A comparative study of the brain stem auditory response in mammals

Rapid hearing threshold assessment with modified auditory brainstem response protocols in dogs

Introduction

Auditory brainstem response (ABR) is the gold standard for hearing testing in dogs. ABR is commonly used in puppies to diagnose congenital sensorineural deafness. Long test times limit the use for a more comprehensive hearing screening in veterinary practice. This study aimed to establish a super-fast hearing screening protocol in dogs.

Methods

Hearing thresholds were routinely measured with a mobile device designed for newborn hearing screening in 90 dogs. We introduced modifications of the ABR protocol, e. g., a binaural test mode, higher stimulus rates, a broadband chirp stimulus, and an algorithm for automatic peak V detection in a stepwise fashion. Hearing thresholds were then measured with fast protocols utilizing either 30 Hz click or 90 Hz broadband chirp stimuli with 80, 60, 40, 30, 20, 10, 0 and -10 dBnHL stimulation intensities. Interrater reliability, agreement between click and chirp hearing thresholds and correlations with clinical characteristics of the dogs were assessed.

Results

Using all innovations, the test time for hearing threshold assessment in both ears was reduced to 1.11 min (mean). The chirp stimulus accentuated both, peak V and the subsequent trough, which are essential features for judgement of the hearing threshold, but preceding peaks were less conspicuous. Interrater reliability and agreement between click and chirp hearing threshold was excellent. Dogs >10 years of age and dogs with abnormal hearing score or otitis score had significantly higher hearing thresholds than younger dogs (p ≤ 0.001) or dogs without abnormalities (p < 0.001).

Conclusion

The results demonstrate that modifications in ABR protocols speed-up test times significantly while the quality of the recordings for hearing threshold assessment is maintained. Modified ABR protocols enable super-fast hearing threshold assessment in veterinary practice.

Evidence for the origin of the binaural interaction component of the auditory brainstem response

The binaural interaction component (BIC) represents the mismatch between auditory brainstem responses (ABR) obtained with binaural stimulation and the sum of ABRs obtained with monaural left and right stimulation. It is generally assumed that the BIC reflects binaural integration. Its potential use as a diagnostic tool, however, is hampered by the lack of direct evidence about its origin. While an origin at the initial site of binaural integration seems likely, there is no general agreement on the contribution of the two primary candidate nuclei, the lateral and medial superior olives (LSO and MSO, respectively). Here, we recorded local field potentials (LFP) and responses of units in the LSO and MSO of Mongolian gerbils (Meriones unguiculatus), presenting clicks with an interaural time or level difference (ITD and ILD, respectively), while simultaneously recording ABR. We determined the BIC from the ABR and, importantly, from LFP and responses of units in the LSO and MSO. If stimulus-induced changes in the ABR-derived BIC have their source in the LSO and/or MSO, we expect coherent changes in the unit-derived and the ABR-derived BIC. We find that BIC obtained from LSO units exhibits the same ITD and ILD dependence as the ABR-derived BIC. Neither BIC obtained from MSO units nor LFP-derived BIC recorded in either LSO or MSO did. The data thus strongly suggest that it is the activity of LSO units in the gerbil that is decisive for the generation of the ABR-derived BIC, determining its properties.

Use of the guinea pig in studies on the development and prevention of acquired sensorineural hearing loss, with an emphasis on noise

Guinea pigs have been used in diverse studies to better understand acquired hearing loss induced by noise and ototoxic drugs. The guinea pig has its best hearing at slightly higher frequencies relative to humans, but its hearing is more similar to humans than the rat or mouse. Like other rodents, it is more vulnerable to noise injury than the human or nonhuman primate models. There is a wealth of information on auditory function and vulnerability of the inner ear to diverse insults in the guinea pig. With respect to the assessment of potential otoprotective agents, guinea pigs are also docile animals that are relatively easy to dose via systemic injections or gavage. Of interest, the cochlea and the round window are easily accessible, notably for direct cochlear therapy, as in the chinchilla, making the guinea pig a most relevant and suitable model for hearing. This article reviews the use of the guinea pig in basic auditory research, provides detailed discussion of its use in studies on noise injury and other injuries leading to acquired sensorineural hearing loss, and lists some therapeutics assessed in these laboratory animal models to prevent acquired sensorineural hearing loss.

Adaptation and spectral enhancement at auditory temporal perceptual boundaries - Measurements via temporal precision of auditory brainstem responses

In human and animal auditory perception the perceived quality of sound streams changes depending on the duration of inter-sound intervals (ISIs). Here, we studied whether adaptation and the precision of temporal coding in the auditory periphery reproduce general perceptual boundaries in the time domain near 20, 100, and 400 ms ISIs, the physiological origin of which are unknown. In four experiments, we recorded auditory brainstem responses with five wave peaks (P1 –P5) in response to acoustic models of communication calls of house mice, who perceived these calls with the mentioned boundaries. The newly introduced measure of average standard deviations of wave latencies of individual animals indicate the waves’ temporal precision (latency jitter) mostly in the range of 30–100 μs, very similar to latency jitter of single neurons. Adaptation effects of response latencies and latency jitter were measured for ISIs of 10–1000 ms. Adaptation decreased with increasing ISI duration following exponential or linear (on a logarithmic scale) functions in the range of up to about 200 ms ISIs. Adaptation effects were specific for each processing level in the auditory system. The perceptual boundaries near 20–30 and 100 ms ISIs were reflected in significant adaptation of latencies together with increases of latency jitter at P2-P5 for ISIs < ~30 ms and at P5 for ISIs < ~100 ms, respectively. Adaptation effects occurred when frequencies in a sound stream were within the same critical band. Ongoing low-frequency components/formants in a sound enhanced (decrease of latencies) coding of high-frequency components/formants when the frequencies concerned different critical bands. The results are discussed in the context of coding multi-harmonic sounds and stop-consonants-vowel pairs in the auditory brainstem. Furthermore, latency data at P1 (cochlea level) offer a reasonable value for the base-to-apex cochlear travel time in the mouse (0.342 ms) that has not been determined experimentally.

The Physiological Basis and Clinical Use of the Binaural Interaction Component of the Auditory Brainstem Response

The auditory brainstem response (ABR) is a sound-evoked noninvasively measured electrical potential representing the sum of neuronal activity in the auditory brainstem and midbrain. ABR peak amplitudes and latencies are widely used in human and animal auditory research and for clinical screening. The binaural interaction component (BIC) of the ABR stands for the difference between the sum of the monaural ABRs and the ABR obtained with binaural stimulation. The BIC comprises a series of distinct waves, the largest of which (DN1) has been used for evaluating binaural hearing in both normal hearing and hearing-impaired listeners. Based on data from animal and human studies, the authors discuss the possible anatomical and physiological bases of the BIC (DN1 in particular). The effects of electrode placement and stimulus characteristics on the binaurally evoked ABR are evaluated. The authors review how interaural time and intensity differences affect the BIC and, analyzing these dependencies, draw conclusion about the mechanism underlying the generation of the BIC. Finally, the utility of the BIC for clinical diagnoses are summarized.

The maturation state of the auditory nerve and brainstem in rats exposed to lead acetate and supplemented with ferrous sulfate

Introduction

The literature has reported the association between lead and auditory effects, based on clinical and experimental studies. However, there is no consensus regarding the effects of lead in the auditory system, or its correlation with the concentration of the metal in the blood.

Objective

To investigate the maturation state of the auditory system, specifically the auditory nerve and brainstem, in rats exposed to lead acetate and supplemented with ferrous sulfate.

