Synchronous auditory nerve activity in the carboplatin-chinchilla model of auditory neuropathy

Two hallmark features of auditory neuropathy (AN) are normal outer hair cell function in the presence of an absent/abnormal auditory brainstem response (ABR). Studies of human AN patients are unable to determine whether disruption of the ABR is the result of a reduction of neural input, a loss of auditory nerve fiber (ANF) synchrony, or both. Neurophysiological data from the carboplatin model of AN reveal intact neural synchrony in the auditory nerve and inferior colliculus, despite significant reductions in neural input. These data suggest that (1), intact neural synchrony is available to support an ABR following carboplatin treatment and, (2), impaired spike timing intrinsic to neurons is required for the disruption of the ABR observed in human AN.

Characteristics of patients with gaze-evoked tinnitus

OBJECTIVE

The authors describe symptoms and population characteristics in subjects who can modulate the loudness and/or pitch of their tinnitus by eye movements.

STUDY DESIGN

Data were obtained by questionnaire.

SETTING

The study was conducted at a university center and a tertiary care center.

PATIENTS

Respondents had the self-reported ability to modulate their tinnitus with eye movements.

RESULTS

Ninety-one subjects reported having gaze-evoked tinnitus after posterior fossa surgery involving the eighth nerve. Eighty-seven of them underwent removal of a vestibular schwannoma (acoustic neuroma), two had bilateral eighth nerve tumors (one underwent bilateral tumor removal; the other unilateral tumor removal), one underwent removal of a cholesteatoma, and one underwent removal of a glomus jugulare tumor. Seventeen subjects who had never had posterior fossa surgery reported gaze-evoked tinnitus. Of those with vestibular schwannomas, tumor size ranged from small (<2 cm) to large (>4 cm). The gender distribution was 48.3% male and 51.7% female. In 77% of patients, the gaze-evoked tinnitus was localized to the surgical ear or side of head; 21.8% had bilateral tinnitus that was louder in the surgical ear or side of head. In 86 of 87 subjects, loudness of tinnitus changed with eye movement. Eye movement away from the central (eyes centered) position increased the loudness of tinnitus in all 86 subjects who responded to this question. Seventy-three of 85 (85.9%) patients indicated that pitch changed with eye movement, with pitch increasing in 64/72 (88.9%) of them. Eighty-three of 87 (95.4%) patients reported total loss of hearing in the surgical ear. Seventy of 83 (84.3%) patients reported facial nerve problems immediately after surgery, 52 of 87 (60%) reported persistent facial weakness, and 16 of 87 (18.4%) patients reported persistent double vision. In those 17 subjects with gaze-evoked tinnitus and no posterior fossa surgery, the majority of respondents (14/17, 82.4%) were male.

CONCLUSIONS

Gaze-evoked tinnitus after cerebellar pontine angle surgery is more common than was previously believed. In addition, posterior fossa surgery is not a prerequisite for the development of gaze-evoked tinnitus. It is likely that gaze-evoked tinnitus is a manifestation of functional reorganization. Gaze-evoked tinnitus could result from an unmasking of brain regions that respond to multiple stimulus/response modalities, and/or from anomalous cross-modality interactions, perhaps caused by collateral sprouting.

Sensitivity of unanesthetized chinchilla auditory system to noise burst onset, and the effects of carboplatin

The gross near-field responses of the auditory nerve and inferior colliculus to noise burst stimuli were recorded through intracranially implanted electrodes in six unanesthetized chinchillas. Responses were studied as a function of stimulus plateau amplitude and rise time, both before and after a systemic dose of 75 mg/kg of carboplatin. Both recording sites showed sensitivity to stimulus level and rise time. Increases in stimulus level and decreases in stimulus rise time each produced increases in the response magnitude, and decreases in response latency. When the stimuli were re-specified as rate of pressure change at sound onset (Pa/s), the amplitude and latency of responses at each site were found to be a direct function of rate of sound pressure change. These data provide the first confirmation in unanesthetized animals of previous single unit observations in barbiturate-anesthetized cats. Carboplatin treatment resulted in a 20-80% loss of inner hair cells, a modest threshold elevation, and a 50-75% reduction in peak response amplitudes. The general patterns of sensitivity to stimulus level and rise time were not markedly affected by carboplatin, nor was the fashion in which response parameters (amplitude and latency) were ruled by rate of pressure change at sound onset.

Cochlear microphonics and otoacoustic emissions in chronically de-efferented chinchilla

The effects of eliminating the olivocochlear bundle (OCB) on cochlear electromechanical properties were examined by measuring cochlear microphonics (CM) and distortion product otoacoustic emissions (DPOAEs) in chronically de-efferented chinchillas. The OCB fibers to the right ears were successfully sectioned in six out of 15 adult chinchillas via a posterior paraflocular fossa approach. At the end of the experiment, these ears were histologically verified as being deprived of both lateral and medial OCB fibers. The opposite (left) ears from the animals served as controls. Following de-efferentation, changes of the inter-modulation distortion components (2f(1)-f(2), f(2)-f(1), 3f(1)-2f(2), 3f(2)-2f(1)) varied, depending on the frequencies and levels of the stimuli. DPOAE amplitudes to low-level stimuli were within the 95% confidence intervals around mean DPOAE amplitudes of the control ears at all the frequencies (1-8 kHz). At high stimulus levels, DPOAE amplitudes increased by 5-20 dB at 1 and 2 kHz while remaining in the normal range at 4 and 8 kHz. In contrast, the CM input/output functions to stimuli from 1 to 8 kHz were significantly reduced by approximately 40-50% at all input levels. The results suggest that the OCB may play a role in modulating electrical properties of the outer hair cells and in reducing the magnitude of cochlear distortion to high-level stimuli.

