Measurement of Ocular Vestibular Evoked Myogenic Potentials: Nasion Reference Montage as an Alternative to the Clinical Standard Montage.

OBJECTIVE: To compare two novel electrode montages for ocular, vestibular evoked myogenic potential using single-nasion reference electrodes with the clinical standard montage. STUDY DESIGN: Randomized crossover experiment. SETTING: Tertiary referral center. PARTICIPANTS: Sixty healthy participants. INTERVENTION: Normal hearing and vestibular function were confirmed with an extensive test-battery. All ocular, vestibular evoked myogenic potential settings were measured with air-conducted tone bursts at 100-dB normal hearing level and a frequency of 500 Hz. Three electrode montages were measured in randomized order: the clinical standard montage ("S"), the nasion reference montage ("N"), and the nasion reference montage with a more lateral active electrode ("L"). Upgaze was standardized to 35 degrees. MAIN OUTCOME MEASURES: Detection rate, latency of N1 and P1, peak-to-peak amplitude of N1 and P1, signal-to-noise ratio (SNR), asymmetry ratio (AR), concordance of expert assessment, and reliability. RESULTS: All electrode montages showed detection rates greater than 90%. Latencies for "L" were shorter than for "S" and "N." Amplitudes and SNR for "S" and "N" were higher than for "L," whereas the values for "S" and "N" did not differ significantly. For AR, no significant differences between the montages were assessed. Concordance of experts ranged from 78% for "L" and 89.8% for "N." All montages provided excellent day-to-day reliability (intraclass correlation coefficient ≥0.9) for amplitudes and SNR. CONCLUSIONS: Montage N could be a useful alternative to the clinical standard montage: although results are roughly equivalent, montage N requires one less electrode to do so.

Vestibular Evoked Myographic Correlation.

This work started from the hypothesis that the physiological processes giving rise to the vestibular evoked myogenic potential (VEMP) can be induced not only by transient sounds but also by a continuous stimulation with a stochastic signal. The hypothesis is based on the idea that the number of motor unit action potentials (MUAPs) decreases after a momentary amplitude increase of the effective stimulus, whereas a momentary amplitude decrease has the opposite effect. This concept was theoretically analyzed by assuming that the effective stimulus is closely related to the envelope of the stimulus actually presented. The analysis led to the prediction that the cross-correlation function of the effective stimulus and the measured electromyogram (EMG) has VEMP-like properties. Experiments confirmed this prediction, thus providing evidence of a novel electrophysiological response: the vestibular evoked myographic correlation (VEMCorr). The methodological approach corresponded to a conventional VEMP study, except that the stimulus (delivered with a hand-held minishaker) comprised not only a series of 500-Hz tone pulses (classical VEMP measurement, for comparison) but also sequences of narrow-band noise with a center frequency of 500 Hz (VEMCorr measurement). Each of the 12 test persons showed a clear VEMCorr. Moreover, VEMP and VEMCorr largely resembled each other, as predicted. Apparently they are two different expressions of a more general mechanism that leads to a roughly linear relationship between stimulus envelope and expectation of the EMG. Future applications of the VEMCorr could exploit that a continuous-stimulation paradigm allows for varying the center frequency of the stimulus without changing the relative bandwidth.

Disruption of the auditory response to a regular click train by a single, extra click.

It has been hypothesized that the steady-state response to a periodic sequence of clicks can be modeled as the superposition of responses to single clicks. Here, this hypothesis is challenged by presenting an extra click halfway between two consecutive clicks of a regular series, while measuring the auditory evoked field. After a solitary click at time zero, the click series sounded from 100 to 900 ms, with the extra click presented around 500 ms. The silent period between two stimulus sequences was 310-390 ms (uniformly distributed) so that one stimulation cycle lasted, on average, 1250 ms. Five different click rates between 20 and 60 Hz were examined. The disturbance caused by the extra click was revealed by subtracting the estimated steady-state response from the joint response to the click series and the extra click. The early peaks of the single-click response effectively coincide with same-polarity peaks of the 20-Hz steady-state response. Nevertheless, prediction of the latter from the former proved impossible. However, the 40-Hz steady-state response can be predicted reasonably well from the 20-Hz steady-state response. Somewhat surprisingly, the amplitude of the evoked response to the extra click grew when the click rate of the train was increased from 20 to 30 Hz; the opposite effect would have been expected from research on adaptation. The smaller amplitude at lower click rates might be explained by forward suppression. In this case, the apparent escape from suppression at higher rates might indicate that the clicks belonging to the periodic train are being integrated into an auditory stream, possibly in much the same manner as in classical stream segregation experiments.

