Use of somatosensory-evoked potentials and cognitive event-related potentials in predicting outcomes of patients with severe traumatic brain injury

Functional and Prognostic Assessment in Comatose Patients: A Study Using Somatosensory Evoked Potentials

Aim

The functional prognosis of patients after coma following either cardiac arrest (CA) or acute structural brain injury (ABI) is often uncertain. These patients are associated with high mortality and disability. N20 and N70 somatosensory evoked potentials (SSEP) are used to predict prognosis. We evaluated the utility of SSEP (N20–N70) as an early indicator of long-term prognosis in these patients.

Methods

This was a retrospective cohort study of patients (n = 120) admitted to the intensive care unit (ICU) with a diagnosis of coma after CA (n = 60) or ABI (n = 60). An SSEP study was performed, including N20 and N70 at 24–72 h, after coma onset. Functional recovery was assessed 6–12 months later using the modified Glasgow scale (mGS). The study was approved by our local research ethics committee.

Results

In the CA and ABI groups, the absence of N20 (36% of CA patients and 41% of ABI patients; specificity = 100%) or N70 (68% of CA patients and 78% of ABI patients) was a strong indicator of poor outcome. Conversely, the presence of N70 was an indicator of a good outcome (AC: specificity = 84.2%, sensitivity = 92.7%; ABI: specificity = 64.2% sensitivity = 91.3%).

Conclusion

Somatosensory evoked potentials are useful early prognostic markers with high specificity (N20) and sensitivity (N70). Moreover, N70 has additional potential value for improving the prediction of good long-term functional outcomes.

Clinical Trial Registration:

[https://clinicaltrials.gov/], identifier [2018/01/001].

The Added Value of Somatosensory Potential N70 in Neurological Prognosis After Coma by Acute Brain Structural Injury: A Retrospective Study

Background:

Coma after acute brain structural injury (ABI) are associated with high mortality and disability. Somatosensory evoked potentials (SSEP) N20 and N70 are used to predict prognosis.

Purpose:

We assessed the utility of SSEP (N20-N70) as an early indicator of long-term functional prognosis in these patients.

Methods:

We conducted a retrospective cohort study of patients admitted to the intensive care unit (ICU) with a diagnosis of coma after ABI (n=60). An SSEP study including N20 and N70 was performed 24–72 hours after coma onset. Functional recovery was evaluated 6 to 12 months later using the Modified Glasgow Scale (mGS). The study was approved by our local research ethics committee.

Results:

The absence of N20 (41% specificity=100%) or N70 (78%) was a strong indicator of a poor outcome. In contrast, the presence of N70 was an indicator of a good outcome (specificity=64.2% sensitivity=91.3%).

Conclusion:

SSEP N20 and N70 are useful early prognostic markers with high specificity (N20) and sensitivity (N70). N70 has potential additional value for improving the prediction of good functional outcomes in the long term.

Automatic Sensory Predictions: A Review of Predictive Mechanisms in the Brain and Their Link to Conscious Processing

The human brain has the astonishing capacity of integrating streams of sensory information from the environment and forming predictions about future events in an automatic way. Despite being initially developed for visual processing, the bulk of predictive coding research has subsequently focused on auditory processing, with the famous mismatch negativity signal as possibly the most studied signature of a surprise or prediction error (PE) signal. Auditory PEs are present during various consciousness states. Intriguingly, their presence and characteristics have been linked with residual levels of consciousness and return of awareness. In this review we first give an overview of the neural substrates of predictive processes in the auditory modality and their relation to consciousness. Then, we focus on different states of consciousness - wakefulness, sleep, anesthesia, coma, meditation, and hypnosis - and on what mysteries predictive processing has been able to disclose about brain functioning in such states. We review studies investigating how the neural signatures of auditory predictions are modulated by states of reduced or lacking consciousness. As a future outlook, we propose the combination of electrophysiological and computational techniques that will allow investigation of which facets of sensory predictive processes are maintained when consciousness fades away.

The Combination of N60 with Mismatch Negativity Improves the Prediction of Awakening from Coma

BACKGROUND

Our objective was to evaluate the use of event-related potentials and the middle-latency somatosensory evoked potential (MLSEP) for the prediction of awakening in coma, determine the evaluation day that evoked potentials (EPs) best predict an awakening outcome, and determine whether the mismatch negativity (MMN) combined with the MLSEP, when recorded at 7 days after coma, improved the prediction of awakening from coma.

METHODS

Design prospective blinded cohort study. Setting neurointensive care unit of a university hospital. Patients 113 consecutive patients who were severely comatose, whose etiologies of coma included stroke (65 patients), hypoxic-ischemic encephalopathy (28 patients), intracranial infection (6 patients), and other (14 patients). Interventions none. Measurements we gathered Glasgow Coma Scale scores and recorded EPs for all patients who were comatose at 7, 14, and 30 days after coma onset, unless the patients returned to consciousness. The EPs examined included the MLSEP, the middle-latency auditory evoked potential, the N100, and the MMN. With telephone follow-up after 3 months, the patients were classified as awakening or nonawakening according to Glasgow Outcome Scale.

RESULTS

When predicting an awakening outcome, at least the unilateral presence of the N60 had the highest sensitivity (82.7%), whereas the presence of the MMN showed the highest specificity (82.0%). The area under the receiver operating characteristic curve for the EPs were high at 7 days after coma onset. At 7 days after coma onset, the combination of the N60 and MMN offered good predictive performance for awakening (area under the receiver operating characteristic curve = 0.852, 95% confidence interval 0.765-0.940), with increased sensitivity (70.0%) and improved specificity (91.7%).

CONCLUSIONS

The N60 and MMN were the strongest prognostic factors for an awakening outcome. Furthermore, at 7 days after coma onset, the combination of the N60 and MMN improved the prediction of an awakening outcome in patients who were comatose.

Clinical neurophysiology of neurologic rehabilitation

Clinical neurophysiologic testing provides valuable support in predicting outcome in the setting of disorders of consciousness (DOC), including coma and traumatic brain injury (TBI). Electroencephalography (EEG) and evoked potentials (EP) are simple to apply, inexpensive, safe, and available in most rehabilitation facilities. This chapter reviews the use of EEG and EP in postanoxic coma and TBI. Bilateral absence of cortical somatosensory evoked potentials (SSEP) may be regarded as a predictor of poor outcome in hypoxic brain damage. Flash VEP may be useful to differentiate between good and poor outcome. In addition, low EEG frequencies, burst suppression, and isoelectric EEG patterns prognosticate poor outcomes in hypoxic brain damage. While a loss of cortical SSEP is generally regarded as a negative prognostic sign in the acute phase of hypoxic brain damage, absence of cortical SSEP responses is not necessarily associated with poor outcome in TBI. Event-related potentials (ERPs) can provide support in outcome prediction. In particular, the N100, mismatch negativity, P300, and N400 may improve accuracy of outcome prediction DOC of different etiologies. Some evidence suggests that ERPs may be superior to SSEP in predicting functional and DOC outcomes (Lew et al., 2003). ERPs are measured brain responses resulting from specific cognitive tasks, sensory stimulation, or planned motor activity.

