A comparative study of the brain stem auditory response in mammals

Comparison of cat and human brain-stem auditory evoked potentials

Brain-stem auditory evoked potentials (BAEPs) elicited by clicks were recorded from both humans and cats. The responses of the two species were compared as functions of click level, click rate, ear stimulated, and electrode position. Since the BAEPs appear to have both high- and low-frequency components, the responses were filtered to analyze these components separately. The similarities and differences in the behavior of the peaks of the two species support the view that the first three (positive and negative) high-frequency peaks which are comparably numbered have similar generators, but the later comparably numbered peaks do not. The presence of binaural interaction beginning with P4 and PV suggests a correspondence between peaks P4 through P5 in cat with PV through PVI, respectively, in human. The similarity in behavior of these peaks also support this correspondence. Furthermore, when conduction times are estimated from interpeak latencies, this correspondence of peaks agrees more closely with the relative pathway lengths in the two species, than does the correspondence based on comparable numbering.

Development of auditory brainstem response to tone pip stimuli in the rat

The course of development of the auditory brainstem response (ABR) in rat was examined. Pure tone pip stimuli of 3, 8 and 40 kHz at intensities up to 106 dB sound pressure level peak equivalent were presented to anesthetized rats on postnatal days (PND) 9, 10, 12, 14, 16, 20, 24, 36, 70 and 96. The peak-to-peak amplitude and the order of appearance of each wave was examined at each stimulus parameter. The first response to presentation of an auditory stimulus was observed on PND 12. This response was demonstrated to be of non-neural origin and presumed to be a summating potential representing hair cell function in the cochlea. The first neural responses, observed on PND 14, to 3- and 8-kHz tones only, appeared as two or more waves. High-frequency (40 kHz) tones did not elicit any substantive neural response on PND 14, but neural activity to this stimulus was observed consistently by PND 16. By PND 20, 4 waves were present consistently at all test frequencies, and by PND 36 the full adult complement of 5 vertex positive waves was recorded. The frequency-dependent sequential appearance of waves during development lends support to data charting the development of frequency mapping in the cochlea. The amplitude of waves I and II at 8 kHz decreased as the animal matured, while waves III-V generally showed an initial increase in amplitude at all test frequencies through PND 24 followed by a gradual amplitude decrease to adult levels. Possible sources for developmental changes in amplitude are discussed.

Short-latency auditory evoked potentials in the monkey. I. Wave shape and surface topography

The components and topography of the short-latency auditory evoked potentials (SLAEPs) in the monkey were analyzed using a combination of surface and epidural electrodes. Nine positive peaks, and a slow negativity following wave 7, are identified in the SLAEP recorded at the vertex referred to the mastoid ipsilateral to a monaural stimulus. Subcomponents of waves 1 and 3 are consistently observed, and other waves are probably compound as well. Barbiturate anesthesia results in latency prolongations which are cumulative across components but does not alter the component sequence. With binaural stimulation there is simple summation of the monaural responses for the components preceding wave 7, but a binaural interaction for wave 7. The polarity and clarity of the various components depend on the recording configuration selected. No single configuration is optimal for the demonstration of all components, and the use of montages with multiple electrodes may increase the information gained from clinical SLAEP recordings in man. The monkey SLAEP components can be grouped based on similarities of their surface topographies, which suggest similar anatomic orientations of their underlying generators. Probable homologues between the components of the simian and human SLAEP, based on temporal sequence, topography, and the effects of variation of the stimulus parameters, are: Jewett's waves I through VII correspond to monkey waves 1, 3, and 5 through 9. Due to differences in the size and shape of the heads, the monkey waves 2 and 4 correspond to negativities in the human SLAEP, labeled IN and IIN after the components which they follow. Another peak, with latency between waves 7 and 8 in the monkey and V and VI in man, is only clearly seen in the mastoid-to-mastoid linkage, contralateral negative.

