Les interactions vestibulo-corticales qui sous-tendent le contrôle de la posture chez les sujets sains

Le système vestibulaire et le cortex moteur participent au contrôle de la posture, mais la nature de leurs interactions est peu documentée. Afin de caractériser les interactions vestibulo-corticales qui sous-tendent le contrôle de l’équilibre en position debout, l’activité électromyographique (EMG) du soléaire (SOL), du tibial antérieur (TA) et du péronier long (PERL) de la jambe droite a été enregistrée chez 14 sujets sains. La stimulation galvanique vestibulaire (GVS) a été appliquée avec la cathode derrière l’oreille droite ou gauche à différents intervalles inter-stimulus (ISIs) avant ou après la stimulation magnétique transcrânienne induisant des potentiels moteurs évoqués (MEPs) au niveau des muscles enregistrés. Lorsque que la cathode était à droite, une inhibition des MEPs a été observée au niveau du SOL à un ISI de 40 et 130 ms et une facilitation des MEPS a été observée au niveau TA à un ISI de 110 ms. Lorsque la cathode était à gauche, une facilitation des MEPs a été observée au niveau du SOL, du TA et du PERL à un ISI de 50, -10 et 0 ms respectivement. L’emplacement de ces interactions sur l’axe neural a été estimé en fonction des ISIs et en comparant l’effet de la GVS sur les MEPs à son effet sur l’EMG de base et sur le réflexe-H. Selon ces analyses, les modulations observées peuvent avoir lieu au niveau spinal ou au niveau supraspinal. Ces résultats suggèrent que les commandes de la voie corticospinale peuvent être modulées par le système vestibulaire à différents niveaux de l’axe neuronal.

Late onset spinal cord ischemia after thoracoabdominal aortic aneurysm open repair

The aim of this study is to evaluate if spinal cord ischemia (SCI), especially its late presentation, and can be correlated to the results of intraoperative evoked potential monitoring (IOM). Methods. This study is a physician-initiated, retrospective, single-center, non-randomized study. Data from all patients undergoing a thoracoabdominal aortic aneurysm surgical repair (TAAA SR) between January 2016 and March 2020 IOM was collected and analyzed. Results. During the study period, 261 patients underwent TAAA SR with MEP/SSEPs monitoring [190 males, 73%; median age 65 (57-71)]. Thirty-seven patients suffered from SCI, for an overall rate of 14% (permanent 9%). When stratifying patients according to the SCI onset, 18 patients presented with an early (11 permanent) and 19 with a late SCI (<24h) (11 permanent). Of 261 patients undergoing TAAA SR with IOM, 15 were excluded due to changes in the upper extremity motor evoked potentials. For the remaining 246, the association between SCI and IOM was investigated: only irreversible IOM loss without peripheral changes have been found to be a risk factor for late onset SCI (p=.006). Furthermore, given that no statistical differences were found between the two groups when no IOM changes were recorded (p=.679), this situation cannot reliably rule out any SCI in our cohort. Independent risk factors for late spinal cord ischemia onset found at multivariate analysis were smoking history (p=.008), BMI>28 (p=.048) and TAAA extent II (p=.009). The irreversible MEP change without peripheral showed a trend of significance (p=.052). Conclusions. Evoked potential intraoperative monitoring is an important adjunct during thoracoabdominal aortic open repair to predict and possibly prevent spinal cord ischemia. Irreversible IOM loss without peripheral changes was predictive of late SCI, therefore more attention should be paid to the postoperative management of this subgroup of patients.

Predicting corticospinal excitability from pre-TMS EEG using convolutional neural networks