Mismatch negativity to predict subsequent awakening in deeply sedated critically ill patients

BACKGROUND

Mismatch negativity (MMN) is the neurophysiological correlate of cognitive integration of novel stimuli. Although MMN is a well-established predictor of awakening in non-sedated comatose patients, its prognostic value in deeply sedated critically ill patients remains unknown. The aim of this prospective, observational pilot study was to investigate the prognostic value of MMN for subsequent awakening in deeply sedated critically ill patients.

METHODS

MMN was recorded in 43 deeply sedated critically ill patients on Day 3 of ICU admission using a classical 'odd-ball' paradigm that delivers rare deviant sounds in a train of frequent standard sounds. Individual visual analyses and a group level analysis of recordings were performed. MMN amplitudes were then analysed according to the neurological status (awake vs not awake) at Day 28.

RESULTS

Median (inter-quartile range) Richmond Assessment Sedation Scale (RASS) at the time of recording was -5 (range, from -5 to -4.5). Visual detection of MMN revealed a poor inter-rater agreement [kappa=0.17, 95% confidence interval (0.07-0.26)]. On Day 28, 30 (70%) patients had regained consciousness while 13 (30%) had not. Quantitative group level analysis revealed a significantly greater MMN amplitude for patients who awakened compared with those who had not [mean (standard deviation) = -0.65 (1.4) vs 0.08 (0.17) μV, respectively; P=0.003).

CONCLUSIONS

MMN can be observed in deeply sedated critically ill patients and could help predict subsequent awakening. However, visual analysis alone is unreliable and should be systematically completed with individual level statistics.

Noninvasive Neuromonitoring: Current Utility in Subarachnoid Hemorrhage, Traumatic Brain Injury, and Stroke

Noninvasive neuromonitoring is increasingly being used to monitor the course of primary brain injury and limit secondary brain damage of patients in the neurocritical care unit. Proposed advantages over invasive neuromonitoring methods include a lower risk of infection and bleeding, no need for surgical installation, mobility and portability of some devices, and safety. The question, however, is whether noninvasive neuromonitoring is practical and trustworthy enough already. We searched the recent literature and reviewed English-language studies on noninvasive neuromonitoring in subarachnoid hemorrhage, traumatic brain injury, and ischemic and hemorrhagic stroke between the years 2010 and 2015. We found 88 studies that were eligible for review including the methods transcranial ultrasound, electroencephalography, evoked potentials, near-infrared spectroscopy, bispectral index, and pupillometry. Noninvasive neuromonitoring cannot yet completely replace invasive methods in most situations, but has great potential being complementarily integrated into multimodality monitoring, for guiding management, and for limiting the use of invasive devices and in-hospital transports for imaging.

Clinical Applications for EPs in the ICU

In critically ill patients, evoked potential (EP) testing is an important tool for measuring neurologic function, signal transmission, and secondary processing of sensory information in real time. Evoked potential measures conduction along the peripheral and central sensory pathways with longer-latency potentials representing more complex thalamocortical and intracortical processing. In critically ill patients with limited neurologic exams, EP provides a window into brain function and the potential for recovery of consciousness. The most common EP modalities in clinical use in the intensive care unit include somatosensory evoked potentials, brainstem auditory EPs, and cortical event-related potentials. The primary indications for EP in critically ill patients are prognostication in anoxic-ischemic or traumatic coma, monitoring for neurologic improvement or decline, and confirmation of brain death. Somatosensory evoked potentials had become an important prognostic tool for coma recovery, especially in comatose survivors of cardiac arrest. In this population, the bilateral absence of cortical somatosensory evoked potentials has nearly 100% specificity for death or persistent vegetative state. Historically, EP has been regarded as a negative prognostic test, that is, the absence of cortical potentials is associated with poor outcomes while the presence cortical potentials are prognostically indeterminate. In recent studies, the presence of middle-latency and long-latency potentials as well as the amplitude of cortical potentials is more specific for good outcomes. Event-related potentials, particularly mismatch negativity of complex auditory patterns, is emerging as an important positive prognostic test in patients under comatose. Multimodality predictive algorithms that combine somatosensory evoked potentials, event-related potentials, and clinical and radiographic factors are gaining favor for coma prognostication.

Somatosensory attention identifies both overt and covert awareness in disorders of consciousness

OBJECTIVE

Some patients diagnosed with disorders of consciousness retain sensory and cognitive abilities beyond those apparent from their overt behavior. Characterizing these covert abilities is crucial for diagnosis, prognosis, and medical ethics. This multimodal study investigates the relationship between electroencephalographic evidence for perceptual/cognitive preservation and both overt and covert markers of awareness.

METHODS

Fourteen patients with severe brain injuries were evaluated with an electroencephalographic vibrotactile attention task designed to identify a hierarchy of residual somatosensory and cognitive abilities: (1) somatosensory steady-state evoked responses, (2) bottom-up attention orienting (P3a event-related potential), and (3) top-down attention (P3b event-related potential). Each patient was also assessed with a clinical behavioral scale and 2 functional magnetic resonance imaging assessments of covert command following.

RESULTS

Six patients produced only sensory responses, with no evidence of cognitive event-related potentials. A further 8 patients demonstrated reliable bottom-up attention-orienting responses (P3a). No patient showed evidence of top-down attention (P3b). Only those patients who followed commands, whether overtly with behavior or covertly with functional neuroimaging, also demonstrated event-related potential evidence of attentional orienting.

INTERPRETATION

Somatosensory attention-orienting event-related potentials differentiated patients who could follow commands from those who could not. Crucially, this differentiation was irrespective of whether command following was evident through overt external behavior, or through covert functional neuroimaging methods. Bedside electroencephalographic methods may corroborate more expensive and challenging methods such as functional neuroimaging, and thereby assist in the accurate diagnosis of awareness. Ann Neurol 2016;80:412-423.

