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PubMed2006

Wiener filter deconvolution of overlapping evoked potentials.

Evoked potentials (EPs) typically contain components that last up to several seconds. In order to save time and study adaptation effects, stimuli are often presented at high stimulation rates. Unfortunately, such protocols often suffer from a response overlap problem. Techniques based on Continuous Loop Averaging Deconvolution (CLAD) have been developed to formulate circular convolution to restore the source EP. These techniques, however, are sensitive to the additive noise and do not perform well when stimulus sequences with poor noise attenuation properties are chosen. This study offers a solution to this problem by applying Wiener theory to obtain an optimal filter that adapts the ratio of noise-to-signal by estimating the power spectra of both noise and signal using non-parametric or parametric methods. Experiments are conducted on simulation data and Auditory Evoked potentials (AEPs) acquired from human subjects to demonstrate the validity of the proposed theory. The results show that the Wiener deconvolution method improves the quality of the responses acquired with sequences with poor noise attenuation in the presence of high noise levels compared to the straight CLAD method. The results are comparable to responses obtained by conventional and Maximum Length Sequence (MLS) methods.

Adult↗
PubMed1989

[Psychoactive drugs and cortical evoked potentials].

Evoked potentials (EP) are an important research instrument in psychopharmacology. They allow to investigate central nervous processes in a time range of milliseconds. Furthermore, they are useful as a link between the basal biochemical or neuroendocrinological and the psychological levels of description. The literature concerning the relationship between EP and the therapeutically important classes of psychoactive drugs (stimulants, neuroleptics, antidepressants, minor tranquilizers/sedatives, lithium salts, carbamazepine) was reviewed. The following two questions were treated separately: 1. What are the effects of psychoactive drugs on EP in healthy subjects and in patients? 2. Are EP suitable as predictors of the clinical response to psychopharmacological treatment? It was found that most effects of psychoactive drugs on the EP are not specific for one class of agents. Furthermore these effects depend on factors concerning the subject (e.g. age, gender, diagnosis, symptomatology, personality, initial value of the EP-variable), the treatment (e.g. dosage, duration), or the context and procedure of the EP recording. Therefore, the importance of evoked potentials for the classification of new psychoactive drugs has been limited until now. However, the EP research has stimulated the development of mostly psychological concepts about the way of action of some psychoactive drugs. The use EP as predictors of the response to psychopharmacological treatment seems to be promising. There are consistent findings concerning the prediction of the clinical response to stimulants and lithium salts.

Cerebral Cortex↗
PubMed1989

Vestibulospinal evoked potential versus motor evoked potential monitoring in experimental spinal cord injuries of cats.

Changes in vestibulospinal evoked potentials (VsEP) and motor evoked potentials (MEP) were examined in 10 cats before and after two different weight-dropping spinal cord injuries. In six animals somatosensory evoked potentials (SEP) were also monitored. The recordings were done from epidural spinal cord electrodes. Before and after severe and light weight-dropping spinal cord injuries all 3 modalities were recorded at the same time intervals till the end of 4th hour postinjury. According to a scoring system, evoked potential changes below and above the level of injury were monitored, and compared with each other. This study showed that the different motor stimulation methods use different descending spinal tracts, and both can be useful as a monitoring tool. Both descending tracts carrying VsEP and MEP had similarly remarkable changes after severe spinal cord injury. These consisted of major deformation, development of an evoked injury potential and complete potential loss. During the 4 hour monitoring period, no case showed EP recovery in the severe injury group. Light spinal cord injury caused somewhat more deterioration in MEPs than VsEP. The higher numbers of severe potential alterations in the lightly injured animals suggest that MEP is a more sensitive method for spinal cord monitoring compared to VsEP and also to SEP. On the other hand, this sensitivity might be a disadvantage during intraoperative monitoring, if MEP alone were used.

Action Potentials↗
PubMed1992

[Magnetic stimulation motor evoked potential in multiple sclerosis. Comparison with visual evoked potentials, brain stem auditory evoked potentials and somatosensory evoked potentials].

This study consists of 45 patients with clinically definite MS, laboratory supported definite MS and clinically probable MS. We compared MEP results with other multimodal evoked potentials (VEP, BAEP and SEP). The abnormal rate of MEP was 87.6%, which was the highest. Abnormal MEP showed prolonged central motor conduction time (CMCT), consistent with pathological change of the demyelination. There was a evident correlation between the abnormal MEP and VEP, which is consistent with the most common MS (Devic Syndrome) in our country.

