Aditi Lahiri

The mental representation of lexical form: A phonological approach to the recognition lexicon ☆

We propose a psycholinguistic model of lexical processing which incorporates both process and representation. The view of lexical access and selection that we advocate claims that these processes are conducted with respect to abstract underspecified phonological representations of lexical form. The abstract form of a given item in the recognition lexicon is an integrated segmental-featural representation, where all predictable and non-distinctive information is withheld. This means that listeners do not have available to them, as they process the speech input, a representation of the surface phonetic realisation of a given word-form. What determines performance is the abstract, underspecified representation with respect to which this surface string is being interpreted. These claims were tested by studying the interpretation of the same phonological feature, vowel nasality, in two languages, English and Bengali. The underlying status of this feature differs in the two languages; nasality is distinctive only in consonants in English, while both vowels and consonants contrast in nasality in Bengali. Both languages have an assimilation process which spreads nasality from a nasal consonant to the preceding vowel. A cross-linguistic gating study was conducted to investigate whether listeners would interpret nasal and oral vowels differently in two languages. The results show that surface phonetic nasality in the vowel in VN sequences is used by English listeners to anticipate the upcoming nasal consonant. In Bengali, however, nasality is initially interpreted as an underlying nasal vowel. Bengali listeners respond to CVN stimuli with words containing a nasal vowel, until they get information about the nasal consonant. In contrast, oral vowels in both languages are unspecified for nasality and are interpreted accordingly. Listeners in both languages respond with CVN words (which have phonetic nasality on the surface) as well as with CVC words while hearing an oral vowel. The results of this cross-linguistic study support, in detail, the hypothesis that the listeners interpretation of the speech input is in terms of an abstract underspecified representation of lexical form.

Bengali intonational phonology

This paper proposes a phonological analysis of the Bengali intonational system, using a descriptive framework developed by Pierrehumbert (1980) and others. Our analysis bears on a number of theoretical points. We argue that the Bengali facts support a typology of intonational tones that includes only pitch accents and boundary tones, and that the docking sites for boundary tones are the phrase edges provided under the theory of the Prosodic Hierarchy (Selkirk 1980). We show that Bengali intonational contours are governed by the obligatory Contour Principle (OCP), which forbids adjacent identical tones. Underlying contours that violate the OCP are converted to permissible surface forms by a phonological rule. We also bring Bengali data to bear on a long-standing controversy concerning phrasal stress: Bengali can be shown to have a default, phonologically assigned phrasal stress pattern; thus phrasal stress assignment cannot be reduced exclusively to focus and other semantic factors.

Neurobiological Evidence for Abstract Phonological Representations in the Mental Lexicon during Speech Recognition

A central issue in speech recognition is how contrastive phonemic information is stored in the mental lexicon. The conventional view assumes that this information is closely related to acoustic properties of speech. Considering that no word is ever pronounced alike twice and that the brain has limited capacities to manage information, an opposing view proposes abstract underspecified representations where not all phonemic features are stored. We examined this proposal using event-related brain potentials, in particular mismatch negativity (MMN), an automatic change detection response in the brain that is sensitive to language-specific phoneme representations. In the current study, vowel pairs were presented to subjects, reversed as standard and deviant. Models not assuming underspecification predict equal MMNs for vowel pairs regardless of the reversal. In contrast, enhanced and earlier MMNs were observed for those conditions where the standard is not phonologically underspecified in the mental representation. This provides the first neurobiological evidence for a featurally underspecified mental lexicon.

A reconsideration of acoustic invariance for place of articulation in diffuse stop consonants: Evidence from a cross‐language study

This study explored the claim that invariant acoustic properties corresponding to phonetic features generalize across languages. Experiment I examined whether the same invariant properties can characterize diffuse stop consonants in Malayalam, French, and English. Results showed that, contrary to theoretical predictions, we could not distinguish labials from dentals, nor could we classify dentals and alveolars together in terms of the same invariant properties. We developed an alternative metric based on the change in the distribution of spectral energy from the burst onset to the onset of voicing. This metric classified over 91% of the stops in Malayalam, French, and English. In experiment II, we investigated whether the invariant properties defined by the metric are used by English-speaking listeners in making phonetic decisions for place of articulation. Prototype CV syllables--[b d] in the context of [i e a o u]--were synthesized. The gross shape of the spectrum was manipulated first at the burst onset, then at the onset of voicing, such that the stimulus configuration had the spectral properties prescribed by our metric for labial and dental consonants, while the formant frequencies and transitions were appropriate to the contrasting place of articulation. Results of identification tests showed that listeners were able to perceive place of articulation as a function of the relative distribution of spectral energy specified by the metric.

