Andrew J. Niemiec

Return of auditory function following structural regeneration after acoustic trauma: Behavioral measures from quail

After measuring baseline behavioral audiograms, three of four behaviorally trained quail and fifteen untrained cohorts were exposed to a 1.5-kHz octave-band noise at 116-dB SPL for 4 h. The trained birds were tested daily following the exposure and showed a steady recovery of absolute sensitivity with a return to normal absolute thresholds by post-exposure days 8-10. Thirteen untrained cohorts were sacrificed after various survival times to evaluate the structural condition of the ear. The cohorts all showed regeneration of sensory cells similar to that seen in chicks. The effects of repeated acoustic trauma on recovery of sensitivity were evaluated by re-exposing the three trained birds and two untrained cohorts 106 days after the first exposure. One of the trained birds was exposed a third time, 113 days following the second exposure. The findings demonstrate that, following acoustic trauma, normal sensitivity returns prior to complete structural regeneration of the sensory epithelium and that repeated acoustic trauma may increase the time course of recovery of normal hearing sensitivity.

Behavioral measures of frequency selectivity in the chinchilla

A simultaneous masking procedure was used to derive four measures of frequency selectivity in the chinchilla. The first experiment measured critical masking ratios (CRs) at various signal frequencies. Estimates of the chinchillas critical bandwidths derived from the CRs were much broader than comparable human estimates, indicating that the chinchilla may have inferior frequency selectivity. The second experiment measured critical bandwidths at 1, 2, and 4 kHz in a band-narrowing experiment. This technique yielded narrower estimates of critical bandwidth; however, chinchillas continued to exhibit poor frequency selectivity compared to man. The third experiment measured auditory-filter shape at 0.5, 1, and 2 kHz via rippled noise masking. Results of the rippled noise masking experiment indicate that auditory filters of humans and chinchillas are similar in terms of shape and bandwidth with chinchillas showing only slightly poorer frequency selectivity. The final experiment measured auditory filter shape at 0.5, 1, 2, and 4 kHz using notched noise masking. This experiment yielded auditory filter shapes and bandwidths similar to those derived from man. The discrepancy between the indirect estimates of frequency selectivity derived from CR and band-narrowing techniques and the direct estimates derived from rippled noise and notched noise masking are explained by taking into account the processing efficiency of the subjects.

Macaque thresholds for detecting increases in intensity: effects of formant structure

Macaque monkeys, like humans, are more sensitive to differences in formant frequency than to differences in the frequency of pure tones (see Sinnott et al. (1987) J. Comp. Psychol. 94, 401-415; Pfingst (1993) J. Acoust. Soc. Am. 93, 2124-2129; Prosen et al. (1990) J. Acoust. Soc. Am. 88, 2152-2158; Sinnott et al. (1985) J. Acoust. Soc. Am. 78, 1977-1985; Sinnott and Kreiter (1991) J. Acoust. Soc. Am. 89, 2421-2429; for summary, see May et al. (1996) Aud. Neurosci. 3, 135-162). In the discrimination of formant frequency, it appears that the relevant cue for macaque monkeys is relative level differences of the component frequencies (Sommers et al. (1992) J. Acoust. Soc. Am. 91, 3499-3510). To further explore the result of Sommers et al., we trained macaque monkeys (Macaca fuscata) to report detection of a change in the spectral shape of multi-component harmonic complexes. Spectral shape changes were produced by the addition of intensity increments. When the amplitude spectrum of the comparison stimulus was modeled after the /ae/ vowel sound, thresholds for detecting a change from the comparison stimulus were lowest when intensity increments were added at spectral peaks. These results parallel previous data from human subjects, suggesting that both human and monkey subjects may process vowel spectra through simultaneous comparisons of component levels across the spectrum. When the subjects were asked to detect a change from a comparison stimulus with a flat amplitude spectrum, the subjects showed sensitivity that was relatively comparable to that of human subjects tested in other investigations (e.g. Zera et al. (1993) J. Acoust. Soc. Am. 93, 3431-3441). In additional experiments, neither increasing the dynamic range of the /ae/ spectrum nor dynamically varying the amplitude of the increment during the stimulus presentation reliably affected detection thresholds.

Constant Stimulus and Tracking Procedures for Measuring Sensitivity

The methods of constant stimuli, adaptive tracking, and transformed tracking are fundamental tools for the study of sensory function of humans and animals. This chapter describes the adaptation of these procedures to the study of auditory sensitivity in the macaque monkey, outlines the primary advantages and disadvantages of each of these techniques, and describes situations in which one of these techniques might be more appropriate than another.

Discrimination of interaural envelope delays: The effect of randomizing component starting phase

The goal of this study was to examine the nature of envelope extraction in the discrimination of high-frequency waveforms on the basis of envelope delay. Threshold interaural envelope delays were measured for complexes consisting of three or five components for which the starting phases of all sinusoids were either sine phase or randomized between intervals of a two-alternative forced-choice (2-AFC) task. The center frequency was 4 kHz and the frequency separation was varied from 25 to 500 Hz. The results showed that thresholds were greater for the phase-randomized conditions than the sine-phase conditions. The phase effect tended to diminish with increasing frequency separation for three-component complexes but not for the five-component complexes. Sensitivity to envelope delay was better for five-component complexes than for three-component complexes at most frequency separations. In general, the results showed superior lateralization performance for conditions in which the envelope fluctuations were greater, a finding that is consistent with models of high-frequency binaural processing that include envelope extraction prior to binaural comparison.