Studies of interaural attenuation to investigate the validity of a dichotic difference tone response recorded from the inferior colliculus in the chinchilla

In a previous paper (Arnold and Burkard, 1998) a dichotic f2-f1 difference tone (DT) auditory evoked potential from the chinchilla inferior colliculus (IC) was measured while presenting f1 (2000 Hz) to one ear and f2 (2100 Hz) to the other ear. This measurement paradigm could be used as a means to study binaural processing in an unanesthetized animal model. However, it is possible that this response is actually generated peripherally, as a result of acoustic crossover. The purpose of the present set of experiments was to investigate whether the dichotic DT is a true binaural phenomenon. Recordings were made from chronically implanted IC electrodes in unanesthetized, monaural chinchillas (left cochlea destroyed). In experiment 1, interaural attenuation (IA) was measured in two ways. First, IA was measured by comparing IC evoked potential thresholds obtained when stimulating the normal right ear and the dead left ear, using tone bursts (0.5-8 kHz). Mean values of interaural attenuation ranged from 50-65 dB across frequency (55 dB at 2000 Hz). Next, the DT was measured monaurally using f1 = 2000 and f2 = 2100 (L1 = L2). By comparing the mean DT input/output functions for monaural stimulation of the right and left ears, a mean value of IA for the tonal pair was estimated (approximately 69 dB). In experiment 2, the DT was measured with right monaural stimulation, while varying the relative levels of the primaries. A small DT could be seen with primary levels up to 30 dB apart, but not for greater level differences. Differences substantially greater than 30 dB would be expected in the crossover situation based upon IA. In experiment 3, the stimuli were presented dichotically (f1 to right ear, f2 to left ear and vice versa, L1 = L2) to determine whether acoustic crosstalk to the normal right ear would generate a DT. No DT was reliably observed in this condition. Taken together, these results suggest that the dichotic DT is a true binaural phenomenon, and not simply attributable to acoustic crossover.

Inner hair cell loss leads to enhanced response amplitudes in auditory cortex of unanesthetized chinchillas: evidence for increased system gain

Carboplatin preferentially destroys inner hair cells (IHCs) in the chinchilla inner ear, while retaining a near-normal outer hair cell (OHC) population. The present study investigated the functional consequences of IHC loss on the compound action potential (CAP), inferior colliculus potential (ICP) and auditory cortex potential (ACP) recorded from chronically implanted electrodes. IHC loss led to a reduction in CAP amplitude that was roughly proportional to IHC loss. The ICP amplitude was typically reduced by IHC loss, but the magnitude of this reduction was generally less than that observed for the CAP. In contrast to the CAP and ICP, ACP amplitudes were generally not reduced following IHC loss. In some animals, the ACP amplitude remained at pre-carboplatin values despite substantial IHC loss. However, in other animals, IHC loss led to an increase ('enhancement') of ACP amplitude. ACP enhancement was greatest at 1-2 weeks post-carboplatin, returning towards baseline amplitudes at 5 weeks post-carboplatin. In other animals, the ACP remained enhanced up to 5 weeks post-carboplatin. We interpret the transient and sustained enhancement of ACP amplitude following partial IHC loss as evidence of functional reorganization occurring at or below the level of the auditory cortex. These results suggest that the gain of the central auditory pathway increases following IHC loss to compensate for the reduced input from the cochlea.

Response magnitude and timing of auditory response initiation in the inferior colliculus of the awake chinchilla

Recent single-unit studies in anesthetized cats have revealed that the latency and strength of transient responses to tone burst stimuli are determined largely by stimulus events in the first few ms of the signal. The present study sought to extend these findings by studying the inferior colliculus potential (ICP) in unanesthetized chinchillas. The ICP magnitude and latency were studied as a function of the plateau amplitude and rise time of noise burst stimuli. ICP amplitude increased with stimulus amplitude and decreased with stimulus rise time. ICP latency decreased with stimulus amplitude and increased with stimulus rise time. The absolute values of the ICP latencies confirmed that it is only the first few ms of the stimulus which determine the timing of response initiation, and therefore, that it is not the plateau level of the stimulus that directly determines the latent period. These data constitute a direct link between earlier single-unit studies in anesthetized animals and brainstem-evoked potential data in animals and man.

