George M. Gerken

Auditory temporal integration and the power function model

The auditory temporal integration function was studied with the objective of improving both its quantitative description and the specification of its principle independent variable, stimulus duration. In Sec. I, temporal integration data from 20 studies were subjected to uniform analyses using standardized definitions of duration and two models of temporal integration. Analyses revealed that these data were best described by a power function model used in conjunction with a definition of duration, termed assigned duration, that de-emphasized the rise/fall portions of the stimuli. There was a strong effect of stimulus frequency and, in general, the slope of the temporal integration function was less than 10 dB per decade of duration; i.e., a power function exponent less than 1.0. In Sec. II, an experimental study was performed to further evaluate the models and definitions. Detection thresholds were measured in 11 normal-hearing human subjects using a total of 24 single-burst and multiple-burst acoustic stimuli of 3.125 kHz. The issues addressed are: the quantitative description of the temporal integration function; the definition of stimulus duration; the similarity of the integration processes for single-burst and multiple-burst stimuli; and the contribution of rise/fall time to the integration process. A power function in conjunction with the assigned duration definition was again most effective in describing the data. Single- and multiple-burst stimuli both seemed to be integrated by the same central mechanism, with data for each type of stimulus being described by a power function exponent of approximately 0.6 at 3.125 kHz. It was concluded that the contribution of the rise/fall portions of the stimuli can be factored out from the rest of the temporal integration process. In Sec. III, the conclusions that emerged from the review of published work and the present experimental work suggested that auditory temporal integration is best described by a power function in conjunction with the assigned duration definition. The exponent for the power function is typically less than 1.0, and varies with frequency and hearing level. Second, a means of empirically assaying the contribution of the rise-fall portions of the stimuli is presented and evaluated. Finally, properties of a central auditory integrator are hypothesized.(ABSTRACT TRUNCATED AT 250 WORDS)

Hypersensitivity to electrical stimulation of auditory nuclei follows hearing loss in cats.

The purpose of the study was to determine if permanent, sound-induced hearing loss altered behaviorally measured thresholds for the detection of electrical stimulation applied to auditory nuclei. Electrodes were placed in cochlear nucleus and inferior colliculus in four cats. Behaviorally measured thresholds for the detection of brief trains of electrical pulses were determined before and after a 48 h exposure to a 1 kHz tone of approximately 110 dB SPL. The mean decrease in electrical stimulation threshold as a result of the sound exposure was 10.4 dB. The ongoing electrical activity (in microV, rms) recorded from the electrodes showed a mean 2.2 dB decrease after the sound exposure. In some electrodes, there was partial recovery towards pre-exposure levels for stimulation threshold and for ongoing activity, but typically, the changes persisted until the animals were terminated 30 days later. The magnitudes of the decreases in stimulation threshold and background activity proved not to be highly correlated. The permanent auditory threshold shift across all cats and all frequencies was 19 dB. This mild hearing loss produced a marked alteration in certain characteristics of the central auditory mechanisms.

Human frequency-following responses to monaural and binaural stimuli

Frequency-following responses, with latencies circa 6 msec, were recorded from five normal-hearing human subjects to brief 500 c/sec tone bursts presented monaurally. The frequency-following responses appear as peaks occurring at 2 msec intervals superimposed on a slow wave (pedestal-like) component. Comparisons were made between the frequency-following responses evoked by binaural and monaural stimuli. The results show that the binaural responses may be interpreted as the sum of two monaural responses. It is concluded, therefore, that there are two independent populations of neurons, each capable of generating a frequency-following response is not a microphonic-like response but rather that the individual waves in the frequency-following response are evoked by the collective activity of phase-locked single units. Finally, on the basis of the distinctness of the individual waves in the frequency-following response, it is concluded that the neural generators of the response must be spatially compact.

Auditory evoked responses in control subjects and in patients with problem-tinnitus.

