Investigation of the physiological basis of summating potential changes in endolymphatic hydrops

An increase in the ratio of the summating potential to the action potential components of the electrocochleogram is known to be a feature of endolymphatic hydrops. We investigated the value of the SP/AP ratio in response to condensation and rarefaction click stimuli delivered separately. In patients with electrophysiological evidence of endolymphatic hydrops there was found to be a significantly greater SP/AP ratio to condensation clicks than rarefaction clicks. This finding supports the hypothesis that the increased SP/AP ratio in hydrops is due to mechanical asymmetry of the basilar membrane.     

Compound action potentials recorded from the intracranial portion of the auditory nerve in man: effects of stimulus intensity and polarity

Compound action potentials (CAP) were recorded from the intracranial portion of the eighth nerve in patients with normal hearing who were undergoing neurosurgical operations for cranial nerve disorders (trigeminal neuralgia and hemifacial spasm). Brain-stem auditory-evoked potentials were recorded intraoperatively to ensure that no noticeable changes occurred in conduction in the auditory nerve as a result of surgical dissections. The CAP recorded from the middle portion of the exposed intracranial portion of the eighth nerve in response to clicks of high intensity (100-110 dB peak equivalent SPL, or pe SPL) had a triphasic shape, as is commonly seen in monopolar recordings from long nerves. A second negative peak (N2) could be identified in some patients. There was little difference in the waveform of the CAP in response to condensation and rarefaction clicks, and in some patients the waveform of the CAP remained the same over a range of stimulus intensities (from 105 to 75 dB pe SPL), whereas in others the negative peak of the CAP became much broader in response to stimuli with intensities of less than 85 dB. In some patients the N2 peak became dominant as the stimulus intensity was decreased. At low stimulus intensities, the response consisted of a single, broad negativity. The latency-intensity curves for the N1 peak had different slopes in different patients. In those individuals in whom there was a noticeable difference between the latency of the N1 peak in response to clicks of opposite polarity, the latency-intensity curves of the responses to rarefaction clicks were steeper than those of the responses to condensation clicks, and the latency of the N1 peak to condensation clicks became shorter than that to rarefaction clicks at intensities below 85-90 dB pe SPL. The latency-intensity curves for the N2 peak were usually less steep than those of the N1 peak, but in some patients the curves for these two peaks had similar slopes. The amplitude of the N1 peaks showed a steep increase in click intensities at 95 and 105 dB, and a much less steep course for intensities below 95 dB. The amplitudes of the N2 peak reached a plateau in the range 95-105 dB, and decreased more rapidly than the N1 peak below 95 dB.     

Comparison between auditory brain stem responses evoked by rarefaction and condensation step functions and clicks

In order to evaluate the influence of the trailing edge of clicks on the auditory brain stem response (ABR) in normal ears, rarefaction and condensation step functions (RS and CS) compared to rarefaction and condensation clicks (RC and CC) at an intensity of 70 dB nHL were used. Significant intraindividual differences could be found for the latencies and amplitudes in the RS-CS, RS-RC and RC-CC comparison. However, the mean values of the complete group of test subjects showed no significant differences for the latencies and amplitudes, except the significantly greater amplitudes of wave I and II for R versus C step and R versus C click. Only a tendency to shorter latency for wave VI with R versus C step and click was revealed. These results show that there was no essential influence of the trailing edge of the used R and C clicks on the ABR. The latency of the ABR with excitation of the cochlea by step or click function seemed to be mainly determined by the internal oscillation sequence in the cochlea and not by the stimulus polarity.     

Diagnostic implications of stimulus polarity effects on the auditory brainstem response

The purpose of this study was to determine whether clicks presented in rarefaction or condensation modes produce more accurate diagnostic information. Subjects were 20 consecutive patients who were seen at the Mayo Clinic for unilateral acoustic neuromas. The nontumor ear served as a control to minimize intersubject variability in the latencies. A standard audiologic evaluation was followed by an auditory brainstem response (ABR) test for which the stimuli were rarefaction and condensation clicks. Responses were analyzed for the presence of waves I, III, and V; absolute latencies of waves I, III, and V; interpeak intervals I-III, III-V, and I-V; and interaural latency difference for wave V. The results indicated that measures from both polarities were similar in this set of patients and that neither click polarity provided diagnostic advantages over the other. Recommendations are to collect ABRs to both click polarities individually to obtain the full complement of waves on which to base the diagnostic impression.     