Auditory brainstem response forward-masking recovery functions in older humans with normal hearing

We investigated the auditory brainstem response (ABR) recovery from forward masking using toneburst maskers and probes. Two subject groups matched for hearing thresholds were evaluated: normal-hearing young adults (21-40 years) and older subjects (63-77 years) with normal audiometric thresholds. Stimuli consisted of 1, 4 and 8 kHz tonebursts, with 2-4 cycle rise/fall time and no plateau. Forward maskers were tonebursts of the same frequency, with a 5 ms rise/fall time and a 20 ms plateau time. Probes were presented at 40 dB above threshold, and the forward masker was adjusted to a level that just eliminated the ABR to the 40 dB sensation level toneburst when the probe onset occurred at masker offset. Forward-masker intervals varied from 2 to 64 ms. ABR wave V latencies were similar for the young and old age groups regardless of toneburst frequency. Under forward-masking conditions, wave V latency was prolonged for the shorter intervals, and recovered to baseline latency by 64 ms. The forward-masker recovery functions were nearly identical for the two age groups for the 1 kHz toneburst. In contrast, there were clear differences in the recovery functions for the two age groups for the 4 and 8 kHz tonebursts. Specifically, the mean latency shift was greater for the aged group for forward-masker intervals of 16 ms or less. The two age groups showed identical latency shifts for longer forward-masker intervals. These data demonstrate prolonged recovery from forward masking in older human subjects. As these subjects had audiometric thresholds within normal limits, one plausible interpretation of this finding is that the prolonged recovery time is a manifestation of an aging effect on the central auditory nervous system rather than the periphery.

The auditory evoked potential difference tone and cubic difference tone measured from the inferior colliculus of the chinchilla

The auditory evoked potential f2-f1 difference tone (DT) and the 2 f1-f2 cubic difference tone (CDT) were recorded from electrodes implanted in the inferior colliculus in a group of chinchillas. The purpose of this study was to measure normative aspects of AEP distortion products in awake chinchillas, by comparing the DT and CDT under a variety of stimulus conditions. For experiment 1, f1 was held constant at 1998 Hz, while the f2/f1 ratio was varied from 1.05 to 1.50. Input-output functions were measured over a range of primary tone levels up to 80 dB SPL. The amplitude of the DT was greatest for the smallest f2/f1 ratio, and decreased systematically as f2/f1 ratio increased. DT amplitude was greater than CDT amplitude for all primary tone pairs. Experiment 2 was conducted to determine the effect of f1 frequency upon the DT and CDT for a constant f1-f2 difference frequency of 102 Hz (f1-999, 1998, 4999, and 9998 Hz). The DT input-output functions were overlapping for all f1 frequencies. For the CDT, amplitude decreased with increasing f1 frequency, which corresponded to an increase in CDT frequency. In experiment 3, the relationship between ear of stimulation and inferior colliculus recorded from was investigated. DT input-output functions (f1 = 1998 Hz, DT = 102 Hz) were measured for monaural contralateral, monaural ipsilateral, and dichotic stimulus conditions. DT amplitude was largest for the contralateral condition, followed by the ipsilateral condition. A smaller, dichotic component to the DT was observed as well.

The effects of click level, click rate, and level of background masking noise on the inferior colliculus potential (ICP) in the normal and carboplatin-treated chinchilla

Carboplatin produces a selective loss of inner hair cells in chinchilla, substantially reducing the amplitude of the compound action potential. A key question that arises from these experiments is: What effect does a reduction in IHC-eighth-nerve fiber input have on the central auditory nervous system? This investigation evaluated the inferior colliculus potential (ICP) in chinchillas treated with carboplatin. The left ear was surgically destroyed and a recording electrode was placed in the left inferior colliculus. Following thirteen days of recovery time, the ICP was recorded in the awake animal. Click level was varied from 10-20 to 80 dB pSPL. Click rate was varied from 10 to 1000 Hz using both conventional averaging and a cross-correlation procedure. Broadband masking noise was varied from 30 to 70 dB SPL with click level held constant at 80 dB pSPL. The dependent variables were the positive peak latency and peak-to-following trough amplitude of the evoked potential. Following baseline studies, the animals were administered carboplatin (50 mg/kg IP) and retested two weeks later. Prior to carboplatin administration, there was an increase in ICP latency and a decrease in ICP amplitude with decreasing stimulus level, increasing rate and increasing noise level. Mean ICP threshold was 30 dB pSPL. Following carboplatin administration, there was little change in threshold or peak latencies. In contrast, the amplitude of the ICP was reduced on average by one-third, although this effect varied considerably across animals. The magnitude of this amplitude decrement was not strongly dependent on click level, click rate, or the level of background noise.