Adolescent↗
PubMed1976

Studies on the human evoked electrospinogram. I. The origin of the segmental evoked potentials.

Evoked potentials from the human spinal cord were studied in 39 normal volunteers. Intrathecal recordings from lower cervical and lower thoracic intervertebral levels were made after the supramaximal stimulation of the median, ulnar and posterior tibial nerves, respectively. It was shown that the segmental cord potentials varied in shape and size according to the spatial relationship between the position of the electrode tip and the spinal cord and roots within the vertebral canal. Three main types of segmental evoked responses were obtained. One of them was recorded behind the cord dorsum and around the midline, and was composed of fast, sharp early, and slow, late components. This was called a CD potential and its first component was related to the activity of the ascending dorsal funiculus fibres. The second evoked response was the DR potential, and this triphasic compound action potential of very high amplitude and longer duration had no remarkable late component. It was recorded when the tip of the intrathecal electrode was lateral to the midline within the vertebral canal, and was then related mostly to activity of the spinal roots. Another kind of potential was called PH potential. It had a very small triphasic spike and two later components with prominent negativities being higher than the first spike. This potential might be related to the electrode tip position facing, and close to, the posterior horn of the spinal gray matter. Late components of the segmental evoked potentials were related to the pre- and post synaptic activity of the horizontally oriented fibre within the segmental gray matter of the posterior horn.

Adult↗
PubMed1999

Comparison of transcranial motor evoked potentials and somatosensory evoked potentials during thoracoabdominal aortic aneurysm repair.

OBJECTIVE: To compare transcranial motor evoked potentials (tc-MEPs) and somatosensory evoked potentials (SSEPs) as indicators of spinal cord function during thoracoabdominal aortic aneurysm repair. SUMMARY BACKGROUND DATA: Somatosensory evoked potentials reflect conduction in dorsal columns. tc-MEPs represent anterior horn motor neuron function. This is the first study to compare the techniques directly during thoracoabdominal aortic aneurysm repair. METHODS: In 38 patients, thoracoabdominal aortic aneurysm repair (type I, n = 10, type II, n = 14, type III, n = 6, type IV, n = 8) was performed using left heart bypass and segmental artery reimplantation. tc-MEP amplitudes <25% and SSEP amplitudes <50% and/or latencies >110% were considered indicators of cord ischemia. The authors compared the response of both methods to interventions and correlated the responses at the end of surgery to neurologic outcomes. RESULTS: Ischemic tc-MEP changes occurred in 18/38 patients and could be restored by segmental artery reperfusion (n = 12) or by increasing blood pressure (n = 6). Significant SSEP changes accompanied these tc-MEP events in only 5/18 patients, with a delay of 2 to 34 minutes. SSEPs recovered in only two patients. In another 11 patients, SSEP amplitudes fell progressively to <50% of control without parallel tc-MEP changes or association with cross-clamp events or pressure decreases. At the end of the procedure, tc-MEP amplitudes were 84 +/- 46% of control. In contrast, SSEP amplitudes were <50% of control in 15 patients (39%). No paraplegia occurred. CONCLUSION: In all patients, tc-MEP events could be corrected by applying protective strategies. No patient awoke paraplegic. SSEPs showed delayed ischemia detection and a high rate of false-positive results.

Adult↗
PubMedDate not supplied

The neurophysiology of auditory perception: from single units to evoked potentials.

Evoked electric potential and magnetic field studies have the immense benefit that they can be conducted in awake, behaving humans and can be directly correlated with aspects of perception. As such, they are powerful objective indicators of perceptual properties. However, given a set of evoked potential and/or evoked field waveforms and their source locations, obtained for an exhaustive set of stimuli and stimulus contrasts, is it possible to determine blindly, i.e. predict, what the stimuli or stimulus contrasts were? If this can be done with some success, then a useful amount of information resides in scalp-recorded activity for, e.g., the study of auditory speech processing. In this review, we compare neural representations based on single-unit and evoked response activity for vowels and consonant-vowel phonemes with distinctions in formant glides and voice onset time. We conclude that temporal aspects of evoked responses can track some of the dominant response features present in single-unit activity. However, N1 morphology does not reliably predict phonetic identification of stimuli varying in voice onset time, and the reported appearance of a double-peak onset response in aggregate recordings from the auditory cortex does not indicate a cortical correlate of the perception of voicelessness. This suggests that temporal aspects of single-unit population activity are likely not inclusive enough for representation of categorical perception boundaries. In contrast to population activity based on single-unit recording, the ability to accurately localize the sources of scalp-evoked activity is one of the bottlenecks in obtaining an accessible neurophysiological substrate of perception. Attaining this is one of the requisites to arrive at the prospect of blind determination of stimuli on the basis of evoked responses. At the current sophistication level of recording and analysis, evoked responses remain in the realm of extremely sensitive objective indicators of stimulus change or stimulus differences. As such, they are signs of perceptual activity, but not comprehensive representations thereof.