Influence of stimulus polarity on far-field auditory-evoked electrical responses in the chick

The effects of inverting click phase on far-field peripheral and brainstem auditory-evoked responses (PBARs) were examined in 8 White Leghorn chicks of age 3 wks. Significant latency differences occurred in all major positive peaks (Pla, P2a peripheral; P3a central) in response to rarefaction vs condensation clicks of equal intensity, with condensation clicks producing the shortest latencies (latency differences: Pla = 0.374, P2a = 0.372 and P3a = 0.352 msec, p less than .001). The mean latency shift corresponds to an equivalent sine wave frequency of approximately 1360 c/s, a value close to the spectral peak energy of the click. No differences in interpeak latency values were found nor any significant amplitude effects.     

Compound Action Potentials Recorded from the Intracranial Portion of the Auditory Nerve in Man: Effects of Stimulus Intensity and Polarity

Compound action potentials (CAP) were recorded from the intracranial portion of the eighth nerve in patients with normal hearing who were undergoing neurosurgical operations for cranial nerve disorders (trigeminal neuralgia and hemifacial spasm). Brainstem auditory-evoked potentials were recorded intraoperatively to ensure that no noticeable changes occurred in conduction in the auditory nerve as a result of surgical dissections. The CAP recorded from the middle portion of the exposed intracranial portion of the eighth nerve in response to clicks of high intensity (100-110 dB peak equivalent SPL, or pe SPL) had a triphasic shape, as is commonly seen in monopolar recordings from long nerves. A second negative peak (N²) could be identified in some patients. There was little difference in the waveform of the CAP in response to condensation and rarefaction clicks, and in some patients the waveform of the CAP remained the same over a range of stimulus intensities (from 105 to 75 dB pe SPL), whereas in others the negative peak of the CAP became much broader in response to stimuli with intensities of less than 85 dB. In some patients the N² peak became dominant as the stimulus intensity was decreased. At low stimulus intensities, the response consisted of a single, broad negativity. The latency-intensity curves for the N¹ peak had different slopes in different patients. In those individuals in whom there was a noticeable difference between the latency of the N¹ peak in response to clicks of opposite polarity, the latency-intensity curves of the responses to rarefaction clicks were steeper than those of the responses to condensation clicks, and the latency of the N¹ peak to condensation clicks became shorter than that to rarefaction clicks at intensities below 85–90 dB pe SPL. The latency-intensity curves for the N² peak were usually less steep than those of the N¹ peak, but in some patients the curves for these two peaks had similar slopes. The amplitude of the N¹ peaks showed a steep increase in click intensities at 95 and 105 dB, and a much less steep course for intensities below 95 dB. The amplitudes of the N² peak reached a plateau in the range 95–105 dB, and decreased more rapidly than the N¹ peak below 95 dB.     

Development of low-frequency tone burst versus the click auditory brainstem response

Often ABR threshold testing employs clicks to assess high-frequency hearing, and low-frequency tone bursts to assess low-frequency sensitivity. While a maturation effect has been shown for click stimuli, similar data are lacking for low-frequency toneburst stimuli. Thus, 305 infants ranging in conceptional age (CA) from 33 weeks to 74 weeks were tested. Absolute latencies were measured for wave V at 55, 35, and 25 dB nHL in response to a click and for wave V500 in response to a 500 Hz tone burst. Major wave latency in response to 500 Hz tone bursts decreases with age and do not stabilize by 70 weeks CA. Likewise, waves III and V latencies in response to clicks decrease with age, as has been reported by others, and do not stabilize by 70 weeks CA. Wave I latency produced by clicks did not decrease with age, being mature by 33 weeks CA.     

Effects of stimulus phase on the normal auditory brainstem response.

The purpose of this investigation was to determine the effects of stimulus phase on the latencies and morphology of the auditory brainstem response (ABR) of normal-hearing subjects. Although click stimuli produced equivalent ABR latencies for the rarefaction and condensation phases, the subtraction of the waveforms from the two phases yielded a difference potential. Tone pip stimuli produced polarity differences that were inversely related to stimulus frequency: the higher the frequency, the smaller the ABR latency differences between responses to rarefaction and condensation stimuli, and the smaller the difference potentials. Thus, whereas the latency of click-evoked ABR is dominated by high-frequency responses with equivalent latencies regardless of stimulus phase, low-frequency responses contribute to the overall morphology of the ABR that yields the phasic difference potential. The implications of these findings are discussed with reference to subjects with high-frequency hearing losses.     

Effect of high-frequency hearing loss on compound action potentials recorded from the intracranial portion of the human eighth nerve.