Binaural interaction in the auditory middle latency response of the guinea pig

Binaural interaction in the guinea pig middle latency response (MLR) was studied using two stimulus paradigms: binaural click stimuli and monaural click stimuli with contralateral white noise. During monaural click stimulation, the MLR is largest in amplitude over the contralateral temporal lobe. Binaural click stimulation reduces the amplitude of this response. Monaurally evoked click responses are also altered by the presentation of continuous white noise in the contralateral ear. The addition of white noise results in an increase in amplitude particularly of components A (12 msec) and B (17 msec). These alterations in response amplitude are specific to the MLR with no change occurring in the auditory brain-stem response (ABR). These findings appear to reflect interaction between the generators of middle latency response.

Effects of temperature and elevated intracranial pressure on peripheral and brain stem auditory responses in dogs

Far-field recordings of central (P2 through P4) and peripheral (cochlear microphonic; and compound action potential of the eighth nerve) auditory responses were used to assess changes in auditory function resulting from elevated intracranial pressure. Normative data for eight dogs were obtained. The relationship between response latency and core temperature was examined. A mean slope of -0.17 ms/degrees C resulted for the temperature range of 35.0 to 40.0 degrees C. Systemic arterial pressure was measured in order to identify the cerebral ischemic response. Responses were not altered significantly unless the intracranial pressure approached within 15 to 30 mm Hg of mean systemic arterial pressure. Changes in the response consisted of both enhancement and deterioration during intracranial pressure elevation and were accompanied by increases in systemic arterial pressure during that elevation. Supernormal amplitudes of the action potential also occurred during recovery periods. Results suggest that: (i) during elevated intracranial pressure, changes in both central and peripheral auditory function result from ischemia rather than pressure-induced distortion of the cochlea or central neural assemblies. (ii) Far-field auditory responses may include an O2-dependent cochlear microphonic. (iii) An unknown process causing enhancement of central and peripheral neural responses exists and operates in connection with intracranial hypertension. Possible mechanisms underlying enhancement of response components are discussed.

Deterioration of auditory evoked potentials during cerebellopontine angle manipulations. An interpretation based on an experimental model in dogs

Evoked action potentials from the internal auditory meatus portion of the cochlear nerve (IAM-EAP's) and brain-stem auditory evoked potentials (BAEP's) from the vertex were simultaneously recorded during cerebellopontine angle (CPA) manipulations (retractions of the cerebellar hemisphere and the cochlear nerve) in dogs. The BAEP changes noted in these dogs were the same as those seen in patients. The IAM-EAP's showed graded deterioration related to BAEP changes. The results are as follows: Prolongation of the I-V interpeak latency of BAEP's, the most common finding during CPA manipulations, is the reflection of prolongation of the I-II interpeak latency, which is caused by conduction impairment or block of the nerve impulses between the extracranial portion of the cochlear nerve and the brain stem. The operative manipulations representing stretch or compression injury to the cochlear nerve in the CPA leads to an acute traumatic cranial nerve root lesion--a retrocochlear lesion. The obliteration of all BAEP components including wave I cannot be caused by conduction block. This is caused by occlusion of the internal auditory artery. Wave I of the BAEP's and the P1-N1 complex of the IAM-EAP's are important indicators of cochlear blood flow during surgical intervention. As possible causes of internal auditory artery obstruction, mechanical distortion of the relationship between the anterior inferior cerebellar artery (AICA) and the internal auditory artery at the junctional portion, mechanical vasospasm of the AICA-internal auditory artery complex, and ensuing no-reflow phenomena are discussed. Evoked action potentials are expected to be a useful intraoperative real-time monitor during CPA surgery that can detect rapid changes derived from cochlear artery insufficency. The real-time aspects can overcome some of the disadvantages of BAEP monitoring.

