885096006

and Ashok K. Krishnamurthy (Ohio State Univ., Columbus, OH 43210)

The envclope wcighted average instantaneous frequency (EWAIF) has been used to model listener performance in complcx signal discrim-ination [L. L. Feth, J. Acoust. Soc. Am. Suppl. 1 88, S48 (1990)]. Previous work with EWAIF has involved narrow-band signals that are confined entirely to a single critica! band; consequentIy, it is sufficicnt to compute a single EWAIF value from the signal. Hcre, the application of EWAIF to wideband signals is extended. The signal is first passed through the gamma-tone filter bank of the Patterson-Holdsworth audi-tory sensation processing model [R. Pattcrson, J. Acoust. Soc. Am. Suppl. 1 88, S26 (1990)] and an EWAIF is computed from the output of each channel. This leads to a vector of EWAIF values, which are used in two ways: (I) the EWAIF values from the different channels are combincd back into a composite EWAIF valuc, which is then used to model listener performance in complex sound perception such as profile analysis. comodulation masking release, and modulation mask-ing; and (2) the vcctor of EWAIF values is used as a feature vector in a vowel recognition task. Preliminary results will be presented and fur-ther modifications to the model discussed. [Work supported by a grant from AFOSR.] 5SP28. A new tcchnique for measuring spectral shape discrimination.

Gerald Kidd, Jr. and Christine R. Mason (Dcpt. of Commun. Disord., Boston Univ., 635 Commonwealth Ave., Boston, MA 02215)

A new technique for measuring the ability of listeners to discrimi-nate between sounds on the basis of spectral shape—called "auditory profile analysis'*—is described. In this lechnique, the signal is a scries of intensity inerements and decrements to an equal-level multitone refer-ence spectrum. For any stirmilus trial, the signal is either an intensity inerement to the odd-numbered tones and an intensity deerernent to the even-numbered tones, or vice-vcrsa. The advantage of the technique is that it reduces, by one-half (in a two-down, one-up adaptive procedurę), the rangę of the random, within-trial rove in ovcrall levcl nceded to limit the usefulness of level cues. Using this technique, discrimination performance was measured for a group of normally hearing listeners for broadband, Iow-pass, and high-pass filtered conditions, and for individ-ual hearing-impaired listeners. The results indicated that the new tech-nique appears to be well suited for studying auditory profile analysis in hearing-impaired listeners where the rangę of sound intensities that may be presented is often quite limited. [Work supported by NIH/NIDCD.] 5SF29. Detection of silent temporal gaps in narrow-band noise .stimuli having second-formantlike properties of yoiceless stop/vowel coinbinations. C. Formby (Dept. of Otolaryngol., Johns Hopkins Univ. School of Medicine, Carnegie 442, Baltimore, MD 21205), C. Barker (Univ. of Florida, Gainesville, FL 32611), H. Abbey (Johns Hopkins Univ., Baltimore, MD 21205), and J. Raney (Univ. of Massachusetts, Amhersl, MA 01003)

Delattre et al. [J. Acoust. Soc. Am. 27, 769-773 (1955)] reported that good-quality stop consonant/vowel pairs could be synthesized in two-formanl representation by replacing the initial part of the second-formant (FI) transition with a silent temporal gap. The best silent gap for each stop varied with the timing and extent of the FI transition, which we know is (I) dependent on coarticulation between the stop and vowel and (2) represented acousticalły by the differencc or change in frequency from the stop burst to FI of the foilowing vowel. Motivated by their report, temporal gap detection thresholds were measured in narrow-band noise-burst markers having acoustic characteristics that simulated isołated steady-state FL properties of yoiceless stop /p. t, k/ and vowel /i, x. u, a/ combinations. Gap detection thresholds inereased systcmatically as a function of the difłerence between the simulated stop and vowel FI frequencies, which accountcd for 76% of the yariance in the results. Differences in other stimulus features had little influence on detection. [Research supported by NIH.] 5SP30. Cap thresholds obtained with auditory steady-state responses. Craig A. Champlin (Dept. of Speech Commun., Univ. of Tcxas. Austin. TX 78712)

Humań gap thresholds were obtained electrophysiologically using auditory steady-state responses (ASRs). Three types of stimuli— bandpass, low-pass, and high-pass noise—were presented at three sensation leve!s (30, 50, and 70 dB SL). The cutoff frequency for the latter two noises was 1000 Hz. The otherwise-conlinuous stimuli were inter-rupted 40 times/s, thus creating a repetitive sequence of gaps. The duration of the gap was 0, 2, 4, 6, 8, 10, 15, 20, or 24 ms. The magnitude and phase of the time-averaged ASRs were obtained for each stimulus condition. Gap thresholds obtained with the bandpass and high-pass noises were similar, while thresholds obtained with the low-pass noise were larger. These results are consistent with existing psychophysical data.

5SP3I. Effects of temporal-envelope correlatiun on the strength of yertical fusion of noise-band pairs. P. W. Rappold, L. L. Mendoza, and M. J. Collins (Dept. of Commun. Disord., 163 M&DA Bldg., Louisiana State Univ., Baton Rouge, LA 70803)

yertical fusion is the perceptual grouping of co-occurring auditory signals into a single acoustic event. In contrast, horizontal streaming is the perceptual grouping of auditory signals occurring in a temporal seąuence. In the present experiment, “strength’* of fusion was inferred from the interstimulus interval (ISI) necessary to evoke a predominant percept of horizontal streaming rather than vertical fusion. Perceptual grouping heuristics suggest “strength’’ of fusion is greater for AM noise bands with correlated temporal envelopes than for AM noise bands with uncorrelated temporal envek>pes. It was therefore predicted that a shorter ISI would be necessary to shift the percept from yertical fusion to horizontal streaming when noise-band envelopes were correlated than when they were uncorrelated. This prediction proved accurate for two of four subjeets. The remaining two subjects, however, showed no significant difference between the ISIs needed to shift the percept from fusion to streaming. The results will be discussed in refcrence to impli-cations for the bases of the comodulation masking release phenomenon.

5SP32. Restarting the adapted binaural system in a background of noise. Ervin R. Hafter and Kourosh Sabcri (Dept. of Psychol., Univ. of Califomia, Berkeley, CA 94720)

The rate-dependent reduced binaural sen$itivity to interaural Information coming after the stimulus onset has bccn callcd "binaural adap-tation." It has been shown that an appropriate restarting trigger presented during the ongoing stimulus can end the adaptation, ailowing for lateralization based on information beyond the restart. Effective triggers include short bursts of noise and gaps in the stimulus. It has been reasoned that continuous spectral changes might constantly restart the system and thus preclude adaptation. To test this, click trains have been presented for lateralization in a continuous background noise and with click rates sufficiently high to produce adaptation in the quiet. Our hypothesis was wrong. Whilc binaural thresholds inereased with noise level as the noise spectrum levels rosę from 0 to 20 dB, no change was observed between the forms of the functions obtained in quict and in noise. Similarly, the inclusion of a gap in the center of the train triggered a restart both in the quiet and in the noise background. Thus a spread of energy into quiet bands as has been postulated in the past as an explanation for restarting is insufficient.

5SP33. Aging and discrimination of Interaural time differences. Daniel Ashmead, Marta Tetzeli, David Chandler, and Amy Thomas (Dept. of Psychol., Vanderbilt Univ., Nashyille, TN 37240)

Discrimination of interaural time differences (ITDs) was compared between ten younger adults (18-24 years) and ten older adults (66-80

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J. Acoust. Soc. Am., Voł. 89, No. 4. Pt. 2, April 1991

121st Meeting: Acouslical Society of America

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