Auditory signal detection appears to depend on temporal integration of subthreshold activity in auditory cortex.

The threshold of hearing decreases with increasing sound duration up to a limit of a few hundred milliseconds, whereas other auditory time constants are orders of magnitude shorter. A possible solution to this resolution-integration paradox is that temporal integration occurs more centrally than computations depending on high temporal resolution. But this would require information about subthreshold events in the periphery to reach higher centers. Here we show that this prerequisite is fulfilled. The auditory evoked response to a just perceptible pulse series does basically not depend on whether single pulses are below or above behavioral threshold. The failure to find evidence of temporal integration up to response latencies of 30 ms suggests that the integrator is located more centrally than primary auditory cortex. By using noise to its advantage, the auditory system apparently has established a central integration mechanism that is about as efficient as the peripheral one in the visual system.

Auditory cortex tracks the temporal regularity of sustained noisy sounds.

Neuroimaging studies have revealed dramatic asymmetries between the responses to temporally regular and irregular sounds in the antero-lateral part of Heschl's gyrus. For example, the magnetoencephalography (MEG) study of Krumbholz et al. [Cereb. Cortex 13, 765-772 (2003)] showed that the transition from a noise to a similar noise with sufficient temporal regularity to provoke a pitch evoked a pronounced temporal-regularity onset response (TRon response), whereas a comparable transition in the reverse direction revealed essentially no temporal-regularity offset response (TRoff response). The current paper presents a follow-up study in which the asymmetry is examined with much greater power, and the results suggest an intriguing reinterpretation of the onset/offset asymmetry. The TR-related activity in auditory cortex appears to be composed of a transient (TRon) and a TR-related sustained response (TRsus), with a highly variable TRon/TRsus amplitude ratio. The TRoff response is generally dominated by the break-down of the TRsus activity, which occurs so rapidly as to preclude the involvement of higher-level cortical processing. The time course of the TR-related activity suggests that TR processing might be involved in monitoring the environment and alerting the brain to the onset and offset of behaviourally relevant, animate sources.

The variance modulation associated with the vestibular evoked myogenic potential.

OBJECTIVE: Model considerations suggest that the sound-induced inhibition underlying the vestibular evoked myogenic potential (VEMP) briefly reduces the variance of the electromyogram (EMG) from which the VEMP is derived. Although more difficult to investigate, this inhibitory modulation of the variance promises to be a specific measure of the inhibition, in that respect being superior to the VEMP itself. This study aimed to verify the theoretical predictions. METHODS: Archived data from 672 clinical VEMP investigations, comprising about 300,000 EMG records altogether, were pooled. Both the complete data pool and subsets of data representing VEMPs of varying degrees of distinctness were analyzed. The data were generally normalized so that the EMG had variance one. RESULTS: Regarding VEMP deflection p13, the data confirm the theoretical predictions. At the latency of deflection n23, however, an additional excitatory component, showing a maximal effect around 30 ms, appears to contribute. CONCLUSIONS: Studying the variance modulation may help to identify and characterize different components of the VEMP. In particular, it appears to be possible to distinguish between inhibition and excitation. SIGNIFICANCE: The variance modulation provides information not being available in the VEMP itself. Thus, studying this measure may significantly contribute to our understanding of the VEMP phenomenon.

Modeling the vestibular evoked myogenic potential.

