Comparison of speech-evoked v tone-evoked P300 response: implications for predicting outcomes in patients with traumatic brain injury

Test-Retest of Long Latency Auditory Evoked Potentials (P300) with Pure Tone and Speech Stimuli

Introduction Long latency auditory evoked potentials, especially P300, have been used for clinical evaluation of mental processing. Many factors can interfere with Auditory Evoked Potential - P300 results, suggesting large intra and inter-subject variations.

Objective The objective of the study was to identify the reliability of P3 components (latency and amplitude) over 4–6 weeks and the most stable auditory stimulus with the best test-retest agreement.

Methods Ten normal-hearing women participated in the study. Only subjects without auditory processing problems were included. To determine the P3 components, we elicited long latency auditory evoked potential (P300) by pure tone and speech stimuli, and retested after 4–6 weeks using the same parameters. We identified P300 latency and amplitude by waveform subtraction.

Results We found lower coefficient of variation values in latency than in amplitude, with less variability analysis when speech stimulus was used. There was no significant correlation in latency measures between pure tone and speech stimuli, and sessions. There was a significant intrasubject correlation between measures of latency and amplitude.

Conclusion These findings show that amplitude responses are more robust for the speech stimulus when compared with its pure tone counterpart. The P300 indicated stability for latency and amplitude measures when the test-retest was applied. Reliability was higher for amplitude than for latency, with better agreement when the pure tone stimulus was used. However, further research with speech stimulus is needed to clarify how these stimuli are processed by the nervous system.

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.

Prognostic Value of Evoked Responses and Event-Related Brain Potentials in

The behaviourally unresponsive patient, unable to exhibit the presence of cognition, constitutes a conundrum for health care specialists. Prognostic uncertainty impedes accurate management decisions and the application of ethical principles. An early, reliable prognosis is highly desirable. In this review investigations studying comatose patients with coma of different etiologies were selected. It is concluded that objective prognostication is enhanced by the use of electrophysiological tests. Persistent abnormalities of brainstem auditory evoked potentials and short-latency somatosensory evoked potentials reliably indicate the likelihood of irreversible neurological deficit or death. Meanwhile, the presence of “cognitive” event-related brain potentials (e.g., P300 and mismatch negativity) reflects the functional integrity of higher level information processing and, therefore, the likelihood of capacity for cognition. An approach that combines clinical and electrophysiological values provides optimal prediction of outcome and level of disability.

Changes in behavior of evoked potentials in the brain as a possible indicator of fatigue in people

Many professions place significant mental and/or physical strain on their workers. Some professionals (such as firefighters, soldiers, and pilots) have an inherent responsibility for the safety of others. Making sure that workers in these remain fit for duty is an important health/safety concern for the workers and those they serve. This paper explores the viability of using EEG as a non-invasive, cost efficient method for assessing fatigue, sleep deprivation, physical exertion and stress. Specifically, P300 evoked potentials are generated in response to certain stimuli. Variations in the response characteristics (magnitude, shape, and peak shift) are explored in relation to sleep deprivation, caffeine usage, and physical exertion. Preliminary data suggests that there are quantifiable changes to the P300 response that may be attributed to fatigue.

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.

How can we know if patients in coma, vegetative state or minimally conscious state are conscious?

This paper examines the claim that patients in coma, vegetative state and minimally conscious state may in fact be conscious. The topic is of great importance for a number of reasons--not least ethical. As soon as we know a given creature has any experiences at all, our ethical attitude towards it changes completely. A number of recent experiments looking for signs of intact or partially intact cognitive processing in the various stages of decreased level of consciousness are reviewed. Whether or not vegetative or coma patients are in fact conscious is an empirical issue that we yet do not know how to resolve. However, the simple fact that this is an unresolved empirical issue implies that the standard behavioural assessment is not sufficient to decide what it is like to be these patients. In other words, different and more sophisticated methods are necessary. From a theoretical position, the paper moves on to discuss differences in validity between reports (e.g. verbal) and signals (e.g. brain activations) in the study of consciousness, and whether results from experiments on the contents of consciousness may be of any use in the study of levels of consciousness. Finally, an integrated approach is suggested, which does not separate research in level and content as clearly as in current practice, and which may show a path to improved paradigms to determine whether patients in coma or vegetative state are conscious.

