How clinical neurophysiology may contribute to the understanding of a psychiatric disease such as schizophrenia.

INTRODUCTION: The increasing knowledge about anatomical structures and cellular processes underlying psychiatric disorders may help bridge the gap between clinical manifestations and basic physiological processes. Accordingly, important insights have been brought these last years into a main psychiatric affection, i.e. schizophrenia. MATERIAL AND METHODS: Here we reviewed and described, by comparison to healthy people, different physiological parameters - oculomotor measures, startle response, and cognitive event related potentials, which are altered in schizophrenia, in order to link these physiological parameters to dysfunctional cognitive processes and specific clinical symptoms. RESULTS: Schizophrenic patients displayed: (1) abnormalities in smooth pursuit eye movements and saccadic inhibition during antisaccade tasks that may stem from the same prefrontal "inhibitory" cortical dysfunction; (2) deficits in prepulse inhibition and facilitation suggesting disturbed attentional modulations, which seem also correlated to abnormal patterns of prefrontal activation; and (3) decreased amplitude for cognitive ERP situated all along the continuum of the information processing, suggesting that schizophrenia shows neurophysiological deficits since the level of the sensory cortex and not only disturbances involving associative cortices and limbic structures. DISCUSSION: The heterogeneity of schizophrenic disorders regarding symptomatology, course, and outcome is underlain by various pathophysiological processes that physiological parameters may help define. These alterations may be related to precise cognitive processes that are easily neurophysiologically monitored in order to create more homogeneous subgroups of schizophrenic patients.

Recommendations for the clinical use of somatosensory-evoked potentials.

The International Federation of Clinical Neurophysiology (IFCN) is in the process of updating its Recommendations for clinical practice published in 1999. These new recommendations dedicated to somatosensory-evoked potentials (SEPs) update the methodological aspects and general clinical applications of standard SEPs, and introduce new sections dedicated to the anatomical-functional organization of the somatosensory system and to special clinical applications, such as intraoperative monitoring, recordings in the intensive care unit, pain-related evoked potentials, and trigeminal and pudendal SEPs. Standard SEPs have gained an established role in the health system, and the special clinical applications we describe here are drawing increasing interest. However, to prove clinically useful each of them requires a dedicated knowledge, both technical and pathophysiological. In this article we give technical advice, report normative values, and discuss clinical applications.

Lateralisation of intranasal trigeminal chemosensory event-related potentials.

AIM OF THE STUDY: To determine whether or not chemosensory event-related brain potentials (CSERP) elicited by nociceptive unilateral intranasal (CO2) trigeminal stimulation are lateralized and, if they are, whether this hemispheric lateralization is related to the side of the stimulated nostril. METHODS: Nine healthy right-handed subjects participated to the study. CSERPs were recorded after left or right monorhinal CO2 stimulation. Latency and baseline-to-peak amplitude of each CSERP component were compared across stimulation conditions (left and right nostril), scalp locations (lower-frontal, frontal, mid-temporal, central, posterior-temporal, parietal) and hemispheres (left or right), using a three-factor analysis of variance (ANOVA) for repeated measures. RESULTS: Intranasal trigeminal CO2 stimulation elicited a large N400-P550 complex. This complex was preceded by an earlier N300 component. Whatever the stimulated nostril, N300, N400 and P550 amplitudes were significantly higher on the right as compared to the left hemisphere, at lower-frontal recording sites. The side of chemosensory stimulation (left or right nostril) did not significantly affect CSERP components. CONCLUSIONS: This study showed that in healthy right-handed volunteers with normal olfactory ability, intranasal chemosensory trigeminal stimulation may elicit a series of event-related brain potentials, which all display a significant right-hemisphere predominance, irrespective of the stimulated nostril. The observed lateralization was maximal at lower-frontal recording sites.

The EEG assessment of low-grade hepatic encephalopathy: comparison of an artificial neural network-expert system (ANNES) based evaluation with visual EEG readings and EEG spectral analysis.

