Locus equations were investigated as a potential metric capable of illustrating relational invariance for place of articulation in voiced initial stop consonants independently of vowel context. Locus equations are straight line regression fits to data points formed by plotting onsets of F2 transitions along the y axis and their corresponding midvowel nuclei along the x axis. Twenty subjects, 10 male and 10 female, produced/b/v/t/,/d/v/t/, and/g/v/t/tokens for ten vowel contexts. Each CVC token was repeated in a carrier phrase five times yielding 50 tokens per stop place category. Formant measures were obtained using the MacSpeech Lab II speech analysis system. Extremely linear regression functions were found characterized by distinct slopes and y intercepts as a function of place of articulation. A discriminant analysis using F2onse t and vowel frequencies as predictors showed 82%, 78%, and 67% classification rates for labial, alveolar, and velar place. Using derived slope and y-intercept values as predictors led to 100% classification into stop place categories. A neurobiologically oriented perspective on the invariance issue is developed and a brain-based recognition algorithm for stop place integrating burst and F2 trajectory cues is offered. PACS numbers: 43.71.Es, 43.71.Pc, 43.71.An INTRODUCTIONThe nature of the linguistic entity known as the "phonetic segment" and its higher-order abstraction, the "phonemic category," has long resisted an explicit specification in terms ofinstantiation in the speech signal, and ultimately, its form as a mental representation in the brain. There is even controversy as to whether linguistic units are abstract constructs (Repp, 1981; Hammarberg, 1976), or acoustic-based physical constructs possessing neurobiological reality (Sussman, 1988(Sussman, , 1989. The theoretical difficulty in accounting for the phoneme is due to its lack of transparency in the signal. The differences between a "planned segment" and an "uttered segment," as described by MacNeilage and Ladefoged (1976), succinctly capture this dilemma. The former is envisioned as discrete, static, and context-free, while the latter is characterized as overlapped, dynamic, and contextsensitive. The pervasive coarticulatory nature of speech has contributed to this theoretical puzzle known as the "noninvariance problem"--i.e., perceptual constancy despite physical variation in the signal. The "litmus test" for invariance is place of articulation in stop consonant + vowel utterances as they typify the coarticulated nature of speech.The overall purpose of this research was to present a refocused conceptualization of a traditional, but elusive, candidate for acoustic invariance for place of articulation-the second formant transition as its onset and trajectory vary with the following vowel. The departure point was to seek invariant acoustic patterns related to place of articulation, not for each individual token, but rather for an entire phonemic category. A. Searching for invariance: formant transitionsPerhaps the earliest w...