Auditory brainstem responses (ABRs) and middle latency responses (MLRs) recorded from problem-tinnitus patients were compared with responses from normal hearing, hearing loss, and elderly subjects. Ten stimulus frequencies were presented in counterbalanced sequence and all frequencies were presented before any given frequency was presented again. The variables of importance were problem-tinnitus, hearing loss, subject age and stimulus frequency. Repeated measures analysis of variance showed a significant difference only in the latency of ABR wave 7. The intrinsically high variability in the problem-tinnitus and elderly groups rendered standard statistical analyses ineffective with the sample sizes used. Alternative analyses were employed in which the MLR waves of the normal hearing subjects were taken as the standard against which the other groups were compared. Very large MLR waves occurred in some, but not all, of the subjects in the problem-tinnitus and elderly groups. Different MLR waves were large in different subjects without correspondingly large ABR potentials. These results suggest: (1) selective alteration of MLR generators in different forms of tinnitus; and (2) differing effects of age on auditory physiology. Stimulus frequency and hearing loss contributed to this multivariate picture. Another variable, the average sound pressure level of the long-term acoustic environment, may also be important.

Central denervation hypersensitivity in the auditory system of the cat

Data are reported for seven cats with a total of 29 electrodes permanently placed in or near the cochlear nucleus, the superior olivary complex, the nucleus of the inferior colliculus, and the medial geniculate body. Detection thresholds for pulsate electrical stimuli were measured using an operant behavioral procedure. Electrical stimulation thresholds were measured prior to and following bilateral destruction of the cochleas in all animals. In addition, four of the animals were tested using a site-of-stimulation discrimination prior to and following the cochlear lesion. Finally, hearing loss was evaluated in all cats after the completion of the experiments. Electrical stimulation thresholds showed a mean reduction of 7.9 dB throughout the brain stem auditory system fater cochlear destruction. The ability of the animals to perform the site-of-stimulation discrimination was not permanently impaired by the cochlear lesion. The data indicated the presence of increased sensitivity to electrical stimulation in most regions of the subcortical auditory system, although a lesser effect was found at the thalamic level. It was concluded that stimulation threshold provides an index relevant to the state of auditory neurons proximal to the electrode tip.

Temporal integration of electrical stimulation of auditory nuclei in normal-hearing and hearing-impaired cat.

Temporal integration functions were measured, before and after a sound-induced hearing loss, in 5 cats using trains of electrical pulses applied to auditory nuclei in the brainstem. The 8 stimuli ranged from 1 pulse (0.25 ms duration) to 16 pulses (0.25 ms pulses spaced over 240 ms). The stimuli were applied to inferior colliculus or cochlear nucleus via permanently implanted electrodes. One electrode was tested extensively in each animal to obtain 10 sets of behaviorally-measured electrical detection thresholds counterbalanced across stimuli. The animal was then exposed to a 110 dB SPL, 2 kHz tone for 48 h and pre- and post-exposure audiograms were measured. The mean permanent threshold shift for acoustic stimuli was 48.5 dB. Another 10 thresholds for each of the 8 electrical stimuli were then measured. In the normal hearing animals, the mean slope of the temporal integration function for electrical stimulation was -7.6 dB per factor of 10 pulses. Alternatively, the mean time constant was 139 ms. In the hearing impaired animals, the slope was reduced to -1.5 dB per factor of 10 pulses, which corresponded to a mean time constant of 17 ms. In addition, the hearing impaired animals showed a decreased threshold for the electrical stimuli (stimulation hypersensitivity) as well as reduced variability across electrical stimulation thresholds. The results suggest that a major contribution to temporal integration occurs in inferior colliculus or higher. In addition, the results suggest that the reduction in temporal integration that follows hearing impairment is a peripherally-induced, central effect.