Intra- and extracranially recorded auditory evoked potentials in the cat. I. Source location and binaural interaction

Auditory field potentials (AFPs) were recorded stereotactically from the superior olivary complex and from the exposed inferior colliculus in the Nembutal anaesthetized cat. The brain-stem evoked response (BSER) was recorded simultaneously between an electrode on the dura mater at the vertex and an electrode on one bulla or in the neck musculature. A closed condenser microphone sound system was used to deliver monaural and binaural clicks. The binaural difference potential (BDP) was calculated by subtracting the sum of both monaurally evoked potentials from the binaurally evoked potential. The first binaural interaction was a reduction of BSER wave P4, arising in the superior olivary complex. The large extracellular AFPs generated in both inferior colliculus and superior olivary complex do not correspond to BSER waves, whereas the small volleys preceding the main waves do. These small waves show a much smaller change with recording distance than do the main AFP waves, i.e., the sources and sinks generating these waves appear to be more widely separated. We suggest that, in contrast to extracellular field potentials, the BSER is generated by action potentials in fibre tracts rather than postsynaptic potentials in nuclei. The implications for stimulation and recording laterality of late BSER waves are discussed.

Changes in contralateral brain-stem auditory evoked responses to cerebellopontine angle compression in rats

Eleven rats were subjected to graded compression of the brain stem at the cerebellopontine angle (CPA) following craniectomy and cerebellectomy. Clicks were delivered to the ear contralateral to the compression site, and brain-stem auditory evoked responses (BAER's) were collected before and during compression. With increasing compression, there was an increase in the latency and a decrease in the amplitude of the peaks of the BAER. The later peaks of the BAER (V and VI) demonstrated changes with minimal compression that progressed as compression increased, while changes in the amplitude and latency of the earlier peaks (II, III, and IV) occurred at the higher grades of compression. Following decompression of the brain stem, there was a decrease in the latencies of most of the BAER peaks. Peak amplitude remained depressed following brain-stem decompression. These experimental findings corroborate clinical reports of abnormal BAER's elicited from stimulation of the ear contralateral to a CPA tumor. The relative sensitivity of the later peaks of the BAER to even minimal brain-stem compression explains the prolonged III to V interpeak latency reported in the literature in patients with brain-stem compression from a CPA lesion. The possible mechanisms for the BAER changes that were observed contralateral to compression are discussed.

Auditory brainstem and middle latency responses in non-human primates

Auditory brainstem (ABR) and middle latency responses (MLR) were obtained from each ear in 8 crab-eating macaques, 4 white-handed gibbons, 4 siamangs and 2 orangutans. Macaques ranged in age from 5 days to 15 years with the 6 older animals in age-matched, male-female pairs. From each animal, latency-intensity functions were obtained and multiple MLR recordings were measured at 60 and 70 dB. Latency-intensity functions, interwave intervals, thresholds and percent detectability were calculated for ABR waveforms. Waves II and IV were largest in amplitude and were most consistently detected at low stimulus intensities in all species tested. Waves I and II had adult latencies in the youngest animal tested (5-day-old macaque), while waves III and IV were prolonged in comparison to the 15-month-old macaque, in whom latencies had reached adult values. There were no apparent sex differences in evoked potentials in the age-matched, male-female pairs. A broad, negative MLR at 7-13 ms was observed in all animals. Longer latency MLRs varied among animals of the same species, but were replicable in some individuals, including the youngest macaque (5 days) and orangutan (7 months). These data were compared to responses obtained in humans, other primates and other vertebrates.

Binaural interaction effects on the auditory brainstem response of the cat and kitten

The auditory brainstem response (ABR) is a series of volume conducted potentials that can be recorded from the scalp within 10 ms following auditory stimulation. Differences between ABRs evoked with binaural stimulation and those constructed by summing equivalent numbers of monaural stimulation to each ear indicate the presence of binaural interaction for some ABR potentials but not for others. In the present study, ABR binaural interactions were studied in both cats and kittens. Binaural interactions were not seen for waves 1-3 but were present at the latencies of waves 4, 5 and, in most cases, 6. The sound intensities used were selected to insure that acoustic cross-over, i.e. sound presented monaurally to one ear stimulating the other, did not influence binaural interaction effects. Experiments with cats that had been monaurally deafened confirmed that the effects observed were not due to acoustic cross-over. Systematic manipulation of stimulus rate and intensity produced marked changes in the level of binaural interaction. Increases in stimulation rate from 10 to 100 clicks/s reduced binaural interaction for wave 4 and reversed the direction of binaural interaction for wave 5. Wave 6 was not generally present at rates above 10 clicks/s. Reduction of stimulus intensity reduced binaural interaction for wave 4. Binaural interaction effects were at adult levels in kittens of 20 days for waves 4 and 5. Wave 6 was not present until 30 days of age. These data suggest a possible model of the physiological processes producing binaural interaction which is based on occlusion as seen in other areas of the nervous system. In such a model, convergent input becomes more important in driving the generators of some ABR potentials when the system is stressed (as, for example, by increased stimulus rate), than it is when the system is not stressed.

