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Discrimination of Time Intervals Marked by Brief Acoustic Pulses of Various Intensities and Spectra

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Discrimination of Time Intervals Marked by Brief Acoustic Pulses of Various Intensities and Spectra Perception & Psychophysics 1977, Vol. 21 (2),125-142 Discrimination of time intervals marked by brief acoustic pulses of various intensities and spectra PIERRE L. DIVENYI and WILLIAM F. DANNER Central Institute/or the Deaf, St. Louis, Missouri 63110 Experienced observers were asked to identify, in a four-level 2AFC situation, the longer of two unfilled time intervals, each of which was marked by a pair of 20-msec acoustic pulses. When all the markers were identical, high-level (86-dB SPL) bursts of coherently gated sinusoids or bursts of band-limited Gaussian noise, a change in the spectrum of the markers generally did not affect performance. On the other hand, for I-kHz tone-burst markers, intensity decreases below 25 dB SL were accompanied by sizable deterioration of the discrimination performance, especially at short (25-msec) base intervals. Similarly large changes in performance were observed also when the two tonal markers of each interval were made very dissimilar from each other, either in frequency (frequency difference larger than 1 octave) or in intensity (level of the first marker at least 45 dB below the level of the second marker). Time-difference thresholds in these two latter cases were found to be nonmonotonically related to the base interval, the minima occurring between 40- and 80-msec onset separations. For the past decade, readers of the psychophysics acoustic complexity of speech, the answer to this and speech literature have witnessed a growing question may be very difficult. To appreciate this interest in the perception of temporal aspects of difficulty, one should remember that phonetic seg­ speech (for a review, see Studdert-Kennedy, 1975). ments differ in their intensity, spectral composition, Many of the studies generated by this interest deal and duration; in addition, different spectral regions with the perception of brief time intervals defined within a given segment may have different buildup by speech sounds. Several phenomena that have been and decay characteristics. Indeed, in some instances revealed in this area, ones that relate to the percep­ it may become virtually impossible to determine tion of voice onset time (VOT), vowel transitions, without ambiguity just when the speech segment in syllabic duration, etc., are intriguing from the question begins or ends. There is, however, another psychoacoustical point of view. Yet, for two reasons, reason why temporal-perceptual phenomena in these phenomena, as well as most of the others speech are difficult to explain in psychophysical related to speech perception, do not lend themselves terms: the relative paucity of studies dealing with easily to detailed psychophysical analysis. First, the psychophysics of short time intervals, especially the prerequisite for such an analysis would be the unfilled intervals, that are bounded by simple definition of what the exact timing cues are in a given acoustic (i.e., nonspeech) markers. In our opinion, speech sound ensemble. Because of the enormous processes that govern temporal perception of speech and speech sounds cannot be completely understood as long as our knowledge about the processes This research has been supported by a grant from the National Institute for Neurological Diseases and Stroke (NS 03856). The governing auditory perception of time intervals authors wish to acknowledge the assistance they received from marked by nonspeech sounds is incomplete. The James J. Bieker throughout the long months of experimental present study represents an attempt to parametrically work, and that of their colleagues at CID for the stimulating investigate the discrimination of brief unfilled time discussions that helped to shape the ideas expressed in the article. I. J. Hirsh, D. A. Ronken, and C. S. Watson critically commented intervals. At the focus of the study is the question on earlier versions of the paper. We wish to thank Professor of how much time discrimination is influenced by M. Taibleson and W. J. Kelly for their efforts to scrutinize the the manipulation of the spectrum and intensity of mathematical derivation of the model at the first stages of its acoustic markers. conception, and W. M. Fisher for making his numerical analysis programs available. At later stages of the mathematical model's One parameter of particular importance in speech development, H. S. Colburn spared no time and effort to point is its spectrum. Our main source of information out and help to correct several inaccuracies. P. L. Divenyi's present with respect to spectral effects on time discrimina­ address: Neurophysiology-Biophysics Laboratories, Veterans tion is the classical study