Detection of pure-tone amplitude modulation as a function of sensation level from 8 to 14 kHz.

Amplitude modulation detection thresholds were obtained for pure-tone stimuli of 8, 10, 12 and 14 kHz at 5 dB intensity increments from 10 to 65 dB sensation level. Performance at 8 and 10 kHz was a non-monotonic function of sensation level for all four subjects with the largest difference limen measured near 30 dB sensation level and optimal performance at the highest sensation level (60 dB). Weber fractions at 12 and 14 kHz appear dependent on each subject's high frequency hearing profile; i.e., the difference limens remain high and either increase or remain essentially constant at high sensation levels only when the frequencies tested are near a particular subject's upper limit of hearing.

Audibility of short-duration tone-glides as a function of rate of frequency change.

Thresholds for rising and falling tone-glides were determined against a background of 50-6000 Hz noise at a level of 60 dB re 20 microPa. Glides were centered around 2000 Hz and changed frequencies linearly at rates of 24, 48, 96 and 192 Hz/ms; tone-glide durations were 5, 10, 20 and 40 ms. Results demonstrate a rate-dependent asymmetry in the detectability of rising and falling tone-glides, with rising tone-glides detected at lower signal intensities for the higher rates of frequency change (i.e., 96 and 192 Hz/ms).

Histochemical evidence that cochlear efferents are carried with the inferior vestibular nerve in guinea pigs.

Histochemical methods were employed to determine the course of the olivocochlear bundle (OCB) within the vestibular nerve of guinea pigs. Following transection of the inferior vestibular nerve, cholinesterase staining in the cochlea was greatly reduced. Transection of the superior vestibular nerve, however, yielded no detectable change in staining. It is concluded that the cochlear efferent innervation in guinea pigs is carried in the inferior vestibular nerve, at the point of entry into the medial bulla.

Conditioning the auditory system with continuous vs. interrupted noise of equal acoustic energy: is either exposure more protective?

The purpose of this study was to test the hypothesis that differences exist in the amount of protection provided by prior sound conditioning with continuous vs. interrupted, moderate-level noise. Differences were determined by monitoring the changes that occurred in cubic (2f1-f2) distortion product otoacoustic emission (DPOAE) amplitude growth functions subsequent to a traumatizing noise exposure (105 dB SPL, 1.0-2.0 kHz octave band noise presented 24 h per day for 3 days) in guinea pigs which had been conditioned with either continuous (89 dB SPL, 1.0-2.0 kHz octave band noise presented 24 h per day for 11 days) or interrupted noise (95 dB SPL, 1.0-2.0 kHz octave band noise presented on a 6-h 'on'/18-h 'off' schedule for 11 days) of equal acoustic energy. Results suggest that there are significant differences in the degree of protection provided by prior sound conditioning with the continuous and interrupted schedules of moderate-level noise used in this study. Specifically, the interrupted conditioning protocol afforded some degree of protection against the damaging effects of the traumatizing noise exposure, limited to frequencies above the noise exposure band. Conversely, there was a lack of any consistent and sizable protective effect found across the entire test frequency range for the continuous sound conditioning protocol.

Auditory brainstem responses in neonatally sound deprived CBA/J mice.

Auditory brainstem responses (ABR) from ten normal (3 males, 7 females) and ten environmentally sound deprived CBA/J mice (6 males, 4 females) were elicited by 30 dB HL click stimuli delivered via specially constructed, matched insert earphones. The mice were tested at 44-49 days of age under chloral hydrate sedation using an electrode montage of vertex/linked bilateral bullae/presacrum. The deprived mice were killed immediately after testing and serial transverse sections were prepared from their brains. Examination of globular cells of the ventromedial ventral cochlear nuclei revealed significantly smaller cross-sectional areas of these cells than in previously studied normal mice. Although monaural thresholds did not differ between the two groups, significantly shorter mean latencies were observed in the ABRs of the sound deprived group in wave I as well as in the interpeak latencies of the later ABR components (I-IV, I-V, and III-V). These electrophysiologic alterations in brainstem conduction presumably reflect the anatomical changes demonstrated in neonatally sound deprived mice.

Development of the VIIIth nerve compound action potential evoked by low-intensity tone pips in the Mongolian gerbil.