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.

Subject's own name as a novel in a MMN design: a combined ERP and PET study

With a view to elaborating a clinical tool to assess cognitive functions in brain-damaged patients, we had previously displayed characteristic patterns of ERPs (32 electrodes) in awake healthy persons in response to their own name (SON) presented as a novel in a passive oddball paradigm. In the present combined ERP and PET study, in an attempt to identify brain correlates of duration MMN and response to SON uttered by a familiar (FV) or an unknown voice (NFV), we used a block design protocol as close as possible to the aforementioned SON protocol. ERP data showed robust duration MMN and novelty P3 in response to SON similar to our previous results. The PET technique did not allow true MMN generators to be disclosed, but blocks with duration deviants elicited an increase of activation in the right temporal pole as compared with the control condition with no deviants, supporting the hypothesis of right hemispheric dominance in early sound discrimination. For SON contrasts, robust cerebral blood flow activation present over temporal, frontal and parietal cortices, in the hippocampus and in the precuneus could be associated with speech, novelty and self-recognition processing. Familiar and unfamiliar voices activated the prefrontal cortex differently, suggesting different retrieval processes, although corresponding ERP responses could not be differentiated.

Self-consciousness in non-communicative patients

The clinical and para-clinical examination of residual self-consciousness in non-communicative severely brain damaged patients (i.e., coma, vegetative state and minimally conscious state) remains exceptionally challenging. Passive presentation of the patient's own name and own face are known to be effective attention-grabbing stimuli when clinically assessing consciousness at the patient's bedside. Event-related potential and functional neuroimaging studies using such self-referential stimuli are currently being used to disentangle the cognitive hierarchy of self-processing. We here review neuropsychological, neuropathological, electrophysiological and neuroimaging studies using the own name and own face paradigm obtained in conscious waking, sleep, pharmacological coma, pathological coma and related disorders of consciousness. Based on these results we discuss what we currently do and do not know about the functional significance of the neural network involved in "automatic" and "conscious" self-referential processing.

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.

Improved prediction of awakening or nonawakening from severe anoxic coma using tree-based classification analysis*

OBJECTIVE

To evaluate the utility of sensory and event-related evoked potentials for the prediction of awakening/nonawakening in severe anoxic coma and to design a decision tree helping decision for any patient in this condition.

DESIGN

Prospective cohort study.

SETTING

Clinical neurophysiology unit and intensive care unit of a French university hospital.

PATIENTS

Sixty-two consecutive severe comatose patients after out-of-hospital cardiac arrest.

INTERVENTIONS

None.

MEASUREMENTS AND MAIN RESULTS

We gathered clinical variables and recorded the somatosensory, auditory, and cognitive evoked potentials within an average period of 8 days after cardiac arrest. The patients were followed for 12 months and classified as awake or nonawake (permanent vegetative state or death). The statistical study included measurements of specificity, sensitivity, and positive and negative predictive value for each clinical and electrophysiologic variable recorded at the early stage of coma. Furthermore, a tree-based classification analysis was performed.All patients in whom somatosensory evoked potentials or middle-latency auditory evoked potentials were abolished did not awaken (100% specificity). All patients in whom mismatch negativity (MMN) was present awakened (100% specificity). MMN was superior to somatosensory evoked potentials for the prediction of awakening and had the best specificity and positive predictive value for awakening. On the decision tree, the awakening/nonawakening explicative variables were, by order of importance, MMN, pupillary reactivity, and somatosensory evoked potentials.