Magnetic Brain Response Mirrors Extraction of Phonological Features from Spoken Vowels

This study further elucidates determinants of vowel perception in the human auditory cortex. The vowel inventory of a given language can be classified on the basis of phonological features which are closely linked to acoustic properties. A cortical representation of speech sounds based on these phonological features might explain the surprisingly inverse correlation between immense variance in the acoustic signal and high accuracy of speech recognition. We investigated timing and mapping of the N100m elicited by 42 tokens of seven natural German vowels varying along the phonological features tongue height (corresponding to the frequency of the first formant) and place of articulation (corresponding to the frequency of the second and third formants). Auditoryevoked fields were recorded using a 148-channel whole-head magnetometer while subjects performed target vowel detection tasks. Source location differences appeared to be driven by place of articulation: Vowels with mutually exclusive place of articulation features, namely, coronal and dorsal elicited separate centers of activation along the posterior-anterior axis. Additionally, the time course of activation as reflected in the N100m peak latency distinguished between vowel categories especially when the spatial distinctiveness of cortical activation was low. In sum, results suggest that both N100m latency and source location as well as their interaction reflect properties of speech stimuli that correspond to abstract phonological features.

Distinctive features: Phonological underspecification in representation and processing

Abstract Phonological variation of any sort (determined by speech styles, phrasing, or morphophonological rules) affecting the shapes of words and morphemes are a matter of concern for theories of speech perception and language comprehension. To come to grips with parsing the speech stream, accessing the lexicon and ultimately recognizing words, both representational as well as processing issues must be considered. The central questions in the research presented here are: W hat is represented in the mental lexicon? H ow is it represented? H ow is the speech signal parsed and information mapped onto the mental lexicon? In this paper we will address four issues within the framework of our Featurally Underspecified Lexicon model (FUL): (a) our assumptions concerning distinctive feature organization defined by phonological, perceptual and acoustic constraints; (b) specification of features in the mental lexicon (based on universal and language specific requirements); (c) extracting distinctive features from the signal; (d) mapping features from the signal to the lexicon. We claim that phonological features are extracted from the variable acoustic signal based on broad acoustic properties. A three-way matching algorithm maps these features onto highly abstract phonological mental representations. We provide evidence from synchronic phonological analyses, language change, psycholinguistic and neurolinguistic data.

PALATALIZATION AND CORONALITY

Publisher Summary This chapter focuses on palatalization and coronality. It describes three major types of assimilatory processes that are often known as palatalizations. An analysis of these palatalization processes supports the characterization of vowels and consonants by a unitary set of features. In all the palatalization processes, the coronal plays a role in one way or another. Where velar consonants typically become palatoalveolar in the context of front vowels and glides, the change is characterized as a spreading of the coronal node with its dependent. Palatal secondary articulations are best understood as a spreading of [+high], where the surface phonetic form contains the unmarked palatal off-glide. The surface phonetic off-glide that accompanies these palatalized articulations is the coronal [j], which is the default glide in most languages. When [+high] spreads to all places of articulation, the coronal consonants are often articulated with strident release and have been known to develop into strident fricatives.

Cortical representation of vowels reflects acoustic dissimilarity determined by formant frequencies

We studied neuromagnetic correlates of the processing of German vowels [a], [e] and [i]. The aim was (i) to show an influence of acoustic/phonetic features on timing and mapping of the N100 m component and (ii) to demonstrate the retest reliability of these parameters. To assess the spatial configuration of the N100 m generators, Euclidean distances between vowel sources were computed. Latency, amplitude, and source locations of the N100 m component differed between vowels. The acoustically most dissimilar vowels [a] and [i] showed more distant source locations than the more similar vowels [e] and [i]. This pattern of results was reliably found in a second experimental session after at least 5 days. The results suggest the preservation of spectral dissimilarities as mapped in a F(1)-F(2) vowel space in a cortical representation.

Auditory-evoked magnetic field codes place of articulation in timing and topography around 100 milliseconds post syllable onset

This study demonstrates by means of magnetic source imaging how consonants and vowels that constitute a syllable differently affect the neural processing within the auditory cortex. We recently identified a topographically separate processing for mutually exclusive place features in isolated vowels (Obleser et al., in press). Does this mapping principle also hold for stop consonants with differing places of articulation? How is the N100m response to consonant-vowel (CV) syllables affected by the congruency of place information in the consonant and the vowel? Moreover, how is the N100m affected by coarticulation, i.e., the spreading of place features to adjacent phonemes? By systematically varying phonological information in the consonant as well as in the vowel of CV syllables, we were able to reveal a difference in N100m syllable source location along the anterior-posterior axis due to mutually exclusive places of articulation in the vowel of the syllable. We also found a change in source orientation rather than source location due to the same mutually exclusive features in the onset of the syllable. Furthermore, the N100m time course of the brain response delivered important complementary information to identify the phonological features present in the speech signal. Responses to all syllable categories originated in the perisylvian region anterior to the source of a band-passed noise stimulus. The systematic variation of both consonantal and vocalic place features and the study of their interaction on auditory processing proves to be a valuable method to gain more insight into the elusive phenomenon of human speech recognition.

Fronted Velars, Palatalized Velars, and Palatals

Articulatory and acoustic characteristics of various stop consonants in Czech, Hungarian, English, and Russian are compared: velars before back and before front vowels, palatalized velars, and palatals. The articulatory data consist of X-ray tracings and palatograms taken from the literature. The acoustic data consist of LPC spectra of brief intervals at stop release and at vowel onset. These data indicate that all of these consonant types are distinct. Contextual fronting of velars is a gradient effect, less extreme than phonemic palatilization of velars. True palatals are even further forward on the palate and contrast with contextually fronted velars before front vowels. Thus these consonant types should not be collapsed by feature systems.