Maturation of the cochlea and afferent auditory units is reflected by changes in VIIIth nerve compound action potential (CAP) parameters. We studied auditory nerve CAPs evoked by low-intensity stimuli in Mongolian gerbils (Meriones unguiculatus) ranging in age from 22 to 92 days after birth. The gerbil CAP development is characterized by marked changes in latency, threshold, and amplitude during the first few weeks of life. CAP latency and CAP threshold reach adult-like values at about 1 month of age. In contrast, the CAP amplitude continues to grow in size even after 2 months. This dichotomy suggests that the development of afferent auditory nerve function in the gerbil is preceded by maturation of the mechanical processes of the middle ear and cochlea.

Development of contralateral suppression of the VIIIth nerve compound action potential (CAP) in the Mongolian gerbil.

We studied whether same-frequency contralateral tones of 65 dB pSPL (peak Sound Pressure Level) suppress the VIIIth nerve compound action potential (CAP) evoked by 40-45 dB pSPL tone pips in the Mongolian gerbil from 22 to 92 days after birth (DAB). The primary stimuli were tone pips of 1, 2, 4, 8, and 10 kHz; only the 1 kHz CAP amplitude was suppressed significantly by tones of the same frequency. The suppression was seen at 22 DAB, and underwent little relative change with development.

Measures of auditory evoked potentials during optokinetic stimulation.

In this investigation auditory brainstem responses (ABR) elicited by click stimuli were recorded before, during, and after optokinetic stimulation in subjects that were (N = 10) or were not (N = 10) prone to visually induced motion sickness. The latency of Wave I, and the I-III and I-V interwave intervals were measured. A significant increase in the I-III interwave interval occurred only during optokinetic stimulation. Neither the Wave I latency nor the interwave interval differed with respect to subject groups and this factor did not interact with any other variables. These results suggest that optokinetic stimulation may alter neural activity in the region of the superior olivary complex, a structure known to be important in sound-source localization.

The effects of perceived motion on sound-source lateralization.

Lateralization of dichotic click pairs differing in time of onset was studied under conditions of angular acceleration, optokinetic stimulation, and gaze fixation. Data obtained from 20 subjects with normal hearing indicate poorer left-right judgment performance for small time differences, as well as shifts in subjective simultaneity, for all experimental conditions relative to control conditions. In addition, response times increased for the experimental conditions. The results suggest that real or apparent motion may affect an individual's ability to process one of the major cues for sound-source localization--binaural time difference--under conditions similar to those encountered in vehicular motion.

Recall performance of children failing memory portions of a speech-language-memory screening battery.

Recall performance of first- and third-grade children who failed memory portions of a speech-language-memory screen was examined using digit and consonant-vowel (CV) stimulus sets. Children passing the screening battery had more accurate recall performance than children failing the screening battery. Systematic variations in performance as a function of stimulus set (i.e., digit vs CV) were noted in the groups: significantly lower recall was noted for CV items than for digit items in all groups passing or failing the screening battery. Data indicate children failing the screening battery differed quantitatively, rather than qualitatively, from children passing the screening battery. These data further suggest the difficulties experienced by children failing the screening battery may be related to deficits in the initial coding of auditory information.

Temporal integration of two-component tone glides.

Temporal integration of complex signals composed of two linearly combined tone glides was studied in a 60-dB SPL noise background. When both glide tones increased or decreased in frequency, integration functions differed for durations shorter than 50 ms in a manner similar to results obtained for single-component tone glides. That is, rising two-component glides were detected at lower thresholds than were falling two-component glides. However, when one component increased in frequency and the other decreased, thresholds were the same, regardless of which component increased and which decreased.

Intensity difference limens at high frequencies.

Thresholds for amplitude modulation detection were obtained from four subjects at frequencies of 2, 4, 6, 8, and 10 kHz for sensation levels of 15, 30, 45, and 60 dB and modulation rates of 2, 4, and 8 Hz. High-frequency difference limens calculated from amplitude modulation thresholds were found to change nonmonotonically as a function of sensation level, independent of modulation rate. This nonmonotonic relation stemmed mainly from a gradual reduction of the difference limen at the lowest sensation level with increasing frequency. Difference limens for pulsed tone discrimination were also measured in two of the subjects at 2, 6, and 10 kHz and sensation levels of 15, 30, 45, and 60 dB. The relation between intensity discrimination and sensation level was similar to that found for amplitude modulation detection. These findings are interpreted as indicating that the nonmonotonic relation between sensation level and intensity resolution is a general characteristic of stimulus processing at higher frequencies.