The amplitude of motor evoked potentials (MEPs) elicited by transcranial magnetic stimulation (TMS) of the primary motor cortex (M1) shows a large variability from trial to trial, although MEPs are evoked by the same repeated stimulus. A multitude of factors is believed to influence MEP amplitudes, such as cortical, spinal and motor excitability state. The goal of this work is to explore to which degree the variation in MEP amplitudes can be explained by the cortical state right before the stimulation. Specifically, we analyzed a dataset acquired on eleven healthy subjects comprising, for each subject, 840 single TMS pulses applied to the left M1 during acquisition of electroencephalography (EEG) and electromyography (EMG). An interpretable convolutional neural network, named SincEEGNet, was utilized to discriminate between low- and high-corticospinal excitability trials, defined according to the MEP amplitude, using in input the pre-TMS EEG. This data-driven approach enabled considering multiple brain locations and frequency bands without any a priori selection. Post-hoc interpretation techniques were adopted to enhance interpretation by identifying the more relevant EEG features for the classification. Results show that individualized classifiers successfully discriminated between low and high M1 excitability states in all participants. Outcomes of the interpretation methods suggest the importance of the electrodes situated over the TMS stimulation site, as well as the relevance of the temporal samples of the input EEG closer to the stimulation time. This novel decoding method allows causal investigation of the cortical excitability state, which may be relevant for personalizing and increasing the efficacy of therapeutic brain-state dependent brain stimulation (for example in patients affected by Parkinson’s disease).

Auditory and visual event-related potential alterations in fragile X syndrome

Le syndrome du X fragile (SXF) est la première cause héréditaire de déficience intellectuelle et également la première cause monogénique d’autisme. Le SXF est causé par l'expansion de la répétition du nucléotide CGG sur le gène FMR1, ce qui empêche l’expression de la protéine FMRP. L’absence du FMRP mène à une altération du développement structurel et fonctionnel de la synapse, ce qui empêche la maturation des synapses induite par l’activité et l’élagage synaptique, qui sont essentiels pour le développement cérébral et cognitif. Nous avons investigué les potentiels reliés aux événements (PRE) évoqués par des stimulations fondamentales auditives et visuelles dans douze adolescents et jeunes adultes (10-22) atteints du SXF, ainsi que des participants contrôles appariés en âge chronologique et développemental. Les résultats indiquent un profil des PRE altéré, notamment l’augmentation de l’amplitude de N1 auditive, par rapport aux deux groupes contrôle, ainsi que l’augmentation des amplitudes de P2 et N2 auditifs et de la latence de N2 auditif. Chez les patients SXF, le traitement sensoriel semble être davantage perturbé qu’immature. En outre, la modalité auditive semble être plus perturbée que la modalité visuelle. En combinaison avec des résultats anatomique du cerveau, des mécanismes biochimiques et du comportement, nos résultats suggèrent une hyperexcitabilité du système nerveux dans le SXF.

Ocular vestibular evoked myogenic potentials (oVEMP) using air conducted sound: Effect of body position on threshold

The primary objective of this research study is to determine if various body positions for ocular vestibular evoked myogenic potential (oVEMP) testing demonstrate differentiation of the saccule and utricle through threshold responses.

Modulation of visually evoked cortical fMRI responses by phase of ongoing occipital alpha oscillations

Using simultaneous electroencephalography as a measure of ongoing activity and functional magnetic resonance imaging (fMRI) as a measure of the stimulus-driven neural response, we examined whether the amplitude and phase of occipital alpha oscillations at the onset of a brief visual stimulus affects the amplitude of the visually evoked fMRI response. When accounting for intrinsic coupling of alpha amplitude and occipital fMRI signal by modeling and subtracting pseudo-trials, no significant effect of prestimulus alpha amplitude on the evoked fMRI response could be demonstrated. Regarding the effect of alpha phase, we found that stimuli arriving at the peak of the alpha cycle yielded a lower blood oxygenation level-dependent (BOLD) fMRI response in early visual cortex (V1/V2) than stimuli presented at the trough of the cycle. Our results therefore show that phase of occipital alpha oscillations impacts the overall strength of a visually evoked response, as indexed by the BOLD signal. This observation complements existing evidence that alpha oscillations reflect periodic variations in cortical excitability and suggests that the phase of oscillations in postsynaptic potentials can serve as a mechanism of gain control for incoming neural activity. Finally, our findings provide a putative neural basis for observations of alpha phase dependence of visual perceptual performance.

