M. Hoke

Increased auditory cortical representation in musicians.

Acoustic stimuli are processed throughout the auditory projection pathway, including the neocortex, by neurons that are aggregated into ‘tonotopic’ maps according to their specific frequency tunings. Research on animals has shown that tonotopic representations are not statically fixed in the adult organism but can reorganize after damage to the cochlea or after training the intact subject to discriminate between auditory stimuli. Here we used functional magnetic source imaging (single dipole model) to measure cortical representations in highly skilled musicians. Dipole moments for piano tones, but not for pure tones of similar fundamental frequency (matched in loudness), were found to be enlarged by about 25% in musicians compared with control subjects who had never played an instrument. Enlargement was correlated with the age at which musicians began to practise and did not differ between musicians with absolute or relative pitch. These results, when interpreted with evidence for modified somatosensory representations of the fingering digits in skilled violinists, suggest that use-dependent functional reorganization extends across the sensory cortices to reflect the pattern of sensory input processed by the subject during development of musical skill.

Relationship of transient and steady-state auditory evoked fields ☆

Transient and steady-state auditory evoked fields (AEFs) to brief tone pips were recorded over the left hemisphere at 7 different stimulus rates (0.125-39 Hz) using a 37-channel biomagnetometer. Previous observations of transient auditory gamma band response (GBR) activity were replicated. Similar rate characteristics and equivalent dipole locations supported the suggestion that the steady-state response (SSR) at about 40 Hz represents the summation of successive overlapping (10 Hz) middle latency responses (MLRs). On the other hand, differences in equivalent dipole locations and habituation effects suggest that the magnetically recorded GBR is a separate phenomenon which occurs primarily at low stimulus rates and is unrelated to either the magnetically recorded MRL or SSR.

Identification of sources of brain neuronal activity with high spatiotemporal resolution through combination of neuromagnetic source localization (NMSL) and magnetic resonance imaging (MRI)

The locations of the origin of wave M100 of the auditory evoked magnetic field in response to tone bursts of different carrier frequencies, obtained through dipole localization methods (DLM), were related to cerebral structures, displayed by coronal MRI (magnetic resonance imaging) tomograms of the respective subjects. This was done by displaying the landmarks which served as reference for the neuromagnetic measurements in MRI tomogram (reference plane). All calculated source locations project exactly onto the transverse temporal gyri (Heschl) in which the primary auditory cortex, the supposed origin of wave M100, is located. The results highlight the exceptional capabilities of a combination of these 2 non-invasive, high-resolution techniques for functional diagnosis.

Objective evidence of tinnitus in auditory evoked magnetic fields

The waveforms of the auditory evoked magnetic field in normal-hearing individuals and patients suffering from tinnitus are distinctly different. In tinnitus patients, the magnetic wave M200 (corresponding to the electric wave P200, or P2) is delayed and only poorly developed or even completely missing, while the amplitude of the magnetic wave M100 (corresponding to the electric wave N100, or N1) is significally augmented. A very characteristic feature turned out to be the amplitude ratio of the two waves M200 and M100. Below the age of 50, the amplitude ratio M200/M100 represents a clear-cut criterion to distinguish between tinnitus patients and individuals without tinnitus. In tinnitus patients, the ratio is less than 0.5, independent of age, whereas, in young and middle-aged normal-hearing individuals, it is greater than 0.5. Since in normal-hearing individuals the average amplitude ratio decreases linearly with age, the clusters of amplitude ratios of the two groups begin to overlap beyond the age of 50. The hypothesis is put forward that the decrease of the average amplitude ratio in normal-hearing individuals reflects a degenerative process, probably initiated by multiple exogenous and endogenous factors, which leads to sustained neural activity in the generators of wave M200 and eventually gives rise to the sensation of tinnitus. The absence or poor development of wave M200 is a concomitant phenomenon, resulting from the involved generators being less responsive to external stimuli.

Weighted averaging — theory and application to electric response audiometry☆

A weighted averaging technique has been developed to overcome the shortcomings of conventional, unweighted averaging in the case of nonstationary noise. The technique is based on the linear least-mean-square estimate of a periodic signal in a simple model of non-stationary noise. This estimate weights each recorded epoch according to the magnitude of the noise within the epoch. In the estimation of a known signal, the weighted averaging procedure yielded smaller root-mean-square errors in comparison with the normal unweighted average. The weighted averaging procedure offers many advantages over conventional averaging or averaging with automatic gain control preamplifiers.

Possibilities and limitations of weighted averaging

A statistical analysis of a weighted averaging procedure for the estimation of small signals buried in noise (Hoke et al. 1984a) is given. The weighting factor used by this method is in inverse proportion to the variance estimated for the noise. It is shown that, compred to conventional averaging, weighted averaging can improve the signal-to-noise ratio to a high extent if the variance of the noise changes as a function of time. On the other hand, uncritical application of the method involves the danger that the signal amplitude is underestimated. How serious this effect is depends on the number of degrees of freedom available for the estimation of the weighting factor. The effect can be neglected, if this number is sufficiently increased by means of an appropriate preprocessing.