Responses of chopper units in the ventral cochlear nucleus of the anaesthetised guinea pig to clicks-in-noise and click trains

Auditory brainstem responses (ABRs) have been measured with clicks, clicks masked by noise, click trains and pseudorandom maximum length sequences (MLS) of clicks. To investigate the neuronal populations contributing to the ABR under these stimulation conditions, we measured the extracellular responses of ventral cochlear nucleus (VCN) units in the urethane-anaesthetised guinea pig. We studied 23 chopper, 7 primary-like and 7 onset units. This report focuses on the responses from chopper units. The probability of discharge for chopper units increased with increasing click level reaching nearly 100% in many units, over a range of about 20-30 dB. Following each response to a click there was a 5-10 ms suppression of the spontaneous or noise evoked activity. As the level of the noise was increased over a range of 20-30 dB, the response to the clicks gradually decreased leading to a complete abolition of the click response at high noise levels. In a few units, low level noise produced a facilitation of the response to single clicks. In response to constant level equally spaced click trains, discharge probability increased with increasing minimum pulse interval (MPI), approaching 100% for MPIs of 4-8 ms in some units. The recovery afforded by the gaps in the MLS train often resulted in higher discharge probability for MLS than click trains with the same MPI, while response probabilities for MLS and click trains were similar when compared at equivalent average click rates. At short MPIs (0.5 and 1.0 ms), peri stimulus time histograms in response to click trains resembled those to best frequency (BF) tones and noisebursts, with chopping peaks unrelated to unit BF. VCN units show highly synchronised and reliable responses to click trains, MLS trains and clicks masked by noise. The decrease in discharge rate and increase in latency of chopper units with decreasing click level, increasing click rate and increasing masker level parallel the peak amplitude and latency changes observed in the auditory brainstem response.

Effects of stimulus rate on the feline brain-stem auditory evoked response during development. II. Peak amplitudes

The effects of click rate on brain-stem auditory evoked response (BAER) peak amplitudes were investigated in adult cats and kittens in four age groups: 10, 15, 20, and 30 post-natal days. Conventional BAERs were obtained at rates of 5, 10, 30, 60, and 90 Hz. Maximum length sequences (MLSs) were used to obtain responses at rates of 83, 125, 250, 500, and 1000 Hz. For both techniques, clicks were presented at 90 dB pSPL, and at 20 dB above threshold (20 dB SL). BAERs were observed for all age groups, at all click rates. However, not all peaks could be identified in all animals, with peak absence more common for the youngest kittens at the higher rates. BAER waves i through iv decreased in amplitude with increasing click rate for all age groups. In general, the smallest peak amplitudes were observed for the youngest animals (10- to 15-day-old) studied, while the largest amplitudes were found for the adult, 20- or 30-day-old animals, regardless of rate. When amplitudes were normalized to eliminate the influences of differences in absolute values due to developmental stage (i.e., made proportional to the amplitude observed at 5 Hz), the younger age groups exhibited enhanced sensitivity to the effects of stimulus rate. However, exceptions to this trend were observed. In general, amplitudes of BAER peaks obtained with the MLS procedure were smaller than those obtained using conventional averaging with a similar average click rate, regardless of age. Finally, the most efficient method for obtaining a response at a criterion signal-to-noise ratio was typically by the use of conventional averaging at a rate of 60 or 90 Hz, for all peaks and age groups, Overall, these data suggest that higher stimulus rates produce greater neural adaptation resulting in reduced BAER peak amplitudes. The mechanism responsible for the acquisition of adult-like adaptation properties appears to develop during the early post-natal period in the cat.

Effects of stimulus rate on the feline brain-stem auditory evoked response during development. I. Peak latencies

The effects of stimulation rate on the brain-stem auditory evoked response (BAER) of developing and adult cats were investigated. Age ranged from 10-post-natal days to young adults. Clicks were presented at levels of 90 dB pSPL and 20 dB above each animal's click threshold (20 dB SL). For all animals, a conventional BAER rate series was obtained at rates of 5, 10, 30, 60, and 90 Hz. BAERs were also obtained using pseudorandom pulse sequences called maximum length sequences (MLSs). The minimum time between pulses, the minimum pulse interval (MPI), included 0.5, 1, 2, 4, and 6 ms, which correspond to average rates of 1000, 500, 250, 125, and 83 Hz, respectively. Dependent variables included the latencies of the first four BAER peaks, labeled i through iv. MLS BAERs were indistinguishable from conventional BAERs at all ages studied. In general, for both conventional and MLS BAERs, peak latencies and the i-iv interval increased with increasing stimulus rate. Although absolute peak latencies and the i-iv interval decreased systematically with age, the relative shift in latency and i-iv interval induced by increasing stimulus rate decreased during development. The enhanced sensitivity to stimulus rate observed at younger ages was not the consequence of the threshold improvement that occurs during development since similar observations were made when stimuli were presented at a constant absolute level (90 dB pSPL) or at a constant level above threshold (20 dB SL). In addition, successive BAER peaks exhibited progressively larger latency shifts with increasing stimulus rate at all ages studied. These data suggest that higher stimulus rates produce greater neural adaptation resulting in prolonged BAER peak latencies. Furthermore, the effects of adaptation are cumulative across synapses, and the mechanisms responsible for the acquisition of adult-like adaptation properties develop during the early post-natal period in the cat.