OBJECTIVE: The EEG provides an objective staging of hepatic encephalopathy (HE), but its interpretation may be biased by inter-observer variability. This study aims at comparing an entirely automatic EEG classification of HE based on an artificial neural network-expert system procedure (ANNES) with visual and spectral analysis based EEG classifications. METHODS: Two hundred and thirty-eight consecutive cirrhotic patients underwent closed-eye EEG. They were followed up for up to one-year to detect bouts of overt HE and death. The EEG was classified by ANNES, qualitative visual reading, main basic rhythm frequency and spectral analysis. The classifications were assessed on the basis of: (i) match with liver function, (ii) prognostic value and (iii) repeatability. RESULTS: All classifications were found to be related to the severity of liver failure, with cognitive findings and a history of previous bouts of HE. All of them had prognostic value on the occurrence of overt HE and on survival. The ANNES based classification was more repeatable than the qualitative visual one, and had the advantage of detecting low power EEG, but its efficiency in analyzing low-grade alterations was questionable. CONCLUSIONS: An entirely automatic - ANNES based - EEG classification of HE can improve the repeatability of EEG assessment, but further improvement of the device is required to classify mild alterations. SIGNIFICANCE: The ANNES based EEG grading of HE needs further improvements to be recommended in clinical practice, but it is already sufficient for detecting normal and clearly altered EEG tracings.

Assessment of olfactory and trigeminal function using chemosensory event-related potentials.

GOALS: To give an overview on the theoretical and practical applications of chemosensory event-related potentials. METHODS: Chemosensory event-related potentials (ERPs) may be elicited by brief and precisely defined odorous stimuli. Based on the principles of air-dilution olfactometry, a stimulator was developed in the late 1970s, which allows stimulation of the olfactory neuroepithelium and the nasal mucosa with no concomitant mechanical stimulation. Chemosensory ERPs were obtained after stimulation of the olfactory nerve (olfactory ERPs) or the trigeminal nerve (somatosensory or trigeminal ERPs). The characteristics of the stimulator for chemosensory research as well as the variables influencing the responses are discussed in this paper. RESULTS: Implementation and normative data from our department are reported with different clinical examples from otorhinolaryngologic clinic. The bulk of the evoked response consists of a large negative component (often referred to as N1), which occurs between 320 and 450 ms after stimulus onset. This component is followed by a large positive component, often referred to as P2, occurring between 530 and 800 ms after stimulus onset. Absence of olfactory ERPs and presence (even with subtle changes) of somatosensory ERPs is a strong indicator of the presence of an olfactory dysfunction. CONCLUSIONS: This review examines and discusses the methods of chemosensory stimulation as well as the electrophysiological correlates elicited by such stimuli. The clinical applications of chemosensory ERPs in neurology and otorhinolaryngology are outlined.

Analysis of EEG tracings in frequency and time domain in hepatic encephalopathy.

Spectral EEG analysis in hepatic encephalopathy (HE) is usually performed disregarding the effect of epoch length, statistical errors and equipment noise. A study on these items was carried out. In addition, spectral analysis and a new analysis, performed in time domain, were compared in the assessment of HE. The EEG tracings of 73 cirrhotic patients with HE were analyzed. Artifact-free periods of about 1 min were selected. Equipment noise was measured by short-circuiting all the electrodes. The equipment noise was notable below 1.5 Hz; the best epoch length was 4s and the statistical errors were minimal for the band with the highest relative power. Nineteen per cent of the tracings were unstable. The spectral values were found to be related to liver function and to the degree of HE, whereas the relationship with psychometric variables was poor. The indexes computed by time-domain analysis were found to be better related to psychometric findings. We have provided information on the optimisation of spectral EEG analysis and presented a time-domain analysis giving results related to psychometric tests and liver function.

Event-related potentials (ERPs) in ecstasy (MDMA) users during a visual oddball task.

Ecstasy is the common name for a drug mainly containing a substance identified as 3,4-methylenedioxymethamphetamine (MDMA). It has become popular with participants in "raves", because it enhances energy, endurance and sexual arousal, together with the widespread belief that MDMA is a safe drug [Byard, R.W., Gilbert, J., James, R., Lokan, R.J., 1998. Amphetamine derivative fatalities in South Australia. Is "ecstasy" the culprit? Am. J. Forensic Med. Pathol. 19, 261-265]. However, it is suggested that this drug causes a neurotoxicity to the serotonergic system that could lead to permanent physical and cognitive problems. In order to investigate this issue, and during an ERP recording with 32 channels, we used a visual oddball design, in which subjects (14 MDMA abusers and 14 paired normal controls) saw frequent stimuli (neutral faces) while they had to detect as quickly as possible rare stimuli with happy or fearful expression. At a behavioral level, MDMA users imply longer latencies than normal controls to detect rare stimuli. At the neurophysiological level, ERP data suggest as main result that the N200 component, which is involved in attention orienting associated to the detection of stimulus novelty (e.g. [Campanella, S., Gaspard, C., Debatisse, D., Bruyer, R., Crommelinck, M., Guerit, J.M., 2002. Discrimination of emotional facial expression in a visual oddball task: an ERP study. Biol. Psychol. 59, 171-186]), shows shorter latencies for fearful rare stimuli (as compared to happy ones), but only for normal controls. This absence of delay was interpreted as an attentional deficit due to MDMA consumption.