CONCLUSIONS

There is a need to predict early and accurately awakening or nonawakening in postanoxic comas. Using sensory and cognitive evoked potentials to assess the functional condition of the brain, a prognostic tree for the prediction of awakening/nonawakening in severe anoxic coma has been designed. It is applicable to any patient in this condition and offers the possibility to predict with very high probability awakening when MMN, the earliest component of event-related potentials, is present and nonawakening when MMN and pupillary light reflex are absent or cortical components of somatosensory evoked potentials are abolished.

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.

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.

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.

Electrophysiologic Abnormalities of Auditory and Visual Information Processing in Patients with Traumatic Brain Injury

OBJECTIVE

Through both auditory and visual modalities, we sought to explore the efficacy of event-related potentials in detecting residual cognitive impairments in patients with traumatic brain injury (TBI).

DESIGN

Control subjects and TBI patients with favorable recovery were recruited. Pure tone and primary color discrimination tasks were utilized to elicit auditory and visual event-related potentials, respectively. All subjects were instructed to push a response button when they detected the target stimuli. Both behavioral and electrophysiologic responses were obtained simultaneously. We analyzed the event-related potential waveforms and examined the differences in amplitude, latency, behavioral reaction time, and response accuracy.

RESULTS

A total of 11 TBI patients and 11 control subjects were tested. Results showed that (1) TBI patients had significantly lower P300 amplitude in both auditory (11.2 vs. 22. 7 microV) and visual (11.6 vs. 20.9 microV) domains, (2) TBI patients had significantly longer P300 latency in both auditory (355 vs. 294 msecs) and visual (376 vs. 341 msecs) modalities, and (3) although there was no significant difference in response accuracy (97.7%vs. 100%), reaction time for both auditory and visual tasks were significantly longer in TBI patients (auditory, 404 vs. 277 msecs; visual, 397 vs. 346 msecs).

CONCLUSION

Although TBI patients with good recovery showed similar response accuracy when compared with control subjects, they demonstrated significantly poorer performance in both electrophysiologic and behavioral responses. Diminished amplitudes and prolonged latencies in P300 responses indicate impaired organization and categorization of incoming sensory information; prolonged behavioral reaction times suggest slowing in the response execution process. Clinical and theoretical implications and goals for continued research are discussed.

Clinical applications of event-related potentials in brain injury

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

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

OBJECTIVE

This study was performed to evaluate the usefulness of somatosensory-evoked potentials (SEPs) and cognitive event-related potentials (ERPs) in predicting functional outcomes of severe traumatic brain injury patients.

DESIGN

Prospective study of 22 patients with severe traumatic brain injury. Demographic information, Glasgow Coma Scale, and electrophysiologic measurements were recorded. Functional outcomes, as quantified by the Glasgow Outcome Scale-Extended, were obtained.

RESULTS

Bilateral absence of median nerve SEP was strongly predictive of the worst functional outcome. The specificity and positive predictive value of absent SEP for predicting death or persistent vegetative state at 6 mo after traumatic brain injury were as high as 100%. If the definition of unfavorable outcome was expanded to include Glasgow Outcome Scale-Extended 1-4, absence of ERP was equivalent to the absence of SEP in specificity and positive predictive value. On the other hand, normal ERPs showed higher sensitivity and negative predictive value for prognosticating the best outcomes compared with normal SEPs. If the definition of favorable outcome was expanded to include Glasgow Outcome Scale-Extended 5-8, ERP was still superior to SEP for prognosticating good outcome. Interestingly, the highest sensitivity and negative predictive value for favorable outcomes were associated with the presence of any discernible waveform.

CONCLUSIONS

Although median nerve SEP continues to make reliable prediction of ominous outcome in severe traumatic brain injury, the addition of the speech-evoked ERPs may be helpful in predicting favorable outcomes. The strength of the latter test seems to complement the weakness of the former.

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

OBJECTIVE

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

MATERIAL AND METHODS

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

RESULTS

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