Leen De Taeye

The effect of vagus nerve stimulation on response inhibition

In the current study, we explored whether vagus nerve stimulation (VNS) in patients with epilepsy, which is believed to increase norepinephrine (NE) levels via activation of the locus coeruleus, would positively affect response inhibition. Moreover, we tried to identify the dynamics of the underlying neural processes by investigating event-related potentials (ERPs) and pupil size. Patients performed a stop-signal task once when stimulation was switched on and once when it was switched off. We found a correlational pattern suggesting that patients who clinically benefit more from VNS treatment also show a larger behavioral advantage, in terms of faster response inhibition, when the vagus nerve is being stimulated. Event-related potential (ERP) results suggested more pronounced reactive inhibition when stimulation was switched on, independent of the individual amount of seizure reduction. Transient go-locked pupil size was increased from go trials to successful stop trials to unsuccessful stop trials but without displaying a clear VNS effect, which however, might relate to limited sensitivity. We conclude that VNS likely has a positive effect on response inhibition, at least in patients with epilepsy that benefit clinically from the treatment, presumably relating to enhancements of response-inhibition mechanisms and, therefore, identify enhanced response inhibition as a possible cognitive benefit of VNS.

Event-Related Potentials Reveal Preserved Attention Allocation but Impaired Emotion Regulation in Patients with Epilepsy and Comorbid Negative Affect

Patients with epilepsy have a high prevalence of comorbid mood disorders. This study aims to evaluate whether negative affect in epilepsy is associated with dysfunction of emotion regulation. Event-related potentials (ERPs) are used in order to unravel the exact electrophysiological time course and investigate whether a possible dysfunction arises during early (attention) and/or late (regulation) stages of emotion control. Fifty epileptic patients with (n = 25) versus without (n = 25) comorbid negative affect plus twenty-five matched controls were recruited. ERPs were recorded while subjects performed a face- or house-matching task in which fearful, sad or neutral faces were presented either at attended or unattended spatial locations. Two ERP components were analyzed: the early vertex positive potential (VPP) which is normally enhanced for faces, and the late positive potential (LPP) that is typically larger for emotional stimuli. All participants had larger amplitude of the early face-sensitive VPP for attended faces compared to houses, regardless of their emotional content. By contrast, in patients with negative affect only, the amplitude of the LPP was significantly increased for unattended negative emotional expressions. These VPP results indicate that epilepsy with or without negative affect does not interfere with the early structural encoding and attention selection of faces. However, the LPP results suggest abnormal regulation processes during the processing of unattended emotional faces in patients with epilepsy and comorbid negative affect. In conclusion, this ERP study reveals that early object-based attention processes are not compromised by epilepsy, but instead, when combined with negative affect, this neurological disease is associated with dysfunction during the later stages of emotion regulation. As such, these new neurophysiological findings shed light on the complex interplay of epilepsy with negative affect during the processing of emotional visual stimuli and in turn might help to better understand the etiology and maintenance of mood disorders in epilepsy.

A Preclinical Study of Laryngeal Motor-Evoked Potentials as a Marker Vagus Nerve Activation.

Vagus nerve stimulation (VNS) is a treatment for refractory epilepsy and depression. Previous studies using invasive recording electrodes showed that VNS induces laryngeal motor-evoked potentials (LMEPs) through the co-activation of the recurrent laryngeal nerve and subsequent contractions of the laryngeal muscles. The present study investigates the feasibility of recording LMEPs in chronically VNS-implanted rats, using a minimally-invasive technique, to assess effective current delivery to the nerve and to determine optimal VNS output currents for vagal fiber activation. Three weeks after VNS electrode implantation, signals were recorded using an electromyography (EMG) electrode in the proximity of the laryngeal muscles and a reference electrode on the skull. The VNS output current was gradually ramped up from 0.1 to 1.0 mA in 0.1 mA steps. In 13/27 rats, typical LMEPs were recorded at low VNS output currents (median 0.3 mA, IQR 0.2-0.3 mA). In 11/27 rats, significantly higher output currents were required to evoke electrophysiological responses (median 0.7 mA, IQR 0.5-0.7 mA, p < 0.001). The latencies of these responses deviated significantly from LMEPs (p < 0.05). In 3/27 rats, no electrophysiological responses to simulation were recorded. Minimally invasive LMEP recordings are feasible to assess effective current delivery to the vagus nerve. Furthermore, our results suggest that low output currents are sufficient to activate vagal fibers.

