order to assess brain function and awakening prognosis. Recently Evoked PotentialsRelated Events (ERP) has been used in coma for identifying patients in minimally conscious state.
The main objective of this study is clarify the correlation between the patterns of ERP and levels of physical and cognitive disabilities due to brain injury. We use the classical paradigm of "oddball" and assessing the degree of disability by Glasgow Outcome Scale (GOS).
Congedo et al. (2008). Thus, it is important to remove noise in order to en- hance signal, and to perform feature extraction in order to feed the classifica- tion algorithm with relevant features.
Several methods based on Independent Component Analysis have been proposed to enhance the Signal to Noise Ratio (SNR) and to remove arte- facts. However, these methods are not specifically designed to separate brain activities and they are supervised. Indeed, after decomposition in different components, it is necessary to select (manually or thanks to spatio-temporal prior) components containing evoked potentials. In this work, Event-RelatedPotentials (ERPs) are considered and an unsupervised denoising method is used. It is based on the xDAWN algorithm Rivet et al. (2009), which has been specifically conceived to maximize the SNR of ERPs.
Computational modeling is commonly focused on the replication of behavioral data. In this article, we ex- plore an additional dimension, namely, modeling ERPs recorded during the AB. Due to the novelty of this ap- proach, there is no established methodology for gen- erating vERPs. Throughout this work, our philosophy is to use the most straightforward method while keep- ing our approach as close as possible to the mechanisms that are assumed to occur in the brain. It is obvious, however, that vERPs remain a coarse approximation of hERPs. Some factors that influence hERPs, such as the distortion of the signal by the scalp, are not addressed. Due to these limitations, one can realistically only expect to obtain a qualitative rather than a quantitative match to the data. Nevertheless, vERPs from the ST 2 model seem to allow us to make sensible predictions about the ERPs measured from the human scalp.
Auditory evoked potentials and their clinical significance
Auditory evoked potentials (AEPs) are a subclass of event- relatedpotentials (ERPs). ERPs are defined as brain responses which are time-locked to some event, such as a sensory stimulus. Averaged ERPs are thought to originate from synchronous activity in pyramidal cells in the activated areas. ERPs result mainly from the summation of cortical excitatory and inhibitory post-synaptic potentials triggered by the release of neurotransmitters such as GABA and glutamate into the synaptic cleft . Recent neurophysiological evidence supports the notion that the features of ERPs result from activity in several cortical sources that are intrinsically connected . The change in amplitude of the AEPs in response to various sound pressure levels (SPLs) is referred to as loudness dependence of auditory evoked potentials (LDAEP) , and is considered as a measure of serotonergic activity. [25–28] Moreover, there are hints of the influence of other neurotrans- mitters such as dopamine and nitric oxide [29,30]. Literature suggests that a pronounced LDAEP of the N1/P2 components reflects low central serotonergic neurotransmission . The inverse relationship between LDAEP and central serotonergic activity has been shown by different methods and in different psychiatric disorders [27,31–35].
