Neuroethological investigations of mammalian and avian auditory systems have documented species-specific specializations for processing complex acoustic signals that could, if viewed in abstract terms, have an intriguing and striking relevance for human speech sound categorization and representation. Each species forms biologically relevant categories based on combinatorial analysis of information-bearing parameters within the complex input signal. This target article uses known neural models from the mustached bat and barn owl to develop, by analogy, a conceptualization of human processing of consonant plus vowel sequences that offers a partial solution to the noninvariance dilemma – the nontransparent relationship between the acoustic waveform and the phonetic segment. Critical input sound parameters used to establish species-specific categories in the mustached bat and barn owl exhibit high correlation and linearity due to physical laws. A cue long known to be relevant to the perception of stop place of articulation is the second formant (F2) transition. This article describes an empirical phenomenon – the locus equations – that describes the relationship between the F2 of a vowel and the F2 measured at the onset of a consonant-vowel (CV) transition. These variables, F2 onset and F2 vowel within a given place category, are consistently and robustly linearly correlated across diverse speakers and languages, and even under perturbation conditions as imposed by bite blocks. A functional role for this category-level extreme correlation and linearity (the “orderly output constraint”) is hypothesized based on the notion of an evolutionarily conserved auditory-processing strategy. High correlation and linearity between critical parameters in the speech signal that help to cue place of articulation categories might have evolved to satisfy a preadaptation by mammalian auditory systems for representing tightly correlated, linearly related components of acoustic signals.

Locus equations are linear regressions of the onset of F2 transitions on their offsets. These functions vowel-normalize the F2 transitions such that they are able to characterize consonantal place categories. The purpose of this research was to determine if compensatory articulation due to bite blocks would alter the normally linear relationship between F2 transition onset and offset frequencies or alter the differential slopes and y intercepts of locus equations as a function of stop place. Six speakers, three male and three female, each produced /bVt/, /dVt/, and /gVt/ tokens for ten vowel contexts under normal and bite block conditions. Extremely linear and practically identical scatterplots were obtained in the two speaking conditions. No adaptation to the bite blocks was found when comparing locus equations derived from the initial versus the final bite block trial. Results are discussed in relation to the "orderly output constraint," which postulates a perceptual function for linearly related F2 transition end points within consonantal place categories.

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Identification curves were estimated for the English consonants /b,d,g/ using five-formant CV synthetic stimuli comprehensively sampling the F2 onset-F2 vowel acoustic space in the vicinity of /b,d,g/ locus equations [H. Sussman et al., J. Acoust. Soc. Am. 90, 1309-1325 (1991)]. The stimuli included 10 English monophthongal vowel contexts, 11 levels of F2 onset per vowel, and 3 levels of F3 onset orthogonally varied with the F2 variables (10 vowels x 11 F2 onsets x 3 F3 onsets = 330 stimuli). After brief training, each of six subjects, three male and three female, was presented eight trials of each of the stimuli, one or two trials per day over a period of several days. Systems of identification curves were visualized as identification surfaces situated in locus equation acoustic space and were overlaid with acoustic data from five male speakers in order to judge the degree of correspondence between perception and acoustic data. A chi square analysis was also performed in order to quantify the correspondence between the observed perception data and expected frequencies derived from the acoustic data. The results, when interpreted in terms of a dominance hierarchy hypothesis, strongly indicate F2 onset and F2 vowel, in combination, serve as important cues for stop consonant place of articulation.

Identification curves were estimated for the English consonants /b, d, g/ using five-formant CV synthetic stimuli comprehensively sampling the F2 onset×F2 vowel acoustic space in the vicinity of Sussman’s /b, d, g/ locus equations [H. Sussman et al., J. Acoust. Soc. Am. 90, 1309–1325 (1991)]. The stimuli included ten English monophthongal vowel contexts, eleven levels of F2 onset per vowel, and three levels of F3 onset orthogonally varied with the F2 variable (ten vowels×eleven F2 onsets×three F3 onsets=330 stimuli). After brief training, each subject was presented several repetitions of each of the above 330 stimuli, two repetitions per day, over a period of several days. Subjects were instructed to choose one of five identifications: Initial consonant /b/, /d/, /g/, or /w/ or ‘‘no consonant.’’ Systems of identification curves were visualized as surfaces floating over Sussman’s locus equation space in order to judge the degree of correspondence between his acoustic results and these perception results. The current perception results are also compared to those of a similar experiment utilizing two-formant syntheses [A. Liberman et al., Psychol. Monogr. 68, 1–13 (1954)].