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M Scherg

Publications and source records attributed to M Scherg.

At least 55 records · Page 3Linked to original sources
PubMed1991

[Analysis of the generators of early cortical somatosensory evoked potentials (N. medianus) using dipole source analysis: initial results].

There is still much controversy about the contribution of the brainstem, the thalamus and the somatosensory and motor areas of the various scalp recorded peaks of the somatosensory evoked potentials (SEP) after median nerve stimulation. This study addressed the generator problem of the scalp recorded potentials using brain electric source analysis. In 11 normal subjects median nerve SEPs were recorded from 32 locations. The brain-electric-source-analysis revealed a minimum of 5 sources with overlapping activities in the interval of 12-35 ms post stimulus. The initial deflections were in the time range of the scalp peaks (P14, P18, N20, P22, N30), but there was no single source to fully explain a scalp peak except for the brainstem source of P14. The other sources appeared to reflect activities of the thalamo-cortical-pathway (P18), of the somatosensory areas 3b (N20) and 1 (P22) and of a fifth source (contribution maximal around 30 ms) with no consistent location. The close location of multiple sources makes the precise separation and localisation of the various sources quite difficult in individual data sets.

Electroencephalography↗
PubMed1989

Diagnosis of unilateral telencephalic hearing disorders. Evaluation of a simple psychoacoustic pattern discrimination test.

A new psychoacoustic pattern discrimination test (PPDT) has been validated for the diagnosis of telencephalic hearing disorders. In this test, regular sequences of noise bursts or click trains are presented dichotically, and randomly occurring monaural changes in intensity or click pattern have to be discriminated. The PPDT was administered to 48 control subjects and to 62 patients with circumscribed cerebrovascular lesions. Involvement of telencephalic auditory structures (TAS) was assessed from CT scans. Abnormality in the PPDT was highly correlated with the incidence of a TAS lesion. The most prominent abnormality consisted of an increased error rate (missed discriminations) on the ear contralateral to the TAS lesion, comparable to the ear effect described for former dichotic tests. In normals, no ear dominance, which might have confounded the interpretation of lesion effects in patients, was observed for our test material. Also, the influence of peripheral hearing loss on test results was small. Criteria for the clinical evaluation of the PPDT were developed and yielded a good sensitivity (76.1%) when related to the CT scan data. Abnormality in the PPDT was also confirmed by reduced auditory evoked dipole source potentials in the lesioned hemisphere. Disturbances in auditory language comprehension in aphasic patients were not significantly related to a positive PPDT result, but a questionnaire about hearing difficulties revealed a close association of PPDT abnormality and auditory perceptual disturbances. These occurred only in difficult hearing environments, for example, the cocktail party situation. It must be concluded that unilateral TAS lesions lead to auditory perceptual impairment and communication problems, which should be given adequate attention during neuropsychological rehabilitation.

Adult↗
PubMed1987

Peripheral origin of BAEP wave II in a case with unilateral pontine pathology: a comparison of intracranial and scalp records.

In a patient with unilateral hearing impairment due to a predominantly ipsilateral pontine pathology, BAEP peak II in a vertex to ipsilateral mastoid derivation (BAEP-II) was preserved on stimulation of the affected ear. All later activity was lost. Intracranial recording from electrodes intraoperatively placed over both cochlear nuclei on the floor of the IVth ventricle showed potentials with size and wave form comparable to the scalp records. Thus only peripheral sources, distant from the ventricular electrode, seem to contribute to scalp BAEP-II. Upon stimulation of the unaffected ear considerably larger intracranial activity was seen with peak 2-4 wave shapes obviously different from the scalp BAEP II-IV. It is concluded that intracranial electrodes sense a complex field distribution predominantly originating in structures to which they are in close electrical connection. This activity, however, need not necessarily project to the scalp.

Brain Neoplasms↗
PubMed1986

Psychoacoustic and electrophysiologic correlates of central hearing disorders in man.

