A neural mapping hypothesis to explain why velar stops have an allophonic split.

Velar stops are phonetically characterized by two allophones-a palatal "g" when produced in front-vowel contexts and a velar "g" when produced in back-vowel contexts. Acoustic characterizations of velar stops based on plotting the onset and offset frequencies of the second formant transition clearly reveal two acoustically separated allophonic groups. Using cross-linguistic acoustic data, it is shown that alveolar stops fall precisely within the acoustic void created by the velar allophones. A neural-based mapping hypothesis is put forth claiming that the bimodal distribution of velars is an instance of articulatory behavior acting in the service of auditory representation.

An acoustic analysis of the development of CV coarticulation: a case study.

This study analyzed stop consonant-vowel productions from babbling to meaningful speech in a single female child spanning the period from age 7 months to age 40 months. A total of 7,888 utterances (3,103 [bV], 3,236 [dV], and 1,549 [gV]) were analyzed to obtain frequencies at F2 onset and F2 at vocalic center for each utterance. A linear regression line ("locus equation") was fit to the cluster of F2 coordinates per stop place category produced during each month. The slope of the regression lines provided a numerical index of vowel-induced coarticulation on consonant productions. Labial, alveolar, and velar CV productions followed distinct articulatory paths toward adult-like norms of coarticulation. Inferences about the gradual emergence of segmental independence of the consonant and vowel in the three stop place environments were made from locus equation scatterplots and mean F2 onset and F2 midvowel frequencies obtained across babbling, early words, and natural speech.

The effect of speaking style on a locus equation characterization of stop place of articulation.

Locus equations were employed to assess the phonetic stability and distinctiveness of stop place categories in reduced speech. Twenty-two speakers produced stop consonant + vowel utterances in citation and spontaneous speech. Coarticulatory increases in hypoarticulated speech were documented only for /dV/ and [gV] productions in front vowel contexts. Coarticulatory extents for /bV/ and [gV] in back vowel contexts remained stable across style changes. Discriminant analyses showed equivalent levels of correct classification across speaking styles. CV reduction was quantified by use of Euclidean distances separating stop place categories. Despite sensitivity of locus equation parameters to articulatory differences encountered in informal speech, stop place categories still maintained a clear separability when plotted in a higher-order slope x y-intercept acoustic space.

Linear correlates in the speech signal: the orderly output constraint.

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.

The perceptual relevance of locus equations.

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.

An investigation of stop place of articulation as a function of syllable position: a locus equation perspective.

Locus equations were employed to phonetically describe stop place categories as a function of syllable-initial, -medial, and -final position. Ten speakers, five male and five female, produced a total of 2700 CVC and 4500 VCV utterances that were acoustically analyzed to obtain F2 onset, F2 vowel, and F2 offset frequencies for locus equation regression analyses. In general, degree of coarticulation, as indexed by locus equation slope, was reduced for post-vocalic (VC) stops relative to pre-vocalic stops (pooled data from initial and medial positions), but significant differences were observed as a function of stop consonant. All stops showed significantly reduced R2 values and increased standard errors of estimate for VC relative to CV productions. Separability of stop place categories in a higher-order slope X y-intercept acoustic space also diminished for VC vs CV stop productions. The degradation of classic locus equation form (high correlation and linearity) for VC relative to CV productions was attributed to greater articulatory precision in the production of pre-vocalic compared to post-vocalic stops. This greater articulatory precision was interpreted as reflecting a greater need to normalize vowel context-induced variability of the F2 transition for syllable onset relative to final stops. The decline in acoustic lawfulness of syllable-final stops is discussed in terms of coarticulatory interactions and expected perceptual correlates.

Locus equations as phonetic descriptors of consonantal place of articulation.