Associating spontaneous with evoked activity in a neural mass model of cat visual cortex

Spontaneous activity of the brain at rest frequently has been considered a mere backdrop to the salient activity evoked by external stimuli or tasks. However, the resting state of the brain consumes most of its energy budget, which suggests a far more important role. An intriguing hint comes from experimental observations of spontaneous activity patterns, which closely resemble those evoked by visual stimulation with oriented gratings, except that cortex appeared to cycle between different orientation maps. Moreover, patterns similar to those evoked by the behaviorally most relevant horizontal and vertical orientations occurred more often than those corresponding to oblique angles. We hypothesize that this kind of spontaneous activity develops at least to some degree autonomously, providing a dynamical reservoir of cortical states, which are then associated with visual stimuli through learning. To test this hypothesis, we use a biologically inspired neural mass model to simulate a patch of cat visual cortex. Spontaneous transitions between orientation states were induced by modest modifications of the neural connectivity, establishing a stable heteroclinic channel. Significantly, the experimentally observed greater frequency of states representing the behaviorally important horizontal and vertical orientations emerged spontaneously from these simulations. We then applied bar-shaped inputs to the model cortex and used Hebbian learning rules to modify the corresponding synaptic strengths. After unsupervised learning, different bar inputs reliably and exclusively evoked their associated orientation state; whereas in the absence of input, the model cortex resumed its spontaneous cycling. We conclude that the experimentally observed similarities between spontaneous and evoked activity in visual cortex can be explained as the outcome of a learning process that associates external stimuli with a preexisting reservoir of autonomous neural activity states. Our findings hence demonstrate how cortical connectivity can link the maintenance of spontaneous activity in the brain mechanistically to its core cognitive functions.

Analysis of transient otoacoustic emissions and brainstem evoked auditory potentials in neonates with hyperbilirubinemia

A hiperbilirrubinemia é tóxica às vias auditivas e ao sistema nervoso central, deixando sequelas como surdez e encefalopatia. OBJETIVOS: avaliar a audição de neonatos portadores de hiperbilirrubinemia, utilizando-se a pesquisa das emissões otoacústicas evocadas transientes (EOAET) e dos potenciais evocados auditivos do tronco encefálico (PEATE). Estudo prospectivo. CASUÍSTICA E MÉTODOS: Constituíram-se dois grupos: GI (n-25), neonatos com hiperbilirrubinemia; GII (n-22), neonatos sem hiperbilirrubinemia e sem fatores de risco para surdez. Todos os neonatos tinham até 60 dias de vida e foram submetidos à EOAET e ao PEATE. RESULTADOS: 12 neonatos de GI e 10 de GII eram meninas e 13 de GI e 12 de GII eram meninos. As EOAET estavam presentes em todas as crianças, porém com amplitudes menores em GI, especialmente nas frequências de 2 e 3KHz (p < 0,05). No PEATE, observou-se discreto prolongamento de PV e de LI-V em GI. As alterações observadas nesses testes não se correlacionaram aos níveis séricos da bilirrubinemia. CONCLUSÕES: em neonatos portadores de hiperbilirrubinemia, menores amplitudes das EOAET e discreto prolongamento de PV e de LI-V foram constatados indicando comprometimento coclear e retrococlear das vias auditivas, salientando-se a importância da utilização e da interpretação minuciosa de ambos os testes nessas avaliações.

Differential effect of elevated intralabyrinthine pressure on ocular vestibular evoked myogenic potentials elicited by air conducted sound and bone conducted vibration

Recently, ocular vestibular evoked myogenic potentials (oVEMP) have emerged as a tool for assessment of utricular function. They are short-latency myogenic potentials which can be elicited in response to vestibular stimulation, e.g. by air-conducted sound (ACS) or bone-conducted vibration (BCV) (reviewed in (Kantner and Gurkov, 2012)). Otolithic afferent neurons trigger reflexive electromyographic activity of the extraocular muscles which can be recorded beneath the eye contralateral to the stimulated ear by use of surface electrodes.

Event-related brain potentials in the study of visual selective attention

Event-related brain potentials (ERPs) provide high-resolution measures of the time course of neuronal activity patterns associated with perceptual and cognitive processes. New techniques for ERP source analysis and comparisons with data from blood-flow neuroimaging studies enable improved localization of cortical activity during visual selective attention. ERP modulations during spatial attention point toward a mechanism of gain control over information flow in extrastriate visual cortical pathways, starting about 80 ms after stimulus onset. Paying attention to nonspatial features such as color, motion, or shape is manifested by qualitatively different ERP patterns in multiple cortical areas that begin with latencies of 100–150 ms. The processing of nonspatial features seems to be contingent upon the prior selection of location, consistent with early selection theories of attention and with the hypothesis that spatial attention is “special.”