Compound action potentials (CAP) were recorded from the exposed intracranial portion of the eighth nerve to stimulation with click sounds in patients with sensorineural high-frequency hearing loss who underwent microvascular decompression (MVD) operations to treat trigeminal neuralgia (TN). In patients with normal hearing the CAP recorded in that way is characterized by a negative peak, preceded by a small positivity and followed by a positivity and sometimes a second negative peak. In patients with high-frequency hearing loss the CAP also usually had an initial sharp negative peak in response to clicks of high intensity (105 to 110 dB Pe SPL), similar to findings in patients with normal hearing, but in patients with high-frequency hearing loss the initial negative peak was often followed by a slow negative deflection. The latency of the initial negative peak in the CAP in patients with high-frequency hearing loss was longer than the latency of this peak in patients with normal hearing, but the difference in latencies of this peak to condensation and rarefaction clicks was small. When the stimulus intensity was lowered the amplitude of the initial peak decreased, and the CAP became dominated by a broad negative peak with a latency of 6 to 8 ms. In 11 of 15 patients with severe high-frequency hearing loss, a series of quasi-periodic waves was superimposed on the CAP. The frequency of these waves varied between 500 and 1200 Hz, and the waves could be detected between 6 and 16 ms after presentation of the click stimulus. These waves were usually present in the response to stimuli in the intensity range from 75 to 110 dB Pe SPL. Only 4 of 17 patients with normal hearing had similar waves.     

Effects of click duration of the latency of the early evoked response

The effect of click duration on the latency of waves I, III, and V was investigated by testing 20 normal-hearing subjects at 60 dB HL using electric pulses of 25, 50, 100, 200, and 400 microseconds. Alternating condensation and rarefaction clicks were used. The results revealed similar and nonsignificant latency differences for the 25-, 50-, and 100-microseconds pulses. However, the 100 microseconds duration is preferred to the 25-microseconds pulse because the latter reduced the maximum measurable hearing loss by about 13 dB. The results also showed that latencies increased approximately 0.10 ms as duration increased from 100 to 200 microseconds and by 0.20 ms when duration increased from 100 to 400 microseconds. Although such differences by themselves are small, they can combine with other stimulus or recording variables to be clinically significant. Therefore, it is important to control click duration when normative data are generated. A second experiment was conducted to assess the interaction of polarity (condensation, rarefaction, and alternating) and pulse duration (100 and 400 microseconds) on the wave V latency. These data revealed no latency differences among polarities at either duration.     

Contribution of click frequency bands to the human binaural interaction components

The purpose of this study was to determine the contribution of click frequency bands (broad-band, >2000 Hz, <2000 Hz and <1000 Hz) to binaural interaction components (BICs) of the human auditory brainstem evoked potentials (ABEPs). The human BICs were studied by subtracting the potentials to binaural clicks from the algebraic sum of monaurally evoked potentials to either ear. Effective frequency bands were derived using clicks alone or clicks with ipsilateral or binaural masking noise, high- or low-pass filtered at different cut-off frequencies. Analysis included single-channel vertex-cervical spinous process VII derivation of BIC and ABEP, as well as estimating the single, centrally located dipole equivalent of the surface activity from three orthogonally positioned electrode pairs, using the three-channel Lissajous' trajectory (3-CLT) analysis. All BIC 3-CLTs included three major components (labeled BdII, BeI, and BeII) approximately corresponding in latency to IIIn, V and VI ABEP peaks. All apex latencies of BIC 3-CLT, except BeI, were longer in response to <2000 Hz and <1000 Hz (low-frequency) effective clicks. Apex amplitude of components BeI and BeII of BIC 3-CLT were smaller with low-frequency effective clicks than with broad-band or high-frequency (>2000 Hz) clicks. We suggest that binaural interaction component BeI is mainly tuned to high frequencies, showing no frequency effect on latency, and decreasing in amplitude with decreasing click high frequency content. In contrast, BdII and BeII of the human BICs are evoked more synchronously by high-frequency binaural inputs, but are also sensitive to low frequencies, increasing in latency according to the cochleotopic activation pattern. These differences between BIC components may reflect their roles in sound localization.     

