James A. Bashford

Spectral redundancy: Intelligibility of sentences heard through narrow spectral slits

The intelligibility of word lists subjected to various types of spectral filtering has been studied extensively. Although words used for communication are usually present in sentences rather than lists, there has been no systematic report of the intelligibility of lexical components of narrowband sentences. In the present study, we found that surprisingly little spectral information is required to identify component words when sentences are heard through narrow spectral slits. Four hundred twenty listeners (21 groups of 20 subjects) were each presented with 100 bandpass filtered CID ( “everyday speech ”) sentences; separate groups received center frequencies of 370, 530, 750, 1100, 1500, 2100, 3000, 4200, and 6000 Hz at 70 dBA SPL. In Experiment 1, intelligibility of single 1/3-octave bands with steep filter slopes (96 dB/octave) averaged more than 95% for sentences centered at 1100, 1500, and 2100 Hz. In Experiment 2, we used the same center frequencies with extremely narrow bands (slopes of 115 dB/octave intersecting at the center frequency, resulting in a nominal bandwidth of l/20 octave). Despite the severe spectral tilt for all frequencies of this impoverished spectrum, intelligibility remained relatively high for most bands, with the greatest intelligibility (77%) at 1500 Hz. In Experiments 1 and 2, the bands centered at 370 and 6000 Hz provided little useful information when presented individually, but in each experiment they interacted synergistically when combined. The present findings demonstrate the adaptive flexibility of mechanisms used for speech perception and are discussed in the context of the LAME model of opportunistic multilevel processing.

Increasing the intelligibility of speech through multiple phonemic restorations

Outside of the laboratory, listening conditions are often less than ideal, and when attending to sounds from a particular source, portions are often obliterated by extraneous noises. However, listeners possess rather elegant reconstructive mechanisms. Restoration can be complete, so that missing segments are indistinguishable from those actually present and the listener is unaware that the signal is fragmented. This phenomenon, called temporal induction (TI), has been studied extensively with nonverbal signals and to a lesser extent with speech. Earlier studies have demonstrated that TI can produce illusory continuity spanning gaps of a few hundred milliseconds when portions of a signal are replaced by a louder sound capable of masking the signal were it actually present. The present study employed various types of speech signals with periodic gaps and measured the effects upon intelligibility produced by filling these gaps with noises. Enhancement of intelligibility through multiple phonemic restoration occurred when the acoustic requirements for TI were met and when sufficient contextual information was available in the remaining speech fragments. It appears that phonemic restoration is a specialized form of TI that uses linguistic skills for the reconstruction of obliterated speech.

Melodic and Nonmelodic Sequences of Tones: Effects of Duration on Perception

Familiar melodic phrases were played repetitively with note durations ranging from 40 msec to 3.6 sec. Recognition required note durations approximating those normally used for playing melodic themes (roughly 150 msec to 1 sec per note). Additional experiments with nonmelodic sequences of tones indicated that different rules applied for nonmelodic patterns: Permuted orders of the same items could be distinguished from each other at all durations employed (10 msec to 5 sec per item). Recognition of different arrangements occurred not only when each tone differed in pitch, but also when all tones had the same pitch but differed in timbre. It was concluded that the durational limits for melodic recognition are not based on perceptual limits applicable to tonal patterns in general, but rather reflect special rules governing melodic organization. Hypotheses concerning the bases for these rules are suggested.

Tweaking the lexicon: Organization of vowel sequences into words

The ability of listeners to distinguish between different arrangements of the same three vowels was investigated for repeating sequences having item durations ranging from 10 msec (single glottal pulses) up to several seconds/vowel. Discrimination was accomplished with ease by un-trained subjects at all item durations. From 30 through 100 msec/vowel, an especially interesting phenomenon was encountered: The sequences of steady-state vowels were organized into words, with different words heard for the different arrangements of items. In a second experiment, repeating sequences of random arrangements of 10 40-msec vowels were employed. When sets of four such sequences were presented to listeners, distinctive words were heard, which permitted each arrangement to be discriminated from the others. In addition, minimal differences (reversing the order of a single contiguous pair of vowels) in the 10-item sequences could be detected via verbal mediation. Hypotheses are offered concerning mechanisms responsible for these results.

Müller-Lyer Illusions: Their Origin in Processes Facilitating Object Recognition

Perspective theories for the Müller-Lyer illusions have considered them to reflect constancies normally permitting the viewer to compensate for distance in estimating the true size of three-dimensional objects. Our experiments suggest a rather different perspective theory involving object recognition. A novel experimental procedure was used. Each of the separate groups of thirty subjects viewed only one of nineteen experimental drawings based on the Müller-Lyer figures. They judged the overall length of the shaft and divided it into subjectively equal quarters. The major findings were that the effect of each component angle was generally independent of other angles and independent of any overall perspective appearance of the figure. The shortening effect produced by an acute angle was limited to the contiguous quarter, whüe the lengthening effect of an obtuse angle extended undiminished from the contiguous quarter to the next quarter. Perspective photographs are shown, demonstrating that these angle-induced changes in apparent length can compensate selectively for the different perspective distortions occurring for the component parts of a single three-dimensional object. Such expansions and contractions provide no direct help in estimating overall size or distance of the object. But they do make possible recognition of the actual shape of the object and its orientation to the viewer, as well as producing errors when judging the length of Müller-Lyer figures.