Animals↗
PubMed1994

Influence of age on cerebral potentials evoked by oesophageal balloon distension in humans.

The recording of viscerosensory cerebral evoked potentials is a new field in the research on gastrointestinal perception. The aim of our study was to assess the relationships between age and peak amplitudes and latencies of cerebral potentials evoked by balloon distension of the human oesophagus. In 16 healthy volunteers (aged 21-59 years), cerebral evoked potentials were recorded from a midline scalp electrode, using a series of 50 rapid balloon inflations with 13 ml of air. Peak to peak amplitudes (N1-P1, P1-N2) and peak latencies (N1, P1, N2) were assessed. Inverse correlations were found between age and N1-P1 amplitude (P < 0.05), and between age and P1-N2 amplitudes (P < 0.05). N1 and P1 latencies were significantly longer in elderly patients (N1: P < 0.05; P1: P < 0.05). Amplitudes and peak latencies of cerebral potentials evoked by balloon distension of the oesophagus are age-dependent. In cerebral evoked potential studies, patients and healthy controls should be age-matched.

Adult↗
PubMed2000

Brainstem auditory evoked potentials and somatosensory evoked potentials in cerebral malaria--a prognostic significance.

OBJECTIVE: To study the changes in brainstem auditory evoked potentials (BAEPs) and somatosensory evoked potentials (SSEPs) in cerebral malaria and to see their prognostic significance. METHODS: BAEPs and right median nerve SSEPs were performed in 25 adult patients of strictly defined cerebral malaria in acute stage in a semi-dark, sound proof chamber on four channel computerized multi-basis OTE-Biomedica machine in department of neurology, SP Medical College, Bikaner. RESULTS: The abnormalities of BAEPs were delayed peak latency of wave III in 13/25 (52%) and wave V in 20/25 (80%) patients and delayed interpeak latencies (IPLs) of wave I-III in 9/25 (36%), wave I-V in 15/25 (60%) and wave III-V in 12/25 (48%) patients. In SSEPs delayed N20 was seen in 11/25 (44%); delayed IPLs of N13-N20 (central conduction time; CCT) in 12/25 (48%) patients. Distorted N20 was recorded in 12/25 (48%) patients. Both N13-N20 IPLs in SSEPs and wave III-V IPLs in BAEPs were delayed in five patients and all of them expired. Delayed N13-N20 with normal III-V IPLs was present in seven patients and two of them died, whereas delayed III-V IPLs with normal N13-N20 was present in seven patients, and one of them expired. In remaining six patients both the parameters were normal and one of them died. CONCLUSIONS: The values of BAEPs and SSEPs were abnormal in patients of cerebral malaria and it was observed that BAEPs/SSEPs alone was not useful for predicting the outcome of coma, whereas abnormalities in both was predictive of worst prognosis. The changes in evoked potentials (BAEPs and SSEPs) could be due to either interruption of conduction in central pathways because of structural changes due to petechial hemorrhages and malarial granuloma at multiple levels in the brain including brainstem or due to metabolic abnormalities.

Adolescent↗
PubMed1990

Brainstem auditory evoked potentials and somatosensory evoked potentials in pontine haemorrhage. Correlations with clinical and CT findings.

Electrophysiological studies were performed on 17 patients with pontine haemorrhage. Most had had massive hypertensive bleeding, leading to death in 12 of them within 3 months. Of the 5 surviving patients, 2 had a poor outcome and 3 a moderately good outcome. Brainstem auditory evoked potentials (BAEPs) and somatosensory evoked potentials (SEPs) were recorded in all patients, mostly with serial recordings. BAEPs were pathological in all patients. The most important finding was a reduction in amplitude or loss of waves. With a loss of waves after wave II bilaterally all patients died; the 2 patients with a normal amplitude of waves I-V at least on one side, survived in good condition. Further prognostic conclusions were not possible. The correlation with the clinical state was limited and was best for small unilateral tegmental haemorrhages. Eleven of the 17 patients suffered bilateral loss of the N20 component of the median nerve SEP. All these patients died. In patients with unilateral loss of the SEP the outcome could be favourable even if the bleeding extended across the midline. 'Subcortical' SEPs were not significantly altered. EEG findings in 15 and visual evoked potentials in 4 patients showed preserved forebrain electrical activity even in patients in poor condition.