Methods

30 weanling male rats (Rattus norvegicus, Wistar) were distributed into six groups of five animals each and exposed to one of two concentrations of lead acetate (100 or 400 mg/L) and supplemented with ferrous sulfate (20 mg/kg). The maturation state of the auditory nerve and brainstem was analyzed using Brainstem Auditory Evoked Potential before and after lead exposure. The concentration of lead in blood and brainstem was analyzed using Inductively Coupled Plasma-Mass Spectrometry.

Results

We verified that the concentration of Pb in blood and in brainstem presented a high correlation (r = 0.951; p < 0.0001). Both concentrations of lead acetate affected the maturation state of the auditory system, being the maturation slower in the regions corresponding to portion of the auditory nerve (wave I) and cochlear nuclei (wave II). The ferrous sulfate supplementation reduced significantly the concentration of lead in blood and brainstem for the group exposed to the lowest concentration of lead (100 mg/L), but not for the group exposed to the higher concentration (400 mg/L).

Conclusion

This study indicate that the lead acetate can have deleterious effects on the maturation of the auditory nerve and brainstem (cochlear nucleus region), as detected by the Brainstem Auditory Evoked Potentials, and the ferrous sulphate can partially amend this effect.

The effects of click and masker spectrum on the auditory brainstem response of bottlenose dolphins (Tursiops truncatus)

Two experiments were performed that investigated the effects of (1) click level and (2) continuous broadband noise on the binaural auditory brainstem response (ABR) of normal-hearing and hearing-impaired bottlenose dolphins (Tursiops truncatus). In addition to spectrally uncompensated clicks and noise, stimuli were digitally compensated to achieve "white" spectra (flat spectral density level) or "pink" spectra (spectral density level rolling off at -3 dB/octave). For experiment 1, in all spectral conditions, ABR peak latencies increased and peak amplitudes decreased with decreasing click level, but interwave intervals changed little. Latency-intensity function (LIF) slopes ranged from -3 to -11 μs/dB. The LIF slopes of ABR peaks evoked by uncompensated clicks were steeper in dolphins with hearing loss. Click level was held constant during experiment 2, and the effect of bilaterally delivered broadband masking noise on the ABR was investigated. Clicks and noise were filtered to create a pink click/noise condition and a white click/noise condition. With increasing levels of masking noise, peak latencies increased (although only P1-P4 white reached significance), peak amplitudes decreased, and interpeak intervals increased (although not significantly). These effects are compared to results reported for terrestrial mammals, and implications for auditory health assessment and biosonar function are discussed.

The contribution of inferior colliculus activity to the auditory brainstem response (ABR) in mice

In mice, the auditory brainstem response (ABR) is frequently used to assess hearing status in transgenic hearing models. The diagnostic value of the ABR depends on knowledge about the anatomical sources of its characteristic waves. Here, we studied the contribution of the inferior colliculus (IC) to the click-evoked scalp ABR in mice. We demonstrate a non-invasive correlate of the IC response that can be measured in the scalp ABR as a slow positive wave P₀ with peak latency 7-8 ms when recorded with adequate band-pass filtering. Wave P₀ showed close correspondence in latency, magnitude and shape with the sustained part of evoked spiking activity and local field potentials (LFP) in the central nucleus of the IC. In addition, the onset peaks of the IC response were related temporally to ABR wave V and to some extent to wave IV. This relation was further supported by depth-dependent modulation of the shape of ABR wave IV and V within the IC suggesting generation within or in close vicinity to the IC. In conclusion, the slow ABR wave P₀ in the scalp ABR may represent a complementary non-invasive marker for IC activity in the mouse. Further, the latency of synchronized click-evoked activity in the IC supports the view that IC contributes to ABR wave V, and possibly also to ABR wave IV.

Effects of aging on peripheral and central auditory processing in rats

Hearing loss is a hallmark sign in the elderly population. Decline in auditory perception provokes deficits in the ability to localize sound sources and reduces speech perception, particularly in noise. In addition to a loss of peripheral hearing sensitivity, changes in more complex central structures have also been demonstrated. Related to these, this study examines the auditory directional maps in the deep layers of the superior colliculus of the rat. Hence, anesthetized Sprague-Dawley adult (10 months) and aged (22 months) rats underwent distortion product of otoacoustic emissions (DPOAEs) to assess cochlear function. Then, auditory brainstem responses (ABRs) were assessed, followed by extracellular single-unit recordings to determine age-related effects on central auditory functions. DPOAE amplitude levels were decreased in aged rats although they were still present between 3.0 and 24.0 kHz. ABR level thresholds in aged rats were significantly elevated at an early (cochlear nucleus - wave II) stage in the auditory brainstem. In the superior colliculus, thresholds were increased and the tuning widths of the directional receptive fields were significantly wider. Moreover, no systematic directional spatial arrangement was present among the neurons of the aged rats, implying that the topographical organization of the auditory directional map was abolished. These results suggest that the deterioration of the auditory directional spatial map can, to some extent, be attributable to age-related dysfunction at more central, perceptual stages of auditory processing.

Albumin administration protects against bilirubin-induced auditory brainstem dysfunction in Gunn rat pups

BACKGROUND

Free bilirubin (Bf), the unbound fraction of unconjugated bilirubin (UCB), can induce neurotoxicity, including impairment of the auditory system, which can be assessed by brainstem auditory evoked potentials (BAEPs). We hypothesized that albumin might reduce the risk of neurotoxicity by decreasing Bf and its translocation into the brain.

AIM

To determine the effects of albumin on BAEPs and brain bilirubin content in two Gunn rat pup models of acute hyperbilirubinemia.

METHODS

We used Gunn rat pups, which have a deficiency of the bilirubin-conjugating enzyme UGT1A1. We induced haemolysis by injection of phenylhydrazine (phz) into 14-days old pups. Subsequently, pups were treated with either i.p. human serum albumin (HSA; 2.5 g/kg; n = 8) or saline (control, n = 8). We induced acute neurotoxicity by injecting 16-days old pups with sulphadimethoxine (sulpha) and treated them with either HSA (n = 9) or saline (control, n = 10). To assess bilirubin neurotoxicity, we used the validated BAEP method and compared relevant parameters; i.e. peak latency values and interwave interval (IWI) between peak I and peak II, a marker of acute neurotoxicity.

RESULTS

Phz and sulpha significantly increased IWI I-II by 26% and 29% (P < 0.05) in the haemolysis and the displacement model, respectively. Albumin completely prevented the increase of IWI I-II in either model. The beneficial effect of albumin in the displacement-model by means of normal BAEPs was in line with less bilirubin in the brain (NS). Interestingly, in the haemolysis model the accumulation of total bilirubin in the brain was unaltered, and BAEPs still appeared normal. This might advocate for a role of brain Bf which was calculated and showed that albumin treatment non-significantly reduces Bf concentrations in brain, compared with saline treatment.

CONCLUSIONS

Albumin treatment is neuroprotective in acute hyperbilirubinemia in Gunn rat pups. Our present results underline the importance of functional diagnostic test of neurotoxicity above biochemical concentrations.

