P300 response under active and passive attentional states and uni- and bimodality stimulus presentation conditions

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.

Towards neurophysiological assessment of phonemic discrimination: context effects of the mismatch negativity

OBJECTIVE

This study focusses on the optimal paradigm for simultaneous assessment of auditory and phonemic discrimination in clinical populations. We investigated (a) whether pitch and phonemic deviants presented together in one sequence are able to elicit mismatch negativities (MMNs) in healthy adults and (b) whether MMN elicited by a change in pitch is modulated by the presence of the phonemic deviants.

METHODS

Standard stimuli [i] were intermixed with small, medium or large pitch deviants or with pitch deviants of the same magnitude together with small and large phonemic deviants, [y] and [u], respectively.

RESULTS

When pitch and phonemic deviants were presented together, only the large pitch and phonemic contrasts elicited significant MMNs. When only pitch deviants were presented, the medium and large pitch contrasts elicited significant MMNs. The MMNs, in response to the medium and large pitch contrasts, were of similar magnitude across the two contexts.

CONCLUSIONS

Pitch and phonemic deviants can be tested together provided the pitch contrast is relatively large.

SIGNIFICANCE

A combined neurophysiological test of phonemic and pitch discrimination, as measured by the MMN, is a time-effective tool that may provide valuable information about the underlying cause of poorly specified phonemic representations in clinical populations.

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.

Sedation modulates recognition of novel stimuli and adaptation to regular stimuli in critically ill adults

BACKGROUND

Responsiveness of critically ill patients is affected by disease and by therapy. Subjective tests of responsiveness (e.g., modified Ramsay score) reflect global integrity of a response arc that includes transduction, perception, classification, and an overt response. The performance of individual components is usually not assessed.

OBJECTIVE

To determine whether an association exists among sedative medications and the brain's component information processing and adaptive abilities.

DESIGN

Initial observational study of a convenience sample.

SETTING

A surgical intensive care unit of a university hospital.

PATIENTS

A total of 22 endotracheally intubated, mechanically ventilated patients chemically sedated with narcotics and a benzodiazepine (n = 12), or narcotics and propofol (n = 10) using sedation protocols.

INTERVENTIONS

None.

MEASUREMENTS AND FINDINGS

Patients were presented flashes of light and pulses of sound at fixed or random intervals. Patients receiving low doses of fentanyl and propofol adapted to stimuli presented at fixed intervals and retained the response to novelty. Patients receiving higher doses of those medications did not respond to novelty and responded to stimuli presented at fixed intervals as if they were novel. Patients receiving fentanyl and benzodiazepines had generally weaker responses to all stimuli. Moreover, their responses to fixed and random stimuli were similar. At low doses, the patients retained responses to novelty and adapted to fixed, whereas at higher doses the opposite was observed: the response to novelty was lost and the patients responded to fixed stimuli as if they were novel. These observations suggest that the sedative medications generally accelerate the physiologic decay of stimuli as they engage the brain's information processing and adaptive abilities and further suggest that different sedative medications may have different effects on the brain.

CONCLUSIONS

Commonly administered sedative medications may alter the brain's biophysical state and thereby modulate specific aspects of the brain's information processing and adaptive functions. These functions can be interrogated even when the patient is seemingly unresponsive. If this observation is confirmed in subsequent prospective controlled randomized trials, electrophysiologic interrogation of the brain's information processing and adaptive capacities could serve as an adjunct to clinical assessment of responsiveness and management of sedative medications.

