Event related potentials recorded in patients with locked-in syndrome

A review on the performance of brain-computer interface systems used for patients with locked-in and completely locked-in syndrome

Patients with locked-in syndrome (LIS) and complete locked-in syndrome (CLIS) own a fully functional brain restricted within a non-functional body. In order to help LIS patients stay connected with their surroundings, brain-computer interfaces (BCIs) and related technologies have emerged. BCIs translate brain activity into actions that can be performed by external devices enabling LIS patients to communicate, leading to an increase in their quality of life. The past decade has seen the rapid development of BCIs that have the potential to be used for patients with locked-in syndrome, from which a great deal is tested only on healthy subjects and not on actual patients. This study aims to (1) provide the readers with a comprehensive study that contributes to this growing area of research by exploring the performance of BCIs tested specifically on LIS and CLIS patients, (2) give an overview of different modalities and paradigms used in different stages of the locked-in syndrome, and (3) discuss the contributions and limitations of BCIs introduced for the LIS and CLIS patients in the state-of-the-art and lay a groundwork for researchers interested in this field.

Breaching Subjects' Thoughts Privacy: A Study with Visual Stimuli and Brain-Computer Interfaces

Brain-computer interfaces (BCIs) started being used in clinical scenarios, reaching nowadays new fields such as entertainment or learning. Using BCIs, neuronal activity can be monitored for various purposes, with the study of the central nervous system response to certain stimuli being one of them, being the case of evoked potentials. However, due to the sensitivity of these data, the transmissions must be protected, with blockchain being an interesting approach to ensure the integrity of the data. This work focuses on the visual sense, and its relationship with the P300 evoked potential, where several open challenges related to the privacy of subjects' information and thoughts appear when using BCI. The first and most important challenge is whether it would be possible to extract sensitive information from evoked potentials. This aspect becomes even more challenging and dangerous if the stimuli are generated when the subject is not aware or conscious that they have occurred. There is an important gap in this regard in the literature, with only one work existing dealing with subliminal stimuli and BCI and having an unclear methodology and experiment setup. As a contribution of this paper, a series of experiments, five in total, have been created to study the impact of visual stimuli on the brain tangibly. These experiments have been applied to a heterogeneous group of ten subjects. The experiments show familiar visual stimuli and gradually reduce the sampling time of known images, from supraliminal to subliminal. The study showed that supraliminal visual stimuli produced P300 potentials about 50% of the time on average across all subjects. Reducing the sample time between images degraded the attack, while the impact of subliminal stimuli was not confirmed. Additionally, younger subjects generally presented a shorter response latency. This work corroborates that subjects' sensitive data can be extracted using visual stimuli and P300.

Impact of putamen stroke on task context updating: Evidence from P300 brain waves

According to the context updating theory, the oddball P300 component indexes brain activities underlying revision of the mental representation induced by incoming stimuli. It involves an attention-driven comparison process that evaluates the representation of the previous event in working memory. Delayed latencies have been reported for various types of stroke such as unilateral thalamic stroke. We investigated memory updating effects in patients with putamen stroke. Two groups of patients with putamen and thalamic stroke were recruited along with two control groups of young and old healthy participants. Auditory and visual P300 were elicited respectively in a two-stimulus oddball paradigm. The auditory P300 peak latencies were significantly longer in patients with a putamen lesion than in the aged and young control groups and the same pattern was found in the thalamus-lesioned patient. The delayed auditory P300 component in both patient groups but neither control group suggests impairment of memory updating in patients with putamen stroke comparable with thalamic stroke. Our study illustrates the important role of subcortical structures subserved in context updating.