The binaural click-evoked auditory brainstem response of the California sea lion (Zalophus californianus)

Auditory brainstem responses (ABRs) elicited by high-amplitude [100 dB re 20 μPa, peak-to-peak equivalent sound pressure level (peSPL)] aerial broadband clicks were collected from seven California sea lions in order to provide a basic description of short-latency auditory evoked potentials in this species. The waveform of the ABR was similar to that of other mammals, comprising seven positive and six negative characteristic waves. Variability in the amplitudes and latencies of waves was higher among subjects than the variability in within-subject repeated measurements. ABRs to progressively attenuated clicks were collected for three additional sea lions. Wave amplitudes decreased and latencies increased with decreasing stimulus level, with only the sixth positive wave visible near threshold (35-40 dB peSPL). Based on observations of wave latency as a function of stimulus amplitude, the sixth positive wave of the ABR is equivalent to the clinically important "wave V" identified in studies with humans. The current results provide information on the basic electrophysiology of the pinniped auditory system, including the processes that underlie brainstem auditory steady-state responses used to measure frequency-specific hearing sensitivity.

Profile of minocycline neuroprotection in bilirubin-induced auditory system dysfunction

Excessive hyperbilirubinemia in human neonates can cause permanent dysfunction of the auditory system, as assessed with brainstem auditory evoked potentials (BAEPs). Jaundiced Gunn rat pups (jjs) exhibit similar BAEP abnormalities as hyperbilirubinemic neonates. Sulfadimethoxine (sulfa) administration to jjs, which displaces bilirubin from serum albumin into tissues including brain, exacerbates acute toxicity. Minocycline administered prior to sulfa in jjs protects against BAEP abnormalities. This study evaluates the neuroprotective capabilities of minocycline HCl (50 mg/kg) administered 30 or 120 min after sulfa (200 mg/kg) in 16 days old jjs. BAEPs are recorded at 6 or 24 h post-sulfa. Abnormal BAEP waves exhibit increased latency and decreased amplitude. The sulfa/saline treated jjs exhibited a significantly increased interwave interval between waves I and II (I-II IWI) and significantly decreased amplitudes of waves II and III compared to the saline/saline jjs. The minocycline 30 min post-sulfa (sulfa/mino+30) group was not significantly different from the saline/saline control group, indicating neuroprotection. The minocycline 120 min post-sulfa (sulfa/mino+120) group had a significantly decreased amplitude of wave III at both 6 and 24h. These studies indicate that minocycline has a graded neuroprotective effect when administered after acute bilirubin neurotoxicity.

[Guinea pig as often object to otoneurological experimental examinations]

INTRODUCTION

The aim of this research was to describe nomograms of brainstem auditory evoked potentials, oscillating stimulation of the vestibular organs and ultrastructural research methodology of a spiral organ of guinea-pigs.

MATERIALS AND METHODS

For the experimental purposes 60 guinea-pigs of both sexes, weight 250-300 grams, were used as research materials. After general anesthesia with Ketalar dosed 100 milligrams per kilogram and with Fentanyl dosed 0.1 milliliter per kilogram of the body mass, brainstem auditory evoked potentials were registered and the oscillating chair test (no prior anesthesia). Was placed on an assistant's knees, who was sitting on a chair. Then the guinea-pigs were anesthetized with 35 per cent solution of chloral hydrate intraperitoneally (dosed 1 milliliter per kilogram of the body mass). For electron microscopic research membranous helix was collected bilaterally by the method of micropreparation. Qualitative observation was conducted in the Philips' electron microscope (EM 300).

RESULTS

On the basis of the obtained results it was concluded that the time of transmission between elements P1-P2 is constant and figures between 3.1 and 3.6 ms independently from a force of stimulation. Vestibular organs of a guinea-pig are more sensitive to high accelerations (17-12 degree/s2) than human vestibularl organs. However, their reaction is much weaker to lower accelerations (12-0 degree/s2).

CONCLUSIONS

The processed nomograms of the brainstem auditory evoked potentials, oscillating stimulation of vestibular organs and methodology of ultrastructural research in guinea-pigs' spiral organs are expected to facilitate using them in experimental research.

Minocycline blocks acute bilirubin-induced neurological dysfunction in jaundiced Gunn rats

BACKGROUND

Extreme hyperbilirubinemia is treated with double volume exchange transfusion, which may take hours to commence. A neuroprotective agent that could be administered immediately might be clinically useful. Minocycline, an anti-inflammatory and anti-apoptotic semisynthetic tetracycline, prevents hyperbilirubinemia-induced cerebellar hypoplasia in Gunn rats. Acute brainstem auditory evoked potential (BAEP) abnormalities occur after giving sulfadimethoxine to 16-day-old jaundiced Gunn rats to displace bilirubin into tissue including brain.

OBJECTIVE

To assess whether minocycline is neuroprotective in this model of acute bilirubin encephalopathy.

METHODS

We recorded BAEPs at baseline and 6 h after injecting sulfadimethoxine. Minocycline 0.5 mg/kg (n = 4), 5 mg/kg (n = 9), 50 mg/kg (n = 9) or 500 mg/kg (n = 3, all died) was administered 15 min before sulfadimethoxine (0 h). Controls received saline followed by either sulfadimethoxine (n = 13) or saline (n = 7).

RESULTS

At 6 h total plasma bilirubin decreased from 10.84 +/- 0.88 mg/dl (mean +/- SD) to 0.70 +/- 0.35 mg/dl (p <10(-9)) in all sulfadimethoxine-injected groups. At 6 h, there was complete protection against decreased amplitudes of BAEP waves II and III and increased I-II and I-III interwave intervals (brainstem conduction times corresponding to I-III and I-V in humans) with 50 mg/kg minocycline, and partial protection with lower doses.

CONCLUSIONS

Minocycline 50 mg/kg 15 min prior to an intervention that normally produces acute bilirubin neurotoxicity is neuroprotective in jaundiced Gunn rat pups. Further studies are needed to investigate the temporal course and mechanism of neuroprotection. Minocycline, administered immediately, may be clinically useful in treating extreme neonatal hyperbilirubinemia and preventing kernicterus. We believe our model provides an efficient in vivo model to screen and evaluate new agents that are neuroprotective against bilirubin toxicity and kernicterus.

Biliverdin-induced brainstem auditory evoked potential abnormalities in the jaundiced Gunn rat

Brainstem auditory evoked potential (BAEP) abnormalities occur in jaundiced Gunn rats given sulfadimethoxine to displace bilirubin bound to serum albumin, releasing it into the tissues. One problem with the model is that after displacement, plasma bilirubin levels drop and do not correlate with neurological dysfunction. In this report, we administered biliverdin, the immediate precursor of bilirubin, in 15- to 17-day-old Gunn rat pups to create an improved model of bilirubin-induced neurological dysfunction. Total plasma bilirubin (TB) levels were measured with a Leica bilirubinometer. Biliverdin (40 mg/kg) or phosphate-buffered saline (PBS) was administered either once and BAEPs recorded 8 h later or twice, 12 h apart, and BAEPs recorded 24 h after the initial injection. A single biliverdin injection produced a significantly decreased amplitude of BAEP wave III, 1.21+/-0.25 vs. 0.49+/-0.27 microV (control vs. biliverdin). The two-injection paradigm resulted in a significantly elevated TB (9.9+/-1.2 vs. 14.9+/-3.1 mg/dl; control vs. biliverdin), significant increases in I-II (1.15+/-0.08 vs. 1.42+/-0.09 ms) and I-III (2.17+/-0.08 vs. 2.5+/-0.13 ms) interwave intervals and a decrease in the amplitude of wave III (1.36+/-0.30 vs. 0.38+/-0.26 microV). Additionally, there were significant correlations between TB and the amplitude of wave III (r2=0.74) and TB and the I-III interwave interval (r2=0.51). In summary, biliverdin administration in jaundiced Gunn rat pups produces BAEP abnormalities consistent with those observed in the sulfadimethoxine model and human newborn hyperbilirubinemia and resulted in increased plasma bilirubin levels that correlate with the degree of neurological dysfunction.