Automatic evaluation of the exteroceptive suppression of the temporalis muscle activity.

AIM: To set up a reliable automated method of allowing to identify the ES1 and ES2 periods of exteroceptive suppression of the temporal muscle (EST), thereby reducing their intra- and inter-individual variability. MATERIALS AND METHODS: Analyses were performed in 79 healthy subjects with strict inclusion and exclusion criteria. Each individual underwent two separate examinations 1 week apart. Activity of the left and right temporal muscles was recorded in response to unilateral stimulation of the left and right labial commissures. Wave forms were successively rectified, averaged, and filtered; the intersections of the resulting curve with values corresponding to 50%, 60%, 70%, 75%, and 80% of the control period (20 ms preanalysis time) were automatically determined. RESULTS: All subjects reached a 80% level of attenuation for ES2. The values of ES2 durations were normally distributed. The mean ES1 and ES2 durations didn't vary between the two recording session, and there was a good individual reproducibility from one session to the other. Though relatively high, the inter-individual variability was slightly lower in the second than in the first session. CONCLUSIONS: Computed analysis of EST may contribute to decrease inter- and intra-individual test variability. Letting patient habituate through a first recording session could increase test sensitivity, too. The obtained values for ES2 duration are normally distributed and well-reproducible at both the population and individual level. We suggest taking into account both ES2 duration and level of attenuation.

Recording click-evoked myogenic potentials (CEMPs) with a setup for brainstem auditory evoked potentials (BAEPs).

AIM OF THE STUDY: Click-evoked myogenic potentials (CEMPs) originate in the sternocleidomastoid (SCM) muscle through a reflex loop involving the sacculus and the vestibular nerve. In this study we suggest that they can be picked up from the mastoid reference used for auditory evoked potential (AEP) recording by using standard filters for brainstem AEPs (BAEPs). They consist of a P13-N20 complex. METHODS: Fifty normal subjects were investigated. Recordings were performed with the same setting as that used for conventional BAEPs but without artifact rejection and using a wide time window (100 ms). Unilateral auditory stimulations were used. All acquisitions were performed in both sitting and supine positions. In nine subjects CEMPs and BAEPs were simultaneously recorded at both earlobe and both SCM muscles. RESULTS: In all subjects, a CEMP P13-N20 component could be evidenced in sitting, but not in supine position at both the ipsilateral earlobe and the ipsilateral SCM muscle. Its latency was 0.7 ms lower at the earlobe. It obeyed the same relationship to stimulus intensity at both earlobe and SCM muscle. CONCLUSION: These results demonstrate the possibility to get simultaneous information on the brainstem auditory pathways and on a reflex probably mediated through the sacculus and the vestibular nerve. Further studies on patients with selective vestibular nerve impairment should be conducted to confirm the clinical utility of this approach.

Microsurgical results with large vestibular schwannomas with preservation of facial and cochlear nerve function as the primary aim.

OBJECTIVE: To evaluate our microsurgical results in dealing with vestibular schwannomas (VS) greater than or equal to 30 mm when preservation of cranial nerve function was considered more important than total tumour removal. METHODS: Sixteen consecutive cases were operated on by the same neurosurgeon according to a prospective protocol using intraoperative neuro-monitoring (IONM) based on electromyographic and brain stem auditory evoked potential recordings. Facial nerve function was evaluated on the House-Brackmann Scale and cochlear nerve function on the Gardner-Robertson Scale. Someone not involved in the clinical management of our patients collected all data. RESULTS: Fifteen patients showed facial nerve (FN) function of House-Brackmann grade (HBG) I or II at one year postoperatively and one kept the HBG IV she had preoperatively. Two patients of four maintained a cochlear nerve function of Gardner-Robertson grade (GRG) II. The tumour excision rates were: total, 68.7%; near total, 6.3%; subtotal, 18.7%, and partial, 6.3%. The average follow-up was 55 months (1-106). Three patients underwent radiotherapy later with growth stabilisation and no additional morbidity. CONCLUSION: When dealing with VS greater than or equal to 30 mm, microsurgery guided by IONM, with a rate of total or near-total tumour excision of about 75%, can retain socially acceptable facial nerve function (HBG I or II) in all cases and serviceable hearing (GRG I or II) in two cases out of four. Maintaining serviceable cranial nerve function should take precedence over total tumour excision.