Inhibition of sustained gamma oscillations (35-80 Hz) by fast transient responses in cat visual cortex.

Interactions between stimulus-induced oscillations (35-80 Hz) and stimulus-locked nonoscillatory responses were investigated in the visual cortex areas 17 and 18 of anaesthetized cats. A single square-wave luminance grating was used as a visual stimulus during simultaneous recordings from up to seven electrodes. The stimulus movement consisted of a superposition of a smooth movement with a sequence of dynamically changing accelerations. Responses of local groups of neurons at each electrode were studied on the basis of multiple unit activity and local slow field potentials (13-120 Hz). Oscillatory and stimulus-locked components were extracted from multiple unit activity and local slow field potentials and quantified by a combination of temporal and spectral correlation methods. We found fast stimulus-locked components primarily evoked by sudden stimulus accelerations, whereas oscillatory components (35-80 Hz) were induced during slow smooth movements. Oscillations were gradually reduced in amplitude and finally fully suppressed with increasing amplitudes of fast stimulus-locked components. It is argued that suppression of oscillations is necessary to prevent confusion during sequential processing of stationary and fast changing retinal images.

Spreading and synchronization of intrinsic signals in visual cortex of macaque monkey evoked by a localized visual stimulus

Spatio-temporal maps of the occipital cortex of macaque monkeys were analyzed using optical imaging of intrinsic signals. The images obtained during localized visual stimulation (IS) were compared with the images obtained on presentation of a blank screen (IB). We first investigated spontaneous variations of the intrinsic signals by analyzing the 100 IBs for each of the three cortical areas. Slow periodical activation was observed in alternation over the cortical areas. Cross-correlation analysis indicated that synchronization of spontaneous activation only took place within each cortical area, but not between them. When a small, drifting grating (2degreesX2degrees) was presented on the fovea. a dark spot appeared in the optical image at the cortical representation of this retinal location. It spread bilaterally along the border between V1 and V2, continuing as a number of parallel dark bands covering a large area of the lateral surface of V1. Cross-correlation analysis showed that during visual stimulation the intrinsic signals over all of the three cortical areas were synchronized, with in-phase activation of V1 and V2 and anti-phase activation of V4 and V1/V2. The significance of these extensive synergistic and antagonistic interactions between different cortical areas is discussed. (C) 2003 Elsevier B.V. All rights reserved.

Visual feedback alters the variations in corticospinal excitability that arise from rhythmic movements of the opposite limb

Augmented visual feedback can have a profound bearing on the stability of bimanual coordination. Indeed, this has been used to render tractable the study of patterns of coordination that cannot otherwise be produced in a stable fashion. In previous investigations (Carson et al. 1999), we have shown that rhythmic movements, brought about by the contraction of muscles on one side of the body, lead to phase-locked changes in the excitability of homologous motor pathways of the opposite limb. The present study was conducted to assess whether these changes are influenced by the presence of visual feedback of the moving limb. Eight participants performed rhythmic flexion-extension movements of the left wrist to the beat of a metronome (1.5 Hz). In 50% of trials, visual feedback of wrist displacement was provided in relation to a target amplitude, defined by the mean movement amplitude generated during the immediately preceding no feedback trial. Motor potentials (MEPs) were evoked in the quiescent muscles of the right limb by magnetic stimulation of the left motor cortex. Consistent with our previous observations, MEP amplitudes were modulated during the movement cycle of the opposite limb. The extent of this modulation was, however, smaller in the presence of visual feedback of the moving limb (FCR omega(2) =0.41; ECR omega(2)=0.29) than in trials in which there was no visual feedback (FCR omega(2)=0.51; ECR omega(2)=0.48). In addition, the relationship between the level of FCR activation and the excitability of the homologous corticospinal pathway of the opposite limb was sensitive to the vision condition; the degree of correlation between the two variables was larger when there was no visual feedback of the moving limb. The results of the present study support the view that increases in the stability of bimanual coordination brought about by augmented feedback may be mediated by changes in the crossed modulation of excitability in homologous motor pathways.