Electrophysiological correlates of azimuth and elevation cues for sound localization in human middle latency auditory evoked potentials

OBJECTIVE

To study, in humans, the effects of sound source azimuth and elevation on primary auditory cortex binaural activity associated with sound localization.

DESIGN

Middle Latency Auditory Evoked Potentials (MLAEPs) were recorded from three channels, in response to alternating polarity clicks, presented at a rate of 5/sec, at nine virtual spatial locations with different azimuths and elevations. Equivalent dipoles of Binaural Interaction Components (BICs) of MLAEPs were derived from 15 normally and symmetrically hearing adults by subtracting the response to binaural clicks at each spatial location from the algebraic sum of responses to stimulation of each ear alone. The amplified potentials were averaged over 4000 repetitions using a dwell time of 78 micro sec/address/channel. Variations in magnitudes, latencies and orientations of the dipole equivalents of cortical activity were noted in response to the nine spatial locations.

RESULTS

Middle-latency BICs included six major components corresponding in latency to the vertex-neck recorded components of MLAEP. A significant decrease of equivalent dipole magnitude was observed for two of the components: Pa2 in response to clicks in the backward positions (medium and no elevation); and Nb in response to clicks in the back and front positions (medium and no elevation) in the midsagittal plane. In the coronal plane, Pa2 equivalent dipole magnitude significantly decreased in response to right-horizontal (no elevation) clicks. Significant effects on equivalent dipole latencies of Pa2 were found for backward positions (no elevation) in the midsagittal plane. No significant effects on Pa2 and Nb equivalent dipole orientations were found across stimulus conditions.

CONCLUSIONS

The changes in equivalent dipole magnitudes and latencies of MLAEP BICs across stimulus conditions may reflect spectral tuning in binaural primary auditory cortex neurons processing the frequency cues for sound localization.

Origin of the binaural interaction component in wave P4 of the short-latency auditory evoked potentials in the cat: evaluation of serial depth recordings from the brainstem

There is no general agreement on the origin of the binaural interaction (BI) component in auditory brainstem responses (ABRs). To study this issue the ABRs to monaural and binaural clicks with various interaural time differences (ITDs) were simultaneously recorded from the vertex and from a recording electrode aiming at the superior olive (SO) in cats. Electrode path was along the fibers of the lateral lemniscus (LL). Binaural difference potentials (BDPs), which were computed by subtracting the sum of the two monaural responses from the binaural response, were obtained at systematic depths and across a range of ITD values. It was observed that only a specific BDP deflection recorded at the level at which lemniscal fibers terminate in the nuclei of LL coincided in time with the most prominent BDP in the cat's vertex-recorded ABRs, the BDP in their wave P4. As ITD was increased, the latency shifts and amplitude decrements of the scalp-recorded far-field BDP wave exactly followed those recorded at this lemniscal near-field BDP locus. The data support our hypothesis that the BI component in wave P4 results from a binaural reduction in dischargings of axons ascending in the LL, with this reduction due to contralateral inhibition of the discharge activity of the inhibitory-excitatory units in the lateral nucleus of the SO. Furthermore, at the level of the SO, the BDP in the responses to contra-leading binaural clicks always had larger magnitudes than those evoked by ipsi-leading ones. This bilateral asymmetry is consistent with the view that the BDP in scalp-recorded ABRs is related to the function of sound lateralization.

Brainstem auditory evoked potentials (BAEPs) in the cynomolgus macaque monkey. Equivalence with human BAEPs and proposal of a new nomenclature

Several groups have studied brainstem auditory evoked potentials (BAEPs) in non-human primates. However, the nomenclature of the waves elicited and their correspondence with human waves I-V differ among authors. BAEPs were recorded from six anaesthetised young cynomolgus macaques (Macaca fascicularis), using different sound stimuli parameters. A constant pattern of four main waveforms was present in all the animals with stimulus intensities over 60 dB SPL, although up to four smaller waveforms were observed in some of the individuals. Latency values increased with decreasing stimulus intensities and with increasing repetition rates. These results were similar to the BAEPs observed in other species of macaques. Although an approximate equivalence between human and monkey BAEPs is possible, some discrepancies suggest that there may be generators which contribute to different waves in both species. This is the reason for our proposal of a new nomenclature for BAEP waveforms in monkeys, following a descriptive order with Arabic numerals preceded by the letter M.

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.

Binaural masking level difference in human binaural interaction components

OBJECTIVE

The purpose of this study was to compare the effects of monaural and binaural broadband masking noise on binaural interaction components (BICs) of the human auditory brain stem evoked potentials (ABEPs).

DESIGN

The BICs of the human ABEPs were studied by subtracting the potentials to binaural clicks from the algebraic sum of monaurally evoked potentials to clicks alone or to clicks with ipsilateral monaural or binaural broadband masking noise. Alternating polarity, 11/sec clicks were presented at 65 dB nHL, and noise was presented at 45 dB nHL. Analysis included peak-to-prestimulus baseline amplitudes and latencies of BICs' peaks and troughs from the vertex-mastoid (A) and vertex-neck (Z) channels. In addition, 3-channel Lissajous' trajectory (3-CLT) analysis, estimating the single, centrally located dipole equivalent of surface activity, was performed on data recorded from three orthogonally positioned electrode pairs. 3-CLT measures included apex latency, amplitude, and orientation, as well as planar segment duration, size, shape, and orientation.

RESULTS

All BICs 3-CLTs included five main components (labeled BdI, BdII, BdIII, BeI, and BeII). In general, apex latencies were longer with masking noise. However, BdII and BeI apex latencies were shorter with binaural than with ipsilateral monaural masking noise. Apex amplitude and planar segment size of component BeI, as well as P1 peak amplitude in BICs of the Z-channel records, were larger with binaural than with monaural noise. No significant difference between the monaural and binaural noise conditions was found in durations, shapes, and orientations of planar segments of BICs 3-CLT, nor in peak latency of BICs in the A- and Z-channel records.

CONCLUSIONS

We suggest that these effects on the latency and amplitude of BICs reflect binaural processing in the human brain stem. In particular, the larger amplitudes and shorter latencies of P1 and BeI with binaural than with ipsilateral monaural masking may be associated with the psychophysical effect of binaural masking level difference.

Interaural delay-dependent changes in the binaural difference potential in cat auditory brainstem response: implications about the origin of the binaural interaction component

Auditory brainstem responses (ABRs) evoked by dichotic clicks with 12 different interaural delays (ITDs) between 0 and 1500 microsecond(s) were recorded from the vertices of 10 cats under ketamine anesthesia. The so-called binaural difference potential (BDP), considered to be an indicator of binaural interaction (BI), was computed by subtracting the sum of the two monaural responses from the binaural one. The earliest and most prominent component of BDP was a negative deflection (DN1) at a latency between 4 and 4.8 ms. Like all the other components of BDP, DNI was also due to binaural reduction rather than enhancement of the corresponding ABR wave, P4 in this case. Furthermore, the way its latency increased as a function of ITD was also not compatible with what would be predicted by the delay-line coincidence detector models based on the excitatory-excitatory units in the medial superior olive (MSO). We therefore proposed an alternative hypothesis for the origin of this BI component based on the inhibitory-excitatory (IE) units in the lateral superior olive (LSO). The computational model designed closely simulated the ITD-dependent attenuation and latency shifts observed in DN1. It was therefore concluded that the origin of this BI component in the cat's vertex-ABR could be the lateral lemniscal output of the LSO, although the delay lines which have been shown to exist also in the mammalian brain may play an important role in encoding ITDs.

