John B. Mott

Speech perception ability and psychophysical tuning curves in hearing‐impaired listeners

Performance-intensity functions for monosyllabic words were obtained as a function of signal-to-noise ratio for broadband and low-pass filtered noise. Subjects were 11 normal-hearing listeners and 13 hearing-impaired listeners with flat, moderate sensorineural hearing losses and good speech-discrimination ability (at least 86%) in quiet. In the broadband-noise condition, only small differences in speech perception were noted between the two groups. In low-pass noise, however, large differences in performance were observed. These findings were correlated with various aspects of psychophysical tuning curves (PTCs) obtained from the same individuals. Results of a multivariate analysis suggest that performance in broadband noise is correlated with filter bandwidth (Q10), while performance in low-pass noise is correlated with changes on the low-frequency side of the PTC.

Neural correlates of nonmonotonic temporal acuity for voice onset time

Human and chinchilla listeners exhibit nonmonotonic temporal acuity for speech sounds differing in voice onset time (VOT). Characteristics of the neural discharge pattern or of the stimuli themselves that might account for the pattern of temporal acuity have not been described. Responses of chinchilla auditory-nerve fibers to syllables from an alveolar VOT continuum were measured. Peak discharge rates and peak response latencies elicited by the syllables with the shortest and longest VOTs were highly variable across groups of neurons with similar characteristic frequencies. For VOTs from the middle of the continuum, peak responses were larger, and response latencies were nearly constant across the same group of neurons. Overall, the magnitude and temporal variability of the responses of populations of primary auditory neurons varied nonmonotonically with VOT, consistent with the pattern of psychophysical temporal acuity for these syllables exhibited by humans and chinchillas. Spectral analyses suggested by the pattern of neural responses indicated that synchronous or correlated spectral cues were available over a wider bandwidth for those syllables from the middle of the continuum for which the neural representation was least variable.

Behavior of spontaneous otoacoustic emissions following intense ipsilateral acoustic stimulation

Following presentation of brief, intense pure tones spontaneous otoacoustic emissions (SOAEs) were reduced in frequency and/or amplitude. The effects were highly tuned with exposures between 1/8 and 5/8 of an octave below the SOAE producing the maximum changes. Exposure frequencies above the SOAE had no effect. The degree of tuning observed depended upon the post-exposure time sampled, with sharpness maximal between 3 and 120 s post-exposure. The effects increased nonlinearly as exposure level and duration were increased. The recovery functions were biphasic, the first phase being rapid and non-monotonic over about 2 min, while the second phase was monotonic and slow, sometimes taking several hours. These data are consistent with changes in outer hair cell (OHC) function and support the hypothesis that OHC changes underlie behavioral temporary threshold shift (TTS).

Effects of the Temporal Properties of a Masker upon Simultaneous-Masking Patterns

Fletcher’s (1940) critical band hypothesis suggests that maskers with equal power within a critical band around the signal frequency ought to produce equal amounts of masking. Simultaneous-masking patterns (SMP’s) for pure-tone and narrowband-noise maskers of equal power and similar center frequency (Egan and Hake, 1950; Carter and Kryter, 1962) indicate that the critical band hypothesis is an over-simplification. At masker levels above 60 dB SL filtered, narrowband-noise and pure-tone maskers were found to mask differently as a function of frequency. Pure-tone maskers were found to be more effective when the probe was 1/2 octave or more above the masker frequency while narrowband-noise maskers were more effective when the probe was at or very near the masker frequency. In the frequency region below the masker, pure-tone and narrow-band noise maskers produced nearly equivalent amounts of masking. More recently, Buus (1985) has shown that pure-tone and broadband-noise maskers, which produced nearly equivalent amounts of masking, produced more masking on the high-frequency side of the masking pattern than did narrowband-noise.

Effects of auditory fatigue on psychophysical estimates of cochlear nonlinearities

Two-tone-suppression (2TS) effects and cubic-difference-tone (CDT) levels were measured using a forward-masking procedure, before and after exposure to an intense tonal stimulus. Changes in 2TS effects were consistent with a change in the nonlinearity, such that the system became more linear, in addition to changes due to attenuation of the primary stimuli F1 and F2. Changes in CDT levels could be accounted for on the basis of a temporary threshold shift at F1 and F2. These results indicate at least a partial dissociation of the mechanisms underlying 2TS and CDT generation.