The role of P300 event-related potentials in the cognitive recovery after the stroke

The aim of this study was to elucidate the effects of an ischemic stroke on the amplitude and latency of the P300 wave and evaluate their changes over a prospective 1-year follow-up period. We recorded the P300 wave using an auditory oddball paradigm in 60 consecutive brain infarct patients at baseline (i.e., within 4 weeks after the stroke), after 3 months, after 12 months and in 30 healthy control subjects. The P300 latencies in stroke patients were significantly longer and the P300 amplitudes were significantly smaller than those of the control group. The latency of P300 showed a highly significant average improvement 12 months after the stroke compared to the baseline. There was no significant change observed for the P300 amplitude during the same period. The P3 latency is initially more increased in the patients with hemispheric brain infarction but shows a better recovery compared to the patients with brainstem infarction. Also, the results of the P300 latency of patients with the left-sided lesions was significantly longer compared to the patients with right-sided lesions on the beginning of the study but not 3 and 12 months after the stroke. The results of our study show the importance of P300 event-related potentials in the detection and follow-up of cognitive changes after ischemic stroke.

Inter-individual variability of oscillatory responses to subject's own name. A single-subject analysis

In previous studies event-related potentials and oscillations in response to subject's own name have been analyzed extensively on group-level in healthy subjects and in patients with a disorder of consciousness. Subject's own name as a deviant produces a P3. With equiprobable stimuli, non-phase-locked alpha oscillations are smaller in response to subject's own name compared to other names or subject's own name backwards. However, little is known about replicability on a single-subject level. Seventeen healthy subjects were assessed in an own-name paradigm with equiprobable stimuli of subject's own name, another name, and subject's own name backwards. Event-related potentials and non-phase locked oscillations were analyzed with single-subject, non-parametric statistics. No consistent results were found either for ERPs or for the non-phase locked changes of oscillatory activities. Only 4 subjects showed a robust effect as expected, that is, a lower activity in the alpha-beta range to subject's own name compared to other conditions. Four subjects elicited a higher activity for subject's own name. Thus, analyzing the EEG reactivity in the own-name paradigm with equiprobable stimuli on a single-subject level yields a high variance between subjects. In future research, single-subject statistics should be applied for examining the validity of physiologic measurements in other paradigms and for examining the pattern of reactivity in patients.

Improving the clinical assessment of consciousness with advances in electrophysiological and neuroimaging techniques

In clinical neurology, a comprehensive understanding of consciousness has been regarded as an abstract concept - best left to philosophers. However, times are changing and the need to clinically assess consciousness is increasingly becoming a real-world, practical challenge. Current methods for evaluating altered levels of consciousness are highly reliant on either behavioural measures or anatomical imaging. While these methods have some utility, estimates of misdiagnosis are worrisome (as high as 43%) - clearly this is a major clinical problem. The solution must involve objective, physiologically based measures that do not rely on behaviour. This paper reviews recent advances in physiologically based measures that enable better evaluation of consciousness states (coma, vegetative state, minimally conscious state, and locked in syndrome). Based on the evidence to-date, electroencephalographic and neuroimaging based assessments of consciousness provide valuable information for evaluation of residual function, formation of differential diagnoses, and estimation of prognosis.

Transient post-traumatic locked-in syndrome: a case report and a literature review

INTRODUCTION

Post-traumatic locked-in syndrome may be particularly difficult to recognize, especially when it follows a state of coma and presents the clinical feature of a "total" locked-in syndrome.

PATIENT AND METHODS

A 56-year-old male with a closed head injury was admitted in intensive care unit (ICU) with GCS=4 (V1, M2, E1). Computed tomography (CT) scan disclosed a limited subarachnoid haemorrhage in the sylvian region without any brain oedema or ventricular shift. The GCS did not change until day 6. At the same time EEG showed a reactivity to acoustic stimuli consisting in the paradoxical appearance of a posterior rhythm in alpha range (10-12c/s), blocked by passive eye opening. Early cortical components (N20-P25) of somatosensory evoked potentials were normal on both hemispheres; middle components were also clearly evident. Magnetic resonance imaging of the brain showed both diffuse and midbrain axonal injuries, particularly in a strategic lesion involving both cerebral peduncles. Event related potentials showed N2 and P3 components to stimulation by rare tones.

CONCLUSIONS

A comprehensive multimodal neurophysiological approach, using the more informative tests and the proper time of recording, should be included in protocols for patients with severe head trauma, in order to establish the actual patient's clinical state and to avoid that a locked-in syndrome state be mistaken for prolonged coma, vegetative state, minimally conscious state or akinetic mutism. Neurophysiological evaluation before discharge from ICU can be a baseline evaluation useful for the follow-up of low-responsive patients in the neuro-rehabilitation unit.