Monaural phase discrimination by macaque monkeys: Use of multiple cues

Research examining the discrimination of monaural phase change has suggested that temporal envelope shape, which varies with phase, may be an important cue. Much of that research employed stimuli consisting of three components, a center frequency (Fc), which is varied in phase, and an upper and lower sideband separated from the carrier by some frequency (delta F). As the phase of the center component is varied, both temporal envelope and temporal fine structure change. The present research explored the salience of both envelope and fine structure as cues in a phase discrimination task. Monkeys were trained to report detection of a change from a three-tone complex with 90 degrees starting phase for the center component to one in which the starting phase was smaller. In general, for the values of Fc tested, thresholds for phase change decreased as delta F increased. When tested with comparison stimuli that had a temporal envelope closely matched to that of the standard, but 0 degree starting phase, subjects had difficulty discriminating these stimuli from the standard for smaller delta F, but readily discriminated them at larger delta F values. These findings suggest that temporal envelope is a critical cue in discrimination of three-tone complexes on the basis of the starting phase of the center component at small values of delta F, but that other cues are used at larger delta F values.

ASSESSMENT OF EYE DOMINANCE THROUGH RESPONSE TIME

The present study examined the effectiveness of a new technique for the assessment of eye dominance. 41 undergraduates of both sexes completed the Edinburgh Handedness Inventory, as well as two conventional performance indexes of eye dominance. Then each subject was positioned before the video monitor of a microcomputer and asked to view the monitor through a device that presented a different area of the monitor screen to each eye. In these two screen areas the digits 0 through 9 were randomly presented. Within every three trials the first two trials presented a digit to both eyes, and on the third trial a digit was presented to both eyes or to only one eye. There were 20 presentations of each kind: to both eyes, right eye, or left eye. The subject indicated whether the viewed digit was larger than or less than 4.5. This cognitive decision task was a constant in all trials. Analyses were performed on the third-trial response times, specifically, on the response time using right eye minus response time using left eye. For right-eyed people this was a positive number. As predicted, handedness, as assessed by the Edinburgh instrument, was significantly correlated with eyedness, as assessed by the response time procedure. The two conventional indexes of eyedness were also correlated with eyedness as assessed by the response time procedure. This procedure, then, provides a cardinal-scale measurement of degree of eye dominance.

Psychophysically derived auditory filters in chinchillas

A positive‐reinforcement behavioral tracking technique was used to measure detection thresholds for a pure‐tone signal (f0) presented against a background of notched noise in a simultaneous masking paradigm. Notched noise was generated by band‐reject filtering a white noise at a rolloff of 96 dB/oct. The spectrum level of the noise was 43 dB. Detection thresholds were measured for a number of conditions in which the notch width was varied from 0 to 0.8 times f0 in steps of 0.1 times f0. The derivatives of the masking functions were computed to obtain the filter functions [R. D. Patterson, J. Acoust. Soc. Am. 59, 640–654 (1976)]. Behavorial data from the simultaneous masking paradigm indicate that auditory filter shapes of chinchillas and humans are similar, both having a simple bandpass characteristic. However, the chinchillas auditory filters are more broadly tuned than those of humans, reflecting poorer frequency selectivity. [Work supported by NIH‐NIDCD center grant.]

The effect of envelope power on the ability to lateralize three‐tone complexes on the basis of interaural envelope delays

In the present experiment, envelope power was varied by setting the starting phases of the three components of a complex 0°−0°−0° (high), 0°−45°−0° (medium), or 0°−90°−0° (low). Threshold interaural envelope delays were measured in a 2‐AFC task as a function of frequency spacing (Δf = 25, 50, 100, and 200 Hz) with the carrier frequency (fc) set to 4000 Hz. The level of each component was 50 dB SPL, and the signal duration was 200 ms with 10‐ms linear rise/decay times. For comparison, thresholds were also measured for SAM 4000‐Hz tones with reduced depths of modulation so that the effects of decreasing modulation depth and decreasing envelope power could be directly compared. Reducing envelope power had only a negligible effect on threshold interaural envelope delays for Δfs of 200 and 100 Hz but a large and systematic effect at 50 Hz and especially at 25 Hz. For three of the five observers, the 0°−90°−0° Δf = 25‐Hz condition was so difficult that 75% correct could not be reached with delays as large as 2...

Lateralization on the basis of interaural envelope delays: The effect of component starting phase

Threshold interaural envelope delays were measured with a 2 AFC paradigm as a function of modulation frequency (fm = 25, 50, 100, 200, 300, 400, and 500 Hz) for three‐ and five‐component complexes whose center frequencies (fc) were equal to 4000 Hz. Comparisons were made between thresholds obtained when the starting phases were randomized and when they were all set to zero. The level of each component was approximately 50 dB SPL, and the signal duration was 200 ms with 10‐ms linear rise/decay times. While reductions in depth of modulation elevate threshold envelope delays [G. B. Henning, J. Acoust. Soc. Am. 55, 84–90 (1974)], no such effects were found for phase randomization even when modulation rates were low and all components fell within a critical band. This seems surprising in light of the fact that both reduce depth of modulation and phase randomization reduce the peak factors of the waveforms, which is commonly believed to interfere with entrainment by the peripheral auditory system. Efforts to re...