Measuring the vestibular evoked myogenic potential (VEMP) promises to become a routine method for assessing vestibular function, although the technique is not yet standardized. To overcome the problem that the VEMP amplitude depends not only on the inhibition triggered by the acoustic stimulation of the vestibular end organs in the inner ear, but also on the tone of the muscle from which the potential is recorded, the VEMP is often normalized by dividing through a measure of the electromyogram (EMG) activity. The underlying idea is that VEMP amplitude and EMG activity are proportional. But this would imply that the muscle tone is irrelevant for a successful VEMP recording, contradicting experimental evidence. Here, an analytical model is presented that allows to resolve the contradiction. The EMG is modeled as the sum of motor unit action potentials (MUAPs). A brief inhibition can be characterized by its equivalent rectangular duration (ERD), irrespective of the actual time course of the inhibition. The VEMP resembles a polarity-inverted MUAP under such circumstances. Its amplitude is proportional to both the ERD and the MUAP rate. The EMG activity, by contrast, is proportional to the square root of the MUAP rate. Thus, the normalized VEMP still depends on the muscle tone. To avoid confounding effects of the muscle tone, the standard deviation of the EMG could be considered. But the inhibition effect on the standard deviation is small so that the measuring time would have to be much longer than usual today.

Auditory temporal processing in healthy aging: a magnetoencephalographic study.

BACKGROUND: Impaired speech perception is one of the major sequelae of aging. In addition to peripheral hearing loss, central deficits of auditory processing are supposed to contribute to the deterioration of speech perception in older individuals. To test the hypothesis that auditory temporal processing is compromised in aging, auditory evoked magnetic fields were recorded during stimulation with sequences of 4 rapidly recurring speech sounds in 28 healthy individuals aged 20 - 78 years. RESULTS: The decrement of the N1m amplitude during rapid auditory stimulation was not significantly different between older and younger adults. The amplitudes of the middle-latency P1m wave and of the long-latency N1m, however, were significantly larger in older than in younger participants. CONCLUSION: The results of the present study do not provide evidence for the hypothesis that auditory temporal processing, as measured by the decrement (short-term habituation) of the major auditory evoked component, the N1m wave, is impaired in aging. The differences between these magnetoencephalographic findings and previously published behavioral data might be explained by differences in the experimental setting between the present study and previous behavioral studies, in terms of speech rate, attention, and masking noise. Significantly larger amplitudes of the P1m and N1m waves suggest that the cortical processing of individual sounds differs between younger and older individuals. This result adds to the growing evidence that brain functions, such as sensory processing, motor control and cognitive processing, can change during healthy aging, presumably due to experience-dependent neuroplastic mechanisms.

Auditory brainstem response at the detection limit.

Specific predictions regarding the level dependence of auditory evoked responses near the detection limit were made in a companion modeling study (Lütkenhöner, J Assoc Res Otolaryngol 9:102-121, 2008). Here, these predictions are experimentally tested for auditory brainstem responses (ABR) to Gaussian-shaped 4-kHz tone pulses (full width at half maximum = 0.5 ms) that were presented at sound levels close to the subjective threshold. In the average of over about one million stimulus repetitions (repetition period = 16 ms), the amplitude of ABR wave V showed a smooth transition from a proportional to a logarithmic growth with increasing sound intensity. The latter type of growth corresponds to a linear increase with respect to sound level measured in decibels. Alternatively, the ABR amplitude near the detection limit may be considered a linear function of sound pressure, although-according to the model-this is only an approximation. Data and model are consistent with the view that a sensory threshold does not exist for the auditory modality, in accordance with signal detection theory. Even so, the model may be used to define a quasithreshold that is comparable to the subjective threshold.

Auditory evoked field at threshold.

Auditory evoked responses are widely used for estimating electrophysiological thresholds, but the relationships to psychophysical thresholds are not necessarily straightforward. Among the aspects that are not well understood is the near-threshold intensity dependence of the evoked response. Here, we investigated wave N100m of the auditory evoked field. The stimulus was a 1-kHz tone with an effective duration of about 110 ms. Up to 10 dB above the psychophysical threshold, the level was varied in steps of 2dB; further measurements were done at 15, 20, 30, and 40 dB SL. Lower levels were presented with higher probability, to partially compensate for the expected signal-to-noise ratio reduction with decreasing level. The latency of the N100m could be characterized as a transmission delay and an integration time. The level dependence of the latter was consistent with the assumption of an almost perfectly operating sound-pressure integrator. The N100m amplitude increased roughly linearly with the level in dB (thus, as a logarithmic function of intensity), showing signs of saturation at higher levels.