Sentence Processing in Traumatic Brain Injury: Evidence From the P600

PURPOSE

Sentence processing can be affected following a traumatic brain injury (TBI) due to linguistic or cognitive deficits. Language-related event-related potentials (ERPs), particularly the P600, have not been described in individuals with TBI history.

METHOD

Four young adults with a history of closed head injury participated. Two had severe injuries, and 2 had mild-moderate injuries more than 24 months prior to testing. ERPs were recorded while participants read sentences designed to be grammatically correct or incorrect. Participants also completed cognitive and sentence comprehension measures.

RESULTS

One participant with TBI was significantly different than the control group on several behavioral sentence measures and 1 cognitive measure. However, none of the participants with TBI had a reliable P600 effect. Nonparametric bootstrapping indicated that the ERP was reliable in 10 control participants but no participants with TBI history.

CONCLUSIONS

There were few behavioral differences between individuals with TBI history and the control group, though all reported subjective difficulty with reading. The P600 was absent in the TBI group in this study. Given the heterogeneity of individuals with TBI and the difficulty in assessing subtle language impairments, exploring the P600 further may provide useful insight into language processing difficulties.

May clinical neurophysiology help to predict the recovery of neurological early rehabilitation patients?

BACKGROUND

So far, the role of clinical neurophysiology in the prediction of outcome from neurological and neurosurgical early rehabilitation is unclear.

METHODS

Clinical and neurophysiological data of a large sample of 803 early rehabilitation cases of the BDH-Clinic Hessisch Oldendorf in Northern Germany have been carefully reviewed. Most patients (43.5%) were transferred to rehabilitation after stroke, mean age was 66.6 (15.5) years. Median somatosensory (SEP), auditory (AEP) and visual evoked potentials (VEP) along with EEG recordings took place within the first two weeks after admission. Length of stay (LOS) in early rehabilitation was 38.3 (37.2) days.

RESULTS

Absence of SEP on one or both sides was associated with poor outcome, χ2 = 12.98 (p = 0.005); only 12.5% had a good outcome (defined as Barthel index, BI ≥50) when SEP were missing on both sides. In AEP, significantly longer bilateral latencies III were observed in the poor outcome group (p < 0.05). Flash VEP showed that patients in the poor outcome group had a significantly longer latency III on both sides (p < 0.05). The longer latency III, the smaller BI changes (BI discharge minus admission) were observed (latency III right r = -0.145, p < 0.01; left r = -0.206, p < 0.001). While about half of the patients with alpha EEG activity belonged to the good outcome group (80/159, 50.3%), only 39/125 (31.2%) with theta and 5/41 (12.2%) with delta rhythm had a favourable outcome, χ2 = 24.2, p < 0.001.

CONCLUSIONS

Results from this study suggest that loss of median SEP, prolongation of wave III in AEP and flash-VEP as well as theta or delta rhythms in EEG are associated with poor outcome from neurological early rehabilitation. Further studies on this topic are strongly encouraged.

Assessing consciousness with auditory event-related potential during coma recovery: a case study

To observe changes in mismatch negativity (MMN) and P300 in a patient transitioning from a vegetative state (VS) to a minimally conscious state (MCS). One patient with intracerebral hemorrhage and an 8-month disease course was evaluated as being in the VS using the Coma Recovery Scale-Revised. Two weeks after the patient was admitted to the hospital, another evaluation was performed, and the patient was determined to be in an MCS. Using the Oddball paradigm, pure tone and name stimuli were presented to the patient to study event-related potentials (ERPs). A 15-week clinical follow-up was carried out, and four ERP examinations were performed at 2 days and 2, 6, and 15 weeks after admission. One healthy individual was assessed as a control participant. MMN and P300 were elicited in all four ERP examinations. MMN and P300 may occur earlier than believed in patients in persistent VS and MCS; their predictive values for prognosis need to be further confirmed by follow-up studies on a large clinical sample.

Traumatic brain injury detection using electrophysiological methods

Measuring neuronal activity with electrophysiological methods may be useful in detecting neurological dysfunctions, such as mild traumatic brain injury (mTBI). This approach may be particularly valuable for rapid detection in at-risk populations including military service members and athletes. Electrophysiological methods, such as quantitative electroencephalography (qEEG) and recording event-related potentials (ERPs) may be promising; however, the field is nascent and significant controversy exists on the efficacy and accuracy of the approaches as diagnostic tools. For example, the specific measures derived from an electroencephalogram (EEG) that are most suitable as markers of dysfunction have not been clearly established. A study was conducted to summarize and evaluate the statistical rigor of evidence on the overall utility of qEEG as an mTBI detection tool. The analysis evaluated qEEG measures/parameters that may be most suitable as fieldable diagnostic tools, identified other types of EEG measures and analysis methods of promise, recommended specific measures and analysis methods for further development as mTBI detection tools, identified research gaps in the field, and recommended future research and development thrust areas. The qEEG study group formed the following conclusions: (1) Individual qEEG measures provide limited diagnostic utility for mTBI. However, many measures can be important features of qEEG discriminant functions, which do show significant promise as mTBI detection tools. (2) ERPs offer utility in mTBI detection. In fact, evidence indicates that ERPs can identify abnormalities in cases where EEGs alone are non-disclosing. (3) The standard mathematical procedures used in the characterization of mTBI EEGs should be expanded to incorporate newer methods of analysis including non-linear dynamical analysis, complexity measures, analysis of causal interactions, graph theory, and information dynamics. (4) Reports of high specificity in qEEG evaluations of TBI must be interpreted with care. High specificities have been reported in carefully constructed clinical studies in which healthy controls were compared against a carefully selected TBI population. The published literature indicates, however, that similar abnormalities in qEEG measures are observed in other neuropsychiatric disorders. While it may be possible to distinguish a clinical patient from a healthy control participant with this technology, these measures are unlikely to discriminate between, for example, major depressive disorder, bipolar disorder, or TBI. The specificities observed in these clinical studies may well be lost in real world clinical practice. (5) The absence of specificity does not preclude clinical utility. The possibility of use as a longitudinal measure of treatment response remains. However, efficacy as a longitudinal clinical measure does require acceptable test-retest reliability. To date, very few test-retest reliability studies have been published with qEEG data obtained from TBI patients or from healthy controls. This is a particular concern because high variability is a known characteristic of the injured central nervous system.

Microdialysis Monitoring of CSF Parameters in Severe Traumatic Brain Injury Patients: A Novel Approach

Background: Neuro-intensive care following traumatic brain injury (TBI) is focused on preventing secondary insults that may lead to irreversible brain damage. Microdialysis (MD) is used to detect deranged cerebral metabolism. The clinical usefulness of the MD is dependent on the regional localization of the MD catheter. The aim of this study was to analyze a new method of continuous cerebrospinal fluid (CSF) monitoring using the MD technique. The method was validated using conventional laboratory analysis of CSF samples. MD-CSF and regional MD-Brain samples were correlated to patient outcome.