Effects of intense sound exposure on spontaneous otoacoustic emissions

The effects of intense sound exposure on spontaneous otoacoustic emissions (SOAE) were measured in normally hearing human subjects as a function of exposure duration and frequency region. Consistent with the TTS literature, exposures one‐half to one‐quarter oct below the SOAE frequency have the largest effect. A 30‐s, 105‐dB SPL exposure shifts the SOAE frequency downward, while having little effect on SOAE amplitude. Recovery is biphasic: In the first minute post‐exposure, the SOAE frequency changes rapidly; over the next 20 to 30 min, the SOAE slowly returns to pre‐exposure values. Exposures of 15 and 60 s have similar effects. These results suggest that alterations in cochlear mechanics can be produced by stimuli that do not typically produce TTS. [Work supported by NINCDS grants R15 NS23202‐01 (SJN & CAC) and T‐32‐NS07257 (JBM).]

Interactions between nearophysiology and speech distrimination

Psychophysical approaches have provided much information about the processing of speech sounds by humans and also by laboratory animals. These studies establish the essential functions of the auditory system that must be explained and define the limits over which they operate. However, processing mechanisms cannot be discovered by psychophysical approaches. Details about mechanisms must come from neurophysiological studies. This view has guided this investigation of the processing of voice onset time (VOT), a complex acoustic cue that differentiates between consonants such as /d/ and /t/. The peripheral neural representation of VOT syllables was studied, asking which (if any) properties of the neural responses correlated with the results of psychophysical experiments conducted by Kuhl and Miller [J. Acoust. Soc. Am. 63, 905–917 (1978); 70, 340–349 (1981)]. The psychophysical findings provided a standard against which hypotheses about the representation of VOT information could be evaluated. As a result, i...

“Confusion effects” in simultaneous masking

A “confusion effect,” the inability to discriminate the probe from the masker in certain stimulus configurations, has been well‐documented in forward masking [e.g., D. Neff, J. Acoust. Soc. Am. 78, 1966–1976 (1985)]. Similar effects are reported for simultaneous masking. Bimodal thresholds in simultaneous masking are observed when the masker envelope fluctuates and the probe duration is shorter than the mean interval between envelope peaks. The magnitude of the confusion effect, as measured by the difference between threshold estimates for a given masker and probe combination, is similar in simultaneous and forward masking. However, while confusion effects in forward masking are typically observed when the probe is near the masker frequency, bimodal thresholds in simultaneous masking are observed across a wider frequency region. This result does not favor a confusion hypothesis, since the spectral difference between masker and probe should reduce or eliminate confusion effects. [Work supported by NINCDS g...

Effects of noise exposure on forward masking

The forward‐masking paradigm is used extensively in the determination of psychophysical tuning curves (PTC). PTCs are, in turn, used as indicators of auditory frequency selectivity. Our previous work on post‐noise exposure determinations of frequency selectivity using PTCs indicate that there may be temporary, frequency‐dependent changes in forward masking effects just after exposure to noise. The current study was designed to determine the nature of post‐noise exposure effects on the time course of forward masking as well as the forward masking function. Forward masking functions were obtained 2, 4, 8, and 16 min after listeners were exposed to an octave band of noise (1200–2400 Hz) at 100 dB SPL for 5 min. For control conditions, the 5‐min exposure was reduced in level to 40 dB. Masker and probe signals were tones of 300‐ and 20‐ms duration, respectively. Their frequency was 3000 Hz. Delta t, measured from masker offset to probe onset, varied from 10–100 ms. Results indicate that some of the variability observed in post‐noise PTCs may be related to changes in forward masking effects. [Work supported by funds from the University of Kansas General Research Fund.]

TTS and psychophysical suppression effect

Robertson and Johnstone [J. Acoust. Soc. Am. 69, 1096–1098 (1981)] found that in single eighth nerve fibers, exposure to a brief, intense pure‐tone stimulus in the region of the suppressor frequency produced changes in two‐tone suppression without affecting the frequency threshold tuning curve. We wished to see if a corresponding effect could be observed in human psychophysical performance. Quiet thresholds, forward‐masked thresholds, and suppression were measured before and after exposure to brief, intense tonal stimuli. The suppressor frequency was above the masker frequency and the exposure stimulus was at or below the suppressor frequency. Postexposure reductions in suppression were observed only when temporary threshold shift (TTS) occurred at the suppressor frequency. However, changes in suppression do not appear to be the result of a simple attenuation of suppressor or masker level. Pre‐ and postexposure masked thresholds were not found to be different, despite significant TTS at the masker frequen...