Limits of auditory patterns discrimination for patterns with various durations and numbers of components.

In three experiments, listeners' abilities to detect changes in randomly generated tonal sequences were determined for sequences or "patterns" ranging in total duration from 62.5 ms to 2 s. Experiment 1 utilized an adaptive-tracking procedure, with n, the number of pattern components, as the dependent variable, and included a variety of spectral and temporal discrimination tasks with isochronous patterns. When the to-be-detected change was the only variation on a given dimension (e.g., the presence or location of a brief pause), patterns were discriminable when the absolute duration of the changed element, or pause, exceeded a critical value. However, when each pattern component varied on the dimension of the to-be-detected change (e.g., frequency), discriminability was strongly related to the number of tones in the pattern, and only weakly to the durations of either the target components or the total pattern. This dependence of discrimination performance on n was also demonstrated with anisochronous patterns in experiment 2. Experiment 3 revealed the same dependence of performance on the number of components per pattern as did experiments 1 and 2, but with delta f/f as the dependent variable, rather than n. The number of pattern components and the proportional duration of the target components, relative to total pattern duration, were confounded in these experiments. Additional research is therefore required to determine whether number or proportional target-tone duration is the primary determinant of pattern discriminability.

Auditory temporal acuity in relation to category boundaries; speech and nonspeech stimuli.

Experiments were conducted to determine the underlying resolving power of the auditory system for temporal changes at the onset of speech and nonspeech stimuli. Stimulus sets included a bilabial VOT continuum and an analogous nonspeech continuum similar to the "noise-buzz" stimuli used by Miller et al. [J. Acoust. Soc. Am. 60, 410-417 (1976)]. The main difference between these and earlier experiments was that efforts were made to minimize both the trial-to-trial stimulus uncertainty and the cognitive load inherent in some of the testing procedures. Under conditions of minimal psychophysical uncertainty, not only does discrimination performance improve overall, but the local maximum, usually interpreted as evidence of categorical perception, is eliminated. Instead, discrimination performance for voice onset time (VOT) or noise lead time (NLT) is very accurate for short onset times and generally decreases with increasing onset time. This result suggests that "categorization" of familiar sounds is not the result of a psychoacoustic threshold (as Miller et al. have suggested) but rather of processing at a more central level of the auditory system.

Central factors in the discrimination and identification of complex sounds.

The paper by Jesteadt and Norton [J. Acoust. Soc. Am. 78, 365-374 (1985)] described certain similarities between psychophysical and physiological measures of frequency selectivity. Although the hearing of naturally occurring sounds is dependent upon these peripherally based relationships, recent research has shown that other, more central, processes are also strongly involved in the perception of complex acoustic events. The present paper describes research on the discrimination of complex sounds other than those of speech or music. In contrast to the more peripherally determined limits on the listener's sensitivity for single tones and other simple stimuli, the processing of complex sounds requires the interaction of peripheral and central mechanisms. These issues are discussed in relation to recent studies of the responses of the cochlea to speech stimuli. It is suggested that the peripheral processor may be relatively transparent to the essential spectral-temporal properties of speech, whereas more central processing severely limits the rates and amount of information that can be extracted from complex sounds.