Materials and Methods: A total of 14 patients suffering from severe TBI were analyzed. They were monitored using (1) a MD catheter (CMA64-iView, n = 7448 MD samples) located in a CSF-pump connected to the ventricular drain and (2) an intraparenchymal MD catheter (CMA70, n = 8358 MD samples). CSF-lactate and CSF-glucose levels were monitored and were compared to MD-CSF samples. MD-CSF and MD-Brain parameters were correlated to favorable (Glasgow Outcome Score extended, GOSe 6–8) and unfavorable (GOSe 1–5) outcome.

Results: Levels of glucose and lactate acquired with the CSF-MD technique could be correlated to conventional levels. The median MD recovery using the CMA64 catheter in CSF was 0.98 and 0.97 for glucose and lactate, respectively. Median MD-CSF (CMA 64) lactate (p = 0.0057) and pyruvate (p = 0.0011) levels were significantly lower in the favorable outcome group compared to the unfavorable group. No significant difference in outcome was found using the lactate:pyruvate ratio (LPR), or any of the regional MD-Brain monitoring in our analyzed cohort.

Conclusion: This new technique of global MD-CSF monitoring correlates with conventional CSF levels of glucose and lactate, and the MD recovery is higher than previously described. Increase in lactate and pyruvate, without any effect on the LPR, correlates to unfavorable outcome, perhaps related to the presence of erythrocytes in the CSF.

Will time heal? A long-term follow-up of severe disorders of consciousness

Objective

Little is known about the long-term outcome of patients with disorders of consciousness (DOCs) such as unresponsive wakefulness syndrome (UWS) or minimally conscious state (MCS). We describe the disease course of a large group of DOC patients 2–14 years after brain damage.

Methods

In 102 patients (59 UWS, 43 MCS), clinical and demographic variables from disorder onset were related to the patients' outcomes 2–14 years after discharge. Etiology, age at event, time since onset, gender, and home care versus institutional care were assessed as predictors and similarities and differences between UWS and MCS determined.

Results

Seventy-one percent of the patients had passed away or showed no improvement in condition. Twenty-nine percent regained consciousness and developed some communicative capacities. The time a syndrome persisted did not predict clinical outcome in either condition. Six patients regained consciousness after more than 3 years. Of these, five had been UWS (42% of recovered UWS, three traumatic origins, one tumor, one hypoxia) and one MCS (5% of recovered MCS, traumatic origin). In UWS, younger patients, those cared for at home, and in tendency those with traumatic origins, were more likely to recover. In MCS, no reliable outcome predictors were found.

Interpretation

Current predictors are too vague for single patient predictions. This study identifies a subgroup of late-recovering patients, casting doubt on the 12-month boundary, after which UWS is stated to be permanent. Routine reexamination, use of more reliable outcome predictors and research determining optimal care settings are needed to inform the crucial decisions made for these patients.

Retinal ganglion cell damage in an experimental rodent model of blast-mediated traumatic brain injury

PURPOSE

To evaluate retina and optic nerve damage following experimental blast injury.

METHODS

Healthy adult mice were exposed to an overpressure blast wave using a custom-built blast chamber. The effects of blast exposure on retina and optic nerve function and structure were evaluated using the pattern electroretinogram (pERG), spectral domain optical coherence tomography (OCT), and the chromatic pupil light reflex.

RESULTS

Assessment of the pupil response to light demonstrated decreased maximum pupil constriction diameter in blast-injured mice using red light or blue light stimuli 24 hours after injury compared with baseline in the eye exposed to direct blast injury. A decrease in the pupil light reflex was not observed chronically following blast exposure. We observed a biphasic pERG decrease with the acute injury recovering by 24 hours postblast and the chronic injury appearing at 4 months postblast injury. Furthermore, at 3 months following injury, a significant decrease in the retinal nerve fiber layer was observed using OCT compared with controls. Histologic analysis of the retina and optic nerve revealed punctate regions of reduced cellularity in the ganglion cell layer and damage to optic nerves. Additionally, a significant upregulation of proteins associated with oxidative stress was observed acutely following blast exposure compared with control mice.

CONCLUSIONS

Our study demonstrates that decrements in retinal ganglion cell responses can be detected after blast injury using noninvasive functional and structural tests. These objective responses may serve as surrogate tests for higher CNS functions following traumatic brain injury that are difficult to quantify.

Prognosis in prolonged coma patients with diffuse axonal injury assessed by somatosensory evoked potentia

A total of 43 prolonged coma patients with diffuse axonal injury received the somatosensory evoked potential examination one month after injury in the First Affiliated Hospital, School of Medicine, Zhejiang University in China. Somatosensory evoked potentials were graded as normal, abnormal or absent (grades I–III) according to N20 amplitude and central conduction time. The outcome in patients with grade III somatosensory evoked potential was in each case unfavorable. The prognostic accuracy of grade III somatosensory evoked potential for unfavorable and non-awakening outcome was 100% and 80%, respectively. The prognostic accuracy of grade I somatosensory evoked potential for favorable and wakening outcome was 86% and 100%, respectively. These results suggest that somatosensory evoked potential grade is closely correlated with coma severity and degree of recovery. Somatosensory evoked potential is a valuable diagnostic tool to assess prognosis in prolonged coma patients with diffuse axonal injury.

Arousal modulates auditory attention and awareness: insights from sleep, sedation, and disorders of consciousness

The interplay between attention and consciousness is frequently tested in altered states of consciousness, including transitions between stages of sleep and sedation, and in pathological disorders of consciousness (DoC; the vegetative and minimally conscious states; VS and MCS). One of the most widely used tasks to assess cognitive processing in this context is the auditory oddball paradigm, where an infrequent change in a sequence of sounds elicits, in awake subjects, a characteristic EEG event-related potential called the mismatch negativity, followed by the classic P300 wave. The latter is further separable into the slightly earlier, anterior P3a and the later, posterior P3b, thought to be linked to task-irrelevant "bottom-up" and task-oriented "top-down" attention, respectively. We discuss here the putative dissociations between attention and awareness in DoC, sedation and sleep, bearing in mind the recently emerging evidence from healthy volunteers and patients. These findings highlight the neurophysiological and cognitive parallels (and differences) across these three distinct variations in levels of consciousness, and inform the theoretical framework for interpreting the role of attention therein.