Adult↗
PubMedDate not supplied

[Electrophysiologic follow-up (brainstem acoustic evoked potentials and visual evoked potentials) in chronic uremic patients treated with various therapeutic protocols].

The aim of our study was to evaluate the possibility of recording also sub-clinical alterations of the Central Nervous System during chronic uremia by means of the study of Pattern-Reversal Visual Evoked Potentials (P-R V.E.Ps) and Brainstem Acoustic Evoked Potentials (B.A.E.Ps), to study their behaviour in answer to different kinds of treatment and follow their possible modifications during the time. With this purpose the uremic patients were divided into therapeutic groups: only dietetic and pharmacological treatment, conventional hemodialysis, hemodiafiltration, peritoneal dialysis and well functioning kidney transplant. A first record of Evoked Potentials was made in 1985 and the values were compared with those obtained by an homogeneous control group. The same patients were evaluated 3 years later and the two records results were compared. In both cases the most important percentage of altered values were found in the group under only dietetic and pharmacological treatment, the less important in the transplanted patients and these data were stable at the follow-up. So that Evoked Potentials are a good test in chronic uremia and the best therapy able to hinder the alterations of Central Nervous System arising during chronic renal failure would seen to be kidney transplant.

Adult↗
PubMedDate not supplied

Visual evoked potential & brainstem auditory evoked potentials in acute attack & after the attack of migraine.

PURPOSE: To study the effect on visual evoked potential (VEP) and brainstem auditory evoked potential (BAEP) at time of acute attack and after 7 days of the attack of migraine. MATERIAL AND METHODS: We studied BAEP and pattern reversal VEP in 25 patients during acute attack and after 7 days of the attack. The diagnosis of migraine was established according to criteria given by international headache society (IHS). Peak and interpeak latencies (IPL's) of BAEP and P100 latency of VEP were the main criteria to judge abnormalities. RESULTS: There were prolonged peak and interpeak latencies in BAEP and prolonged peak latency (P100) in VEP at the time of acute attack of migraine. The data of these abnormal recording were highly significant. After 7 days when the attack was over, we recorded the BAEP and VEP again. The observation obtained at this time was comparable to normal values. CONCLUSIONS: From the observation of this study we can safely conclude that in acute attacks of migraine there may be some pathological changes in different areas of brain and brainstem, producing changes in evoked potential which are statistically highly significant. However, these changes are reversible, as the values of BAEP & VEP on 7th day after the attack were comparable to those observed in normal healthy control.

Acute Disease↗
PubMed1997

[The effect of volatile anesthetics on EEG, auditory evoked potentials and somatosensory evoked potentials in dogs].

We compared the changes in EEG and evoked potentials among dogs anesthetized with halothane (H), isoflurane (I) or enflurane (E). The mean frequency of EEG with H was higher than that with I and E at 1 and 2 MAC. The amplitude of the 5th wave of auditory brain stem response with H was larger than that with I and E at 2 MAC. In terms of the Pa wave of auditory middle latency response, the amplitude with H was larger than that with E at 1 MAC, and the incidence of the wave with H was higher than that with I and E at 2 MAC. The amplitudes of N2 and N3 waves of somatosensory evoked potentials with H and I tended to decrease with increasing MAC, whereas those with E increased temporarily at 1 MAC. The inhibitory effects on EEG and evoked potentials were different among three volatile anesthetics.

Anesthetics, Inhalation↗
PubMed2003

Sensitivity of laser-evoked potentials versus somatosensory evoked potentials in patients with multiple sclerosis.