Evaluation of central hearing disorders in neuropsychologic patients is handicapped by their insufficient ability to describe auditory deficits and by the lack of easily applicable audiological tests. A novel psychoacoustic discrimination test (PDT) was developed to determine ear asymmetries in the discrimination of changes in intensity, frequency, or temporal structure of regularly presented dichotic stimuli. In 19 of 21 patients with lesions of the auditory cortex or the acoustic radiation according to CT scan evaluation a higher error score was observed for target stimuli presented at the ear contralateral to the side of brain infarction (6 right, 15 left). In the remaining 2 and in 3 other patients with lesions sparing auditory structures no significant ear asymmetries were seen. This may indicate that auditory perception is reduced in patients with only one intact auditory cortex or one intact acoustic radiation, possibly because of a limitation in information processing capacity. Auditory evoked potential results are presented for a normal subject and two patients to illustrate electrophysiologic correlates of central hearing disorders. Using a transformation of scalp into dipole source activity (Scherg and von Cramon 1986), a unilateral loss of middle latency activity was found in case A, who had a lesion of the left acoustic radiation. The extended lesion of the right auditory cortex in case B resulted in a loss of both middle and late latency dipole source potentials of the right temporal lobe. In both cases a corresponding increase in the PDT error score on the contralateral ear was found.

Adult↗
PubMed1986

Evoked dipole source potentials of the human auditory cortex.

A new description of evoked potential activity in terms of 'dipole source potentials' is presented, based on the physical laws relating intracranial electrical activity and scalp potentials. Recorded at a sufficiently distant electrode, the electrical activity of a spatially restricted region can be approximated uniquely by a time varying dipole vector field with stationary equivalent dipole location. Each of its 3 projections on a 3-dimensional coordinate system presents an accordingly defined 'dipole source component.' Magnitudes of these components are functions of time, named 'dipole source potentials.' The 2-dimensional coronal scalp distribution of middle and late AEPs, obtained in 15 normal subjects, could best be decomposed into tangential and radial dipole source components, originating from the auditory structures in both temporal lobes. Initial tangential activity (N19t-P30t) appeared to arise in primary auditory cortex, and initial radial activity (N27r-P39r) in secondary auditory cortex, in view of the similarity with intracranial records (Celesia 1976). Unilateral lesions of the acoustic radiation abolished ipsilateral MAEP dipole source potentials. Lesions involving AI/II and AAI also abolished the LAEP source potentials in the damaged hemisphere. The normal dipole source potentials in the intact hemisphere fully explained scalp distributions in these patients. In cases with assumed interruption of primary cortical input, presence of late dipole source potentials, which were delayed by 20-30 msec, probably reflected cortical activation via commissural fibres.

Adult↗
PubMed1986

P3b peak latency dependency on the point of information delivery within a stimulus.

An acoustic matching experiment was conducted consisting of three quasirandom conditions in which the sequence of tones comprising the second stimulus was changed at the beginning, at the end, or not at all. Latency of maximum positivity (P3b) was measured for each subject between Pz and the averaged mastoids. Evaluation of mismatch conditions demonstrated that peak latency depended on the point of information delivery within the stimulus. Further, peak latency within both mismatch conditions, when adjusted for the timing of tone sequence change, was approximately equally delayed in comparison to match. The implications of the results for information processing models of attention are discussed.

Acoustic Stimulation↗
PubMed1985

A new interpretation of the generators of BAEP waves I-V: results of a spatio-temporal dipole model.