Binaural interaction measured behaviorally and electrophysiologically in young and old adults

12 young men (mean age = 39.1 years) and 12 older men (mean age = 69.4 years) presenting some degree of sensorineural hearing loss were chosen so that the two groups were audiometrically matched. Behavioral tests of binaural interaction (BI) included binaural fusion, rapidly alternating speech perception, and masking level difference (at 500 Hz). There was a tendency toward better performance by the younger subjects for each of these tests, but these comparisons fell short of statistical significance. Auditory brainstem responses (ABRs) and middle-latency responses (MLRs) to clicks were recorded in both monaural and binaural modes, with off-line derivation of BI components. Latencies of wave V (in ABR) and waves Pa and Nb (in MLR) were prolonged in the older subjects for all conditions; this effect was independent of degree of hearing loss. BI components did not differ substantially between the two groups, except for a 0.45-ms latency difference for the 'N1' peak, as expected from the raw ABR data. MLRs for both groups showed a pattern of BI which was different from that seen in young subjects with normal hearing, in that latency for wave Pa in MLR was not reduced in the binaural condition. The young subjects showed the expected reduction of binaural wave Pa amplitude compared to monaural sum responses, while the older subjects, as a group, did not. No significant correlations were found between behavioral and electrophysiological tests.

Neurone response latency in the inferior colliculus in relation to the auditory brainstem responses (ABR) in the guinea pig

In order to determine the possible source of the later vertex positive waves of the guinea pig auditory brainstem response (ABR), we have measured field potentials on the surface and at depths through inferior colliculus whilst making concurrent recordings of single unit responses (using identical broadband click stimuli). We find neurones, restricted to the antero-ventral inferior colliculus, with characteristic frequencies in excess of 12 kHz which have well synchronized onset responses short enough (c. 5 ms) to contribute to the P5 wave of the ABR.

Harmaline effects on the sensory-motor reactivity: modifications of the acoustic startle pattern

The effects of harmaline, an indoleamine and a MAOI, were tested on the acoustic startle pattern. EMG measures of the startle reflex, the pinna reflex as well as the characters of the vertex evoked responses to brief intense tone bursts (60 msec, 110 dB, 8000 Hz) were simultaneously studied in 4 alert guinea-pigs. The basic experimental design was a 4 by 4 latin square, with the treatments being given at 2 day intervals. The four harmaline-HCl treatments were isotonic saline, 0.25, 5.0, and 10.0 mg/kg. Compared with saline baselines, all the doses resulted, throughout the 60 min session, in overall high significant depressions of the startle reflex, the pinna reflex and the initial wave of the acoustic evoked potential at the vertex. In contrast, harmaline had little or no influence on amplitude and latency of the late wave of the vertex response. The effects of harmaline on the general behavior of the guinea-pig are also reported. These results may support an involvement of serotonergic systems in the modulation of the sensory-motor reactivity at the brainstem level. Nevertheless, the probably more complex cortical processes involved in startle responsiveness do not appear univocally affected by the indoleamine drugs such as harmaline.

