJECTIVE

Our purpose was to characterize the dorsal contour of the corpus callosum during the first 2 years of life by defining the prevalence, onset and trajectory of notching on midsagittal T1-weighted images.

MATERIALS AND METHODS

We reviewed retrospectively 1,157 consecutive patients between birth and 2 years of age. Corpus callosum morphology was evaluated and described. A notch was defined as a dorsal concavity of at least 1 mm in depth along the dorsal surface of the corpus callosum. Patient age as well as notch depth, location, number and presence of the pericallosal artery in the notch were noted.

RESULTS

Two hundred thirty-three notches were identified in 549 patients: 36 anterior, 194 posterior and 3 patients with undulations. A statistically significant (R²=0.53, Beta=0.021, P=0.002) positive correlation between posterior notch prevalence and age in months was noted. A positive correlation between age and depth of the posterior notch was also statistically significant (r=0.32, n=179, P≤0.001). A trend for increased anterior notch prevalence with age was identified with significant correlation between visualized pericallosal artery indentation and anterior notching (r=0.20, n=138, P=0.016). Sub-analysis of the first month of life showed corpus callosum notching was not present.

CONCLUSION

The presence of posterior notching increased significantly with age and was more frequent than that of anterior notching. Corpus callosum notching was absent in the first week of life, building on prior studies suggesting corpus callosum notching is acquired. This study provides baseline data on normative corpus callosum notching trajectories by age group during early life, a helpful correlate when associating corpus callosum morphology with disease.

Anodal occipital tDCS enhances spontaneous alpha activity

Transcranial direct current stimulation (tDCS) is a form of brain stimulation technique that modulates neuronal excitability changes in targeted cerebral areas through a constant low current. The existing studies mainly concentrated in tDCS effects on motor cortex. The number of tDCS studies targeting visual area is sparse. And parameters of tDCS on the visual cortex are not well optimized yet. Therefore, this study explored the effect of anodal occipital tDCS in eyes-open resting state to disclose possible modulation to spontaneous brain activity by electroencephalography (EEG). Fifteen healthy subjects were involved in this study. Each subject endured sham and anodal tDCS in turn. 2 mA tDCS was applied over 21 min with Oz-Cz montage. Amplitudes of spontaneous brain activities were evaluated for each experimental condition. Compared with pre-stimulation and sham tDCS, anodal tDCS caused an obvious increment in parieto-occipital alpha activity. These results demonstrated electrophysiological changes in EEG oscillations induced by anodal occipital tDCS, and would help to improve the understanding of modulation of tDCS-induced visual cortex excitability changes in humans.

Unilateral Application of Cathodal tDCS Reduces Transcallosal Inhibition and Improves Visual Acuity in Amblyopic Patients

Objective: Amblyopia is a neurodevelopmental disorder characterized by visual acuity and contrast sensitivity loss, refractory to pharmacological and optical treatments in adulthood. In animals, the corpus callosum (CC) contributes to suppression of visual responses of the amblyopic eye. To investigate the role of interhemispheric pathways in amblyopic patients, we studied the response of the visual cortex to transcranial Direct Current Stimulation (tDCS) applied over the primary visual area (V1) contralateral to the “lazy eye.”

Methods: Visual acuity (logMAR) was assessed before (T₀), immediately after (T₁) and 60’ following the application of cathodal tDCS (2.0 mA, 20’) in 12 amblyopic patients. At each time point, Visual Evoked Potentials (VEPs) triggered by grating stimuli of different contrasts (K90%, K20%) were recorded in both hemispheres and compared to those obtained in healthy volunteers.

Results: Cathodal tDCS improved visual acuity respect to baseline (p < 0.0001), whereas sham polarization had no significant effect. At T₁, tDCS induced an inhibitory effect on VEPs amplitudes at all contrasts in the targeted side and a facilitation of responses in the hemisphere ipsilateral to the amblyopic eye; compared with controls, the facilitation persisted at T₂ for high contrasts (K90%; Holm–Sidak post hoc method, p < 0.001), while the stimulated hemisphere recovered more quickly from inhibition (Holm–Sidak post hoc method, p < 0.001).

Conclusions: tDCS is a promising treatment for amblyopia in adults. The rapid recovery of excitability and the concurrent transcallosal disinhibition following perturbation of cortical activity may support a critical role of interhemispheric balance in the pathophysiology of amblyopia.

Tracking the Functional Development of the Corpus Callosum in Children Using Behavioral and Evoked Potential Interhemispheric Transfer Times

Visual functions requiring interhemispheric transfer exhibit a long developmental trajectory up to age 12, which might be constrained by corpus callosum maturation. Here, we use electrophysiological and behavioral crossed-uncrossed differences (CUDs) in a visual Poffenberger paradigm to estimate the interhemispheric transfer time (IHTT)-a measure of corpus callosum maturation-in 7-year-old children and adults. Adults' electrophysiological CUDs were faster than 7-year-olds'. Behavioral CUDs did not differ and proved to be unreliable in a 6-month follow-up test. These findings suggest that the corpus callosum still undergoes development at the age of 7 that can only reliably be traced with neuroscientific methods.

Reorganization of Visual Callosal Connections Following Alterations of Retinal Input and Brain Damage

Vision is a very important sensory modality in humans. Visual disorders are numerous and arising from diverse and complex causes. Deficits in visual function are highly disabling from a social point of view and in addition cause a considerable economic burden. For all these reasons there is an intense effort by the scientific community to gather knowledge on visual deficit mechanisms and to find possible new strategies for recovery and treatment. In this review, we focus on an important and sometimes neglected player of the visual function, the corpus callosum (CC). The CC is the major white matter structure in the brain and is involved in information processing between the two hemispheres. In particular, visual callosal connections interconnect homologous areas of visual cortices, binding together the two halves of the visual field. This interhemispheric communication plays a significant role in visual cortical output. Here, we will first review the essential literature on the physiology of the callosal connections in normal vision. The available data support the view that the callosum contributes to both excitation and inhibition to the target hemisphere, with a dynamic adaptation to the strength of the incoming visual input. Next, we will focus on data showing how callosal connections may sense visual alterations and respond to the classical paradigm for the study of visual plasticity, i.e., monocular deprivation (MD). This is a prototypical example of a model for the study of callosal plasticity in pathological conditions (e.g., strabismus and amblyopia) characterized by unbalanced input from the two eyes. We will also discuss the findings of callosal alterations in blind subjects. Noteworthy, we will discuss data showing that inter-hemispheric transfer mediates recovery of visual responsiveness following cortical damage. Finally, we will provide an overview of how callosal projections dysfunction could contribute to pathologies such as neglect and occipital epilepsy. A particular focus will be on reviewing noninvasive brain stimulation techniques and optogenetic approaches that allow to selectively manipulate callosal function and to probe its involvement in cortical processing and plasticity. Overall, the data indicate that experience can potently impact on transcallosal connectivity, and that the callosum itself is crucial for plasticity and recovery in various disorders of the visual pathway.