Brain-stem auditory evoked potential (BAEP) scalp distribution, recorded in 10 normal subjects, was analysed using a spatio-temporal dipole model. This model can simulate surface wave forms due to overlapping activity from multiple dipolar sources within a 3-shell head model. The conventional 5-peak (I-V) hypothesis could not fully account for the experimental BAEP distribution, particularly around waves I- and III-. When the temporal course of dipole strength was modelled according to a triphasic compound action potential, 6 dipolar sources were sufficient to fit all BAEP wave forms. Location, orientation and latency of the dipoles indicated generation of dipole 1 at the distal end of the auditory nerve (AN), of dipole III in, or near to, the cochlear nucleus (CN), of dipole III- in the trapezoid body and of dipoles IV and V in the superior olivary complexes and in the lateral lemnisci. Dipole I-, peaking only 0.65 msec after wave I, is suggested to result from the electric inhomogeneity at the porus acusticus internus. Wave II required no extra model dipole, but was attributable, in part, to the second peak of dipole I. Estimation of latencies based on AN conduction velocity, synaptic delay and differences in AN length amongst species confirmed that second order neuronal activity cannot arise before wave III. Second order axons, spreading widely through the brain-stem, apparently cause major contributions also to waves III-, IV and V. The new source hypothesis must leave open questions concerning the amount of contribution from third order neurones or from ipsi- versus contralateral structures to the IV/V wave complex.

Adult↗
PubMed1985

Two bilateral sources of the late AEP as identified by a spatio-temporal dipole model.

A new spatio-temporal dipole model is presented, which enables prediction and analysis of scalp potential wave forms due to spatio-temporal overlap of multiple generators. Each generator is thought to represent a local neural subset, the electric activity of which can be modelled by an equivalent dipole with stationary location and orientation closely related to the spatial organization of the neural subset. The temporal course of dipole magnitude is assumed to depict the external far field due to the compound discharge processes of the generator. Simulations of uni- and bilateral dipoles within the temporal lobe, oriented vertically and horizontally, demonstrate how spatio-temporal overlap mag bring about the 'vertex response' of the late AEP and the wave form changes observed over temporal sites. Analyses of late AEPs reported for a coronal chain of electrodes by Peronnet et al. (1974) and Vaughan et al. (1980) revealed that the wave forms in the 60-250 msec range could be perfectly matched at all electrodes by model wave forms due to 2 bilateral sources within the temporal lobe. Their locations, orientations and their latency difference of about 30 msec suggest consistently that the sequential activation of primary and secondary auditory cortices is the predominant source to the late AEPs.

Brain Mapping↗
PubMed1984

Brain-stem auditory-evoked potentials in post-comatose patients after severe closed head trauma.

Brain-stem auditory-evoked potentials (BAEPs) were recorded in 35 post-comatose patients after severe closed head trauma. Compared to the normal BAEPs obtained from 30 normal controls, significant individual abnormality could be detected in only 2 patients. This abnormality consisted of lateral asymmetry, but not prolonged interpeak latencies or abnormal amplitude ratios. Further, no differences were found between groups as regards interpeak latencies. A significant reduction of BAEP amplitudes was seen in the patient group, being most pronounced in the 13 patients with hearing loss. It is concluded that the BAEP does not sufficiently reveal brain-stem lesions expected to persist after severe closed head trauma, presumably because the structures generating waves IV and V are not permanently affected. The amplitude reductions observed may be explained by the diffuse cochlear damage known to occur concurrently with head trauma.

Adolescent↗
PubMed1984

Effects of high-pass filter frequency and slope on BAEP amplitude, latency and wave form.

The effects of increases in analogue high-pass filter frequency and slope on the BAEP were investigated in a group of 20 subjects. Frequency effects were additionally compared to the wide-band recorded signal and slope effects to a zero-phase shift filter, all filters being digitally simulated. Absolute amplitude, latency and interpeak latency were significantly reduced by both progressive filter frequency and steepness. Additionally, different effects on the ascending and descending limbs of waves I, III and V were observed which explain wave form distortions. The results suggest that filtering should be limited to, at most, 6 dB/oct and 100 Hz frequency.

Adult↗
PubMed1983

Frequency specificity of simultaneously recorded early and middle latency auditory evoked potentials.