Multimodal neuroimaging approaches to disorders of consciousness

Advances in neuroimaging techniques hold significant promise for improving understanding of disorders of consciousness arising from severe brain injuries. We review neuroimaging studies of the vegetative state (VS) and minimally conscious state (MCS), and findings in an unusual case of late emergence from MCS. Multimodal neuroimaging studies using positron emission tomography techniques, functional magnetic resonance imaging, and quantitative electroencephalography and magnetoencephalography quantify variations of residual cerebral activity across these patient populations. The results suggest models to distinguish the pathophysiologic basis of VS and MCS. Less clear are potential brain mechanisms underlying late recovery of communication in rare MCS patients. Diffusion tensor magnetic resonance imaging studies and recent experimental findings suggest that structural remodeling of the brain following severe injury may play a role in late functional recoveries. More generally, relatively long time courses of recovery following severe brain injury emphasize the need to develop markers for identifying patients who may harbor potential for further meaningful recovery. Introduction of neuroimaging into the clinical evaluation process will require developing frameworks for longitudinal assessments of cerebral function. Although limited in number, available studies already provide important insights into underlying brain mechanisms that may help guide development of such assessment strategies.

P300-based brain computer interface: Reliability and performance in healthy and paralysed participants

OBJECTIVE

This study aimed to describe the use of the P300 event-related potential as a control signal in a brain computer interface (BCI) for healthy and paralysed participants.

METHODS

The experimental device used the P300 wave to control the movement of an object on a graphical interface. Visual stimuli, consisting of four arrows (up, right, down, left) were randomly presented in peripheral positions on the screen. Participants were instructed to recognize only the arrow indicating a specific direction for an object to move. P300 epochs, synchronized with the stimulus, were analyzed on-line via Independent Component Analysis (ICA) with subsequent feature extraction and classification by using a neural network.

RESULTS

We tested the reliability and the performance of the system in real-time. The system needed a short training period to allow task completion and reached good performance. Nonetheless, severely impaired patients had lower performance than healthy participants.

CONCLUSIONS

The proposed system is effective for use with healthy participants, whereas further research is needed before it can be used with locked-in syndrome patients.

SIGNIFICANCE

The P300-based BCI described can reliably control, in 'real time', the motion of a cursor on a graphical interface, and no time-consuming training is needed in order to test possible applications for motor-impaired patients.

Apallic syndrome is not apallic: is vegetative state vegetative?

Initial conceptualisation about the nature of vegetative state (VS) assumed at least temporary loss of the entire cortical functioning. Since a broad range of stimulus-related cortical activations was demonstrated in VS patients, this simplified idea is not tenable any longer, but no alternative concept emerges instead. Two recent hypotheses, empirically testable and well grounded, could fill this vacuum: (1) In VS, isolated cortical areas may work, but their integration into a distributed network is lacking. (2) In VS, complex stimulus processing is limited to primary sensory and motor areas; the co-ordination between them and the secondary and tertiary areas is lacking. To test these hypotheses, we estimated the frequency of occurrence of late event-related potential components P3 and N400, presumably indicating activity of complex distributed networks including high-level sensory and associative areas. Both components occurred in VS with above-chance frequencies, but less frequently than in two control groups. Besides these frequent normal brain activations, some VS patients exhibit highly significant but abnormal activations, whose functional meaning remains unclear. A methodological analysis leads to the conclusion that any neurophysiological assessment of VS patients is biased toward under-, rather than over-estimation, of their remaining information processing abilities.

The locked-in syndrome : what is it like to be conscious but paralyzed and voiceless?