Post-stimulatory effects on the auditory brain stem response: partial-masking and enhancement ☆

The auditory brain stem response (ABR) was recorded in human subjects using the stimulus configuration of a tone-on-tone forward-masking paradigm, but with all stimuli at suprathreshold levels. A masking stimulus preceded, by delta t msec, the probe stimulus which elicited the ABR. The latency vs. delta t functions for waves III and V were essentially parallel to each other and were interpreted in terms of a partial forward-masking effect, possibly originating in the cochlea. Likewise, the amplitude function for wave III showed varying degrees of decrement as a function of delta t that was also compatible with a partial forward-masking interpretation. In contrast, wave V showed an amplitude increment relative to the unmasked wave that was maximal for delta t values of 15 and 45 msec. The amplitude increment in wave V was termed enhancement and was interpreted as a central process governed by the timing of sound sequences.

Summating-Potential/Action-Potential Ratio in Normal Ears: Effects of Dehydration

INTRODUCTION Studies using electrocochleography (ECoG) in patients with Menieres disease and in patients with perilymph fistula have demonstrated abnormally large summating-potential (SP) components compared with the amplitude of the action-potential (AP). The possible influence of normal physiological variation, methodological differences, and test-retest variations are often difficult to interpret. The purpose of this study is to specify what constitutes a clinically important change in the SP/AP ratio in normal hearing subjects under the condition of dehydration induced by urea. MATERIALS AND METHODS Baseline ECoG was performed on 20 normal hearing volunteer subjects after an 8 hour fast. Urea, 20 g, was administered orally, following which recordings were made at 60 and 90 minutes. The majority of subjects were found to have maximal SP/AP changes at 60 minutes post dehydration; therefore, only these results were included in the final analysis. Subsets of data were statistically evaluated. RESULTS In all ears but one, the preingestion SP/AP ratio was less than or equal to 0.37. Male and female data were shown to comprise separate populations both before and following dehydration. The 95% upper limit for baseline SP/AP ratio was 0.39 for males and was 0.25 for females. The gender-related difference remained after dehydration. The mean male SP/AP ratio showed a statistically significant but slight decrease whereas the female SP/AP ratio was unaltered. CONCLUSION These data demonstrate a statistically significant gender-related difference in SP/AP ratio in normals. The upper limit of normal is 0.39 for males and 0.25 for females. A basis for the gender difference is puzzling and requires further investigation. Dehydration produces a statistically significant reduction in SP/AP ratio in men. This change was not observed in women.

Behavioral thresholds for electrical stimulation applied to auditory brainstem nuclei in cat are altered by injurious and noninjurious sound

Each of three young-adult female cats with normal hearing received a total of eight permanent electrodes which were implanted bilaterally in cochlear nucleus (CN) and inferior colliculus (IC). Three experiments were performed using behaviorally measured thresholds for electrical stimulation of CN and IC. In Expt. 1, electrical stimulation thresholds (in dB re 1.0 microA) were obtained in the presence of a continuous tone of moderate intensity and in quiet. In comparison with quiet, electrical stimulation thresholds measured during tone were lower by as much as 15 dB (stimulation hypersensitivity). In Expt. 2, a brief exposure to an intense sound produced a temporary threshold shift (TTS) for acoustic stimuli but only produced small changes in electrical stimulation threshold. The acoustic stimuli used in Expts. 1 and 2 were termed noninjurious since no permanent hearing loss was produced. Expt. 3 employed an exposure to a white noise that resulted in a mean permanent threshold shift (PTS) of 34.1 dB for acoustic stimulation. The PTS was accompanied by a mean stimulation hypersensitivity of 9.6 dB. Comparing Expts. 1 and 3, it was shown that the transient hypersensitivity produced by the noninjurious continuous tone correlated strongly with the permanent hypersensitivity that was produced by the PTS. In regard to the origin of stimulation hypersensitivity, the suggestion is made that it is an indication of a physiological change localizable perhaps in the auditory nuclei of the upper brainstem.

Magnetically Coupled Stimulation of the Ossicular Chain: Measures in Kangaroo Rat and Man

A ferrite core coil is used to drive a small magnet fastened to the ossicular chain in kangaroo rat and man. The cochlear potentials recorded in kangaroo rat to such a drive are the same as when acoustic stimuli are used. When magnetic coupling is used to drive the human ossicular chain, the subject hears sounds that are indistinguishable from sounds generated through earphones.