Middle and long latency auditory evoked potentials in cat. I. Component definition and dependence on behavioral factors

Middle (10-50 ms) and long (50-600 ms) latency auditory evoked potentials (AEPs) were investigated in artificially respired, muscle-paralyzed cats. Similarity to human potentials of comparable latencies was examined in two ways: (1) the similarity of waveform features such as peak amplitude, polarity and latency, and (2) the effects of task-related variables on these various waveform features. Four behavioral variations of a classical pupillary conditioning paradigm were used to vary attention and arousal. Twelve peaks and troughs were identified in the AEP: P10, N13, P17, N22, P31, N41, P55, N70, N100, N140, P260 and N520. Principal components analysis (PCA) defined 7 AEP components, some of which spanned several peaks. Analysis both of peak latencies and amplitudes, and of principal component scores, revealed differential effects of the behavioral manipulations on these components: those with latencies longer than 50 ms were strongly influenced by behavioral variations, while earlier components were relatively immune to these effects. On the basis of these findings, several relationships between cat and human AEP components were suggested. Specifically, peaks P10-P41 in the cat were thought related to human middle latency components, cat P55 to human P50, cat N140 to human N300, and cat P260 to human P300. Cat N520 was comparable to several long latency components in humans. No obvious correspondences between cat AEP components and human N90 and P170 were identified.

Evoked potentials in rats with misonidazole neurotoxicity. I. Brain stem auditory evoked potentials

Central neurotoxicity produced in rats by daily administration of 300 mg/kg of misonidazole (MISO) 5 times/week for 4-5 weeks (total dose = 6.0 gm/kg) was evaluated weekly wit brain stem auditory evoked potentials (BAEPs). Compared to untreated control rats, all treated rats had a prolongation of the I-IV interpeak latency (p less than 0.005) at a mean of 13.2 +/- 2.7 days at a cumulative dose of approximately 4.0 gm/kg of MISO per rat. In some rats, the I-III and I-II interwave latencies were prolonged and waves III and IV were lost. Control rats did not show any significant alteration in BAEP latency or amplitude. Histopathologic examination of the brain stems of treated rats showed that necrotic lesions were present primarily in the nuclei of the tegmentum of the fourth ventricle, with scattered nuclear involvement in the cerebellar roof nuclei, inferior olive, and nucleus of the spinal tract of the trigeminal nerve. The cerebral cortex appeared to be normal in all treated rats. Changes in BAEPs caused by central neurotoxicity correlated with the histopathologic findings. We conclude that BAEPs are a sensitive method for evaluating MISO central neurotoxicity in the rat model.

Trigeminal evoked potentials in the cat

Electrical stimulation of the infraorbital nerve in the cat resulted in a series of far-field evoked potentials at the vertex. The wave form of these potentials is similar to the auditory brain stem evoked response and the somatosensory far-field response; it is multicomponent, the amplitudes of individual components are of the order of 1 microV, and the latencies are all less than 4 msec. The anatomical origins of these evoked potentials were investigated. The results indicated that contributions due to the afferent trigeminal nerve, the principal sensory nucleus, and the spinal trigeminal nucleus are involved.

Effect of interaural intensity differences on binaural summation of brainstem auditory-evoked potentials

With binaural stimulation the two specific parameters that can modify the neural responses substantially are the intensity differences and the time of arrival of the stimulus at the two ears; these events on electrophysiological evidence, being mediated at the superior olivary complex and at the inferior colliculus, the origins of Waves III and V respectively. Comparison of the brainstem auditory-evoked potentials to monaural stimulation with that due to binaural stimulation reveals a significant increase only in the amplitude of Wave V. The amount of increase is reduced from 68.7% with equal intensities at the two ears to 44% with a 10-dB interaural intensity difference. With a 20-dB interaural intensity difference the amplitude of Wave V reverts to that due to monaural stimulation. The 68.7% greater amplitude for Wave V was not due to effective increase in loudness sensation on binaural stimulation (which is equivalent to a monaural increase in intensity of 3 dB) but to binaural summation. In contrast to Wave V, Wave III decreased in amplitude below that resulting from monaural stimulation with intensity differences of 10 and 20 dB at the two ears. This supports the suggestion of inhibitory processes at that level of the brainstem. As only Wave V shows binaural summation the region of inferior colliculus is suggested as the site for the occurrence of this phenomenon. Binaural interaction can also be demonstrated within a limited range with intensity differences at the two ears.