2f1-f2 distortion product otoacoustic emissions in White Leghorn chickens (Gallus domesticus): effects of frequency ratio and relative level

The effects of primary tone frequency ratio (f2/f1 ratio) and relative level (L2/L1) on the amplitude of the cubic difference tone (CDT: 2f1-f2) distortion product otoacoustic emissions (DPOAEs) were investigated in adult White Leghorn chickens (Gallus domesticus). In experiment 1, 9 f2/f1 ratios ranging from 1.05 to 1.8 were investigated. Measurements were obtained from both ears of 4 chickens at 7 f1 frequencies ranging from 0.8 to 4.0 kHz. The primary tones were equal in level, and varied from 20 to 80 dB SPL. The mean CDT amplitude increased with increasing primary tone level once the measurement noise floor was exceeded. The input/ output functions assumed one of two shapes: one in which there was a systematic increase in DPOAE amplitude with increasing primary tone level, and the other in which there was a plateau in the input/output function near 65-70 dB SPL. At the highest primary tone level (80 dB SPL), there was a decrease in the CDT amplitude with increasing f2/f1 ratio. At high primary tone levels, the f2/f1 ratio which produced the largest CDT was 1.05 or 1.1, while at lower primary tone levels the largest CDT occurred at f2/f1 ratios of 1.2-1.3. In experiment 2, L2 was held constant at 70 dB SPL, and L1 varied from 50 to 80 dB SPL. For f1 frequencies of 0.8 and 3.2 kHz, there was an increase in the CDT amplitude with increasing L1, followed by an asymptote at higher levels. In contrast, for 1.6 and 2.0 kHz f1 frequencies, the amplitude increased, plateaued and then increased again at higher levels. Informal measurements suggest that spontaneous otoacoustic emissions (SOAEs) are rarely seen in chickens. However, a reliable SOAE was observed in 1 chicken, which could be suppressed by external sounds and anoxia.

Comparison of the envelope following response in the Mongolian gerbil using two-tone and sinusoidally amplitude-modulated tones

Two-tone (TT) and sinusoidally amplitude-modulated (SAM) signals, although differing in spectra, are both periodic; the period corresponds to the difference between the two frequencies (f2,1 = f2-f1) in the former and to the frequency of the modulation tone (fmod) in the latter. Here the results of a study comparing the steady-state electrophysiologic responses to TT and SAM stimuli recorded from Nembutal-anesthetized Mongolian gerbils are reported. In the first experiment a modulation rate transfer function (MRTF) was obtained for each stimulus type by setting the SAM carrier frequency (fc) and f1 of the TT signal at the same frequency while fmod and f2,1 were covaried. MRTFs were obtained for f1s and fcs of 1, 3, and 5 kHz, with envelopes which varied between 50 and 500 Hz in 50-Hz increments. Stimuli were presented at 75 dB peak sound-pressure level (pSPL). Responses to the two stimulus types yielded MRTFs which were very similar and generally low pass in shape. In the second experiment responses to the TT and SAM signals were recorded in the presence of a continuous interfering tone of 85-dB pSPL which was varied between 650 Hz and 3 kHz. In these experiments a maximum reduction in the response to the TT and SAM signals, measured at f2,1 and fmod as well as at fc and f1, occurred within a narrow frequency band above the frequency of the probe carrier and a broader region of reduced response extending to higher frequencies. This reduction in response was asymmetrical, spreading more to high than to low frequencies. The similarity of both MRTFs and interference response patterns supports the view that the envelope following responses to TT and SAM stimuli are manifestations of the same nonlinear phenomena.

The brain-stem auditory-evoked response in the big brown bat (Eptesicus fuscus) to clicks and frequency-modulated sweeps

Three experiments were performed to evaluate the effects of stimulus level on the brain-stem auditory-evoked response (BAER) in the big brown bat (Eptesicus fuscus), a species that uses frequency-modulated (FM) sonar sounds for echolocation. In experiment 1, the effects of click level on the BAER were investigated. Clicks were presented at levels of 30 to 90 dB pSPL in 10-dB steps. Each animal responded reliably to clicks at levels of 50 dB pSPL and above, showing a BAER containing four peaks in the first 3-4 ms from click onset (waves i-iv). With increasing click level, BAER peak amplitude increased and peak latency decreased. A decrease in the i-iv interval also occurred with increasing click level. In experiment 2, stimuli were 1-ms linear FM sweeps, decreasing in frequency from 100 to 20 kHz. Stimulus levels ranged from 20 to 90 dB pSPL. BAERs to FM sweeps were observed in all animals for levels of 40 dB pSPL and above. These responses were similar to the click-evoked BAER in waveform morphology, with the notable exception of an additional peak observed at the higher levels of FM sweeps. This peak (wave ia) occurred prior to the first wave seen at lower levels (wave ib). As the level of the FM sweep increased, there was a decrease in peak latency and an increase in peak amplitude. Similarity in the magnitude and behavior of the i-iv and ib-iv intervals suggests that wave ib to FM sweeps is the homolog of the wave i response to click stimuli. Experiment 3 tested the hypothesis that wave ia represented activity emanating from more basal cochlear regions than wave ib. FM sweeps (100-20 kHz) were presented at 90 dB pSPL, and broadband noise was raised in level until the BAER was eliminated. This "masked threshold" occurred at 85 dB SPL of noise. At masked threshold, the broadband noise was steeply high-pass filtered at five cutoff frequencies ranging from 20 to 80 kHz. Generally, wave ia was eliminated for masker cutoff frequencies of 56.6 kHz and below, while wave ib was typically observed for masker cutoffs down to 28.3 kHz. The results of these three experiments are compared and contrasted with data from other mammalian BAER studies.