The locked-in syndrome (pseudocoma) describes patients who are awake and conscious but selectively deefferented, i.e., have no means of producing speech, limb or facial movements. Acute ventral pontine lesions are its most common cause. People with such brainstem lesions often remain comatose for some days or weeks, needing artificial respiration and then gradually wake up, but remaining paralyzed and voiceless, superficially resembling patients in a vegetative state or akinetic mutism. In acute locked-in syndrome (LIS), eye-coded communication and evaluation of cognitive and emotional functioning is very limited because vigilance is fluctuating and eye movements may be inconsistent, very small, and easily exhausted. It has been shown that more than half of the time it is the family and not the physician who first realized that the patient was aware. Distressingly, recent studies reported that the diagnosis of LIS on average takes over 2.5 months. In some cases it took 4-6 years before aware and sensitive patients, locked in an immobile body, were recognized as being conscious. Once a LIS patient becomes medically stable, and given appropriate medical care, life expectancy increases to several decades. Even if the chances of good motor recovery are very limited, existing eye-controlled, computer-based communication technology currently allow the patient to control his environment, use a word processor coupled to a speech synthesizer, and access the worldwide net. Healthy individuals and medical professionals sometimes assume that the quality of life of an LIS patient is so poor that it is not worth living. On the contrary, chronic LIS patients typically self-report meaningful quality of life and their demand for euthanasia is surprisingly infrequent. Biased clinicians might provide less aggressive medical treatment and influence the family in inappropriate ways. It is important to stress that only the medically stabilized, informed LIS patient is competent to consent to or refuse life-sustaining treatment. Patients suffering from LIS should not be denied the right to die - and to die with dignity - but also, and more importantly, they should not be denied the right to live - and to live with dignity and the best possible revalidation, and pain and symptom management. In our opinion, there is an urgent need for a renewed ethical and medicolegal framework for our care of locked-in patients.

Clinical applications of event-related potentials in brain injury

ERPs may extend the battery of neurophysiologic tests currently available for determining the functional integrity of the central nervous system and the capacity of cognition in patients with brain injury. The use of stimuli relevant for the patient can enhance the probability to record these waves in unconscious patients and in patients with cognitive impairment and enhance the predictive value on outcome. The experimental data in these patients still are not sufficient, however, to standardize the indications of ERPs in clinical practice. Their limitations, mainly the variability also present in normal individuals and the limited standardization and validation, must be considered, and they must be judged cautiously as a prognostic index. Nevertheless, ERPs might be applied as a useful supplement to neuropsychologic assessment.

Cognitive processing in completely paralyzed patients with amyotrophic lateral sclerosis

Cognitive functions have never been studied before in completely paralyzed patients, probably due to the lack of the appropriate method. In this study, three male patients in the last stage of amyotrophic lateral sclerosis (ALS) are examined. They were totally locked-in, i.e., incapable of any voluntary response including eye movements. Given this complete lack of motor expression, the method of event-related brain potentials (ERPs) was used. The ERP recording was carried out at the patients' bedside. In one patient indications of preserved cognitive abilities including adequate language comprehension were obtained. The data of the second patient also demonstrate a high degree of intactness of complex cognitive functions, but some ERP phenomena were consistently abnormal, which may be interpreted as a consequence of disease-related degenerative processes. In the third patient no electrocortical response which might indicate higher cortical processing was found. This first ERP study of totally paralyzed patients shows that some of them can possess high, perhaps even normal, information processing capacity after a long period of severe immobility and artificial ventilation.

Event-related brain potentials in a patient with akinetic mutism

The clinical pattern of complete akinetic mutism (AM) was observed in a patient with a bilateral infarction of the anterior cerebral arteries extending to the rostral cingulate cortex and the supplementary motor areas. Since the patient was unable to produce any overt response, event-related brain potentials (ERPs) were used to obtain information about cortical processing of stimuli. Oddball tasks with simple acoustical stimuli and semantic categories were used. Verbal processing was further assessed by comparing event-related potentials to words compatible versus incompatible to the semantic context. Although the pattern of cortical responses was abnormal, differential responses were clearly obtained to semantically different word classes. Thus, the hypothesis about cortical non-responsivity of AM patients, drawn from several previous reports, was not supported. An ERP examination in AM patients can deliver information about their mental state, provided that the stimuli and tasks possess a wide range of informational complexity and motivational value.