This experiment tested whether locus equation coefficients, slope, and y-intercept could serve as indices of place of articulation for obstruents sharing the same place of articulation across different manner classes. Locus equations for 22 speakers were derived from CV/t/ words with initial voiced stop /d/, voiceless aspirated stop /t/, nasal /n/, voiced fricative /z/, and voiceless fricative /s/ preceding 10 vowel contexts. Post hoc tests revealed /d/ = /z/ = /n/ for slope means. Voiced /d/ and voiceless /t/ were also equivalent when F2 transition onset measurement points were equated. Scatterplots of locus equation coefficients revealed three nonoverlapping and distinct clusters when the diverse coronal group was compared with labials and velars. A discriminant analysis using slope and y-intercept as predictors successfully categorized all five coronals into one alveolar group with 87.1% accuracy. The collective results support the contention that locus equations can serve as effective phonetic descriptors of consonant place of articulation across manner classes.

Consonant-vowel interdependencies in babbling and early words: preliminary examination of a locus equation approach.

Consonant-vowel productions at two distinct stages of language development were studied in a single female child. At 12 months canonical babbling syllables (N = 144) identified by a panel of listeners as comprising [bV], [dV], and [gv] tokens were acoustically analyzed by measuring F2 transition onset and F2 midvowel frequencies and plotting their relationship as locus equations for each stop category. A regression analysis performed on these scatterplots revealed differential slopes and y-intercepts as a function of stop place. The same analysis was performed 9 months later on CV utterances (N = 243) produced as syllable-initial segments of real words by the same child. Whereas labial and velar locus equation parameters moved toward more adult-like values, alveolar slope and y-intercept moved away from adult values and more in the direction of decreased coarticulation between vowel and consonant. There was greater scatter of data points around the regression line for production of words compared to babbling. These results are compared to locus equations obtained from 3-5-year-olds and adults. Locus equations appear to be useful as an empirical developmental probe to document how CV productions gradually approach adult categorical standards.

Locus equations derived from compensatory articulation.

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.

An investigation of vowel organization in speakers with severe and profound hearing loss.

Vowel auditory formant distances were obtained from speakers with hearing loss to investigate how perceptual constraints affect the contrastiveness and intelligibility of their spoken vowels. These distances were evaluated in relation to the 3-Bark critical distance principle for vowel height and place as described by Syrdal (1985) and Syrdal and Gopal (1986). Seven speakers with profound hearing loss, 10 with severe hearing loss, and seven with normal hearing produced the vowels /u/, /i/, /I/, /ae/, /a/, and /--/ in an /hVt/ context. Vowel formants and fundamental frequencies were obtained with acoustic spectrographic and LPC analysis and converted to Bark values to establish auditory formant distances. Confusion matrices were constructed from normal listeners' identifications of recorded vowel productions. When frequency data were transformed to a Bark auditory scale, increasing convergence of vowel targets was obtained with increase in hearing loss. Percent correct identifications of the vowels produced by the three groups reflected speaker group differences seen in vowel contrastiveness/overlap in auditory phonetic space. Four levels of performance based on error incidence and type were determined. F1-F0 by F3-F2 Bark distance coordinate plots of a given speaker's vowel space reflected the differential intelligibility scores shown by confusion matrices of individual speakers from the four performance levels. Vowel organization by speakers with hearing loss was influenced by (a) formant critical distance, and (b) formant audibility. The least audible formants, F2 and F3, showed the greatest effects of severe and profound hearing loss. F1 and F0 showed further change with the most profound losses and revealed individual differences as well.

The perception and production of rhyme in normal and developmentally apraxic children.

