Patrick Marquis

Evoked potentials recorded from the auditory cortex in man: evaluation and topography of the middle latency components

The goal of this study is to determine and localize the generators of different components of middle latency auditory evoked potentials (MLAEPs) through intracerebral recording in auditory cortex in man (Heschls gyrus and planum temporale). The present results show that the generators of components at 30, 50, 60 and 75 msec latency are distributed medio-laterally along Heschls gyrus. The 30 msec component is generated in the dorso-postero-medial part of Heschls gyrus (primary area) and the 50 msec component is generated laterally in the primary area. The generators of the later components (60-75 msec) are localized in the lateral part of Heschls gyrus that forms the secondary areas. The localization of N100 generators is discussed.

Single-trial analysis of oddball event-related potentials in simultaneous EEG-fMRI

There has recently been a growing interest in the use of simultaneous electroencephalography (EEG) and functional MRI (fMRI) for evoked activity in cognitive paradigms, thereby obtaining functional datasets with both high spatial and temporal resolution. The simultaneous recording permits obtaining event‐related potentials (ERPs) and MR images in the same environment, conditions of stimulation, and subject state; it also enables tracing the joint fluctuations of EEG and fMRI signals. The goal of this study was to investigate the possibility of tracking the trial‐to‐trial changes in event‐related EEG activity, and of using this information as a parameter in fMRI analysis. We used an auditory oddball paradigm and obtained single‐trial amplitude and latency features from the EEG acquired during fMRI scanning. The single‐trial P300 latency presented significant correlation with parameters external to the EEG (target‐to‐target interval and reaction time). Moreover, we obtained significant fMRI activations for the modulation by P300 amplitude and latency, both at the single‐subject and at the group level. Our results indicate that, in line with other studies, the EEG can bring a new dimension to the field of fMRI analysis by providing fine temporal information on the fluctuations in brain activity. Hum Brain Mapp, 2007.

Electric source imaging in temporal lobe epilepsy

The objective of this study was to determine the validity of interictal spike (IIS) source localization in temporal lobe epilepsies (TLE) using stereoelectroencephalography as a validating method. Twenty patients with drug-resistant TLE were studied with high-resolution EEG and stereoelectroencephalography. Sixty-four scalp channels, a realistic head model, and different algorithms were used. For each patient, the intracerebral interictal distribution was studied and classified into one of three groups: L (mainly lateral), ML (mediolateral), and M (medial). In group L (three patients), surface IIS were recorded with a high signal-to-noise ratio. Source localizations designated all or part of the intracerebral interictal distribution. In group ML (11 patients), 8 patients had surface IIS, only 5 of which were localizable. High-resolution EEG permitted localization of the more lateral portion and definition of its rostrocaudal extension. A common pattern was identified in three patients with a predominant role of the temporal pole. In group M (six patients), four patients had rare surface IIS, none of which were localizable. Surface EEG does not record IIS limited to medial temporal lobe structures. In TLE with a mediolateral or a lateral interictal distribution, only the lateral component is detectable on surface EEG and accurately localizable by source localization tools.

Electric Source Imaging in Frontal Lobe Epilepsy

Summary: The objective of this study was to determine the validity of interictal spike (IIS) source localization in frontal lobe epilepsies (FLE) using stereoelectroencephalography as a validating method. Ten patients with drug-resistant FLE were studied with high-resolution EEG and stereoelectroencephalography. Sixty-four scalp channels, a realistic head model, and different algorithms were used. For each patient, the intracerebral interictal distribution was studied and classified into one of three groups: lateral, medial, and mixed (latero-medio-basal). Surface IIS were abundant or subcontinuous for 8 of 10 FLE patients. In lateral and medial groups, intracerebral interictal activities were accurately localized. In the mixed group, source localizations designated a part of the intracerebral interictal distribution. A high degree of source localization accuracy is obtained in FLE. False-positive results were never obtained, but the extent of interictal activity could be underestimated by source localization results. Geometrical and cytoarchitectonic characteristics of the generator appear crucial to explain why medial frontal IIS (anterior para-cingulate gyrus and anterior cingulate gyrus) may be localizable whereas only the lateral orbitofrontal IIS seems to be localizable.

Source localization of scalp-EEG interictal spikes in posterior cortex epilepsies investigated by HR-EEG and SEEG

Purpose:  To determine the validity of scalp‐electroencephalography (EEG)‐interictal spike (IIS) source localization in posterior cortex epilepsies (PCE).