Predicting Outcome after Traumatic Brain Injury

The purpose of this review is to look at the most recent work carried out on predicting outcome after traumatic brain injury (TBI). TBI is a leading cause of death and disability but prediction of long-term outcome for individual patients is difficult. In particular, predicting outcome in the first few hours or days after injury is limited by the paucity of scoring systems or clinical models available. Many clinical variables have been studied to determine if they may play a role in outcome prediction, including age, admission Glasgow Coma Score and pupillary reactivity. Newer variables being studied include serum biomarkers, abnormalities seen on magnetic resonance imaging and data obtained from evoked potentials and electroencephalography studies. There are many factors that impact on outcome and a perfect model is yet to be developed. Models must take into account the economic status of the country in which the trauma occurs. It is important that less affluent nations are not left behind in the search for accurate prognostic modelling.

Measuring interconnection of the residual cortical functional islands in persistent vegetative state and minimal conscious state with EEG nonlinear analysis

OBJECTIVES

The aim of the study is to investigate the cortical response to painful and auditory stimuli for subjects in persistent vegetative state (PVS) and minimal conscious state (MCS), and measure the interconnection of the residual cortical functional islands with electroencephalographic (EEG) nonlinear dynamic analysis (NDA).

METHODS

Thirty PVS subjects, 20 MCS subjects and 30 subjects in normal conscious state (NCS) were involved in the study. EEG was recorded under three conditions: eyes closed, auditory stimuli and painful stimuli. EEG nonlinear index of cross-approximate entropy (C-ApEn) was calculated for all subjects.

RESULTS

Interconnection of local and distant cortical networks of patients in PVS was generally suppressed, and painful or auditory stimulation could hardly cause any activation of associative cortices. Instead, interconnection of local cortical networks of patients in MCS improved significantly. The only significant difference with the NCS existed in the unaffected distant cortical networks.

CONCLUSIONS

Interconnection of local and distant cortical networks in MCS is superior to that of PVS. NDA could measure interconnection of the residual cortical functional islands with associative cortices in the unconscious patients.

SIGNIFICANCE

NDA can characterise the interconnection of cortical networks for the unconscious state and provide some information of unconsciousness at the awareness level.

Electrophysiological assessments of cognition and sensory processing in TBI: applications for diagnosis, prognosis and rehabilitation

Traumatic brain injuries are often associated with damage to sensory and cognitive processing pathways. Because evoked potentials (EPs) and event-related potentials (ERPs) are generated by neuronal activity, they are useful for assessing the integrity of neural processing capabilities in patients with traumatic brain injury (TBI). This review of somatosensory, auditory and visual ERPs in assessments of TBI patients is provided with the hope that it will be of interest to clinicians and researchers who conduct or interpret electrophysiological evaluations of this population. Because this article reviews ERP studies conducted in three different sensory modalities, involving patients with a wide range of TBI severity ratings and circumstances, it is difficult to provide a coherent summary of findings. However, some general trends emerge that give rise to the following observations and recommendations: 1) bilateral absence of somatosensory evoked potentials (SEPs) is often associated with poor clinical prognosis and outcome; 2) the presence of normal ERPs does not guarantee favorable outcome; 3) ERPs evoked by a variety of sensory stimuli should be used to evaluate TBI patients, especially those with severe injuries; 4) time since onset of injury should be taken into account when conducting ERP evaluations of TBI patients or interpreting results; 5) because sensory deficits (e.g., vision impairment or hearing loss) affect ERP results, tests of peripheral sensory integrity should be conducted in conjunction with ERP recordings; and 6) patients' state of consciousness, physical and cognitive abilities to respond and follow directions should be considered when conducting or interpreting ERP evaluations.

Early prognosis in traumatic brain injury: from prophecies to predictions

Traumatic brain injury (TBI) is a heterogeneous condition that encompasses a broad spectrum of disorders. Outcome can be highly variable, particularly in more severely injured patients. Despite the association of many variables with outcome, prognostic predictions are notoriously difficult to make. Multivariable analysis has identified age, clinical severity, CT abnormalities, systemic insults (hypoxia and hypotension), and laboratory variables as relevant factors to include in models to predict outcome in individual patients. Advances in statistical modelling and the availability of large datasets have facilitated the development of prognostic models that have greater performance and generalisability. Two prediction models are currently available, both of which have been developed on large datasets with state-of-the-art methods, and offer new opportunities. We see great potential for their use in clinical practice, research, and policy making, as well as for assessment of the quality of health-care delivery. Continued development, refinement, and validation is advocated, together with assessment of the clinical impact of prediction models, including treatment response.

Early somatosensory evoked potential grades in comatose traumatic brain injury patients predict cognitive and functional outcome

OBJECTIVES

To relate early somatosensory evoked potential grades from comatose traumatic brain injury patients to neuropsychological and functional outcome 1 yr later; to determine the day (within the first week after traumatic brain injury) that somatosensory evoked potential grade best correlates with outcome; to determine whether somatosensory evoked potential grade improvement in the first week after traumatic brain injury is associated with improved outcome.

DESIGN

Prospective cohort study.

SETTING

Critical care unit at a university hospital.

PATIENTS

Median nerve somatosensory evoked potentials were obtained from 81 comatose patients with traumatic brain injury. Somatosensory evoked potential grades were calculated from results obtained on days 1, 3, and 7 after traumatic brain injury. Glasgow Outcome Scale, Barthel Index, Rivermead Head Injury Follow-up Questionnaire, General Health Questionnaire, Stroop Color-Word Test, Paced Auditory Serial Addition Task, and Symbol-Digit Modalities Test scores were obtained 1 yr after injury.

INTERVENTIONS

None.

MEASUREMENTS AND MAIN RESULTS

Somatosensory evoked potential grade on days 1, 3, and 7 related significantly with Glasgow Outcome Scale and Barthel scores (day 3 better than day 1) but did not relate with Rivermead Head Injury Follow-up Questionnaire or General Health Questionnaire scores. Day 3 and day 7 somatosensory evoked potential grades related significantly with Stroop scores. Day 3 somatosensory evoked potential grades related significantly with Symbol-Digit Modalities Test scores. Patients with bilaterally present but abnormal somatosensory evoked potentials, whose somatosensory evoked potential grade improved between days 1 and 3, had marginally better functional outcome than those without somatosensory evoked potential grade improvement.

CONCLUSIONS

Day 3 somatosensory evoked potential grade related to information-processing speed, working memory, and the ability to attend to tasks 1 yr after traumatic brain injury. Day 3 somatosensory evoked potential grade had the strongest relationship with functional outcome. Somatosensory evoked potential grades were not related to emotional well-being.