As to the time course of EFE processing, when computing the ERPs by averaging and as expected, we found emotion- dependent modulations of the amplitude of the P2–P3 complex as well as the LPP component. Yet, no difference was found between EFEs for the N170, which might be in favor of the part of the literature that views the first stage as a raw structural processing one ( Eimer et al., 2003 ), which might also be linked to stimuli of low arousal ( Almeida et al., 2016 ). Considering the ERPs computed using the regression method, no significant impact of emotion on the brain response elicited exclusively by the presentation of the stimuli was found. This difference with the literature might be explained by the stimuli and paradigm we used. Indeed, Neath-Tavares and Itier (2016) , like most authors interested in this research topic, use prototypical stimuli (whether from POFA or from the MacBrain Face Stimulus Set, Batty and Taylor, 2003 ). This might explain at least in part why we do not have the same impact of valence on EFEs decoding as revealed by ERPs. In fact, with prototypical stimuli, the displayed emotions are overstated and amplified, whereas in the present study, EFE are natural and spontaneous, thus weaken ( Tcherkassof et al., 2013 ; cf. also Wagner et al., 1986 ; Valstar et al., 2006 ). For prototypical stimuli, the actors exaggerate the EFE. For instance, some past studies showed that posed smiles are larger in amplitude and are longer in duration than spontaneous smiles ( Ekman and Friesen, 1982; Cohn and Schmidt, 2004 ; Schmidt et al., 2006 ). Valstar et al. (2006) also showed that characteristics of brow actions (as such as intensity, speed and trajectory) are different between spontaneous and posed EFE. In our case, the filmed persons expressed spontaneously and naturally the EFE. Consequently, we used less “intensified” or “aroused” EFE than other studies based on prototypical EFE ( Tcherkassof et al., 2013 ). Another explanation is that, when the participants freely explore a stimulus, the brain responses to the presentation of the stimulus can be polluted by the subsequent responses to saccades that can occur after only 200 ms post-stimulation. In our case, in addition to using natural stimuli, we analyzed separately the
Uncovering the neural correlates of inattentional deafness
The brain activity involved in processing auditory information has been extensively studied using Electroencephalography (EEG) techniques to measure Event-RelatedPotentials (ERPs), even in an aeronautical context [ 25 – 28 ]. The P300 component, one of the most commonly studied ERPs, reflects the detection of an expected but unpredictable target (the oddball) in a stream of stimuli [ 29 ]. It can be elicited by the “oddball” paradigm. The P300 is typically ob- served in a time window between 300 to 600 ms after the auditory stimulus onset and reflects the occurrence of cognitive and attentional processes (c.f. [ 29 ] and [ 30 ] for a detailed review). When attentional focus deviates from the target, the P300 amplitude significantly decreases [ 31 ]. This link indicates that the P300 component is an excellent candidate to determine whether an auditory stimulus has broken through the attentional barrier. Importantly, it is gen- erally accepted that a distinction can be made between two subcomponents of the P300, name- ly the novelty P3 and the target P3 (also called P3b, or “classical” P3). Novelty P3 is a large positive deflection with a fronto-central scalp distribution that is elicited by novel, non-target stimuli and that mainly reflects involuntary attention shifts to changes in the environment [ 32 , 33 ]. It is functionally related to another subcomponent called P3a, that seems to be more specifically related to deviant auditory non-target events [ 34 ]. In contrast, the P3b, has a more posterior-parietal scalp distribution and a somewhat longer latency than novelty P3 and P3a. The P3b has been regarded as a sign of processes of memory access that are evoked by evalua- tion of stimuli in tasks that require some form of action like a covert or overt response, ecolog- ically closer to a real alarm occurring in a cockpit [ 35 ].
Improving our ability to detect conscious processing in non communicating patients remains a major goal of clinical cognitive neurosciences. In this perspective, several functional brain imaging tools are currently under development. Bedside cognitive event-relatedpotentials (ERPs) derived from the EEG signal are a good candidate to explore consciousness in these patients because: 1) they have an optimal time resolution within the millisecond range able to monitor the stream of consciousness, 2) they are fully non-invasive and relatively cheap, 3) they can be recorded continuously on dedicated individual systems to monitor consciousness and to communicate with patients, 4) and they can be used to enrich patients’ autonomy through brain-computer interfaces. We recently designed an original auditory rule extraction ERP test that evaluates cerebral responses to violations of temporal regularities that are either local in time, or global across several seconds. Local violations led to an early response in auditory cortex, independent of attention or the presence of a concurrent visual task, while global violations led to a late and spatially distributed response that was only present when subjects were attentive and aware of the violations. In the present work, we report the results of this test in 65 successive recordings obtained at bedside from 49 non-communicating patients affected with various acute or chronic neurological disorders. At the individual level, we confirm the high specificity of the ‘global effect’: only conscious patients presented this proposed neural signature of conscious processing. Here, we also describe in details the respective neural responses elicited by violations of local and global auditory regularities, and we report two additional ERP effects related to stimuli expectancy and to task learning, and we discuss their relations to consciousness.