[Traumatic brain injuries in adults: from coma to wakefulness. Neurophysiological data]

OBJECTIVE

To analyse relevant literature and to express an expert point of view concerning the interest of electroencephalography and evoked potentials recordings in the evaluation of severe head trauma in adults in the context of a consensus conference.

MATERIAL AND METHODS

Scientific databases have been checked on the Internet using key-words. The summaries of 340 papers have checked out. Consequently 94 papers have been thoroughly analysed. Fifty-nine of them are cited in the text of this paper.

RESULTS

Electroencephalography (EEG) and evoked potentials (Eps) evaluate the functional status of the brain. They augment the clinical examination. They are non invasive and easy to perform at patient's bedside. The EEG evaluate globally the functional status of the brain but it is very sensitive to sedative and anaesthetic drugs. It can disclose subclinical or electroclinical epileptic seizures. When reactivity to sensory stimulations can be elicited, this can be considered a prognostic indicator for a good outcome. Evoked potentials are less influenced by sedative drugs. There are several types of evoked potentials, each one with a different localizing value. Brainstem auditory evoked potentials (or short-latency Eps) evaluate the auditory nerve and brainstem. When normal they have no specificity. When abnormal they are an indicator of a poor or bad outcome. Somatosensory and auditory middle-latency Eps evaluate the primary cortex. In coma due to traumatic brain injury the presence of primary cortex components is an indicator of a good outcome and its absence is an indicator of a poor outcome at least when there is no focal brain lesion as to have the primary cortex component to be absent. Event-related potentials evaluate associative brain areas. When they are present in a comatose patient they favor the idea that some cognitive processes are active and they have a high positive predictive value for a return to consciousness. The electrophysiological evaluation can help to identify atypical situations and pathologies close to coma, disclose nonconvulsive seizures and localize certain complications or dysfunctions in atypical cases.

Donepezil versus vitamin E in Alzheimer's disease: Part 2: mild versus moderate-severe Alzheimer's disease

SUMMARY

Early studies showed that the latency of P300 (P3) event related potential increases or diminishes when anticholinergic or cholinergic drugs are administered. We tested the hypothesis that new cholinesterase inhibitors like Donepezil (DPZ) may have an effect on the often abnormal P300 of patients with Alzheimer's Disease (AD), and therefore, that P300 recordings might simplify the evaluation of responses to cholinesterase inhibitor in patients with mild and moderate-severe AD. We evaluated 60 patients with AD: 30 patients with "mild" (Mini Mental State Examination 26-19) and 30 patients with "moderate-severe" (Mini Mental State Examination 18-10), according to the National Institute of Neurological and Communicative Disorders and Alzheimer's Disease and Related Disorders Association criteria in comparison with 40 age-matched controls. All subjects underwent P300 recordings and neuropsychologic examinations (Alzheimer's Disease Assessment Scale-Cognition and Wechsler Adult Intelligence Scale) during the 6-month follow-up. Patients were divided into four groups of 15 patients each: Group I DPZ (10 mg/day) and Group I Vitamin E (2000 IU/day) with "mild" AD; Group II DPZ and Group II Vitamin E with "moderate-severe" AD and same drug dosages. In patients treated with Vitamin E, we observed P3 latency increments (delta) by 11.8 +/- 1.8 ms in Group I and by 12.8 +/- 2.8 ms in Group II at 6 months; neuropsychologic test scores significantly worsened at 6 months (p < 0.001) in Group II patients. Donepezil induced significant P3 latency reductions (11.2 +/- 2.4 ms) in nine patients of Group I and all patients of Group II (16.1 +/- 4.0 ms), reaching a maximum at 3 months (23.2 +/- 2.7 ms). Alzheimer's Disease Assessment Scale-Cognition and Wechsler Adult Intelligence Scale scores improved during the same period, and the difference between Vitamin E and DPZ treated patients was highly significant for P3 (analysis of variance) and for P3-Alzheimer's Diseases Assessment Scale-Cognition (analysis of covariance) with p < 0.001 for pooled groups of patients with AD and Group II (DPZ) versus Group II (Vitamin E). Combined P3 event related potentials measurements, neuropsychologic test comparison evidences significant effects of DPZ in mild and in moderate-severe AD.