The protective effects of alpha-melanocyte stimulating hormone on canine brain stem ischemia

OBJECTIVE

To evaluate the influence of alpha-melanocyte stimulating hormone (alpha-MSH), an anti-inflammatory antagonist of the production and action of proinflammatory cytokines, 26 dogs were divided into four groups and exposed to isolated, reversible brain stem ischemia in the presence or absence of alpha-MSH treatment.

METHODS

Brain stem auditory evoked potentials (BAEPs) and regional cerebral blood flow were measured during ischemia and for 5 hours after reperfusion. Group I was composed of five dogs that underwent surgical preparation only. Group II was composed of seven dogs that were exposed to 20 minutes of ischemia without treatment. Group III was comprised of seven dogs exposed to 20 minutes of ischemia with alpha-MSH treatment before and during ischemia. Group IV was composed of seven dogs exposed to 20 minutes of brain stem ischemia with alpha-MSH treatment only during reperfusion.

RESULTS

During the ischemic period, BAEPs were abolished in all animals within 10 minutes. With reperfusion, the BAEPs increased to approximately 36% of baseline in Group II dogs that received no treatment. However, this increase was approximately 63% in animals that received alpha-MSH both before and during ischemia (Group III). In Group IV dogs that received alpha-MSH only during reperfusion, BAEPs were increased approximately 10 to 14% more than in Group II during the late reperfusion period.

CONCLUSION

The improved recovery of BAEPs in dogs treated with alpha-MSH suggests that this peptide may have neuroprotective effects in brain stem ischemia and reperfusion injury. This effect may be caused by an antagonistic action of alpha-MSH on cytokine-induced ischemic brain damage.

Cellular generators of the binaural difference potential in cat

In humans, lateralization and fusion of binaurally presented clicks are correlated with the latency and amplitude of the binaural difference potential (BDP) (e.g., Furst et al., 1985). The BDP is derived by subtracting the brainstem auditory evoked potential (BAEP) for binaural stimulation from the sum of the BAEPs for left and right monaural stimulation. Our aim in this work was to determine the cellular generators of the BDP and thus identify cells that may be crucial for specific types of binaural sound processing. To this end, we injected kainic acid into the superior olivary complex (SOC) or the cochlear nucleus (CN) in cats and examined the effects of the resulting lesions on the click-evoked BDP. Lesions confined to the anterior anteroventral CN (AVCNa) substantially reduced the BDP, while lesions primarily involving more posterior parts of the CN had little or no effect. BDP reductions occurred for lesions involving either high (> 10 kHz) or lower (< 10 kHz) characteristic frequency (CF) regions of the AVCNa (as well as the posterior CN). Lesions involving the SOC reduced the BDP and, in one case, eliminated the high-pass filtered (270 Hz cutoff) BDP. Combining these results with published information about the physiology and anatomy of auditory brainstem cells, we conclude that: (1) spherical cells in the AVCNa are essential for BDP production, (2) the earliest part of the BDP is generated by medial superior olive (MSO) principal cells which receive spherical cell inputs, (3) a later part is probably generated by the cellular targets of MSO principal cells and, (4) the cells involved in BDP generation have CFs above, as well as below, 10 kHz. Since humans, like cats, have a well-developed MSO, we suggest that the MSO may also be essential for BDP production in humans. Thus, perceptual correlates of the BDP, binaural fusion and click lateralization, apparently involve the MSO.

A canine model of acute hindbrain ischemia and reperfusion

Animal models of brain stem ischemia are needed for pathophysiological study and evaluation of treatment; few such models are available currently. A new canine model of hindbrain ischemia and reperfusion is introduced in this article. Through an anterior cervical approach, the basilar artery was surgically exposed in 18 dogs. The posterior communicating and superior cerebellar arteries were embolized with cyanoacrylate glue to isolate the posterior circulation from the anterior circulation. Reversible hindbrain ischemia was induced in 14 dogs by the temporary clipping of the vertebral and ventral spinal arteries for various periods (10-30 min), then the clips were removed and reperfusion was achieved for 5 hours. In all 14 dogs, the hindbrain ischemia was confirmed by the decreased perfusion pressure in the basilar artery (< 10 mm Hg), the diminished regional cerebral blood flow as measured with a laser Doppler flowmeter at the medulla oblongata (< 10 ml/100 g/min), the flattened brain stem auditory evoked potentials, and the increased leakage of Evans blue dye from tissue. These parameters did not change in the four control dogs. The changes in brain stem auditory evoked potentials were closely related to the length of ischemic interval; after 10 minutes of ischemia, reperfusion fully reversed the changes in brain stem auditory evoked potentials, but 20-minute and 30-minute ischemic intervals partially or totally depleted the brain stem auditory evoked potentials. Delayed postischemic hypoperfusion occurred in all five dogs that underwent the 30-minute ischemic interval. The early physiological changes in this model allowed us to estimate the severity of brain stem ischemia and the resulting damage.(ABSTRACT TRUNCATED AT 250 WORDS)

Three-channel Lissajous' trajectory of the binaural interaction components of human auditory middle-latency evoked potentials

Three-channel Lissajous' trajectories (3-CLTs) of the binaural interaction component (BI) of auditory middle latency evoked potentials (AMLEPs) were derived from 14 normally hearing adults by subtracting the response to binaural clicks from the algebraic sum of monaural responses. AMLEPs were recorded in response to 65 dB nHL, rarefaction clicks, presented at a rate of 3.3/s. A normative set of BI 3-CLT measures was calculated and compared with the corresponding measures of simultaneously recorded, single-channel vertex-left mastoid and vertex-neck derivations of BI and of AMLEP to binaural stimulation (B). 3-CLT measures included: apex latency, amplitude and orientation, as well as planar segment duration, orientation, size and shape. The results showed seven main apices and associated planar segments ('Be', 'Bf', 'Bg', 'Bh', 'Bi1', 'Bi2' and 'Bj') in the 3-CLT of BI. Apex latencies of the BI 3-CLT were comparable to peak latencies of the vertex-left mastoid and vertex-neck AMLEP and BI records, both in their absolute values and in intersubject variability. Durations of BI planar segments were approximately 5.0 ms. Apex amplitudes of BI 3-CLT were larger than the respective peak amplitudes of the vertex-mastoid and vertex-neck BI records, while their intersubject variabilities were comparable. The lateralization of BI components may indicate asymmetric processing of binaural auditory input, or may be connected with anatomical asymmetry such as skull thickness. Preliminary analyses did not reveal a clear correlation between the lateralization of the BI component 'Bi2' and the handedness of the subject. We suggest that BI components of AMLEP may be associated with the primary auditory cortex and subcortical ascending structures.