Evidence of pitch processing in the N100m component of the auditory evoked field.

The latency of the N100m component of the auditory evoked field (AEF) is sensitive to the period and spectrum of a sound. However, little attention was paid so far to the wave shape at stimulus onset, which might have biased previous results. This problem was fixed in the present study by aligning the first major peaks in the acoustic waveforms. The stimuli were harmonic tones (spectral range: 800-5000 Hz) with periods corresponding to 100, 200, 400, and 800 Hz. The frequency components were in sine, alternating or random phase. Simulations with a computational model suggest that the auditory-nerve activity is strongly affected by both the period and the relative phase of the stimulus, whereas the output of the more central pitch processor only depends on the period. Our AEF data, recorded from the right hemisphere of seven subjects, are consistent with the latter prediction: The latency of the N100m depends on the period, but not on the relative phase of the stimulus components. This suggests that the N100m reflects temporal pitch extraction, not necessarily implying that the underlying generators are directly involved in this analysis.

Piano tones evoke stronger magnetic fields than pure tones or noise, both in musicians and non-musicians.

Regarding the net firing rate of the auditory nerve, the strongest response is to be expected when the input energy is spread as evenly as possible over the cochlea rather than being concentrated at a particular location. In some respects, this effect seems to be preserved up to the auditory cortex, but conflicting results have been reported as well. Here, we compared the auditory evoked fields (AEF) elicited by a pure tone and two sounds causing a more wide-spread cochlear activation: a piano tone as a representative of a complex tone, and bandpass noise. The stimuli had the same intensity (60 dB above threshold), and the center frequency of the noise corresponded to the fundamental frequency of the tones (1047 Hz, two octaves above middle C). Among the 26 subjects were 11 musicians and 11 persons who never played an instrument. At a latency of about 50 ms (wave P50m), the piano tone and the noise yielded stronger responses than the pure tone, in accordance with the concepts about the auditory periphery. By contrast, around 100 ms (wave N100m), the noise clearly elicited the smallest response, whereas the strongest response was elicited again by the piano tone. Musicians and non-musicians did not significantly differ concerning the responses to pure tones and piano tones. Thus, previous claims that an enhanced response to piano tones indicates use-dependent reorganization in musicians are not supported by the present data.

The effect of cross-channel synchrony on the perception of temporal regularity.

Temporal models of pitch are based on the assumption that the auditory system measures the time intervals between neural events, and that pitch corresponds to the most common time interval. The current experiments were designed to test whether time intervals are analyzed independently in each peripheral channel, or whether the time-interval analysis in one channel is affected by synchronous activity in other channels. Regular and irregular click trains were filtered into narrow frequency bands to produce target and flanker stimuli. The threshold for discriminating a regular target from an irregular distracter click train was measured in the presence of an irregular masker click train in the target band, as a function of the frequency separation between the target band and a flanker band. The flanker click train was either regular or irregular. The threshold for detecting the regular target was 5-7 dB lower when the flanker was regular. The data indicate that the detection of temporal regularity (and thus, pitch) involves cross-channel processes that can operate over widely separated channels. Model simulations suggest that these cross-channel processes occur after the time-interval extraction stage and that they depend on the similarity, or consistency, of the time-interval patterns in the relevant channels.

Mechanisms determining the salience of coloration in echoed sound: influence of interaural time and level differences.

This study investigates whether the salience of the pitch associated with a single reflection of a broadband sound, such as noise, is determined by the monaural information mediated by the stimuli at the two ears, or by the relative locations of the primary sound and the reflection. Pitch strength was measured as a function of the reflection delay and the lateral displacement between the primary sound and the reflection. Thereby, lateral displacement was produced by means of interaural time differences (ITDs) in experiment 1 and interaural level differences (ILDs) in experiment 3. The results from both experiments are in accordance with the assumption that the strength of the pitch associated with a reflection is based on a central average of the internal representations of the stimuli at the two ears. This notion was corroborated by experiment 2, which showed that the results from experiment 1 could be mimicked by simply adding the stimuli from the two ears and presenting the merged stimulus identically to both ears.