OBJECTIVE: Somatosensory evoked potentials (SEPs) play a less important role in the diagnosis of multiple sclerosis (MS) than visually evoked potentials. Since standard SEPs only reflect the dorsal column function, we now investigated spinothalamic tract function in patients with MS using laser-evoked potentials (LEPs). METHODS: LEPs to thulium laser stimuli (3ms, 540 mJ, 5mm diameter) were recorded from 3 midline positions (Fz, Cz, Pz) in 20 patients with MS, and 6 patients with possible but unconfirmed MS. Peak latencies and peak-to-peak amplitude of the vertex potential negativity (N2) and positivity (P2) were evaluated and compared with normative values from 22 healthy control subjects. Median and tibial nerve SEPs were recorded with standard methods. Depending on the results of sensory testing, two skin areas (both hands, both feet, or one hand and foot of the same body side) were assessed in each patient. RESULTS: In group comparisons, LEPs in patients with MS were significantly delayed and reduced in amplitude compared with healthy subjects (P<0.001) or patients with suspected but unconfirmed MS (P<0.05). In intraindividual comparisons within the patients with MS, LEP amplitude was significantly lower (P<0.01) and latencies were significantly longer (N2: P<0.01; P2: P<0.05) for a clinically hypoalgesic skin area than an unaffected control area. On a single case basis, LEPs were abnormal in 12 (60%) and SEPs in 8 (40%) of the patients with MS; combined analysis of LEPs and SEPs raised sensitivity to 75% (15 patients). LEPs were also abnormal for 7 skin areas with clinically normal nociception and thermal sensitivity, indicating subclinical lesions. Standard SEPs detected subclinical lesions in 5 areas with normal tactile sensitivity. CONCLUSIONS: In patients with multiple sclerosis, spinothalamic tract function and LEPs were impaired more often than dorsal column function and SEPs. LEPs also detected subclinical lesions. Combined assessment of LEPs and SEPs can help to document dissemination of demyelinating CNS lesions and thus contribute to the diagnosis of multiple sclerosis.

Adult↗
PubMed1997

[Wavelet transform and its application to brain evoked potentials].

Evoked potentials (EP) is an important bioelectric signal for studying the neural processes, but it is immersed in very strong background noise. Extraction of EP from a single trial has been a topic of great theoretical and clinical interest. In this paper, a new signal processing method--wavelet transform is introduced and several applications are given.

Animals↗
PubMed1991

A beamformer for the acquisition of evoked potentials.

Evoked potentials (EP) contain information about various physiological parameters and the estimation and detection of these signals can aid in the diagnosis of many pathological conditions. However, the signal-to-noise ratio (SNR) for EP measurement is often very low, and thus signal processing techniques must be employed to enhance the SNR. A delay and sum beamformer acquisition system has the potential for significant SNR improvement in EP measurements. In this communication it is shown that an electrode array acquisition system implements a uniform coherent delay and sum beamformer. The performance of the beamformer is characterized in terms of the number of electrodes, and cross-channel correlation. When compared to conventional ensemble averaging, the beamformer reduces the number of response repetitions required to achieve a given SNR by a factor which approaches the number of channels in the acquisition system.

Electrodes↗
PubMed1979

Digital filtering of on-line evoked potentials.

Evoked potentials recorded automatically at frequent intervals are a useful adjunct for monitoring head injury patients; however, unaveraged residual noise due to patient movement and synchronization of th- stimulus to harmonics of the line frequency is sometimes present. A frequency analysis was performed on 23 records with varying degrees of unaveraged noise and the results were used to design a digital filter. The frequency content of the records analyzed was largely contained in a band from D.C. to 200 Hz. A zero phase shift lowpass digital filter with a cutoff frequency of 200 Hz eliminated the majority of noise. Despite the low frequency content, a wide-band amplifier is still required to avoid waveform distortion, particularly latencies. Digital filters can then be applied without causing distortion.

Computers↗
PubMed1991

Optimal a posteriori time domain filter for average evoked potentials.

Evoked potentials measured with scalp electrodes are often described as a deterministic process corrupted by unrelated noise. The common procedure to determine the signal is to average N repetitive measurements. By obtaining additional information from the N measurements, signal detection can be improved. An algorithm that estimates the signal autocorrelation from N given measurements is proposed. The estimator is consistent and unbiased, and its variance tends to 0 as o(N). Two filters that are applied to the average response are introduced. Both depend on the estimation of the signal and the noise autocorrelations. One filter is based on the assumption that the average response is a stationary process. The second filter coefficients are obtained by minimizing the mean squared error (MSE) of an optimal filter of a nonstationary process applied on a single sweep. When a small number of sweeps are averaged, the stationary assumption is adequate, and the MSE of the stationary optimal filter is two to five times less than the MSE of the average response. When a large number of measurements are considered, the error in estimating the autocorrelations decreases. In this case, applying the optimal filter for a nonstationary process leads to a significant improvement in the signal estimation.

Algorithms↗