The effect of broad-band noise on the binaural interaction components of human auditory brainstem-evoked potentials

Three-channel Lissajous trajectories (3-CLTs) of binaural interaction components (BI) of auditory brainstem potentials (ABEPs) were derived from 13 normally hearing adults by subtracting the response to binaural clicks from the algebraic sum of monaurally evoked responses to clicks. ABEPs were recorded in response to 65 dB nHL, alternating-polarity clicks, presented at a rate of 11/s. The procedure was repeated with clicks alone as well as with clicks with broad-band masking noise. Noise was presented at 25 and 45 dB nHL, producing a signal-to-noise ratio of +40 and +20 dB, respectively. All BI 3-CLTs included 6 planar segments (labeled BdI, BdII, BdIII, BeI, BeII and Bf) whose apex latencies, except Bf, increased with increasing noise level above 25 dB nHL, and whose durations, sizes, shapes and orientations did not change across noise levels. There were also significant increases in peak latencies of the BI from single channels vertex-mastoid and vertex-neck with increasing noise level. No significant change was found in the trajectory amplitude of apices, with the exception of apices BdIII and Bf whose amplitudes increased with increasing noise level. We suggest that the paradoxical increase in BI amplitude with masking noise may reflect a binaural enhancement of the effect of noise. The effects observed indicate that, whereas the response to clicks displays occlusion, the response to noise displays spatial facilitation at the brainstem level.

Peaks of brainstem auditory evoked potentials in dogs

The effects of electrode configuration and click polarity on brainstem auditory evoked potentials (BAEP) in dogs were investigated to clarify the inconsistent nomenclature for each peak. Four positive peaks (waves 1, 2, 3 and 4) before a deep negative trough and a fifth positive peak (wave 5) following the trough were the basic components of BAEP in dogs, which were easily identified regardless of recording conditions such as electrode configuration and click polarity. Additional peaks tended to be present when a noncephalic reference electrode and/or single-polarity (rarefaction or condensation) click stimuli were used. The Roman nomenclature for the individual positive peaks of BAEP in dogs is confused owing to variations in the observed waveforms among researchers, but click polarity and/or reference electrode position can explain all the previously reported variations in BAEP waveforms in dogs. When the criteria concerning 'wave V' in the guidelines of BAEP in human beings are applied to avoid further confusion of Roman nomenclature in dogs, it is recommended that the basic five positive peaks (waves 1, 2, 3, 4 and 5 as identified easily with Ai-Vertex configuration and alternating clicks) are designated as waves I, II, III, V and VI, respectively. Wave IV (wave 3b) occurs occasionally before wave V in dogs.

Three-channel Lissajous' trajectory of the binaural interaction components in human auditory brain-stem evoked potentials

The 3-channel Lissajous' trajectory (3-CLT) of the binaural interaction components (BI) in auditory brain-stem evoked potentials (ABEPs) was derived from 17 normally hearing adults by subtracting the response to binaural clicks (B) from the algebraic sum of monaural responses (L + R). ABEPs were recorded in response to 65 dB nHL, alternating polarity clicks, presented at a rate of 11/sec. A normative set of BI 3-CLT measures was calculated and compared with the corresponding measures of simultaneously recorded, single-channel vertex-left mastoid and vertex-neck derivations of BI and of ABEP L + R and B. 3-CLT measures included: apex latency, amplitude and orientation, as well as planar segment duration and orientation. The results showed 3 apices and associated planar segments ("BdII," "Be" and "Bf") in the 3-CLT of BI which corresponded in latency to the vertex-mastoid and vertex-neck peaks IIIn, V and VI of ABEP L + R and B. These apices corresponded in latency and orientation to apices of the 3-CLT of ABEP L + R and ABEP B. This correspondence suggests generators of the BI components between the trapezoid body and the inferior colliculus output. Durations of BI planar segments were approximately 1.0 msec. Apex amplitudes of BI 3-CLT were larger than the respective peak amplitudes of the vertex-mastoid and vertex-neck recorded BI, while their intersubject variabilities were comparable.

Relationship between latency of brainstem auditory-evoked potentials and head size in dogs

Cranium and brainstem dimensions were measured in 32 postmortem dog heads. Positive correlations were found between cranium length (CL) and brainstem length (BL) (r = 0.87), between cranium width (CW) and brainstem width (BW) (r = 0.83), and between cranium distance (CD = CL+CW/2) and brainstem distance (BD = BL+BW/2) (r = 0.91). Positive correlation coefficients were also found between CL and CW (r = 0.90), and between BL and BW (r = 0.85). It was concluded that head size accurately reflected brainstem size. A least squares estimation of the brainstem distance (BD) from CL and CW values was BD = 10.9 + 0.16 (CL+CW/2) (BD, CL and CW in mm). Brainstem auditory evoked potentials (BAEPs) and cranium dimensions were measured in 43 dogs (86 ears) with different head size, body size, sex and age. Wave form, absolute and interpeak latencies and correlation coefficients, relating latencies to cranium dimensions and body weight, were analysed. CL, CW, and CD were positively correlated with body weight (r = 0.93, 0.70 and 0.93, respectively), and CL, CW, and CD were correlated with age (r = 0.33, 0.52, and 0.40, respectively). BAEPs consisted of five distinct positive peaks (I to V). Secondary positive peaks following peaks I and II were seen in 60% (I') and 90% (II') of the recordings. Late waves were recorded in 90% (VI), 50% (VII), and 25% (VIII) of the recordings. Latencies increased with decreasing stimulus intensity level (from 90 dB to 10 dB hearing level, HL), especially for peaks I, II, V, and the I-V interpeak interval.(ABSTRACT TRUNCATED AT 250 WORDS)

The effects of digital filtering on feline auditory brain-stem evoked potentials

The power spectrum of the feline auditory brain-stem evoked potentials (ABEPs) consists of 3 frequency bands, similar to the human wave form, but differing in range. The frequency bands in the feline spectra were separated by notches at 326 Hz and 732 Hz. Click-evoked ABEP from 15 cats were digitally filtered in 3 passbands: (1) below 326 Hz ('slow filter'), (2) between 326 and 732 Hz ('medium filter'); and (3) between 732 and 1790 Hz ('fast filter'). Filtering in each of these bands differentially affected the ABEP components. The vertex positive components are labeled by their order of appearance, i.e., 1, 2, ... 5. Peak 1 is subdivided into 2 subcomponents labeled 1a and 1b. The slow filter was associated with the loss of all components leaving a slow potential shift, i.e., the 'pedestal' peaking at the latency of peak 4. The medium filter was associated with the loss of components 1a, 1b and 2, sparing 3 and 4. The fast filter was associated with the loss of 1b and a diminution of 2. Comparing cat and human ABEP, feline components 2, 3 and 4 behaved precisely the same as the human II, III and V. In contrast to the human I, the feline first component (1a) was not detected with the medium filter. No feline component, following peak 1 in the unfiltered wave form, disappeared with the slow and medium filters, and reemerged with the fast filter (as human IV does).(ABSTRACT TRUNCATED AT 250 WORDS)

The effect of brainstem lesions on brainstem auditory evoked potentials in the cat

Brainstem auditory evoked potentials (BAEPs) were recorded before and after cuts were made in either the midline trapezoid body (TB), the lateral lemniscus (LL), or the combined dorsal and intermediate acoustic striae (DAS/IAS) in 23 anesthetized cats. Monaural and binaural rarefaction clicks were presented at a rate of 10 per s, and the potentials recorded from a vertex electrode referenced to either earbar or to the neck. The potentials were filtered so that fast and slow components could be examined separately and special efforts were exerted to obtain stable conditions so that small changes in waveforms could be significant. Lesions of the DAS/IAS produced negligible changes in either the fast or slow waves. Lesions of the midline TB reduced the amplitudes of peaks P3 through P5, while greatly reducing the amplitude of the slow wave. Complete lesions of the LL always reduced the amplitude of the slow wave. Lesions of the ventral part of the LL were more likely to reduce the amplitude of P4-P5. Our interpretations of these lesion experiments are based on the idea that individual fast peaks of the BAEP represent compound action potentials of fiber pathways. According to this view, only synchronized activity generated in populations of neurons that are both favorably oriented in space and significant in number, will contribute to the fast peak.