The combined effects of forward masking by noise and high click rate on monaural and binaural human auditory nerve and brainstem potentials

OBJECTIVE: To study effects of forward masking and rapid stimulation on human monaurally- and binaurally-evoked brainstem potentials and suggest their relation to synaptic fatigue and recovery and to neuronal action potential refractoriness. METHODS: Auditory brainstem evoked potentials (ABEPs) were recorded from 12 normally- and symmetrically hearing adults, in response to each click (50 dB nHL, condensation and rarefaction) in a train of nine, with an inter-click interval of 11 ms, that followed a white noise burst of 100 ms duration (50 dB nHL). Sequences of white noise and click train were repeated at a rate of 2.89 s(-1). The interval between noise and first click in the train was 2, 11, 22, 44, 66 or 88 ms in different runs. ABEPs were averaged (8000 repetitions) using a dwell time of 25 micros/address/channel. The binaural interaction components (BICs) of ABEPs were derived and the single, centrally located equivalent dipoles of ABEP waves I and V and of the BIC major wave were estimated. RESULTS: The latencies of dipoles I and V of ABEP, their inter-dipole interval and the dipole magnitude of component V were significantly affected by the interval between noise and clicks and by the serial position of the click in the train. The latency and dipole magnitude of the major BIC component were significantly affected by the interval between noise and clicks. Interval from noise and the click's serial position in the train interacted to affect dipole V latency, dipole V magnitude, BIC latencies and the V-I inter-dipole latency difference. Most of the effects were fully apparent by the first few clicks in the train, and the trend (increase or decrease) was affected by the interval between noise and clicks. CONCLUSIONS: The changes in latency and magnitude of ABEP and BIC components with advancing position in the click train and the interactions of click position in the train with the intervals from noise indicate an interaction of fatigue and recovery, compatible with synaptic depletion and replenishing, respectively. With the 2 ms interval between noise and the first click in the train, neuronal action potential refractoriness may also be involved.     

Sensitivity of neurons in the dorsal medullary nucleus of the grassfrog to spectral and temporal characteristics of sound

The responses of 58 dorsal medullary nucleus units to a set of spectrally and temporally structured stimuli were investigated. Responses to tonepips and noise indicated monomodal spectral sensitivities, with diverse response patterns. Phase-locking was strong for frequencies from 0.1 to 0.2 kHz, and in one unit extended up to 0.6 kHz. To clicks, amplitude modulated tonebursts and natural and artificial versions of the mating call various responses were found. Most low-frequency units fired tonically. They showed a non-selective or low-pass rate response to increasing modulation frequency, and a low-pass synchronization behavior to the envelope. A group of mid-frequency units fired phasically and exhibited a band-pass rate characteristic of amplitude modulated tonebursts. Frequently this was combined with a low-pass rate characteristic of click trains. These units hardly responded to the time-reversed mating call, but fired in a time-locked fashion to the pulses of the original mating call, up to a signal-to-noise ratio of 0 dB. This suggests that aspects of pulse envelope and interpulse interval are coded in the dorsal medullary nucleus.     

Effect of click polarity on ABR threshold

This study investigated the effects of click polarity on threshold detectability (threshold level in dB SL) of the auditory brain stem response (ABR). ABRs were obtained from 10 normally hearing adult subjects in response to rarefaction and condensation clicks presented from 0 to 10 dB SL in 2 dB steps. An objective response signal-to-noise estimator, known as Fsp, was the dependent variable. ABR detectability functions (Fsp by dB SL) were not significantly influenced by click polarity. Conclusions can only be drawn for normally hearing adults. For this population, click polarity does not affect threshold detection with the ABR.     

Tympanic electrocochleography: normal and abnormal patterns of response.

Electrocochleography has been widely used in human and animal studies of endolymphatic hydrops. A variety of response patterns have been reported in normal and hydropic ears. Recent clinical studies have focused almost exclusively on the amplitude ratio of the summating potential (SP) and action potential (AP) derived from alternating polarity click responses. In this report normal response patterns are described with a tympanic membrane electrode employing condensation, rarefaction and alternating polarity clicks and tone burst stimulation. A variety of response abnormalities are described in patients with suspected endolymphatic hydrops. The exclusive use of alternating polarity clicks is not adequate to reveal the nature of these abnormalities.     

Auditory Nerve and Brain Stem Responses to Sound Stimuli at Various Frequencies

The responses of the auditory nerve and brain stem auditory nuclei have been recorded with ear lobe and scalp electrodes in response to unfiltered click stimuli which excite mainly high-frequency nerve fibers. In order to overcome this limitation, normal subjects and patients with hearing loss were stimulated with third-octave filtered clicks with center frequencies of 250, 500 and 1 000 Hz. Analysis of the responses suggests that these lower-frequency stimuli excite lower-frequency cochlear nerve fibers. It is concluded that low-frequency filtered clicks can be used to complement unfiltered clicks in order to obtain a more complete objective audiogram.     