Conventional and cross-correlation brain-stem auditory evoked responses in the white leghorn chick: rate manipulations

Rate-dependent changes in the chick brain-stem auditory evoked response (BAER) using conventional averaging and a cross-correlation technique were investigated. Five 15- to 19-day-old white leghorn chicks were anesthetized with Chloropent. In each chick, the left ear was acoustically stimulated. Electrical pulses of 0.1-ms duration were shaped, attenuated, and passed through a current driver to an Etymotic ER-2 which was sealed in the ear canal. Electrical activity from stainless-steel electrodes was amplified, filtered (300-3000 Hz) and digitized at 20 kHz. Click levels included 70 and 90 dB peSPL. In each animal, conventional BAERs were obtained at rates ranging from 5 to 90 Hz. BAERs were also obtained using a cross-correlation technique involving pseudorandom pulse sequences called maximum length sequences (MLSs). The minimum time between pulses, called the minimum pulse interval (MPI), ranged from 0.5 to 6 ms. Two BAERs were obtained for each condition. Dependent variables included the latency and amplitude of the cochlear microphonic (CM), wave 2 and wave 3. BAERs were observed in all chicks, for all level by rate combinations for both conventional and MLS BAERs. There was no effect of click level or rate on the latency of the CM. The latency of waves 2 and 3 increased with decreasing click level and increasing rate. CM amplitude decreased with decreasing click level, but was not influenced by click rate for the 70 dB peSPL condition. For the 90 dB peSPL click, CM amplitude was uninfluenced by click rate for conventional averaging. For MLS BAERs, CM amplitude was similar to conventional averaging for longer MPIs.(ABSTRACT TRUNCATED AT 250 WORDS)

Gerbil brain-stem auditory-evoked responses to maximum length sequences

This study evaluated the feasibility of obtaining brain-stem auditory-evoked responses (BAERs) to trains of clicks with average rates of up to 500 clicks per second in the gerbil, in order to evaluate BAER behavior at rates higher than those possible in conventional averaging, and to evaluate whether these higher rates allow obtaining BAERs more efficiently. Gerbils were anesthetized with Nembutal, and normothermia was maintained with a homeothermic blanket system. Pseudorandom pulse trains, called maximum length sequences (MLSs), were generated by an Ariel DSP-16 board housed in an IBM-AT. Electrical activity was recorded with Grass needle electrodes placed subdermally. This activity was amplified and filtered by a Nicolet Compact Four, and digitized by the A/D of the Ariel DSP-16. Clicks were presented at levels of 50, 60, 70, 80, and 90 dB pSPL. Conventional BAERs were obtained at a rate of 30 Hz. MLS BAERs were obtained by presenting MLS pulse trains which were comprised of 64 pulses. Following collection of the response to MLS pulse trains, the MLS BAER was obtained by cross correlating the response with a recovery sequence. The minimum time between pulses is called the minimum pulse interval (MPI). MLS BAERs were obtained for MPIs of 6, 4, 2, and 1 ms. BAERs were obtained in ten young adult gerbils for all rate by click-level conditions. BAERs were observed in all animals for all click levels and rates. Generally, with increasing click level there were decreases in peak latencies and increases in peak amplitudes. With increasing rate (decreasing MPI), there were increases in peak latencies, increases in the i-iv interval, and decreases in peak amplitudes. Computations suggested the the most efficient rate (i.e., the rate producing a constant signal-to-noise ratio response in the least amount of time) is not necessarily the fastest rate, but depends on the BAER peak and click level.

Effects of noise burst rise time and level on the human brainstem auditory evoked response

An experiment was conducted to examine the effects of noise burst rise time and level on the human BAER. Noise burst levels included 15, 30, 45 and 60 dB nHL, with linear rise times of 0, 0.5, 1.25 and 2.5 ms. With increasing noise burst level, there is a decrease in wave V latency and an increase in peak amplitude. With increasing noise burst rise time, there is an increase in wave V latency and a decrease in wave V amplitude. The slope of the latency/intensity function increases with increasing rise time. The slope of the latency/rise time function increases with decreasing noise burst level. The change in wave V latency associated with changing rise time is less than the change in rise time for all experimental conditions.

A comparison of N1 of the whole nerve action potential and wave i of the brain-stem auditory evoked response in Mongolian gerbil