Auditory brain-stem responses in the rat brain isolated from the body trunk and maintained via cross-circulation

The auditory brain-stem response (ABR) recorded from the vertex of the rat was analyzed before and after the brain was functionally isolated from the body trunk and maintained via cross-circulation. The ABR profile did not change for as long as 30 min after isolating the brain. The finding indicates that this experimental procedure is useful for studying the brain devoid of input from the trunk.

Brain stem auditory-evoked potentials in different strains of rodents

This study was conducted to evaluate variations in brain stem auditory-evoked potentials (BAEPs) among different strains of rodents. BAEPs were recorded by routine procedures from rodents of different strains or species. These included 22 Long-Evans, 28 Wistar and 28 Sprague-Dawley rats, and six hamsters. Within the first 10 ms, there were five consistent and reproducible positive waves of BAEPs in each rodent, named I, II, III, IV and V in correspondence with the nomenclature of waves I-VII in human BAEPs. These BAEPs were also similar to those observed in other vertebrates and in human controls. However, there were variations in waveforms and peak latencies among rodents, even in the rats of the same strain that came from different laboratory centres. At optimal stimulation intensity, usually around 90 dB, the mean latencies of the waves varied as follows: I, 1.23-1.53 ms; II, 1.88-2.28 ms; III, 2.62-2.94 ms; IV, 3.49-3.97 ms; and V, 4.47-5.14 ms. They were significantly different between species, but not in different strains of rats if they came from the same animal centre. The conduction time in the central portion illustrated by interpeak latencies between I and III, III and V, and I and V was dependent on the species (P less than 0.05). When recorded in a soundproof incubator, the minimal hearing threshold showed a significant species difference. The animal BAEP model can be employed for evaluating the physiological function or the pathological conditions of the brain stem. The confirmation of BAEP variations among different species or strains will be helpful in deciding which kind of rodents will be appropriate to serve as animal models for the various purposes of BAEP studies.

Audiogenic seizure severity and hearing deficits in the genetically epilepsy-prone rat

Hearing deficits have been observed in rodents that are susceptible to audiogenic seizures (AGS), including the genetically epilepsy-prone rat (GEPR). AGS susceptibility can be induced in normal animals by treatments that damage the cochlea. In this study, we measured the relative degree of hearing loss in animals from the GEPR substrains that exhibit different degrees of AGS severity and examined the relationship between the deficit and the AGS severity. Auditory brain stem response (ABR) thresholds to clicks in the GEPR substrain that exhibits exclusively maximal AGS severity (GEPR-9) were significantly elevated, and latencies for ABR peaks I, III, and IV were significantly increased as compared to normal Sprague-Dawley rats. ABR thresholds for the substrain of GEPRs were even higher than those in the GEPR-9, and ABR waveforms were distorted. ABR peak IV was significantly longer than normal in the GEPR-3 substrain, as were mean interpeak intervals and central conduction times. These data indicate that significant hearing deficits occur in the GEPR-3 substrain. In non-AGS-susceptible progeny of the GEPR-9 [GEPR-0(9)], ABR thresholds were not significantly different from normal. These data along with studies of ABR thresholds in thyroid-deficient rats suggest that an inverted U-shaped relationship exists between hearing deficit and AGS severity. That is, moderate threshold elevations are associated with increasing AGS severity, but when the hearing deficit exceeds a certain level, a decrement in AGS severity occurs.

Intra-ponto-mesencephalic recording of binaural interaction in human brain-stem auditory evoked potentials

Brain-stem auditory evoked potentials (BAEPs) were recorded at intraparenchymatous sites along a ponto-mesencephalic stereotactic penetration path in a patient with the rare condition of a ponto-medullary lesion which required biopsy but did not grossly alter scalp BAEPs. Click stimuli were applied either monaurally (with contralateral masking noise against acoustic cross-talk; conditions 'R,' 'L') or binaurally (condition 'RL'). A binaural interaction trace ('BI') was derived by subtracting the sum of the monaural from the binaural responses: BI = (RL)-(R + L). Despite failure to obtain significant BI components above noise level for scalp BAEPs, at the lower pons clearly discernible, multiphasic BI activity could be recorded beginning at the peak latency of scalp wave III and extending over approximately the next 4 msec. Its amplitude rapidly fell off with distance toward more rostral, mesencephalic recording sites. In relation to this positive finding, the equivocality among some of the previous studies on the detection of BI components in human scalp BAEPs is tentatively rephrased in terms mainly of a low signal-to-noise ratio and of functional peculiarities introduced by the respective stimulation protocols.

Brain-stem auditory evoked potentials in squirrel monkey (Saimiri sciureus)

To more fully characterize brain-stem auditory evoked potentials (BAEPs) in non-human primates, BAEPs were recorded from chronically implanted epidural electrodes in 10 squirrel monkeys (Saimiri sciureus). The effects of stimulus intensity, repetition rate, and anesthesia (ketamine 20 mg/kg i.m.) on peak latencies and inter-peak intervals were evaluated. Monkey wave forms consisted of approximately 7 peaks (I-VII), each exhibiting similar latencies across sessions, with later peaks exhibiting greater variability. In some subjects, additional peaks (IIa, IIIa) and slow potentials were recorded. The slow potentials provided a substratum for peaks IV through VII. As with human, monkey peaks exhibited systematic changes in latency with changes in stimulus intensity or repetition rate. These shifts included significant decreases in latency with increasing intensity for peaks I-IV and increases in latency with increases in repetition rate for peaks III, V, and VI. Inter-peak intervals were similar to those observed in human. Furthermore, ketamine anesthesia significantly delayed the latencies of most peaks (except I, V, and VII). Some differences between monkey and human BAEPs were evident in the relative amplitude of specific peaks. For example, peak V is typically most prominent in human, while this was true for peak III in monkey. The similarities between unanesthetized monkey and human inter-peak intervals suggest that the times required for impulses to reach particular brain-stem areas are conserved across primate species that vary in brain size. This supports the hypothesis that comparably numbered BAEP peaks in monkey and human index homologous processes. The data also suggest that the differences between animal and human BAEPs commonly reported may result from the use of anesthetics. In summary, unanesthetized monkey BAEPs resemble human BAEPs in morphology, number of peaks, polarity, latency variability, inter-peak intervals, slow potentials superimposed on the high-frequency peaks, and variations in morphology, amplitude, and resolution of peaks as a function of recording site. Thus, unanesthetized monkey BAEPs may be an excellent model for investigating the neural substrates of human BAEP or for determining species differences in acoustic processing among primates.