Binaural interaction in brainstem auditory evoked potentials elicited by frequency-specific stimuli

The frequency specificity of the binaural interaction in brainstem auditory evoked potentials (BAEP) was investigated in ten normal-hearing young adults. A novel stimulus paradigm was devised to reduce the influence of the acoustic reflex (middle ear muscle contraction) on the BAEP, and to minimize the effect of variations in noise level. Sequences of six stimuli (rarefaction clicks or Gaussian-shaped tone pulses with carrier frequencies of 1, 2, 4 and 6 kHz) were periodically presented in the following order: right monaural, left monaural, binaural, left monaural, right monaural, binaural, with an interstimulus interval of 22 ms. Since the sequence of monaural stimuli with binaural stimuli interposed produces a uniform loudness and since the acoustic reflex is a consensual reflex, the relative high stimulus repetition rate (approx. 45/s) causes a muscle contraction which is equal on both sides and rather constant in time. This paradigm turned out to be usable for stimulus intensities as high as 80 dB nHL. The binaural difference potential (BDP) was computed by subtracting the sum of the monaurally (ipsilateral and contralateral) evoked potentials from the binaurally evoked potential. The major binaural interaction occurred in the latency range of BAEP waves V and VI, and there was no evidence of interaction in the earlier portion of the BAEP. Both latency and amplitude of the BDP components were evaluated statistically. The latency of the BDP components - except of the lasted one - showed an almost linear dependence both on stimulus intensity and stimulus frequency. The amplitude grew larger with decreasing frequency, and the visual detection threshold elevated as the stimulus frequency increased. Click stimuli, however, produced the largest amplitudes with lowest visual detection threshold. This novel stimulus paradigm appears to be most suitable for routine clinical investigations since high stimulus intensities can be used.

Possibilities and limitations of magnetic source imaging of methohexital-induced epileptiform patterns in temporal lobe epilepsy patients.

The usefulness of MEG-based techniques in lateralizing and localizing the epileptogenic area was investigated in the present study. Spontaneous and methohexital-induced spikes were studied in a group of 15 patients with temporomesial epilepsy using a 37-channel neuromagnetometer. The accuracy of the magnetic source imaging was compared to the results of electrocorticographic (ECoG) recordings. Differences of drug-induced spike densities in the MEG recordings between both sides confirmed a similar lateralizing power of the MEG and ECoG recordings. Source location analyses based on a moving dipole model resp. a rotating dipole model were performed using a spherical head model. After subdivision of the volume of each patients head, 8 cm3 cubicles containing at least 3 source locations were projected onto the individual MRI scan and resulted in source locations within or close to the presurgically defined primary epileptogenic area only in 3 of the 15 patients. Spike induction by methohexital has the advantage of shortening the recording period as compared to recordings of interictal epileptiform discharges. However, the correlation analyses of spike densities from MEG and ECoG recordings and the source location analyses from MEG recordings indicate that spike generated in deep temporomesial structures may escape the MEG registration.

The magnetic counterpart of the contingent negative variation

The magnetic counterpart of the CNV, the contingent magnetic variation (CMV), was investigated in an Go/No Go design: subjects moved their index finger to the offset of a 4 sec tone of a certain frequency in the Go condition and were asked not to move during presentation of a 4 sec tone of different frequency in the No Go condition. During the preparatory interval, both the CMV and the electrical wave form followed a similar time course and both produced an equally pronounced statistical difference between conditions (Go and No Go). Compared to the variability in the auditory evoked fields, the CMV showed considerably more variance in the field distribution across subjects. The polarity reversal across the temporal surface of the head and the pronounced amplitudes over inferior temporal areas led us to conclude that a significant temporal activity contributes to both the late and the early CMV. However, neither for the early nor for the late CMV component did a single equivalent dipole prove to be a satisfying model. The data are consistent with the suggestion that the earlier as well as the later aspects of the CMV are fed through distributed sources in motoric, sensory and association areas, a distribution with considerable intersubject variability.

Tinnitus remission objectified by neuromagnetic measurements

In a previous paper of ours (Hoke et al., 1989a) the hypothesis was put forward that the amplitude ratio of the two major waves of the auditory evoked magnetic field (AEF), M200/M100, is an objective measure which allows to discriminate between individuals suffering from tinnitus (ratio less than 0.5) and individuals without tinnitus (ratio greater than 0.5). We have now been able to trace the process of tinnitus remission in one exemplary case during a period of 256 days after acute onset of tinnitus (due to a noise trauma), in which the amplitude ratio recovered from 0 to a normal value of approximately 1. This very first objectification of tinnitus remission strongly supports our hypothesis and indicates that AEF may become an indispensable, invaluable tool in both tinnitus research and management.