The phonological competence of four children, aged 5-7 years old, who demonstrated a cluster of symptoms consistent with a diagnosis of developmental apraxia of speech (DAS), was contrasted to that of four normal children. Four rhyming tasks were used to assess the ability of the children to both spontaneously generate rhyming words to targets and to judge the (in)appropriateness of a rhyme in both a word series and forced-choice word pairs. The DAS children revealed a severe deficit in rhyming ability across all tasks and had rhyming abilities markedly inferior to those shown by normal children. The rhyming results were interpreted as possibly indicative of an impoverishment of an internalized phonemic representation system, which precludes accessing and evoking the needed sound image for the vowel + coda for a rhyme. These results, while preliminary in nature, lend support to a conceptualization of DAS as a fundamental disorder of the segmental phonological level of language that impacts on all hierarchically relevant language components. The hypothesis that the underlying etiology of DAS is a developmental dysmorphology of the neural substrates that mediate such basic phonological representational structure is discussed.

A cross-linguistic investigation of locus equations as a phonetic descriptor for place of articulation.

A previous study [H. Sussman, H. McCaffrey, and S. Matthews, J. Acoust. Soc. Am. 90, 1309-1325 (1991)] of American English CV coarticulation showed a remarkably linear relationship between onset frequencies of F2 transitions, plotted on the y axis, in relation to the F2 midvowel "target" frequencies, plotted on the x axis, for CVC tokens with initial [b d g] and ten medial vowel contexts. Slope and y-intercept values of regression functions fit to these scatterplots ("locus equations") were shown to serve as statistically powerful phonetic descriptors of place of articulation. The present study extends the locus equation metric to three additional languages--Thai, Cairene Arabic, and Urdu--having both two and four place contrasts for syllable-initial voiced stops. A total of 14 speakers (Thai = 6, Arabic = 3, Urdu = 5) produced 1740 CVC tokens that were acoustically analyzed using MacSpeech Lab II. Strong linear regression relationships were found for every stop category across all speakers. Slopes and y intercepts systemically varied as a function of place of articulation. Cross-language comparisons of stop place categories were performed but variability of slope and y intercept means tempered conclusions concerning the existence of CV "phonetic hot spots."

Locus equations as an index of coarticulation for place of articulation distinctions in children.

Locus equations were investigated as a phonetic index for children's production of stop + vowel tokens. Locus equations are straight-line regression fits to data points formed by plotting onsets of F2 transitions along the ordinate and their corresponding midvowel nuclei along the abscissa. Such functions for adult speech have been found to be extremely linear with slope and y-intercept values contrastively distinctive across place of articulation. Sixteen children, aged 3-5 years, produced /bVt/, /dVt/, and /gVt/ tokens embedded in a carrier phrase and repeated in randomized order a minimum of three times. Six medial vowel contexts were used [i, I, ae, [symbol: see text], a, u]. Both individual and group mean scatterplots were extremely linear and highly remniscent of adult prototypes. While labial and velar slopes exhibited some degree of overlap, labial versus alveolar and alveolar versus velar slopes were significantly different. All y-intercepts as a function of place of articulation were significantly different. Compared to adult norms, intersubject variability of slope and y-intercept ranges were greater for children. Locus equations can provide a phonetic descriptor for a child's attainment of stop place categories seeking to achieve the adult standard of a balance between coarticulatory adjustments and contrastive distinctiveness.

The representation of stop consonants in three-dimensional acoustic space.

An acoustic study of English initial voiced stops showed that the three place categories are well distinguished on the basis of three properties: second-formant onset frequency (F2 onset), second-formant frequency of the vowel nucleus (F2), and third-formant onset frequency (F3 onset). Ten male and 10 female subjects produced multiple tokens of /bVt/, /dVt/, and /gVt/ with ten different medial vowels. Three-dimensional scatterplots were generated using the above acoustic properties as coordinates. Three nonoverlapping clusters corresponding to the three place categories were evident across all subjects. Discriminant analyses based on F2 onset, F2 vowel, and F3 onset as predictor variables showed a mean correct categorization rate of 84.5% when tested within gender groups. The graphic and statistical results are interpreted as indicating that phonetic categories may be defined and represented in purely acoustic terms if variably valued multiple acoustic properties are relationally examined.

Acoustic correlates of the front/back vowel distinction: a comparison of transition onset versus "steady state".

