Evidence For Categorical Perception Research Articles

Perception of blended timbres in music

We examined perception of artificial timbre blending using recordings of two actual instruments. In Experiment 1, participants heard stimuli comprising different proportions of sounds from an oboe and a trumpet, constructed using both a linear and a logarithmic algorithm, and judged the degree of blending. In Experiment 2, participants chose between an oboe and a trumpet in each blend condition. In both experiments, participants were able to track the degrees of blending between the two anchor points quite accurately. In Experiment 3, participants matched test blends to two target blends in an ABX design and showed no evidence for categorical perception of oboe and trumpet timbres in their judgments. Further, participants with and without musical training showed similar patterns of responding. The findings suggest a high level of sensitivity for timbre coding in auditory perception and also have implications for timbre manipulation as a compositional device and sound morphing techniques.

Dissociating the performance of cortical and subcortical patients on phonemic tasks

To assess cortical and subcortical contributions to phonemic processing, patients with left frontal, temporal–parietal, or cerebellar lesions as well as those with Parkinson’s disease were tested on phonemic identification and production tasks. In Experiment 1, patients and controls were asked to identify syllables on both a voicing and place of articulation continuum. Subcortical patients were relatively unimpaired at this task whereas cortical patients were less accurate at identifying the endpoints of both continua and exhibited little evidence of categorical perception. For Experiment 2, controls and patients were asked to produce syllables. Subcortical patients were able to produce contrastive voice onset times (VOTs) for voicing cognates although VOT of the voiceless phoneme was more variable for cerebellar patients. Cortical patients showed greater overlap in the production of both VOT and formant transition intervals. These results are discussed in terms of the type of computations hypothesized to originate from each neural area.

Major/Minor Triad Identification and Discrimination by Musically Trained and Untrained Listeners

The extent to which a computer-synthesized continuum of major to minor triads was categorically perceived was examined using labeling and discrimination tests. The 32 listeners varied widely in " musicality," assessed by an objective test of basic musical skills. There was a strong positive relationship between musicality and ability to label the major/minor continuum consistently ( measured by the slope of the labeling function). Overall discrimination performance varied only weakly with musicality, although the pattern of discrimination performance across the continuum differed strongly among three listener subgroups, distinguished on the basis of musicality. The most "musical" listeners showed a close relationship between the position of the discrimination peak and the category boundary calculated from the labeling function, a strong indicator of categorical perception. On similar criteria, the evidence for categorical perception was nonexistent in the least musical listeners and moderate in an intermediate group. From the evidence that the extent of categorical perception appears to vary in a graded fashion with the degree of musicality, we conclude that categorical perception can arise primarily through a process of learning.

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.

Categorical or continuous speech perception: A new test

An important question in speech perception is whether listeners have continuous or categorical information about the acoustic signal in speech. Most traditional experimental studies have been interpreted as evidence for categorical perception. It is also argued in the present paper that more recent results taken as evidence against categorical perception are not unequivocally negative. Accordingly, further tests between continuous and categorical views of speech perception are necessary. In the present experiments, listeners were asked for continuous rather than discrete judgments in order to provide a more direct answer to this question. Subjects were asked to rate speech sounds according to where they fell on a particular speech continuum. The continua consisted of stop consonants varying in place (/bæ/to/dæ/) or voicing (/bæ/to/pæ/) or a vowel continuum varying from /i/to/I/. The distributions of rating responses of individual subjects were used to test quantitative models of categorical and continuous perception of acoustic features in speech. The results provide strong evidence against the categorical perception of speech contrasts, and contribute additional evidence for the role of continuous acoustic feature information in speech processing.

Categorical results do not imply categorical perception.