Relationship between auditory perception skills and mismatch negativity recorded in free field in cochlear-implant users

This study investigated the ability of cochlear-implanted patients to discriminate tone bursts in free field using the electrophysiological recordings of mismatch negativity (MMN). Seven cochlear-implanted patients (CIP) and eight control subjects (CS) were tested. Event-related potentials were recorded from either 32 or 64 electrodes in response to binaural stimuli using a passive oddball paradigm. Two stimulus-contrast conditions were used to produce MMN: The standard-tone frequency was fixed at 1 kHz, and the deviant-tone frequency was set at 2 or 1.5 kHz. The results show that response waveforms (N1/P2) are similar in latency and amplitude for CS and CIP, suggesting that pure-tone detection is performed over the same time window in both groups. These waveforms are also similar in left- and right-implanted patients, suggesting that electric stimulation of the auditory nerve activates both hemispheres in profound, bilateral hearing loss. Pure-tone audiograms and word-discrimination scores were also measured in each subject in an anechoic room and their relations with MMN data were examined. Correlations were found between the latency of MMN for a 1.5 kHz deviant and the thresholds obtained for pure-tone detection and word discrimination. MMN appears as a possible complementary clinical tool to objectively assess auditory sensitivity in cochlear-implanted populations. However, further improvements are still necessary before it can be used as a standard clinical examination.

Functional interactions in brain networks underlying epileptic seizures in bilateral diffuse periventricular heterotopia

OBJECTIVE Our aim was to investigate relationships between heterotopic and remote cortical structures at seizure initiation, in a patient with bilateral periventricular nodular heterotopias (BPNH) explored by intracerebral electrodes. METHODS Stereoelectroencephalography (SEEG) was performed in a man with BPNH and refractory epilepsy to investigate the hypothesis of right temporal lobe epilepsy and the possible involvement of heterotopic structures during seizures. SEEG signals were analyzed with quantification of functional coupling between different brain structures during seizures, using nonlinear regression. We have used Z-score transformation of correlation values to reflect the change from the preictal period. Relationships between BPNH and cortical structures were investigated using analysis of stimulation-induced potentials. RESULTS Three spontaneous seizures were recorded and analyzed. Signal analysis of interdependencies in two seizures demonstrated a large initial network involving both heterotopia and cortical structures. Stimulations of heterotopia induced responses in remote cortical structures. CONCLUSIONS Distinct epileptogenic networks were identified, in which leader structures were either the heterotopic or the mesial temporal structures, with functional connections between heterotopic and cortical areas. SIGNIFICANCE These results confirm that a vast epileptogenic network, including heterotopic and cortical neurons, may be responsible for seizure generation in BPNH. This may explain certain surgical failures in this group.

Effects of long-term vigabatrin on somatosensory-evoked potentials in epileptic patients.

Summary: Vigabatrin (gamma vinyl GABA, GVG) has been shown to be an effective antiepiieptic agent. GVG specifically and irreversibly inhibits GABA‐transaminase (GABA‐T). Long‐term animal toxicology studies have demonstrated that GVG can induce nonprogressive, reversible intramyelinic edema in central white matter tracts. The response to GVG varies among species, with rodents being the most dramatic and monkeys showing an equivocal effect even at high doses. The response in dogs is marked and measurable. The detection of these subtle findings requires the use of sophisticated technology. Evoked potentials are becoming reliable and sensitive tools in clinical neurology. This study, involving 54 patients for 11 months, was undertaken to assess the effect and safety of GVG in humans with refractory epilepsy. No data from this investigation indicate prolongation of neuronal conduction time in CNS pathways, suggesting that this agent is safe in humans.

Distinction between the myoclonus-related potential and the epileptic spike in epilepsia partialis continua

Neurophysiological analysis of spontaneous and reflex-triggered myoclonus in a case of epilepsia partialis continua is reported. A central positive wave (P1), part of the high voltage evoked potential and of the spontaneous spike, is held to be responsible for the myoclonic jerk. It is demonstrated that P1 is a distinct phenomenon from both the evoked potential and the epileptic spike, and so should be identified as a myoclonus-related potential (MRP).

A comparison of methods for assessing alpha phase resetting in electrophysiology, with application to intracerebral EEG in visual areas.

There are two competing views on the mechanisms underlying the generation of visual evoked potentials/fields in EEG/MEG. The classical hypothesis assumes an additive wave on top of background noise. Another hypothesis states that the evoked activity can totally or partially arise from a phase resetting of the ongoing alpha rhythm. There is no consensus however, on the best tools for distinguishing between these two hypotheses. In this study, we have tested different measures on a large series of simulations under a variety of scenarios, involving in particular trial-to-trial variability and different dynamics of ongoing alpha rhythm. No single measure or set of measures was found to be necessary or sufficient for defining phase resetting in the context of our simulations. Still, simulations permitted to define criteria that were the most reliable in practice for distinguishing additive and phase resetting hypotheses. We have then applied these criteria on intracerebral EEG data recordings in the visual areas during a visual discrimination task. We investigated the intracerebral channels that presented both ERP and ongoing alpha oscillations (n=37). Within these channels, a total of 30% fulfilled phase resetting criteria during the generation of the visual evoked potential, based on criteria derived from simulations. Moreover, 19% of the 37 channels presented dependence of the ERP on the level of pre-stimulus alpha. Only 5% of channels fulfilled both the simulation-related criteria and dependence on baseline alpha level. Our simulation study points out to the difficulty of clearly assessing phase resetting based on observed macroscopic electrophysiological signals. Still, some channels presented an indication of phase resetting in the context of our simulations. This needs to be confirmed by further work, in particular at a smaller recording scale.