by Henry (1948); it is un­ Administration Hospital, Martinez, California 94553. W. F. fortunate, however, that it had to be conducted with­ Danner's present address: National Bureau of Standards, Washington, D.C. 20234. Requests for reprints should be out the benefit of the presently available stimulus addressed to Research Department, Central Institute for the control. That study showed that discriminability of Deaf, 818 South Euclid, St. Louis, Missouri 63110. the duration of pure tones was somewhat better in 125 126 DIVENYI AND DANNER the middle frequency range (500-2,000 Hz) than at as clear as it is predictable, Our information, especially frequencies either below or above this range. Regret­ about the exact degree of the handicap caused by tably, no study is known to have dealt with a similar low-intensity markers, is incomplete. question for unfilled intervals. Furthermore, there In the following sections, five experiments are is no indication in the literature as to the influence of described, experiments which attempted to explore various noise bandwidths on the discrimination of the effects of intensity and spectral content on the intervals marked by noise bursts, beyond the finding discriminability of the duration of unfilled intervals (Abel, 1972a) that, at equal sound pressure level, marked by short bursts of tones as well as of band­ the durations of tone bursts and of noise bursts is limited Gaussian noise. These effects were studied equally well resolved. For the discrimination of un­ mainly at three base interval durations, each of which filled intervals, not even such a simple tone-vs-noise has its particular importance in speech: 25 msec (a comparison has been made. Therefore, one must typical voiced-voiceless CV category boundary), conclude, on the basis of presently available reports, 80 msec (a typical voice onset time [VOT] for English that questions of whether auditory time discrimina­ unvoiced-plosive-vowel pairs), and 320 msec (a tion is invariant across diverse time-marker spectra typical syllable duration). Investigation of temporal or whether there are certain spectral configurations discriminability at these three principal intervals was which carry macrotemporal information more preceded by two studies in our laboratory (Danner, effectively than others cannot be unambiguously 1975; Divenyi, Danner, & Bieker, 1974) in which a answered. Yet, temporal discrimination of speech fine-grain analysis of discrimination inside the 10­ signals almost invariably involves markers that differ 6OO-msec range was attempted. These previous ex­ in their spectra. Thus, in order to better understand periments demonstrated that the three base durations temporal discrimination of speech sounds, one needs selected for the present study are a representative to learn about effects of spectral changes on the set of the whole continuum of durations within that discriminability of intervals marked by simple range. acoustic signals, such as tone bursts. The common In addition, an attempt is made to integrate the message of the few available reports on this subject present experimental results into a theoretical frame­ appears to be that, as the frequency difference work. Supported by a certain amount of experi­ between two markers increases, detectability of a gap mental data, Creelman (1962) proposed a simple between them (Williams & Perrott, 1972) or dis­ model for auditory time discrimination, a model criminability of a longer (120-800-msec) interval that which was upheld, with some modification, by separates the onsets of the two tones (Divenyi, 1971; Abel (1972a, b). Recently, on the basis of different van Noorden, 1971) generally worsen. However, experimental evidence, Allan and Kristofferson (1974) these latter experiments have not covered the im­ presented a model different from Creelman's. In a portant temporal range below 100 msec. Further­ later section of the present paper, we would like to more, it is not certain whether the detection of a gap show that Creelman's model describes our data and the discrimination of unfilled intervals involve reasonably well and shall propose a somewhat identical mechanisms, for gap detection may well be refined version of his theory. based on the detection of certain gating transients that are present in the gap-condition and absent in PROCEDURES the no-gap-condition. The other parameter that is of great importance The effect of marker intensity on time discrimination was examined in Experiments I and 2, whereas the influence of marker in speech signals and which has been shown to affect spectrum was investigated in Experiments 3, 4, and 5. In Experi­ temporal discrimination is marker intensity. Several ments I, 2, 3, and 4, the markers were bursts of pure tones; in investigators have reported that discrimination of Experiment 5, the intervals were marked by bursts of band­ tonal durations
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