Keywords: biomedical signal processing, spatial filtering, asymptotical performance analysis, brain-computer interface, P300
In cognitive neuroscience, it is useful to explore brain activ- ity through evoked potentials (EP) or event-relatedpotentials (ERP) recorded by electro-encephalography (EEG), e.g. [1, 2]. For instance, ERPs allow to investigate i) the basic functional pathways through early ERPs or EPs as auditory, visual or so- matosensory networks, and ii) cognitive pathways through late ERPs which are more related to memory tasks, execution of attention and emotion. ERP experiments usually involve the presentation of several kinds of stimuli and suppose that there exists a typical spatio-temporal pattern which is time-locked to each kind of stimuli (also called events).
In this context, we aimed at exploring the prognosis value of such a brain activity diagnostic marker that we previously conceived as a specific signature of conscious access to the violation of auditory regularities: the event-related EEG global effect (GE) ( 17 ). More precisely, we designed an auditory paradigm to probe cerebral responses to violations of temporal regularities that are either local in time (intra-trial) or global across several seconds and several trials. Local regularity violations (local effect or LE) led to an early response in the auditory cortex, independent of attention or the presence of a concurrent visual task, whereas global violations led to a late and spatially distributed response. Interestingly, GE was found significant in each of the healthy controls who attended to the series and sounds and counted occurrences of global violations. However, in the absence of instructions, this effect was present exclusively in those subjects who could report the existence of violations of global regularities. We could detect the GE in individual subjects using fMRI and both scalp and intracerebral event-relatedpotentials (ERPs) ( 18 ), and more recently with pupillometry ( 19 ). Applied to DOC patients, our initial logic was to infer that the presence of such a signature of conscious access to a specific perceptual attribute (violations of global regularities) would, by definition, require the patient to be in a conscious state. In other terms, by probing conscious access to violations of the global regularity, we would indirectly probe conscious states
Keywords: EEG, ERP, ERS, ERSP, single-trial, baseline, additive model, multiplicative gain model
Electroencephalography and magnetoencephalography methods have become standard tools to study brain mechanisms. Different approaches have been used to unveil brain electrical activity in rela- tion to sensory, motor, or cognitive events using electrical potential variations recorded either at the scalp level or from intra-cranial electrodes. The study of changes of the ongoing electroencephalo- gram (EEG) in response to stimulation started with event-relatedpotentials (ERP) techniques, which relies on measuring the ampli- tude and latency of post-stimulus peaks in stimulus-locked EEG trial averages. The standard ERP model relies on the hypothe- sis that ERPs consist of stereotyped patterns of stimulus-locked electrical activity, superimposed onto an independent stationary stochastic EEG processes ( Basar and Dumermuth, 1982 ; Luck, 2005 ; Nunez and Srinivasan, 2006 ). In the ERP model, every single- trial contains a noisy version of the grand average ERP, and, when averaging trials, “stationary” or “non-time-locked” background EEG elements of the signal cancel out.
Federica Turi - Nathalie Gayraud – Maureen Clerc
Inria Sophia Antipolis-Mediterranée, Université Côte d'Azur France Introduction:
Brain Computer Interfaces (BCIs) based on visual evoked potentials (VEP)  allow to spell from a keyboard of flashing characters. Among VEP BCIs, code-modulated visual evoked potentials (c-VEPs) are designed for high-speed communication . In c-VEPs, all characters flash simultaneously. In particular, each character flashes according to a predefined 63-bit binary sequence (m-sequence), circular-shifted by a different time lag. For a given character, the m-sequence evokes a VEP in the electroencephalogram (EEG) of the subject , which can be used as a template. This template is obtained during a calibration phase at the beginning of each session. Then, the system outputs the desired character after a predefined number of repetitions by estimating its time lag with respect to the template. Our work avoids the calibration phase, by extracting from the VEP relative lags between successive characters, and predicting the full word using a dictionary.
As regards to aetiology (i.e., basilar thrombo- sis) and functional imaging findings (i.e., preser- vation of supra-tentorial areas), the diagnosis of a ‘total’ LIS, as described by Bauer et al. (1979) was considered. Cognitive auditory event-relatedpotentials which will be outlined in detail in the next section were therefore performed on the 25th, 39th and 49th day after brain insult to demonstrate the existence of voluntary brain responses in this patient. The patient was free of sedative and other centrally acting drugs for at least 24 h. Fourteen days after the last event- relatedpotentials’ recordings (63rd day after brain insult), the first sign of consciousness was observed (i.e., inconsistent but reproducible fin- ger movements). At that moment, she was trans- ferred to a rehabilitation centre where she made slow but progressive improvement. Her left arm power recovered from distal to proximal mus- cles, as has been noted in LIS patients (Richard et al., 1995). Head movements were possible and permitted communication. She smiled in an appropriate context. However, severe emotional lability such as pathologic laughing and crying (i.e., she displayed these emotions involuntarily, often in response to inappropriate stimuli) was noted. She was discharged home 12 months after stroke, fully dependent in all personal activities of daily living. Currently, she still lives with her parents.