[Evoked potentials and post-traumatic evolution]. / Potentiels évoqués et évolution post-traumatique.

Visual, somatosensory, and brainstem auditory evoked potentials provide functional quantitative assessment of the cerebral cortex and brainstem. Their contribution at the acute stage of coma concerns diagnosis, prognosis, and follow-up. Four patterns are observed in traumatic coma: pattern 1=dysfunction of the cerebral cortex, brainstem integrity: good prognosis in more than 80% of cases; pattern 2=midbrain dysfunction: prognosis depends on both the reversibility of midbrain dysfunction and the extent of associated axonal lesions in the hemispheric white matter; pattern 3=pontine dysfunction due to transtentorial herniation: ominous prognosis, this pattern must be early detected by continuous monitoring; pattern 4=brain death: we currently use evoked potentials at the only brain-death confirmatory test, even in sedated patients. The contribution of evoked potentials in vegetative or minimally responsive states concerns the identification of these patients whose state is determined by midbrain dysfunction and the evaluation of persisting cognitive abilities in individual cases.

Somatosensory evoked potentials in the assessment of peripheral neuropathies: Commented results of a survey among French-speaking practitioners and recommendations for practice.

BACKGROUND: Somatosensory evoked potentials (SSEPs) are increasingly performed for the assessment of peripheral neuropathies, but no practical guidelines have yet been established in this specific application. STUDY AIM: To determine the relevant indication criteria and optimal technical parameters for SSEP recording in peripheral neuropathy investigation. METHODS: A survey was conducted among the French-speaking practitioners with experience of SSEP recording in the context of peripheral neuropathies. The results of the survey were analyzed and discussed to provide recommendations for practice. RESULTS: SSEPs appear to be a second-line test when electroneuromyographic investigation is not sufficiently conclusive, providing complementary and valuable information on central and proximal peripheral conduction in the somatosensory pathways. CONCLUSIONS: Guidelines for a standardized recording protocol, including the various parameters to be measured, are proposed. CLINICAL RELEVANCE: We hope that these proposals will help to recognize the value of this technique in peripheral neuropathy assessment in clinical practice.

Refractoriness cannot explain why C-fiber laser-evoked brain potentials are recorded only if concomitant Adelta-fiber activation is avoided.

Co-activation of Adelta- and C-fiber nociceptors by brief cutaneous laser heat stimuli may induce a dual sensation composed of first and second pain but evokes only a single, Adelta-fiber related, late laser-evoked potential (LEP). Yet, when concomitant activation of Adelta-nociceptors is avoided, C-nociceptor activation evokes an ultra-late LEP. As cumulating evidence indicates that late and ultra-late LEPs may share common generators, investigators have hypothesized that when Adelta-fibers trigger a late LEP, the later arriving C-fiber afferent volley cannot trigger an ultra-late LEP because underlying generators are in a 'refractory state'. Better understanding of these interactions could have important consequences regarding the functional significance of LEPs. Therefore, this hypothesis was tested by applying two consecutive laser stimuli to the hand dorsum such as to produce a second Adelta-nociceptor afferent volley arriving at generators during their expected 'refractory period'. Results showed that late LEPs evoked by the second stimulus were not altered and consequently that this hypothesis does not hold. In addition, when stimuli ended the sensory detection task, an ample P600 component was recorded. Studies have shown that this component is probably related to the P3b component described in other sensory modalities. This result provides support to the 'context closure' model hypothesizing that this component reflects the closure of information processing occurring when expectations are terminated. Altogether, these results suggest that late and ultra-late LEPs reflect very general processes, which are mainly related to detection and orientation and constitute only a fraction of the central processing of both nociceptive inputs.

Brain death diagnosis in misleading conditions.