Auditory evoked potentials to spectro-temporal modulation of complex tones in normal subjects and patients with severe brain injury

In order to assess higher auditory processing capabilities, long-latency auditory evoked potentials (AEPs) were recorded to synthesized musical instrument tones in 22 post-comatose patients with severe brain injury causing variably attenuated behavioural responsiveness. On the basis of normative studies, three different types of spectro-temporal modulation were employed. When a continuous 'clarinet' tone changes pitch once every few seconds, N1/P2 potentials are evoked at latencies of approximately 90 and 180 ms, respectively. Their distribution in the fronto-central region is consistent with generators in the supratemporal cortex of both hemispheres. When the pitch is modulated at a much faster rate ( approximately 16 changes/s), responses to each change are virtually abolished but potentials with similar distribution are still elicited by changing the timbre (e.g. 'clarinet' to 'oboe') every few seconds. These responses appear to represent the cortical processes concerned with spectral pattern analysis and the grouping of frequency components to form sound 'objects'. Following a period of 16/s oscillation between two pitches, a more anteriorly distributed negativity is evoked on resumption of a steady pitch. Various lines of evidence suggest that this is probably equivalent to the 'mismatch negativity' (MMN), reflecting a pre-perceptual, memory-based process for detection of change in spectro-temporal sound patterns. This method requires no off-line subtraction of AEPs evoked by the onset of a tone, and the MMN is produced rapidly and robustly with considerably larger amplitude (usually >5 microV) than that to discontinuous pure tones. In the brain-injured patients, the presence of AEPs to two or more complex tone stimuli (in the combined assessment of two authors who were 'blind' to the clinical and behavioural data) was significantly associated with the demonstrable possession of discriminative hearing (the ability to respond differentially to verbal commands, in the assessment of a further author who was blind to the AEP findings). Behavioural and electrophysiological findings were in accordance in 18/22 patients, but no AEPs could be recorded in two patients who had clear behavioural evidence of discriminative hearing. The absence of long-latency AEPs should not, therefore, be considered indicative of complete functional deafness. Conversely, AEPs were substantially preserved in two patients without behavioural evidence of discriminative hearing. Although not necessarily indicative of conscious 'awareness', such AEP preservation might help to identify sentient patients who are prevented by severe motor disability from communicating their perception.

Concurrent visual task effects on evoked and emitted auditory p300 in adolescents

Using an oddball stimulus presentation paradigm, the effects of divided attention on auditory P300s were studied. Auditory attention was either divided or focused, depending on the demands placed on subjects during the performance of a concomitantly presented visual task. Two types of auditory tasks were performed under each of the two auditory attention conditions. In one, subjects responded to infrequently presented high pitched tones (oddball stimuli). In the other they responded to the occasional omission of a stimulus in an otherwise rhythmically presented chain of stimuli. P300s and reaction times were recorded to both the rare tones and the omissions. The Sternberg visual memory task was used to manipulate the subject's auditory attention state. Subjects actively performed the Sternberg task during the divided auditory attention condition, whereas during the focused attention condition they were not required to respond to the visual stimuli. During focused auditory attention, evoked auditory P300s were both larger and faster than their emitted counterparts. During divided attention, auditory P300s were reduced in amplitude but latency was unaffected. Evoked auditory P300s showed evidence of containing P300a as well as P300b components, particularly when attention was shared with the visual task.

Tri-axial recording of event-related potentials during passive cognitive tasks in patients with Alzheimer's disease

Standard methods used to assess cognitive function in patients with Alzheimer's Disease (AD) often use instructions to direct attention and gauge task difficulty, and measure only the output of processing, i.e., the patient's behavioral response. Because this may focus assessment on functions that are observable and to periods when patient comprehension is not compromised, the present study presented stimuli without instruction, manipulated task difficulty by varying stimulus factors, and used the brain's electrical response as the dependent variable. Because the recording electrode's position on the scalp may limit full examination of the voltage distribution of these responses, a Tri-Axial method of recording electrical activity within a Cartesian coordinate system was used. Results suggest attention may inhibit habituation so that inputs can be represented, discriminated and consolidated. For the control group, the levels of task difficulty modulated electrical peaks presumed to reflect the brain's ability to perform these functions. In the AD group, these responses were attenuated or absent, suggesting that dysfunctional attentional processing may underlie response errors often attributed to memory.