Temporal integration of tone glides.

Temporal integration of rising and falling tone glides against a 50-2800-Hz background of noise at a sound pressure level of 60 dB re 20 micronPa was studied in two experiments. Glides were in the frequency ranges 200-700 Hz and 1200-1700 Hz for durations of 5-120 ms. Results indicate an asymmetry in the detectability of rising and falling glides of short duration, with rising glides detected at lower signal intensities in both frequency ranges. These effects are discussed in terms of differences in pattern of frequency analysis of identical, but temporally reversed, waveforms.

Rate discrimination of high-pass-filtered pulse trains.

Difference limens for trains of 30-microseconds pulses were determined for repetition rates of 50, 100, 200, 400, and 800 pulses per second under conditions of no filtering and high-pass filtering (115 dB/oct) with corner frequencies of 2.5, 5.0, 7.5, and 10 kHz. Low-pass-filtered noise was mixed with the trains of impulses to preclude discrimination on the basis of potential low-frequency signal components. Measures were obtained from four trained listeners at a signal level of 30 dB SL relative to individually determined thresholds for each filter condition and repetition rate. The data support the hypothesis that resolution of pulse-train repetition rate involves both temporal- and frequency-based processes--the latter becoming ineffective when frequency resolution of the ear is insufficient to resolve separate harmonics of the signal. Inter- and intra-individual differences are interpreted as reflecting frequency resolution capacity.

Measures of signal processing in persons with sensorineural hearing loss below 6 kHz.

Estimates of threshold, wide- and narrow-band noise masking, frequency and amplitude modulation detection, gap detection, and rate discrimination were obtained from 10 subjects with near-normal hearing at frequencies above 6 kHz, but severe-to-profound hearing losses at lower frequencies. The same measures were obtained from 10 young control subjects with normal hearing sensitivity for all frequencies up to 16 kHz. The hearing-impaired subjects were able to process sounds in the region of near-normal hearing sensitivity as well as the unimpaired control subjects. Performance in the low-frequency, impaired region depended on the lowest frequency of near-normal hearing sensitivity.

Cochlear microphonic responses of the peripheral auditory system to frequency-varying signals.

Cochlear partition displacement responses to rising and falling frequency sweeps were inferred from cochlear microphonic potentials recorded from three basal turn locations in the guinea pig cochlea. Relative phase measures of microphonic potentials recorded from the three locations suggested that displacements of the partition toward either scala vestibuli or scala tympani occurred closely together in time for rising sweeps and were dispersed in time for falling sweeps. These differences in peripheral response patterns to sweeps may explain, in part, asymmetric neural discharges elicited from higher neural centers.

Temporal integration of tone glides by children with auditory-memory and reading problems.

Temporal auditory integration for short-duration, frequency-varying signals was examined in children with deficits in auditory memory and reading. Signals were similar to those previously used: a fixed-frequency tone, a tone glide rising in frequency, and a tone glide falling in frequency. No significant differences were found between the Experimental and Control groups' thresholds as a function of signal class (i.e., fixed-frequency or tone glide) or duration. In addition, a significant asymmetry in the thresholds of short-duration rising and falling tone glides was found for both groups. Falling tone glides were detected at higher intensities than were rising tone glides. Data from this study indicated this particular group of children with auditory memory and reading problems was able to detect simple and complex short-duration signals in a manner similar to normal children.

Effects of background noise level on detection of tone glides.

Detection thresholds were obtained for short duration rising tone glides and falling tone glides for normal hearing listeners in quiet and with varied levels of background, broadband noise. For signal durations 50 ms, or greater, pure tones were detectable at lower levels than were rising and falling tone glides; no level effects were observed. For signal durations less than 50 ms an interaction between background noise level, signal duration, and signal class was observed. The previously reported tendency for short duration rising glides to be detected at lower sound pressures than pure tones or falling tone glides appears to be unique to a limited range of noise levels (40-75 dB).