Age-related changes of evoked potentials

The aim of this review is to analyse the current state of our knowledge on evoked potentials (EPs) in ageing and to report some conclusions on the relation between EPs and elder age. Evoked potentials provide a measure of the function of sensory systems that change during the different stages of life. Each sensory system has its own time of maturation. The individuation of the exact period of life when brain ageing starts is difficult to define. Normally, the amplitude of EPs decreases, and their latency increases from adult to elder life. Many authors speculate that these modifications might depend on neuronal loss, changes in cell membrane, composition or senile plaques present in older patients, but there is no evidence that these changes might modify the cerebral function in healthy aged individuals. This review emphasises some incongruities present in different studies confirmed by daily neurophysiologic practice. Different techniques as event-related desynchronization (ERD), contingent negative variation (CNV) and Bereitschaftspotential, are available to study central neuronal changes in normal and pathologic ageing.

Event-related potentials in patients with total locked-in state due to fulminant Guillain-Barré syndrome

A series of electrophysiological investigations were performed over a 6-month period in two patients affected by fulminant Guillain-Barré polyradiculoneuropathy, who developed an ascending paralysis leading, within 72 h, to flaccid quadriplegia, internal and external ophthalmoplegia, absence of all brainstem reflexes and no respiratory effort: the clinical state resembled brain death. Brain CTs were normal and spinal fluid examination revealed albuminocytological dissociation. All motor nerves tested were unexcitable, whereas sensory responses were markedly abnormal but present. Sequential EEG recordings revealed normal, partially reactive alpha rhythm in both patients. In one patient, normal auditory event-related potentials (ERPs: peak N1, P2, N2, P3, evoked in an 'oddball' paradigm) and CNV-like potentials could be recorded not earlier than the 20th day into the illness. In earlier recordings, N1 and P2 peaks as well as mismatch negativity (MMN) were present over the frontal and central scalp electrodes. This patient has now partially recovered motor functions and no cognitive defects are present, but he has little recollection of the events occurring in the first 2 weeks spent in the ICU, when he was completely paralyzed. The other patient generated normal N1 and P2 ERP peaks, but no N2, P3 and MMN were detected in a series of recordings. He died without having ever regained appropriate behavioral responses. The ERP abnormalities observed raise the matter of the origin of cognitive dysfunction in patients with severe and prolonged de-efferentation/de-afferentation. ERPs allow monitoring the level of alertness and attention and appear more specific than EEG in identifying a state of awareness in patients in which communication is severely impaired as a consequence of neurological disorders.

Medical technology assessment EEG and evoked potentials in the intensive care unit

We review the principal aspects of EEG and evoked potential (EP) neuromonitoring in the intensive care unit. The electrophysiological methods allow functional assessment of comatose patients and can be used (a) as a help to diagnose the origin of coma, (b) as a means to predict outcome, and (c) for monitoring purposes. The combination of the EEG and long-, middle-, and short-latency EPs allows widespread assessment of the cerebral cortex, the brain-stem, and the spinal cord. The EEG and the EP interpretation first requires taking into account non-neurological factors that may interfere with the recorded activities (sensory pathologies, toxic or metabolic problems, body temperature). The sensitivity and the specificity of any neurophysiological technique depend on the etiology of coma. Anoxic comas are associated with a predominantly cortical involvement, while the cortical and brain-stem functions are to be taken into account to interpret the EEG and the EPs in head trauma. The EEG and the EPs can be used to differentiate the comas due to structural lesions from those of metabolic origin, to confirm brain death and help to diagnose psychogenic unresponsiveness or a de-efferented state. While the prognostic value of the EEG is markedly hampered by the widespread use of sedative drugs, it has been possible to design efficient systems based on early- and middle-latency multimodality evoked potentials in anoxic and traumatic comas and, more generally, in all comas associated with an increase of the intracranial pressure. Continuous neuromonitoring techniques are currently under development. They have already been proven useful for the early detection and for the prevention of subclinical seizures, transtentorial herniation, vasospasm, and other causes of brain or spinal-cord ischemia.