Index Terms—Brain computer interface, P300-speller, xDAWN algorithm, spatial enhancement.
I. I NTRODUCTION
Brain-Computer Interfaces (BCI) enable direct communica- tion between the user’s brain and a computer by analysing brain activities measured with electroencephalogram (EEG) for example . Such human-computer interfaces provides a new non-muscular powerful channel for communicating with the external world. BCIs are thus suitable for people that are incapable of any motor functions: e.g., people with severe neuromuscular disorders or ‘locked-in’ syndrome patients , . Present-day BCIs determine the intent of the user from different electrophysiological signals: for instance, the user may control the modulation of some brain waves (e.g., mu or beta rhythms , ) or the BCI may exploit natural automatic responses of the brain to external stimuli (e.g., event-relatedpotentials , , , ). See  for a more detailed review of BCIs.
A recent study has shown a higher prevalence of the auditory P300 evoked by simple tones in an oddball paradigm in healthy subjects (HS) as compared to patients in VS and MCS, but this ERP component showed a low sensitivity to differentiate between VS/UWS and MCS patients ( Real et al., 2016 ). Given the characteristic of LIS patients of presenting with a sub-cortical lesion keeping full consciousness and intact or nearly intact cognitive abilities, it is possible to hypothesize that such a test would be useful for detecting quickly and efficiently the presence of consciousness in these patients. The availability of a reliable and rapid ERP test to differentiate patients with DOC and patients with LIS is an important matter. A previous study has shown that, on average, about two and a half months pass between onset of the brain lesion and diagnosis ( León-Carrión et al., 2002 ). A test, to be used in the context of an acute medical setting, should be sufficiently sensitive to detect signals of conscious mental processing and at the same time short enough to be used despite the fluctuations of vigilance and short attentional span of these patients.
Vowelling priming Semantic priming
The eventrelated potential (ERP) N400 wave is a negative deflection elicited by the brain as a reaction to semantically unexpected words in sentence contexts. Thus, for many languages, such as English, French, Mandarin Chinese, ..., it has been demonstrated that orthographic depth has measurable effects on the morphology, amplitude and time occurrence of this negative ERP component which reflects the interruption of ongoing sentence processing due to the existence of inappropriate words making it senseless [1,2]. In our project, we aim to determine whether semantic priming effects are found in the Arabic language.
The new method presented here is designed to track fluctuations in brain electromagnetic activity, for any given set of frequency bands. Our method has several original features. First, we introduce a methodology for defining a ref- erence set of Gabor time-frequency atoms, or ”template”, that is capable of modelling both low frequency event-relatedpotentials and high frequency oscil- lations. Second, the template is deformed across trials with nonlinear optimiza- tion, which permits to follow accurately the fluctuations of the actual signal and obtain a sparse final representation of the data. Third, the deformations for each trial are constrained using information arising from all the trials, which increases the robustness of the fit even for low SNRs.
This is the first study analyzing transient pattern-reversal VEPs with a single trial method (st-VEP) in episodic migraine patients.
We found that neural activation to single visual stimulus, as reflected by st-VEP, is greater in episodic migraine patients than in healthy subjects. When we searched with MRI for the anatomo-functional correlate of this electrophysiological measure in terms of gray matter volume and functional connectivity, we found that mean st-VEP amplitude was proportional to gray matter volume in the visual cortex as in cerebral areas belonging to the ventral attention network ( 34 ). Indeed, the hub of this network, the right temporo-parietal junction (rTPJ), showed the strongest correlation regarding gray matter volume, and displayed significant functional connections with most of the other st-VEP correlated brain regions. Taken together, these results suggest that cerebral responsiveness to visual stimuli, as assessed with st-VEP, is exaggerated in interictal migraine, and correlated with gray matter volume, not only in the visual cortex, but also in a series of functionally connected brain areas involved in attention control.