The present study seeks to provide empirical support for the assumption that wave i of the gerbil brain-stem auditory evoked response (BAER) corresponds to N1 of the whole nerve action potential (WNAP) by comparing the latency and amplitude of BAER wave i and WNAP N1. Fourteen 3-month old gerbils were anesthetized with Nembutal and Urethane-Dial. Normothermia was maintained by a homeothermic blanket system. BAERs were recorded with Grass needle electrodes placed subdermally. The WNAP was recorded with a silver wire placed in the round window niche. WNAP and BAER were simultaneously recorded with a passband of 100-10,000 Hz. Responses consisted of 500 sweeps, and two responses were obtained for each condition. Clicks were 25-microseconds electrical pulses. Tonebursts were shaped with a Hanning window, with 1-ms rise and fall times. Toneburst frequencies included 1, 2, 4, 8, and 16 kHz. For each stimulus, responses were obtained at levels of 30, 50, 70, and 90 dB pSPL. SPL was measured near the entrance to the ear canal with an Etymotic ER-7C probe microphone. Dependent variables were the latency and amplitude of N1 of the WNAP and wave i of the BAER. The latencies of wave i and N1 were very similar. Mean (across animal) latencies of N1 and wave i were within 70 microseconds for all six stimuli (clicks, tonebursts) and all four levels. Latency/intensity function slopes for N1 and wave i were also very similar, with both dependent variables showing an increasing latency/intensity function slope with decreasing toneburst frequency. The N1/wave i amplitude ratio was computed.(ABSTRACT TRUNCATED AT 250 WORDS)

Contribution of thallium-201-SPECT to the grading of tumorous alterations of the brain

Single photon emission computed tomography (SPECT) with thallium-201-chloride (201Tl) was used in 22 patients to assess the grade of malignancy of brain tumors. Low- and high-grade malignant gliomas could be well differentiated by calculating the Grade Index (GI), i.e., 201Tl uptake in the tumor area relative to a contralateral brain region. Low-grade gliomas (WHO-grade I-II) usually showed a GI of < 1.5. Tumors classified histologically as high-grade malignant (WHO-grade III-IV) had GI values greater than 1.42 and a mean value of 1.89. Until labelled amino-acid tracers for gamma-cameras become commercially available, thallium-201 brain-SPECT can provide an independent and complementary method to CT/MRI for the differential diagnosis of grading of brain tumors. This simple technique can help to reduce sampling errors during needle biopsies of brain tumors, particularly of high-grade lesions incorrectly graded as low-grade tumors due to inadequate biopsy material. In addition, pre- and post-therapy studies can influence the strategy of therapy itself and allow an early detection of recurrences.

The effect of cochlear hearing loss on auditory brain stem response latency

The effect of audiometric configuration on the auditory brain stem response was studied in a large patient sample, and wave I latencies, wave V latencies, and the I-V interwave intervals were compared to those from a previous report. Patients with notched hearing losses showed longer wave V latencies and I-V interwave intervals than those with other audiometric configurations, but the magnitude of the effect was relatively small, and the confidence limit for cochlear diagnosis was essentially the same as that based upon a cochlear hearing loss population without regard to audiometric configuration.

Effects of noiseburst rise time and level on the gerbil brainstem auditory evoked response

The effects of noiseburst rise time and level on the latency and amplitude of waves i and v of the gerbil brainstem auditory evoked response (BAER) were evaluated. For tonal carriers, changes in rise time produce changes in stimulus spectrum. The use of a noise carrier eliminates such confounding spectral changes. Noiseburst levels included 40, 55, 70 and 85 dB SPL. Noiseburst rise times included 0, 0.5, 1.25 and 2.5 ms. With increasing noiseburst level, there is a decrease in peak latency, an increase in peak amplitude and a small increase in the i-v interval. With increasing noiseburst rise time, there is an increase in peak latency, a decrease in peak amplitude and a small increase in the i-v interval. The slopes of wave i and v latency/intensity functions increase with increasing rise time. The changes in peak latencies are always less than the increase in rise time. The slopes of these functions increase with decreasing noiseburst level and are greatest for the change in rise time from 0 to 0.5 ms. These data demonstrate changes in the BAER with rise time manipulation, even in the absence of stimulus spectral changes. It was hypothesized that changes in the BAER with increasing noiseburst rise time were due to a decrease in the effective amplitude of the stimulus. It was found that this hypothesis does not, in isolation, adequately account for the peak latency and amplitude changes found in the present investigation.

Brainstem auditory-evoked response in the rat. Normative studies, with observations concerning the effects of ossicular disruption

Six young adult Sprague-Dawley rats were unilaterally cochleotomized, Brain-stem auditory-evoked responses (BAERs) to clicks and to 1-, 2-, 4-, 8- and 16-kHz tone bursts were obtained. In addition, response thresholds were estimated before and after ossicular disruption in the noncochleotomized ear of 4 animals. With increasing tone burst frequency, there was a decrease in BAER peak latencies as well as a decrease in threshold. With increasing click and tone burst intensity, there was a decrease in peak latencies and an increase in peak amplitudes. BAER peak latency/intensity functions to click stimuli ranged from -.013 to -.018 ms/dB. With increasing tone burst frequency there was a decrease in the slope of the latency/intensity function. Following ossicular disruption, BAER thresholds to clicks were elevated by an average of 49 dB. Threshold shifts to tone burst stimuli were smallest for 1- and 2-kHz tone bursts (35-36 dB) and increased with increasing frequency up to a maximum of 65 dB for 16-kHz tone bursts.