Traumatic brain injury research priorities: the Conemaugh International Brain Injury Symposium

In 2005, an international symposium was convened with over 100 neuroscientists from 13 countries and major research centers to review current research in traumatic brain injury (TBI) and develop a consensus document on research issues and priorities. Four levels of TBI research were the focus of the discussion: basic science, acute care, post-acute neurorehabilitation, and improving quality of life (QOL). Each working group or committee was charged with reviewing current research, discussion and prioritizing future research directions, identifying critical issues that impede research in brain injury, and establishing a research agenda that will drive research over the next five years, leading to significantly improved outcomes and QOL for individuals suffering brain injuries. This symposium was organized at the request of the Congressional Brain Injury Task Force, to follow up on the National Institutes of Health Consensus Conference on TBI as mandated by the TBI ACT of 1996. The goal was to review what progress had been made since the National Institutes of Health (NIH) Consensus Conference, and also to follow up on the 1990's Decade of the Brain Project. The major purpose of the symposium was to provide recommendations to the U.S. Congress on a priority basis for research, treatment, and training in TBI over the next five years.

Repetitive transcranial magnetic stimulation-associated neurobehavioral gains during coma recovery

BACKGROUND

Repetitive transcranial magnetic stimulation (rTMS) is a noninvasive method to induce changes in cortical neural excitability. This report presents findings from the first participant of a safety and efficacy study that examined a therapeutic rTMS protocol for persons with severe traumatic brain injury (TBI).

OBJECTIVE

The primary hypothesis was that there will be no adverse events related to the provision of a 6-week rTMS protocol for persons with severe TBI who remain, at best, in a minimally conscious state for longer than 3 months. The secondary hypothesis was that the rTMS protocol would induce significant neurobehavioral gains during treatment and that these gains would persist at 6-week follow-up.

METHODS

A 6-week rTMS protocol (30 sessions) was delivered to a 26-year-old man who remained in a vegetative state 287 days after severe TBI. Stimulation was directed over the right dorsolateral prefrontal cortex. Repeated safety measures, neurobehavioral assessments, clinical examinations, and evoked potentials (EP) were obtained at baseline, every fifth rTMS session (weekly), and at a 6-week follow-up.

RESULTS

There were no adverse events related to the provision of rTMS treatment. A trend toward significant (P = .066) neurobehavioral gains was temporally related to provision of rTMS. Left-sided brain stem auditory EP wave V latencies and waves I to V interpeak latencies improved along with neurobehavioral gains during provision of rTMS, suggesting that improved neural conduction in the pathway mediated the neurobehavioral improvements.

CONCLUSIONS

Repetitive TMS merits further investigation as a safe therapeutic intervention to alter neural activity, to modulate neural activity, and/or to facilitate recovery in persons with disordered consciousness subsequent to severe TBI.

Cognitive event-related potentials in comatose and post-comatose states

We review the interest of cognitive event-related potentials (ERPs) in comatose, vegetative, or minimally conscious patients. Auditory cognitive ERPs are useful to investigate residual cognitive functions, such as echoic memory (MMN), acoustical and semantic discrimination (P300), and incongruent language detection (N400). While early ERPs (such as the absence of cortical responses on somatosensory-evoked potentials) predict bad outcome, cognitive ERPs (MMN and P300) are indicative of recovery of consciousness. In coma-survivors, cognitive potentials are more frequently obtained when using stimuli that are more ecologic or have an emotional content (such as the patient's own name) than when using classical sine tones.

The Effects of Increasing Stimulus Complexity in Event-Related Potentials and Reaction Time Testing: Clinical Applications in Evaluating Patients with Traumatic Brain Injury

This study compared the effectiveness of P300 event related potentials (ERPs) and reaction time (RT) in discriminating patients with traumatic brain injury (TBI) from healthy control subjects. In particular, we examined how the use of more complex, ecologically relevant stimuli may affect the clinical utility of these tasks. We also evaluated how length of posttraumatic amnesia (PTA) and loss of consciousness (LOC) related to P300 and RT measures in our patient sample. There were 22 subjects (11 patients with TBI and 11 age-matched healthy control subjects). Four stimulus detection procedures were used: two using simple, conventional stimuli (auditory tone discrimination, AT; visual color discrimination, VC), and two using complex, ecologically relevant stimuli in the auditory and visual modalities (auditory word category discrimination, AWC; visual facial affect discrimination, VFA). Our results showed that RT measures were more effective in identifying TBI patients when complex stimuli were used (AWC and VFA). On the other hand, ERP measures were more effective in identifying TBI patients when simple stimuli were used (AT and VC). We also found a remarkably high correlation between duration of PTA and P300 amplitude.

Temporal Stability of Auditory Event-Related Potentials in Healthy Individuals and Patients With Traumatic Brain Injury

Historically, cognitive event-related potentials (ERPs) have received limited acceptance for clinical use due to lack of evidence for their reliability. However, recent advances in computer technology and artifact rejection methods have greatly enhanced the fidelity of ERP measurements. The present study examined the test-retest reliability of ERP measurement by using current data processing methods. We assessed the temporal stability of the most commonly used ERP paradigm, auditory pure-tone "odd ball" detection, and compared it with other commonly used clinical measures reported in the literature. Auditory ERPs were collected in 19 healthy subjects and 7 patients with traumatic brain injury at two time points, 2 days to 2 months apart. Test-retest reliability was calculated for four ERP components: N1, MMN (mismatch negativity), P3, and N4. In healthy subjects, temporal stabilities of these four commonly studied ERP components' amplitude measurements were moderate to high, with intraclass correlations ranging from 0.6 to 0.8. In contrast, in patients with traumatic brain injury, ERPs were stable only for the N1 component (intraclass correlation = 0.7).

Sex differences in two event-related potentials components related to semantic priming

Although sex differences in language processing are well documented in behavioral studies, only a few electrophysiological studies have explored this topic. We analyzed sex differences in two language-related components of event-related potentials (ERPs): the N400 and the Late Positive Complex (LPC). Ten men and 10 women, matched by age and handedness, participated in the study. Two semantic priming paradigms were presented: word pairs (60 congruent and 60 incongruent) and sentences (50 with congruent and 50 with incongruent ending words). In addition, the effect of context was investigated by a comparison between ERP effects obtained in single word priming and sentential priming. The N400 effect was earlier and larger in women, and the LPC effect was larger in men. Furthermore, the LPC effect in men, but not in women, was much larger with sentence priming than with word priming, suggesting that the LPC effect may be more dependent on context in men than in women. The opposite sex difference on the two ERP components indicates different underlying mechanisms. While the LPC effect is thought to be generated by purely explicit mechanisms, such as postlexical integrative processes, the N400 effect may be also governed by prelexical implicit and explicit mechanisms. Our results were consistent with the notion of more automated processing of language in women than in men.