Representation of steady-state visual evoked potentials elicited by luminance flicker in human occipital cortex: An electrocorticography study

&NA; Despite the widespread use of steady‐state visual evoked potentials (SSVEPs) elicited by luminance flicker in clinical and research settings, their spatial and temporal representation in the occipital cortex largely remain elusive. We performed intracranial‐EEG recordings in response to targets flickering at frequencies from 11 to 15 Hz using a subdural electrode grid covering the entire right occipital cortex of a human subject, and we were able to consistently locate the gazed stimulus frequency at the posterior side of the primary visual cortex (V1). Peripheral flickering, undetectable in scalp‐EEG, elicited activations in the interhemispheric fissure at locations consistent with retinotopic maps. Both foveal and peripheral activations spatially coincided with activations in the high gamma band. We detected localized alpha synchronization at the lateral edge of V2 during stimulation and transient post‐stimulation theta band activations at the posterior part of the occipital cortex. Scalp‐EEG exhibited only a minor occipital post‐stimulation theta activation, but a strong transient frontal activation. HighlightsThe spatiotemporal representation of SSVEP in the occipital cortex is largely unclear.The fundamental frequency is represented at the posterior part of the primary visual cortex.The spatial representation of the second harmonic varies with the stimulation frequency.Simultaneous foveal and peripheral flickering stimuli are processed independently.SSVEP stimulation elicits localized alpha band activations at the lateral edge of V2.

Cortical distribution of N400 potential in response to semantic priming with visual non-linguistic stimuli

We conducted a study on visual semantic priming using related and unrelated image pairs while simultaneously recording electroencephalography (EEG) from 27 scalp electrodes and electrocorticography (ECoG) from a mixture of deep brain and subdural grid/strip electrodes in the left and right hippocampus, the right temporo-basal and temporo-lateral cortices, and the left temporal cortex. The EEG data showed a clear centro-parietal, bi-hemispheric N400 effect in response to unrelated image-pairs compared to related ones. Although with ECoG the N400 effect was more widely spread across both hemispheres, compared to linguistic stimuli, it was relatively localized within each ECoG grid as it was present only in some electrodes and, in some cases, even had its polarity reversed. We advocate this could be due to some grids gauging dipoles at different positions when covering sulci and gyri.

Reduced distractor interference during vagus nerve stimulation

Suppressing irrelevant information in decision making is an essential everyday skill. We studied whether this ability could be improved in epileptic patients during vagus nerve stimulation (VNS). VNS is known to increase norepinephrine (NE) in the brain. NE is thought to improve several aspects of cognitive control, including the suppression of irrelevant information. Nineteen epileptic VNS patients executed the Eriksen flanker task twice, both during on and off stimulation. Distractor interference was indexed by the congruency effect, a standard empirical marker of cognitive control. We found a reduced congruency effect during stimulation, which indicates an improved ability to suppress distractor interference. This effect was only found in patients that are clinically determined VNS-responders (n = 10). As VNS increases NE in VNS-responders, our finding suggests a beneficial role of NE in cognitive control. At the same time, it suggests that VNS does not only reduce seizure frequency in epileptic patients, but also improves cognitive control.

Decoding Steady-State Visual Evoked Potentials From Electrocorticography

We report on a unique electrocorticography (ECoG) experiment in which Steady-State Visual Evoked Potentials (SSVEPs) to frequency- and phase-tagged stimuli were recorded from a large subdural grid covering the entire right occipital cortex of a human subject. The paradigm is popular in EEG-based Brain Computer Interfacing where selectable targets are encoded by different frequency- and/or phase-tagged stimuli. We compare the performance of two state-of-the-art SSVEP decoders on both ECoG- and scalp-recorded EEG signals, and show that ECoG-based decoding is more accurate for very short stimulation lengths (i.e., less than 1 s). Furthermore, whereas the accuracy of scalp-EEG decoding benefits from a multi-electrode approach, to address interfering EEG responses and noise, ECoG decoding enjoys only a marginal improvement as even a single electrode, placed over the posterior part of the primary visual cortex, seems to suffice. This study shows, for the first time, that EEG-based SSVEP decoders can in principle be applied to ECoG, and can be expected to yield faster decoding speeds using less electrodes.