This study investigated whether F2 and F3 transition onsets could encode the vowel place feature as well as F2 and F3 "steady-state" measures [Syrdal and Gopal, J. Acoust. Soc. Am. 79, 1086-1100 (1986)]. Multiple comparisons were made using (a) scatterplots in multidimensional space, (b) critical band differences, and (c) linear discriminant functional analyses. Four adult male speakers produced /b/(v)/t/, /d/(v)/t/, and /g/(v)/t/ tokens with medial vowel contexts /i,I, E, ey, ae, a, v, c, o, u/. Each token was repeated in a random order five times, yielding a total of 150 tokens per subject. Formant measurements were taken at four loci: F2 onset, F2 vowel, F3 onset, and F3 vowel. Onset points coincided with the first glottal pulse following the release burst and steady-state measures were taken approximately 60-70 ms post-onset. Graphic analyses revealed two distinct, minimally overlapping subsets grouped by front versus back. This dichotomous grouping was also seen in two-dimensional displays using only "onset" data as coordinates. Conversion to a critical band (bark) scale confirmed that front vowels were characterized by F3-F2 bark differences within a critical 3-bark distance, while back vowels exceeded the 3-bark critical distance. Using the critical distance metric onset values categorized front vowels as well as steady-state measures, but showed a 20% error rate for back vowels. Front vowels had less variability than back vowels. Statistical separability was quantified with linear discriminant function analysis. Percent correct classification into vowel place groups was 87.5% using F2 and F3 onsets as input variables, and 95.7% using F2 and F3 vowel. Acoustic correlates of the vowel place feature are already present at second and third formant transition onsets.

A reassessment of the time-sharing paradigm with ANCOVA.

The purpose of this note is to follow up on the suggestion by Willis and Goodwin (1987) (Neuropsychologia, 25, 719-724) that analysis of covariance is the proper statistical control to eliminate the confound provided by the "initial values problem" inherent in the time-sharing paradigm. The discrepancy between baseline tapping speed of the dominant versus nondominant hand can affect concurrent tapping disruption rates independently of laterality factors. A re-analysis of previous time-sharing data showed that significant laterality main effects and interactions as shown by ANOVA are eliminated with ANCOVA procedures. Thus, when the initial-values problem is statistically controlled, the laterality effect of asymmetrical hand disruption disappears. This result seriously questions the continued use of the time-sharing paradigm as a behavioral index of language lateralization.

Neural coding of relational invariance in speech: human language analogs to the barn owl.

The ability to form perceptual equivalence classes from variable input stimuli is common in both animals and humans. Neural circuitry that can disambiguate ambiguous stimuli to arrive at perceptual constancy has been documented in the barn owl's inferior colliculus where sound-source azimuth is signaled by interaural phase differences spanning the frequency spectrum of the sound wave. Extrapolating from the sound-localization system of the barn owl to human speech, 2 hypothetical models are offered to conceptualize the neural realization of relative invariance in (a) categorization of stop consonants/b, d, g/ across varying vowel contexts and (b) vowel identity across speakers. 2 computational algorithms employing real speech data were used to establish acoustic commonalities to form neural mappings representing phonemic equivalence classes in the form of functional arrays similar to those seen in the barn owl.

The dual task paradigm: speech dominance or manual dominance?

Two hundred and sixty monolinguals divided into subgroups based on gender, handedness, and familial left-handedness, were given a concurrent task, verbal-manual interference paradigm. The primary purpose of the study was to test whether the dual task interference effects were more related to hemispheric speech or manual dominance factors. The dominant hand, regardless of handedness, underwent relatively more tapping interference than the nondominant hand. The obtained results seriously question the basic assumptions underlying the interpretation of the dual task paradigm as an assessment index for hemispheric language lateralization. A possible explanation of this result that is in keeping with known hemispheric language representation proportions in left-handers is offered based on left hemisphere ipsilateral control of the left hand.