The necessity of defining brain death (BD) arose from technological development in medical science. The definition of this concept had practical consequences and opened the way to organ donation from BD patients. Nowadays, the imbalance between the number of organs available for transplantation and the size of the demand is becoming critical. In most laboratories, a BD diagnosis is made according to precise criteria and in a well-defined process. BD diagnosis should be improved, not only to assure the safety and to preserve the human dignity of the patient, but also in order to increase the rate of organ donation. By analysing some epidemiological parameters in BD diagnosis and organ donation, it appears that BD diagnoses can be made more often and more rapidly if one has a reliable, accurate, and safe confirmatory test, especially under misleading conditions (hypothermia, drugs, metabolic disturbances). In our experience, the use of multimodality evoked potentials (MEPs) to confirm a BD diagnosis has many advantages: MEPs can be rapidly performed at the patient's bedside, assess the brain stem as well as the cerebral cortex, and are innocuous for the patient. Moreover, their insensitivity to the aforementioned misleading factors is sufficient to distinguish BD from clinical and EEG states that mimic BD. They give an immediate diagnosis, and no delay is required in BD confirmation if there is sufficient cause to account for BD. MEPs are a safe, accurate, and reliable tool for confirming a BD diagnosis, and their use can improve the organ donation rate while preserving the safety of the patient.

Sensitivity, specificity, and surgical impact of somatosensory evoked potentials in descending aorta surgery.

BACKGROUND: We evaluate the efficiency of multilevel somatosensory evoked potential (SEP) monitoring for intraoperative re-establishment of blood flow to the spinal cord in 63 patients undergoing descending aorta repair. METHODS: The presence of critical vessels in a cross-clamped aortic segment was ascertained by a 15 minute SEP observation period while the segment between the clamps was vented to drain out the collateral flow. RESULTS: SEPs influenced the surgical strategy in 17 cases (27%): use of the Biomedicus in 1 traumatic rupture; critical vessel reimplantation or distal clamp replacement in 13 cases of segmental spinal ischemia; and hastening the procedure or proximal clamp replacement in 3 cases of left carotid ischemia. There were no cases of unexplained multilevel SEP abnormalities. Immediate paraplegia was observed in 2 cases (1 pre-existing; 1 forecast by a 199-minute period of SEP absence due to segmental ischemia); 2 patients presented delayed paraplegias despite unchanged intraoperative SEPs, and 1 case presented a transient paraplegia due to lower motoneuronal involvement. CONCLUSIONS: SEPs efficiently identified critical vessels to be reimplanted in order to avoid immediate paraplegia. However, systematic additional vessel reimplantation, if technically feasible, and prolongation of SEP monitoring during the postoperative period with careful blood pressure control are needed to prevent delayed paraplegia.

Retrospective study of somatosensory evoked potential monitoring in deep hypothermic circulatory arrest.

BACKGROUND: We evaluated the efficiency of median-nerve somatosensory evoked potentials (SEPs) monitoring in determining the optimal level of hypothermia in 62 consecutive patients operated on under deep hypothermic circulatory arrest (CA). METHODS: CA was started at 1 degree C below the temperature at which both brainstem and cortical SEP components disappear. No additional method of cerebral protection was used. RESULTS: New neurological complications were observed in 15 patients: long-lasting in 11 and transient in 4. A retrospective analysis of SEP monitoring identified the origin of the complications in 12 cases: early CA with incomplete cooling due to emergency (3 cases); inefficient retrograde perfusion through the femoral artery during cooling due to the dissection flap effect (4 cases); embolism during rewarming (2 cases); delayed embolism (2 cases); hemorrhagic shock (1 case). In 2 cases, neurological sequelae involved the lower limbs (extracerebral origin). One case without intraoperative SEP changes was neurologically abnormal preoperatively and did not change postoperatively. There were no cases with sequelae due to excessive CA duration. CONCLUSIONS: The use of SEP monitoring to determine the optimal level of hypothermia efficiently prevents neurological sequelae of CA. It helps in monitoring the degree of cerebral protection during cooling (flap effect), and rewarming.

Subacute sensory neuronopathy associated with Sjögren's sicca syndrome.

Sensory subacute neuropathies associated with sicca syndrome without any systemic involvement have been reported rarely. A sixth case is described with what appears to be the first report of muscle and nerve biopsy findings. The histological studies revealed axonal degeneration without vasculitis in the sural nerve, and a slight denervation process and a discrete myositis in the gastrocnemius muscle, reflecting a subtle systemic disorder. The clinical course of a long-standing subacute sensory neuropathy, the biopsy-documented axonal degeneration, and the neurophysiological findings suggest involvement of the dorsal root ganglia.