Answering the call: the influence of neuroimaging and electrophysiological evidence on rehabilitation

Functional recovery after brain damage or disease is dependent on the neuroplastic capability of the cortex and the nonaffected brain. Following cortical injury in the motor and sensory regions, the adjacent spared neural tissues and related areas undergo modifications that are required in order to drive more normal motor control. Current rehabilitation models seek to stimulate functional recovery by capitalizing on the inherent potential of the brain for positive reorganization after neurological injury or disease. This article discusses how neuroimaging and electrophysiological data can inform clinical practice; representative data from the modalities of functional magnetic resonance imaging, diffusion tensor imaging, magnetoencephalography, electroencephalography, and positron emission tomography are cited. Data from a variety of central nervous system disease and damage models are presented to illustrate how rehabilitation practices are beginning to be shaped and informed by neuroimaging and electrophysiological data.

Single-trial extraction of cognitive evoked potentials by combination of third-order correlation and wavelet denoising

The application of a recently proposed denoising implementation for obtaining cognitive evoked potentials (CEPs) at the single-trial level is shown. The aim of this investigation is to develop the technique of extracting CEPs by combining both the third-order correlation and the wavelet denoising methods. First, the noisy CEPs was passed through a finite impulse response filter whose impulse response is matched with the shape of the noise-free signal. It was shown that it is possible to estimate the filter impulse response on basis of a select third-order correlation slice (TOCS) of the input noisy CEPs. Second, the output from the third-order correlation filter is decomposed with bi-orthogonal splines at 5 levels. The CEPs is reconstructed by wavelet final approximation a<inf>5</inf>. We study its performance in simulated data as well as in cognitive evoked potentials of normal rat and Alzheimer's disease (AD) model rat. For the simulated data, the method gives a significantly better reconstruction of the single-trial cognitive evoked potentials responses in comparison with the simulated data. Moreover, with this approach we obtain a significantly better estimation of the amplitudes and latencies of the simulated CEPs. For the real data, the method clearly improves the visualization of single-trial CEPs. This allows the calculation of better averages as well as the study of systematic or unsystematic variations between trials.

Predicting coma and other low responsive patients outcome using event-related brain potentials: a meta-analysis

OBJECTIVE

A meta-analysis was performed to estimate the predictive power (odd ratio, OR) for awakening of auditory event-related potential (ERP) components in low responsive patients with stroke or hemorrhage, trauma, anoxic, post-operative, and metabolic encephalopathy etiologies.

METHODS

We reviewed MEDLINE and analyzed citations for retrieved articles. Logistic regressions were applied on patient samples (Glasgow Coma Scale <12) across and for separate etiologies.

RESULTS

For stroke and hemorrhage the ORs with 95% confidence intervals were: 2.05 [1.12-3.75] (N100), 4.47 [1.92-10.44] (MMN), 10.29 [2.00-52.79] (P300), for trauma: 1.63 [0.70-3.80] (N100), 4.72 [1.35-16.44] (MMN), 12.89 [4.82-34.43] (P300), anoxic: 8.03 [2.83-22.75] (N100), 15.50 [4.27-56.26] (MMN), 5.93 [2.38-14.77] (P300), post-operative: 10.66 [1.98-57.50] (N100), metabolic encephalopathy: 2.12 [0.34-13.13] (N100), 3.60 [0.28-46.36] (MMN), 7.71 [0.75-79.77] (P300), and all etiologies: 2.85 [1.91-4.27] (N100), 6.53 [3.55-12.01] (MMN), and 8.79 [4.88-15.83] (P300). Based on six N100 studies (N=548 patients), five MMN studies (N=470), and six P300 studies (N=313), the N100, MMN, or P300, when present, significantly predicted awakening, P300 and MMN being significantly better predictors than N100.

CONCLUSIONS

The MMN and P300 appear to be reliable predictors of awakening.

SIGNIFICANCE

The prognostic assessment of low responsive patients with auditory ERP should take into account both MMN and P300.

Prognostic value of evoked and event-related potentials in moderate to severe brain injury

Clinicians are often expected to project patients' clinical outcomes to allow effective planning for future care. This can be a challenge in patients with moderate to severe traumatic brain injury (TBI) who are often unable to participate reliably in clinical evaluations. With recent advances in computer instrumentation and signal processing, evoked potentials and event-related potentials show increasing promise as powerful tools for prognosticating the trajectory of recovery and ultimate outcome from the TBI. Short- and middle-latency evoked potentials can now effectively predict coma outcomes in patients with acute TBI. Long-latency event-related potential components hold promise in predicting recovery of higher order cognitive abilities.

Neurorehabilitation issues in states of disordered consciousness following traumatic brain injury

The assessment and care of persons with disorders of consciousness (DOC) following catastrophic traumatic brain injury is often difficult and filled with both challenges and potential controversies. Rates of misdiagnosis of low-level neurological state (LLNS) patients with signs of awareness as being vegetative have been noted to be unacceptably high and call for better education and training regarding the assessment methodologies of individuals with DOC. Clinician knowledge regarding prognostication and neural recovery from LLNSs following traumatic brain injury is often lacking, as is awareness of the neurorehabilitative interventions that can potentially facilitate recovery, as well as minimize morbidity and mortality in this unique population of neurological patients.

Brain responses to a subject's own name uttered by a familiar voice

Hearing one's own first name automatically elicits a robust electrophysiological response, even in conditions of reduced consciousness like sleep. In a search for objective clues to superior cognitive functions in comatose patients, we looked for an optimal auditory stimulation paradigm mobilizing a large population of neurons. Our hypothesis was that wider ERPs would be obtained in response to the subject's own name (SON) when a familiar person uttered it. In 15 healthy awake volunteers, we tested a passive oddball paradigm with three different novels presented with the same probability (P = 0.02): SON uttered by a familiar voice (FV) or by an unknown voice (NFV) and a non-vocal stimulus (NV) which preserved most of the physical characteristics of SON FV. ERP (32 electrodes) and scalp current density (SCD) maps were analyzed. SON appeared to generate more robust responses related to involuntary attention switching (MMN/N2b, novelty P3) than NV. When uttered by a familiar person, the SON elicited larger response amplitudes in the late phase of novelty P3 (after 300 ms). Most important differences were found in the late slow waves where two components could be temporally and spatially dissociated. A larger parietal component for FV than for NFV suggested deeper high-level processing, even if the subjects were not required to explicitly differentiate or recognize the voices. This passive protocol could therefore provide a valuable tool for clinicians to test residual superior cognitive functions in uncooperative patients.