Categorical perception refers to the ability to discriminate between- but not within-category differences along a stimulus continuum. Although categorical perception was thought to be unique to speech, recent studies have yielded similar results with nonspeech continua. The results are usually interpreted in terms of categorical, as opposed to continuous, perception of both speech and nonspeech continua. In contrast, we argue that these continua are perceived continuously, although they are characterized by relatively large increases in discrim-inability near the category boundary. To support this argument, the amplitude rise time of a tone was varied to produce either an increase or a decrease in the intensity during the initial portion of the tone. A bipolar continuum of onset times increasing and decreasing in amplitude yielded traditional categorical results. However, when only half of this continuum was tested, subjects perceived the same sounds continuously. The finding of traditional categorical results along the bipolar continuum, when the sounds were shown to be perceived continuously in another context, argues against the use of traditional categorical results as evidence for categorical perception.

Categorical perception--phenomenon or epiphenomenon: evidence from experiments in the perception of melodic musical intervals.

Categorical perception was investigated in a series of experiments on the perception of melodic musical intervals (sequential frequency ratios). When procedures equivalent to those typically used in speech-perception experiments were employed, i.e., determination of identification and discrimination functions for stimuli separated by equal physical increments), musical intervals were perceived categorically by trained musicians. When a variable-step-size (adaptive) discrimination procedure was used, evidence of categorical perception (in the form of smaller interval-width DL's for ratios at identification category boundaries than for ratios within categories), although present initially, largely disappeared after subjects had reached asymptotic performance. However, equal-step-size discrimination functions obtained after observers had reached asymptotic performance in the adaptive paradigm were not substantially different from those initially obtained. The results of other experiments imply that this dependence of categorical perception on procedure may be related to differences in stimulus uncertainty between the procedures. An experiment on the perception of melodic intervals by musically untrained observers showed no evidence for the existence of "natural" categories for musical intervals.

Disruption of categorical processing in music

Musicians judged 13 tonal intervals under magnitude estimation instructions. The intervals were spaced in 20-cent steps and covered a range of three semitone categories. In experiment I, the subjects were naive with respect to the exact nature of the stimulus set, and under these conditions, evidence for categorical perception was obtained. The psychometric functions were well predicted by these subjects identification of the same stimuli. Stimuli in different categories were judged as perceptually distinct, but the judgment of intonation within a category was poor. That is, these subjects were unable to distinguish among sharp, in-tune, and flat stimuli with the same interval name. In Experiment II, subjects were given specific information concerning the nature of the stimuli. Their judgment of within-category differences failed to improve, and their assignment of the stimuli to the three interval categories became more variable. It appears that musicians are poor at judging within-category differences in intonation, and that attempting to do so disrupts the processing of categorical information.

A Comparison of Speech Sound Discrimination in Man and Monkey

Enhanced discrimination between speech sounds across phoneme boundaries relative to discrimination within phoneme categories based on identification defines categorical perception. The present experiment compared man and Old World monkey on speech sound discrimination and particularly on the ability to perceive categorically. Difference thresholds for three humans and four monkeys were determined on a synthetic speech sound continuum from “BA” to “DA.” Humans were instructed and monkeys conditioned with food reinforcement to indicate a stimulus change from a repetitive “BA” in a direction towards the other phoneme “DA,” or, from “DA” towards “BA.” In addition to percent correct discrimination data, reaction times to stimulus change were recorded. Thresholds, defined as 50% correct discrimination between speech sounds, were typically smaller for humans than for monkeys and tended to occur intraphonemically for humans, but at phoneme boundaries for monkeys. Human reaction times, however, were longer near threshold than were those for monkeys, presumably due to increased time required for discriminating within a phoneme category. In this experiment, therefore, the reaction time data provided better evidence for categorical perception in humans than did percent correct discrimination data. [Research supported by grants from NIH and University of Michigan.]

Speech perception in infants.

Discriminiationi of synthetic speech sounds was studied in 1- and 4-month-old infants. The speech sounds varied along an acoustic dimension previously shown to cue phonemic distinctions among the voiced and voiceless stop consonants in adults. Discriminability was measured by an increase in conditioned response rate to a second speech sound after habituation to the first speech sound. Recovery from habituation was greater for a given acoustic difference when the two stimuli were from different adult phonemic categories than when they were from the same category. The discontinuity in discrimination at the region of the adult phonemic boundary was taken as evidence for categorical perception.