Recovery from adaptation in human compound action potential following forward masking

Recovery from adaptation was studied as changes in compound action potential (CAP) in response to clicks after broadband noise masking in volunteers with normal hearing. CAP was transtympanically elicited from the promontory using a needle electrode. Preceding masking noise and subsequent click stimuli were delivered separately from 2 loud speakers 80 cm from the tested ear. We evaluated the effect of masking noise duration and intensity on CAP recovery from adaptation as a function of delta t, defined as the interval between masking noise offset and click stimulus onset. Results were as follows: At delta t shorter than 200 ms or less, recovery from latency prolongation and amplitude decrease slowed with increasing masking noise duration and intensity. At delta t longer than 200 ms, no significant difference was seen in CAP recovery based on changes in masking noise duration or intensity. Recovery from adaptation, which depended on click intensity, took about 700 ms at a 40 dBnHL click and about 300 ms at a 60 dBnHL click. The influence of the middle ear muscle reflex on CAP recovery patterns is neglected when considering the intensity both masking noise and click stimulus. In CAP recovery, accumulated effects of the relative auditory-nerve refractory period may adversely affect recovery at very short delta t, while recovery from short-term adaptation, assumed involved in cochlear-hair cell synapse function, may be an important factor at delta t longer than 30-40 ms. Based on animal experiments showing different recovery times dependent on different auditory nerve spontaneous rates(SR), we concluded that at delta t of 200 ms or less, CAP recovery reflects high SR neuron activity, while at delta t longer than 200 ms, it mainly reflects that of low SR neurons.     

Compound Action Potential and Cochlear Microphonic Extracted from Electrocochleographic Responses to Condensation or Rarefaction Clicks

In electrocochleography (ECochG) compound action potential (CAP) and summation potential (SP) are usually separated from the cochlear microphonic (CM) by the CM cancellation technique consisting in averaging the responses evoked by rarefaction and condensation clicks. With the aim of analysing the ECochG responses evoked by monophasic clicks, we developed a numerical method based on the theory of optimal filtering, which makes no assumptions about the unknown potentials. The application of the filtering technique to the ECochG recordings obtained from 6 normally hearing children and 10 children with cochlear hearing loss allowed us to perform CAP extraction in cases where CM was not cancelled by the conventional method. Differences in SP amplitude and polarity were found between rarefaction and condensation click-evoked responses in cochlear hearing losses.     

Compound action potential and cochlear microphonic extracted from electrocochleographic responses to condensation or rarefaction clicks

In electrocochleography (ECochG) compound action potential (CAP) and summation potential (SP) are usually separated from the cochlear microphonic (CM) by the CM cancellation technique consisting in averaging the responses evoked by rarefaction and condensation clicks. With the aim of analysing the ECochG responses evoked by monophasic clicks, we developed a numerical method based on the theory of optimal filtering, which makes no assumptions about the unknown potentials. The application of the filtering technique to the ECochG recordings obtained from 6 normally hearing children and 10 children with cochlear hearing loss allowed us to perform CAP extraction in cases where CM was not cancelled by the conventional method. Differences in SP amplitude and polarity were found between rarefaction and condensation click-evoked responses in cochlear hearing losses.     

Auditory evoked potentials from medulla and midbrain in the clawed frog, Xenopus laevis laevis

Auditory evoked potentials (AEPs) to clicks and tonal pulses were recorded from medulla and midbrain in Xenopus laevis laevis. They comprise three components: an initial peak (I) at 2.2-3 ms latency, a fast series of peaks (F) at 5-15 ms latency, and a slow negative wave (S) at 20-40 ms latency. In medullary recordings, the initial peak was largest, whereas in midbrain recordings typically the two other components prevailed. For all components and animals, response threshold at 4 clicks/s was approximately 69 dB SPL. In response to tonal stimuli, AEP amplitudes were maximal at 1.3-2.0 and 3.5 kHz. Raising the click rate to 100/s gradually reduced the amplitude of the I and the first F peaks, whereas later F peaks and the S wave virtually disappeared at 20-40 clicks/s. On the other hand, extending the plateau duration of tonal stimuli from 4 to 10 ms hardly affected the I and F peaks but doubled the S amplitude. This suggests two systems for stimulus processing, a fast system capable to follow clicks up to high repetition rates and a slow system with longer integration time.