Frequency sensitivities of auditory neurons in the cerebellum of the cat

Threshold tuning curves were obtained from neurons in the cerebellar auditory area of the cat. The threshold of the brainstem auditory evoked response was also measured in each animal as a function of sound frequency in order to monitor the overall frequency sensitivity of the auditory periphery. Cerebellar auditory neurons responded to sound stimuli with little discrimination for the sound frequency. The values of Q10dB (a measure of the sharpness of tuning) were less than 2 for most of the neurons in this study. There was no significant difference in the sharpness of tuning for neurons in the various layers of the cerebellar auditory area. Electrophysiological mapping showed that the frequency sensitivity of single neurons did not appear to vary as a function of location within the cerebellar auditory area which includes lobules VI and VII of Larsell. Broad tuning was observed in long-latency (greater than 11 ms) neurons which responded to binaural sound stimuli as well as in short-latency (less than 6 ms) neurons which only responded to monaural sound stimuli. Within each animal, tuning curves of single cerebellar neurons were essentially superimposable onto each other and matched well with the overall frequency sensitivity of the animal as shown by brainstem auditory evoked response. Since the frequency tuning of these neurons appeared to reflect the overall frequency sensitivity of the auditory periphery, auditory neurons in the posterior vermis may receive inputs that involve convergence and integration from the entire length of the cochlea.

Brainstem evoked responses to paired-click stimuli: the use of digital response subtraction

Brainstem evoked responses were obtained to 60-dB nHL clicks and click pairs presented at 27/s. Paired-click intervals ranged from 9.8 to 0.5 ms. 6 normal-hearing young adults served as subjects. Derived responses were obtained by digital response subtraction of single-click from paired-click responses. Wave V was observed to unsubtracted click 2 of the click pair at paired-click intervals down to 4 ms. Wave V of the derived (subtracted) click 2 response was observed for paired-click intervals down to 1 ms. To assess additivity, the click 1 response of the click pair was compared to the single-click response, and the unsubtracted click 2 response was compared to the derived click 2 response. At longer paired-click intervals, wave V latency was greater for click 1 of the click pair than for the single-click response. This latency difference decreases at shorter paired-click intervals and is attributed to adaptation of click 1 by click 2 of the previous pair. Wave V amplitudes for single-click and paired-click click 1 responses were not significantly different at any paired-click interval. The latency and amplitude for unsubtracted click 2 and derived click 2 responses were not significantly different at any paired-click interval. It is concluded that the responses to click 1 and click 2 of the paired clicks are additive, at least at very brief paired-click intervals.

Sound pressure level measurement and spectral analysis of brief acoustic transients

The sound pressure level (SPL) of an acoustic transient can be quantified in several ways. The SPL value obtained is dependent on measurement procedure, in addition to signal and transducer characteristics. The acoustic spectrum of a signal shows sound pressure as a function of frequency. The acoustic spectrum can be determined by the use of analog filtering or by Fourier transformation. A constant electrical signal can produce different acoustic spectra due to varying transfer functions across transducers. Signal center frequency, rise/fall time and plateau influence acoustic spectrum. Recording parameters, such as constant bandwidth versus logarithmic bandwidth filtering, or the time domain windowing function used prior to Fourier transformation, also influence the acoustic spectrum.

The effect of broadband noise on the human brainstem auditory evoked response. I. Rate and intensity effects

A series of experiments investigated the effects of continuous broadband noise (ipsilateral) on wave V of the click-evoked brainstem auditory evoked response (BAER). In general, a broadband noise masker increases the latency and decreases the amplitude of wave V. Varying both click and noise intensity, it was found that noise levels above about 40 dB SPL increase the latency and decrease the amplitude of wave V, regardless of click intensity. The effects of noise on wave V amplitude appear constant across click intensity, whereas the effects of a constant noise level on wave V latency decrease at higher click intensities. Both masking and adaptation increase wave V latency, but their combined effects are occlusive: rate-induced wave V latency shift decreases in the presence of continuous broadband noise. The clinical and theoretical implications of these findings are discussed.

The effect of broadband noise on the human brainstem auditory evoked response. II. Frequency specificity

A series of experiments evaluated the effects of broadband noise (ipsilateral) on wave V of the brainstem auditory evoked response (BAER) elicited by tone bursts or clicks in the presence of high-pass masking noise. Experiment 1 used 1000- and 4000-Hz, 60-dB nHL tone bursts in the presence of broadband noise. With increasing noise level, wave V latency shift was greater for the 1000-Hz tone bursts, while amplitude decrements were similar for both tone-burst frequencies. Experiment 2 varied high-pass masker cutoff frequency and the level of subtotal masking in the presence of 50-dB nHL clicks. The effects of subtotal masking on wave V (increase in latency and decrease in amplitude) increased with increasing derived-band frequency. Experiment 3 covaried high-pass masker cutoff frequency and subtotal masking level for 1000- and 4000-Hz tone-burst stimuli. The effect of subtotal masking on wave V latency was reduced for both tone-burst frequencies when the response-generating region of the cochlear partition was limited by high-pass maskers. The results of these three experiments suggest that most of the wave V latency shift associated with increasing levels of broadband noise is mediated by a place mechanism when the stimulus is a moderate intensity (60 dB nHL), low-frequency (1000 Hz) tone burst. However, the interpretation of the latency shifts produced by broadband noise for 4000-Hz tone-burst stimuli is made more complex by multiple technical factors discussed herein.