Auditory event-related potentials in mentally retarded subjects during active and passive oddball experiments

Auditory event-related potentials were recorded from subjects performing an active and/or a passive oddball task. The subjects belonged to three groups: 27 nonretarded (NR) subjects; 39 "discriminating" retarded (DR) subjects; and 12 "nondiscriminating" retarded (NDR) subjects. With respect to NR subjects, DR subjects had significantly longer latencies for peaks N1, P2, N2 and P3 in the active task and for N2 in the passive task, and NDR subjects had significantly longer latencies for peaks N2 and P3 in the passive task. We conclude: that the generation of P3 may involve both a permanent automatic basis and controlled processes whose intervention depends on the attention paid to the P3-inducing stimuli; and that whether a mentally retarded subject exhibits significantly lengthened P3 latency in a particular task depends on the degree to which the cognitive processes involved in performance of that task are affected by the causes of his or her retardation.

The effects of attention and context on the spatial and magnitude components of the early responses of the event-related potential elicited by a rare stimulus

Sokolov's (1963) model-comparator theory of orienting and attention theorizes that different events underlie passive and active processing of sensory information presented in different contexts. The following study investigates changes in event-related potentials (ERPs) related to this theory by presenting stimuli inter- and intra-modally during passive and active processing tasks. Model-comparator theory proposes that novel events are detected by a mismatch discrimination process made between incoming and previously presented stimuli during preattentive processing. Central processing is engaged when this mismatch is relevant to the organism. To explore how engaging central processing, induced by instructional priming, affects preattentive processing, a 'truly' passive task was compared with a standard active task. This 'truly' passive task is different from the distracted one normally used to control direction of attention in ERP experiments in that it did not instruct subjects attention towards any task. ERP data were modeled as a dipole whose trajectory moved through voltage space. Our results suggest that both the spatial components and the magnitude of the dipole trajectory changed as functions of both passive and active processing and the context in which stimuli were presented. Our results also suggest that the trajectory and magnitudes of certain ERP components reflect processes proposed by model-comparator theory.

Somatosensory evoked potential peak latencies and amplitudes in contralateral and ipsilateral hemispheres in normal and severely traumatized brain-injured subjects

The purpose of this study was to compare in normal and traumatic brain injury (TBI) subjects long latency cortical brain-evoked potential patterns obtained upon stimulation of the median nerves. Quantitative data were analysed involving nine peak latencies and eight amplitudes obtained simultaneously contralaterally and ipsilaterally. Left-right hemispheric differences were also analysed. The following was found: TBI latencies were significantly longer for five of nine peaks (N30, P40, N60, P185, P285). TBI amplitudes were significantly smaller for two of eight amplitudes (P185-N240 and N240-P285). A significant contralateral-ipsilateral latency difference occurred only at P40 where latencies in the contralateral hemisphere are shorter for both normals and TBIs. Significant contralateral-ipsilateral amplitude differences occurred in the four early amplitudes (N30-P40, P40-N60, N60-P105, P105-N140) with amplitudes being smaller on the ipsilateral side. A differential effect, however, was found for amplitudes N30-P40 and P40-N60 where the difference is significantly larger in the contralateral hemisphere for normals but not for TBIs. This suggests that contralateral-ipsilateral amplitude difference can be a marker of extent and severity of injury and may also be helpful in localizing site of injury, particularly interhemispheric or corpus callosal injury. The differential latency and amplitude responses for later peaks occurring in the P300 region suggest sensitivity to detecting impairments in pre-cognitive and early cognitive activities.

Short and long latency auditory evoked potentials in traumatic brain injury patients

Short latency auditory nerve and brainstem evoked responses (BAERs) and long latency cortical auditory evoked responses (CAERs) in 75 long-term traumatic brain injury (TBI) cases were compared. CAERs were found to be significantly correlated with clinical disability as measured by the Disability Rating Scale, while BAERs were not. Also, BAER patterns were consistently and significantly less abnormal and less sensitive to overall dysfunction than CAER patterns. Findings support previous observations that BAERs have relatively little utility for evaluating in surviving TBI patients the degree of overall brain impairment. In general, long latency AEP patterns are better able to reflect the extent and severity of brain dysfunction and overall clinical condition than are short latency AEP patterns in long-term severe TBI patients, and these patterns should be obtained routinely in the evaluation of such patients.