Evoked potentials for the prediction of vegetative state in the acute stage of coma

For comatose patients in intensive care units, it is important to anticipate their functional outcome as soon and as reliably as possible. Among clinical variables the Glasgow Coma Score (GCS) and the patient's pupil reactivity are the strongest predictive variables. Evoked potentials help to assess objectively brain function. Over the past 20 years, numerous studies have assessed their prognostic utility in terms of awakening from coma. Fewer studies, however, have focused upon the utility of evoked potentials in predicting progression to the vegetative state. In this area evoked potentials appear to have a highly predictive value. In anoxic coma the abolition of somatosensory evoked potentials (SEPs) is related to a poor outcome, defined as death or survival in a vegetative state, with a 100% specificity. Following traumatic brain injury, the predictive value for unfavourable outcome is 98.5% when there are no focal injuries likely to abolish SEP cortical components. In contrast, the presence of event-related evoked potentials, and particularly mismatched negativity (MMN), is a strong predictor of awakening and precludes comatose patients from moving to a permanent vegetative state (PVS).

The vegetative and minimally conscious states: current knowledge and remaining questions

In the last 2 decades, the minimally conscious state has been distinguished conceptually from the vegetative state and operational criteria for these diagnoses have been published. Standardized and individualized assessment tools have been developed to assist with the diagnosis of severe disorders of consciousness and the measurement of clinical improvement. The natural course of recovery and the importance of key prognostic predictors have been elucidated. Important advances have also been made in defining the similarities and differences in the pathophysiology of these two states, and functional imaging modalities have begun to explicate the neural substrate underlying the behavioral features of these disorders. Research on the efficacy of treatments for severe disorders of consciousness lags behind, due to the practical and ethical difficulties in executing large rigorously controlled clinical trials. The past and future scientific developments in this area provide an important background for continuing discussions of the ethical controversies surrounding end-of-life decision making and resource allocation.

Late Auditory and Event-Related Potentials Can Be Useful to Predict Good Functional Outcome After Coma

OBJECTIVE

To investigate whether late auditory and event-related potentials, and in particular N100 and mismatch negativity, together with clinical parameters, can help to predict good functional outcome in comatose patients.

DESIGN

Prospective cohort study.

SETTING

Hospital.

PARTICIPANTS

Consecutively sampled comatose patients (N=346) whose etiologies of coma were stroke (125 patients), brain injury (96 patients), anoxia (64 patients), complication of neurosurgery (54 patients), and encephalitis (7 patients).

INTERVENTIONS

Not applicable.

MAIN OUTCOME MEASURES

Glasgow Outcome Scale score at 1 year postonset. Patients in a minimally conscious state and those who awoke and died during the follow-up period were classified separately.

RESULTS

Univariate analysis showed that all variables studied, except brainstem auditory evoked potentials, correlated significantly with functional outcome. Mismatch negativity showed the highest positive predictive value for good outcome. A validated model was obtained with multivariate logistic analysis, including pupillary light reflex, N100, mismatch negativity, etiology, and age.

CONCLUSIONS

Late auditory and event-related potentials, and particularly N100 and mismatch negativity, provide strong prognostic factors for good functional outcome. Furthermore, these components may enhance the accuracy of prognosis when associated with other clinical parameters available at the early stage of coma.

Acute changes in cortical excitability in the cortex contralateral to focal intracerebral hemorrhage in the swine

Injury to the cerebral cortex results in functional deficits not only within the vicinity of the lesion but also in remote brain regions sharing neuronal connections with the injured site. To understand the electrophysiological basis of this phenomenon, we evaluated the effects of a focal intracerebral hemorrhage (ICH) on cortical excitability in a remote, functionally connected brain region. Cortical excitability was assessed by measuring the somatic evoked potential (SEP) elicited by electrical stimulation of the swine snout, which is somatotopically represented in the rostrum area of the primary somatosensory (SI) cortex. The SEP was measured on the SI cortex ipsilateral to the site of ICH and on the contralateral SI cortex during the acute period (< or =11 h) after collagenase-induced ICH. The ICH rapidly attenuated the SEP on the ipsilateral cortex as we reported earlier. Interestingly, the ICH also attenuated the SEP on the contralateral SI cortex. Evoked potentials in the contralateral SI cortex showed a gradual decrease in amplitude during this acute period of ICH. We then investigated whether the interhemispheric connections shared by the contralateral SI and the lesion cortex were responsible for the diminished evoked potentials in the uninjured hemisphere after ICH. A separate group of animals underwent corpus callosal transection prior to electrocorticography (ECoG) recordings and ICH injury. Within hours of hemorrhagic injury, a gradual but marked increase in evoked potential amplitude was observed in the homotopic SI cortex of callosotomized animals as compared to pre-injection recordings. The enhancement suggests that there are additional effects of ICH on remote areas functionally connected to the site of injury. Functional deficits were present in both SI cortices within the first several hours of a unilateral injury indicating that the cessation of brain activity in the lesioned SI is mirrored in the contralateral hemisphere. This electrophysiological depression in the uninjured SI cortex is mediated in part by the interhemispheric connections of the corpus callosum.

Event-related potential measures of consciousness: two equations with three unknowns

This is a brief review of event-related brain potentials (ERPs) as indices of cortical information processing in conditions in which conscious perception of stimuli is supposed to be absent: sleep, coma, vegetative state, general anesthesia, neglect as well as presentation of subliminal or masked stimuli. Exogenous ERP components such as N1 and P2 are much more likely to remain in all these conditions than endogenous components. Further, all varieties of the late posterior positive ERP waves (e.g., P3b, P600, late positive complex) are most difficult to be elicited in such conditions, indicating that the cortical activity underlying the late posterior positivity may have a particularly close relationship to brain mechanisms of conscious perception. Contrary to what might be expected, reliable ERP effects indicating complex analysis of semantic stimulus features (i.e., meaning) can be recorded without conscious awareness, generally, as easy as (in some conditions, even easier than) ERP components related to rather simple physical stimulus features. It should be emphasized, however, that we never should overestimate our confidence about the degree of subjects' unawareness. Particularly in the conditions in which no behavioral response can be obtained (e.g., sleep, coma, anesthesia), residual conscious processing, at least in some subjects and on some trials, cannot be ruled out.