Non-phase locked electroencephalogram (EEG) responses to CO2 laser skin stimulations may reflect central interactions between A partial partial differential- and C-fibre afferent volleys.

OBJECTIVE: By co-activating A partial partial differential- and C-fibre nociceptors, intense CO2 laser heat stimuli produce a dual sensation, composed of first and second pain, but induce only a single A partial partial differential-fibre related late laser evoked potential (LEP). However, when avoiding concomitant activation of A partial partial differential-fibres, C-fibre related ultra-late LEPs are recorded. This poorly understood phenomenon was re-investigated using a method which, unlike time-domain averaging, reveals electroencephalogram (EEG) changes whether or not phase-locked to stimulus onset. METHODS: CO2 laser stimuli were applied to the dorsum of the hand. Reaction-time was used to discriminate between A partial partial differential- and C-fibre mediated detections. Analyses were performed using a method based on the time-frequency wavelet transform of EEG epochs. RESULTS: This study revealed: (1) a novel non-phase-locked component related to the activation of A partial partial differential-fibres occurring at similar latencies as the late LEP; and (2) a widespread post-stimulus event-related desynchronization (ERD) induced by both A partial partial differential- and C-fibres. CONCLUSIONS: A partial partial differential- and C-fibre related LEPs could be electrophysiological correlates of similar brain processes, which, when already engaged by A partial partial differential-fibres, cannot or do not need to be reactivated by the later arriving C-fibre afferent volley. A partial partial differential-fibre related ERD could reflect a transient change of state of brain structures generating these responses.

Intra- and postoperative factors determining neurological complications after surgery under deep hypothermic circulatory arrest: a retrospective somatosensory evoked potential study.

OBJECTIVES: Intraoperative median nerve SEP monitoring uses the disappearance of cortical and brain stem activities as the criterion to determine that brain cooling is sufficient in deep hypothermic circulatory arrest (CA) surgery. This study presents the results of a retrospective SEP analysis of intraoperative events that engendered neurological complications. METHODS: Median nerve SEP monitoring was performed on 58 consecutive patients who underwent surgery under deep hypothermic CA. The monitoring was retrospectively analyzed and compared with intraoperative events, and postoperative symptoms. RESULTS: Intraoperative SEP abnormalities were observed in 9 out of the 16 patients who presented neurological complications. Among the 7 others, the abnormalities either were present preoperatively (n = 2), occurred postoperatively (n = 3) or only involved the lower limbs (n = 2). Although the mere inspection of the intraoperative SEPs was insufficient to determine the origin of the alterations, their comparison with intraoperative events facilitated the identification of hemodynamic factors in 7 cases and embolism in two cases. There were no patients in whom CA per se caused neurological complications. CONCLUSIONS: Intraoperative SEP monitoring helps identify intraoperative events responsible for neurological complications and prevent these in subsequent procedures. However, the neurological complications of deep hypothermic CA can also be due to pre- or postoperative factors that escape the domain of intraoperative monitoring.

ERPs obtained with the auditory oddball paradigm in coma and altered states of consciousness: clinical relationships, prognostic value, and origin of components.

OBJECTIVE: To study the event related potentials (ERPs) in coma and altered states of consciousness, their relationship with the clinical status and coma outcome. METHODS: ERPs were recorded with a passive auditory oddball paradigm in 103 patients. Their probability of occurrence and the peak latencies and amplitudes were studied as a function of the Glasgow Coma Score (GCS). Their relationship with outcome was studied in a subset of 83 patients examined within the first 4 days, and expressed in terms of sensitivity, specificity, and negative or positive prognostic values. RESULTS: When present, the ERPs to rare stimuli consisted of a fronto-central negativity (N(endog), mean latency: 330 ms) and a fronto-central positivity (P(endog), mean latency: 431 ms) following the exogenous N100-P200 complex. Both their probability of occurrence and their latencies and amplitudes were related with the GCS in anoxic and traumatic comas. The N(endog) and P(endog) had high sensitivity with a negative predictive value of 70% and 100%, respectively, but a low specificity, with a positive predictive value of 44% and 41%, respectively. CONCLUSIONS: ERPs can be recorded in some comatose patients and are likely to reflect implicit orienting processes rather than preserved consciousness. Their presence implies a good prognosis but no conclusion can be drawn from their absence.