Early and middle latency auditory evoked potentials were recorded simultaneously with a 2-channel wide-band recording technique. Latency and amplitude distributions of components V, Na and Pa were determined in 20 normal hearing adults for 3 different stimuli (click, plop, 500 Hz tone burst) presented at 70, 30 and 20 dB HL. Normative latency data of the middle latency components are presented. The detectability and amplitude of the Na-Pa complex were considerably larger than those of wave V at 20 and 30 dB HL for the two low frequency stimuli. Ten ears exhibiting high frequency sensorineural hearing loss were examined. In these cases, only click-evoked Na and Pa, but not wave V, were observed down to 20 dB HL. Also, the click latency-intensity function of Na-Pa was found to be similar to the normal low frequency latency functions. The results consistently suggested that the low frequency contributions to wave V cancel in phase because of short duration, whereas the duration of the Na-Pa complex allows for a positive superimposition of all frequency bands. Since these differences in frequency specificity of the EAEP and the MAEP affect latency, care is advised for the interpretation of inter-peak latencies.

Adolescent↗
PubMed1982

Simultaneous recording and separation of early and middle latency auditory evoked potentials.

A method, consisting of wideband (1-5000 Hz) recording, dual artifact rejection (for both EEG and muscle artifacts) and separation of the EAEP by digital high-pass filtering of the averaged signal, is proposed for the simultaneous recording of early and middle latency auditory evoked potentials. The high-pass was realized by a simple recursive filter. The middle (MAEP) latency components were derivable directly from the wideband averaged signal without further filtering, thus avoiding wave form distortions. Through this method additional information on the afferent auditory system can be gained, and auditory threshold determination can be considerably improved. The proposed procedure is implementable on any computer-based EP system and does not require extra amplifiers, filters or separate test runs for the acquisition of the MAEP.

Brain↗
PubMed1982

Distortion of the middle latency auditory response produced by analog filtering.

The effect of analog filtering on the middle latency auditory response (MLAR) was investigated. Substantial waveform distortions and latency shifts were found when using similar filter settings as reported in the literature. The waveform changes amounted to complete polarity reversal of the Na-Pa-Nb pattern due to the high-pass cut-off with a steep slope characteristic. The analog filters were simulated by recursive digital filters and compared with symmetrical digital filters having the same cut-off frequency and slope. MLAR waveform and latencies were scarcely affected by the digital zero-phase shift filters. In contrast, the oscillations in the step function response of the analog filter elucidated how early activity of the MLAR is folded onto later components, leading to a much larger late activity than present physiologically. Since physiological interpretations of the MLAR ought to rely on unbiased recordings, minimal usage of analog filtering is recommended.

Evoked Potentials, Auditory↗
PubMed1980

[Brainstem auditory evoked potentials applied to clinical neurology (author's transl)].

Pathological alterations of the brainstem auditory evoked potential in 5 patients suffering from different kinds of brainstem diseases (reversible tumorous infiltration of the brainstem, multiple sclerosis, mesodiencephalic syndrome, apallic syndrome, braindeath) are shown. The alterations resemble the findings of animal experiments reported by Buchwald and Huang 1975: The particular components of brainstem auditory evoked potentials (waves IV-V, III, II and I) may decrease in amplitude or even completely disappear. The components are affected in a sequential reversed order, i.e. if a particular component has disappeared or shows a significant latency increase, the subsequent components are similarly affected. These observations suggest that the particular components are generated by different structures of the afferent acoustic pathway. This underlines the usefulness of brainstem auditory evoked potential recordings in detecting brainstem disorders.

Adult↗
PubMed1979

Human auditory on- and off-potentials of the brainstem.

Brainstem auditory evoked potentials (BAEP) elicited by bursts of white noise and 2 kHz sine waves were investigated in 3 subjects with normal hearing. The effects of Gaussian-shaped envelopes of varying half-width on the BAEP for stimulus on- and offset are demonstrated. The concept of "virtual trigger time, amplitude or energy" of the acoustic stimulus is proposed for an understanding of the effects of different envelope characteristics on the BAEP. Apart from potentials due to stimulus onset, we also found consistent potential changes after signal switch-off both for bursts of white noise and sinewaves. In the case of white noise these potentials appear to be related to a synchronized decrease in the firing rate of the cochlea and the auditory nerve. In the case of tone-bursts the potentials appear to be related to transient spectral components which elicit a new On-potential during acoustic stimulus offset.

Acoustic Stimulation↗