oriented toward the selection of stimulus types which has to be done across the whole set of voxelwise regressions (Sec- tion 2). The proposed approach is carried out in a variational EM framework (Section 3), which offers a faster alternative to intensive stochastic procedures as used in . Simulated experiments confirm the ability of our model to select the rel- evant conditions while real fMRI data illustrate an enhanced determination of activated brain regions (Section 4). The pro- posed model is referred to as the parsimonious model while the model in which all stimulus types are included is referred to as the complete model.
mutation of FMR1 and the learning disability often found in FXS, are likely to be associated with impairments in mechanisms of information processing (Belmonte & Bourgeron, 2006). Early sensory and cognitive processing can be non-invasively investigated using the EventRelated Potential (ERP) technique that records local field potentials, which are summarized postsynaptic potentials from large groups of neurons (Luck, 2005). Studies investigating ERPs in FXS so far exclusively used oddball paradigms and mostly studied auditory ERPs (Castrèn et al., 2003; St Clair et al., 1987; M. J. Van der Molen et al., 2012a, 2012b). Auditory N1 amplitude has been found to be enhanced in FXS (Castrèn et al., 2003; Rojas et al., 2001; St Clair et al., 1987; M. J. Van der Molen et al., 2012a, 2012b). Results concerning auditory P2 amplitude are inconsistent; in two studies an enhanced P2 amplitude was reported (St Clair et al., 1987; M. J. Van der Molen et al., 2012b), whereas in another study no difference between FXS patients and control group was detected (M. J. Van der Molen et al., 2012a). Mismatch- negativity (MMN) amplitude was found to be decreased (M. J. Van der Molen et al., 2012b), whereas results for auditory N2 amplitude were again inconsistent, stating an increase in amplitude in FXS (M. J. Van der Molen et al., 2012a, 2012b) or no difference between FXS and controls (St Clair et al., 1987). Finally, auditory P3 amplitude was consistently found to be decreased in FXS compared to healthy controls (St Clair et al., 1987; M. J. Van der Molen et al., 2012a, 2012b). Concerning latency, auditory N2 and P3 latency appeared to be increased in FXS patients compared to healthy controls (St Clair et al., 1987; M. J. Van der Molen et al., 2012a, 2012b), whereas no differences were detected in auditory N1, P2 and MMN latency (Castrèn et al., 2003; Rojas et al., 2001; St Clair et al., 1987; M. J. Van der Molen et al., 2012a, 2012b). Only one study investigated visual ERPs in FXS thus far, also using an oddball paradigm and showing an increase in N1 and N2 amplitude and a decrease in P3 amplitude, but no differences in latencies and P2 amplitude (M. J. Van der Molen et al., 2012a). According to this study, the alterations found in stimulus processing in FXS vary across modalities, with the auditory modality being more affected than the visual modality. This seems to match modality differences in performance found in FXS (Sullivan et al., 2007; M. J. W. Van der Molen et al., 2010).
Compositionality is investigated as applied to sentence structure—objects fill argument positions in pred- icates that combine to form sentential meaning—and decompose such into independent constituents. Recent work has identified brain regions correlated with compositionality that may not be decomposable using a task called complement coercion ( Pylkk¨ anen et al. , 2011 ). Subjects were presented with sentences whose meanings were partly implied rather than fully expressed overtly through surface constituents. For example, the sentence The boy finished the pizza is understood as meaning that the pizza was eaten, even though the verb eat does not appear anywhere in the sentence ( Pustejovsky , 1995 ). The presence of pizza, belonging to the category food, coerces the interpretation of finish as finish eating. By contrast, He finished the newspa- per induces the interpretation finish reading. Because syntactic structure in this prior experiment was held constant, the assumption was that coercion reflects incorporation of extra information in the result that is absent in the constituents. Brain activity measured using magnetoencephalography (MEG) showed activity related to coercion in the anterior midline field. This result suggests that there may be some regions that do not exhibit decomposable brain activity but does not rule out the possibility that there are other regions that do.