Brainstem auditory evoked response in the ferret (Mustela putorius)

The effect of click intensity, repetition rate and binaural interaction on the brainstem auditory evoked response (BAER) was examined in sixteen pigmented adult male ferrets. Potentials were recorded from platinum needle electrodes inserted over the vertex and left and right mastoids. Square waves, 100 microseconds in duration, were transduced by earphones enclosed in an assembly designed to fit securely over the ferret's external ear. The BAER in the ferret consists of four prominent vertex-positive peaks (P1-P4) and a fifth peak of smaller amplitude and more variable latency. The mean latencies of P1-P4 at 104 dB peak SPL were 0.96, 1.83, 2.75 and 3.62 ms. Reducing intensity over a 70 dB range resulted in a reduction in amplitude and a corresponding increase in latency ranging between 0.57 and 0.67 ms. Also, increasing click repetition rate resulted in a reduction in amplitude and an increase in latency. With intensity fixed at 104 dB peak SPL comparison of latencies at 10 and 50/s showed a mean increase of 20, 50, 60 and 80 microseconds for P1-P4, respectively. The effect of binaural interaction on the BAER was examined using the procedure of Dobie and Berlin (1979); the response evoked by binaural stimulation was subtracted from the summed left and right monaural responses to obtain a binaural interaction component. Binaural interaction in the ferret gave rise to a distinct vertex-negative wave with a latency similar to P4. An increase in click intensity over a 70 dB range resulted in a monotonic increase in amplitude and a decrease in latency of the binaural interaction component.

Anatomical bases of binaural interaction in auditory brain-stem responses from guinea pig

There is a non-linear interaction of binaural stimulation on auditory brain-stem potentials in both human and animals. The interaction takes the form of the binaurally evoked ABR being of smaller amplitude than the sum of the monaurally evoked ABRs. In the guinea pig this interaction occurs at the time of components P4, N4 and P5. In order to investigate the generator sites of binaural interaction in the ABR, various lesions were made in the brain-stem auditory system in 29 guinea pigs. The effects of those lesions on binaural interaction were as follows: (1) unilateral lesion of lateral lemniscus or bilateral lesions of the inferior colliculi had no significant effect on binaural interaction; (2) transection of the lateral lemnisci bilaterally was associated with a loss of the component of binaural interaction associated in time with N4; (3) a lesion just lateral to the lateral superior olivary complex resulted in an attenuation of the component of binaural interaction associated in time with P4; (4) complete section of the decussating fibers of the trapezoid body or a complete unilateral lesion of the superior olivary complex led to a loss of all components of binaural interaction. These results suggest that binaural interaction in the guinea pig ABR requires the integrity of several distinct portions of the brain-stem auditory pathway, i.e., both lateral lemnisci are required for the interaction occurring at the time of N4; the brain-stem just lateral to the lateral superior olive participates in the interaction at the time of P4. The trapezoid body and superior olivary nucleus are required for binaural interaction at P4, N4 and P5.

Absence of rate-dependent BAEP P5 latency changes in patients with definite multiple sclerosis: possible physiological mechanisms

Brain-stem auditory evoked potential (BAEP) rate studies have been incorporated into evoked potential protocols in an attempt to identify demyelinating lesions. A group of 9 patients with clinically definite MS are described who showed abnormal BAEP P1-P5 interwave latencies at slow repetition rates and failed to demonstrate a significant enhancement of this abnormality following rapid click presentation rates. The lack of rate-dependent P5 latency changes has been hypothesized to represent a less severe form of axonal demyelination. Thus, it may be possible to subclassify or subgroup patients with evidence of brain-stem demyelination based on the presence or absence of BAEP rate-dependent abnormalities.

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

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

Effect of hyperthermia on brain auditory evoked potentials in the conscious sheep

To assess the effect of hyperthermia on brain function of conscious sheep, auditory evoked potentials (AEPs) were studied. Auditory brain-stem potentials (BAEPs) and mid-latency potentials (MLPs) to monaural rarefaction click stimuli were recorded as the potential difference between midline skull screws and mastoid electrodes. Hyperthermia was induced by a combined passive heat and work stress in a climatic chamber. Brain temperature was monitored with a thermistor in the parietal lobe. Hyperthermia resulted in a progressive decrease in the absolute latencies of the BAEPs up to the time of heat-induced collapse. There was a similar decrease in the latencies of MLPs up to a brain temperature of 42.0 degrees C +/- 0.3 degrees C (mean +/- S.D.), while there was prolongation of latencies at higher brain temperatures. The wave form of the BAEPs (I-V) persisted up to the time of heat-induced collapse. In contrast those of MLPs showed reproducible changes in the form of flattening or splitting of the Pa wave at a brain temperature of 41.8 degrees C +/- 0.7 degrees C, reversible with cooling. That temperature was also associated with behavioural changes reversible with cooling. A complete loss of the MLP waves occurred at a brain temperature of 42.9 degrees C +/- 0.6 degrees C which was not reversed with whole body cooling and immediately preceded the heat-induced collapse. This study demonstrates that hyperthermia in the conscious sheep produces potentially damaging effects on the central nervous system once a critical brain temperature (41.8 degrees C +/- 7.0 degrees C) is exceeded and that the MLPs are more sensitive indicators of this damage than BAEPs.

Brain-stem auditory evoked potentials in the alligator. Effects of temperature and hypoxia

Brain-stem auditory evoked potentials (BAEPs) were recorded from young alligators (Alligator mississippiensis), and the effects of hypothermia, hyperthermia and hypoxia on the wave forms were determined. The wave form shape was similar to the human BAEP, although extra waves were routinely seen. The responses were highly repeatable and varied in a predictable manner as a function of stimulus frequency, polarity, intensity, and body temperature. Rarefaction clicks produced longer wave form latencies than condensation clicks. BAEPs were present over the entire temperature range studied (0-36 degrees C). In contrast, mammalian BAEPs disappear over the temperature range of 20-27 degrees C, and seizures occur at 20-21 degrees C. At temperatures below 20 degrees C, the alligator BAEP peak amplitudes decreased with decreased temperature, but latencies only decreased slightly. At temperatures above 20 degrees C the peak amplitudes increased, and the latencies decreased with temperature. Peak I was largely unaffected by temperature change, while peaks IIIa and V increased 0.015 and 0.018 msec/degree C, respectively, at temperatures above 24 degrees C. Transient brain hypoxia, achieved by inverting the alligator, produced a progressive decrease in BAEP waves to an isoelectric amplitude without greatly altered latencies. The reverse sequence of changes was seen during recovery. Postural effects on blood flow were documented in two alligators with implanted flow probes. Carotid artery blood flow decreased 43% with body inversion, in both anesthetized and unanesthetized alligators, but no sequelae from the hypoxia could be detected. Metabolic differences between mammals and the alligator may account for the alligator's resistance to hypothermia, hyperthermia and hypoxia.

Changes in the brainstem auditory evoked response of the rabbit during the first postnatal month

Brainstem auditory evoked responses (BAERs) were recorded in 73 albino rabbits during the first postnatal month. Responses could not be evoked before the ninth day post-term using free field click stimulation at 60 dBHL. The onset of BAERs to these stimuli on or after day 9 was coincident with the onset of behavioural responses to sounds and, in the majority of animals, with eye opening. The onset of BAERs was delayed in animals with low body weight. The intensity required to evoke detectable BAERs in normally grown animals decreased rapidly after day 9 post-term. The most significant changes in the form of the BAER in the first postnatal month were an increase in the amplitude of peak III and the separation of peaks IV and V. Peak I and the negative dip after peak V (Vn) were consistent features of the BAER during development. The latencies of these deflections and the interval between them decreased by approximately 1.5 and 4 ms respectively up to the end of the first month post-term. On days 9 and 10 post-term, stimulation at a higher rate (40 Hz) failed to evoke a BAER in some animals. In other animals the change in stimulation rate from 10 to 40 Hz produced a large increase in the latency of peak V. The unusually large changes in the latencies of peaks and the interpeak intervals during the development of the rabbit indicate that this animal may be particularly suitable for studies of perinatal complications on development of the brainstem when the BAER is to be used as non-invasive measure of neural function.