Multiple phonemic restorations follow the rules for auditory induction

Addition of noise to gaps produced by deletion of speech segments can result in both illusory continuity and increased intelligibility. Earlier findings suggest that the perceptual restoration of speech may represent a linguistic adaptation of a nonverbal phenomenon. “Auditory induction” permits perceptual synthesis of contextually appropriate sounds when deleted segments of a signal are replaced by a potential masker. In Experiment 1, when intelligible narrowband discourse was periodically interrupted by silence and by one of a number of bands of noise having different center frequencies, the illusory restoration of continuity was found to follow the spectral rules for nonverbal auditory induction. In Experiment 2, the durational limit of illusory continuity was determined for several types of broadband speech interrupted by broadband noise. The limiting duration was longest for normal discourse interrupted by noise and was equal to the average word duration of that stimulus. The results suggest that auditory induction becomes coupled with special linguistic mechanisms permitting perceptual synthesis of verbal components from noise, with the size of the segments dependent upon the extent of contextual information.

Spectral restoration of speech: Intelligibility is increased by inserting noise in spectral gaps

In order to function effectively as a means of communication, speech must be intelligible under the noisy conditions encountered in everyday life. Two types of perceptual synthesis have been reported that can reduce or cancel the effects of masking by extraneous sounds: Phonemic restoration can enhance intelligibility when segments are replaced or masked by noise, and contralateral induction can prevent mislateralization by effectively restoring speech masked at one ear when it is heard in the other. The present study reports a third type of perceptual synthesis induced by noise: enhancement of intelligibility produced by adding noise to spectral gaps. In most of the experiments, the speech stimuli consisted of two widely separated narrow bands of speech (center frequencies of 370 and 6000 Hz, each band having high-pass and low-pass slopes of 115 dB/octave meeting at the center frequency). These very narrow bands effectively reduced the available information to frequency-limited patterns of amplitude fluctuation lacking information concerning formant structure and frequency transitions. When stochastic noise was introduced into the gap separating the two speech bands, intelligibility increased for “everyday” sentences, for sentences that varied in the transitional probability of keywords, and for monosyllabic word lists. Effects produced by systematically varying noise amplitude and noise bandwidth are reported, and the implications of some of the novel effects observed are discussed.

Use of speech-modulated noise adds strong "bottom-up" cues for phonemic restoration

When deleted segments of speech are replaced by extraneous sounds rather than silence, the missing speech fragments may be perceptually restored and intelligibility improved. This phonemic restoration (PhR) effect has been used to measure various aspects of speech processing, with deleted portions of speech typically being replaced by stochastic noise. However, several recent studies of PhR have used speech-modulated noise, which may provide amplitude-envelope cues concerning the replaced speech. The present study compared the effects upon intelligibility of replacing regularly spaced portions of speech with stochastic (white) noise versus speech-modulated noise. In Experiment 1, filling periodic gaps in sentences with noise modulated by the amplitude envelope of the deleted speech fragments produced twice the intelligibility increase obtained with interpolated stochastic noise. Moreover, when lists of isolated monosyllables were interrupted in Experiment 2, interpolation of speech-modulated noise increased intelligibility whereas stochastic noise reduced intelligibility. The augmentation of PhR produced by modulated noise appeared without practice, suggesting that speech processing normally involves not only a narrowband analysis of spectral information but also a wideband integration of amplitude levels across critical bands. This is of considerable theoretical interest, but it also suggests that since PhRs produced by speech-modulated noise utilize potent bottom-up cues provided by the noise, they differ from the PhRs produced by extraneous sounds, such as coughs and stochastic noise.

Perception of acoustic iterance: pitch and infrapitch.

Detection of acoustic repetition is considered as a continuum extending from .5 through 16,000 Hz. Perceptual characteristics are mapped for the entire range, using repeated randomly derived waveforms (segments from Gaussian noise) as model stimuli. Contributions from temporal domain (neural periodicity) analysis extend from about .5 through 5,000 Hz and from frequency domain (neural place) analysis from roughly 50 through 16,000 Hz. Within the range of overlapping analyses (50 through 5,000 Hz), it is difficult to separate the effects of temporal cues from place cues. However, by using low-frequency acoustic iteration from 1 through 16 Hz, we were able to study temporal analysis in the absence of place cues to repetition. New perceptual phenomena are reported for the “infrapitch” produced by “infratones,” some of which are analogous to phenomena observed for the pitch produced by tones. It appears useful for theory to consider pitch and infrapitch as a single topic: the perception of acoustic iterance.

Auditory contralateral induction: An early stage in binaural processing

Binaural interaction was studied using headphones presenting signals (tones or filtered speech) to one ear and noises of various spectral compositions to the other. Every half-second, the sides receiving the signal and noise were reversed. The noise was always perceived to alternate from side to side, but the signal appeared to be stationary and diffusely localized about the midsaggital plane when the noise contained the spectral components of the signal at appropriate intensity levels. This delateralization of a monaural signal results from a process called “contralateral induction” (CI). Additional observations indicate that CI corresponds to an early stage in binaural interaction which generally escapes notice because of further perceptual processing.