Martha W. Burton

The Role of Segmentation in Phonological Processing: An fMRI Investigation

Phonological processes map sound information onto higher levels of language processing and provide the mechanisms by which verbal information can be temporarily stored in working memory. Despite a strong convergence of data suggesting both left lateralization and distributed encoding in the anterior and posterior perisylvian language areas, the nature and brain encoding of phonological subprocesses remain ambiguous. The present study used functional magnetic resonance imaging (fMRT) to investigate the conditions under which anterior (lateral frontal) areas are activated during speech-discrimination tasks that differ in segmental processing demands. In two experiments, subjects performed same/ different judgments on the first sound of pairs of words. In the first experiment, the speech stimuli did not require overt segmentation of the initial consonant from the rest of the word, since the different pairs only varied in the phonetic voicing of the initial consonant (e.g., dip-tip). In the second experiment, the speech stimuli required segmentation since different pairs both varied in initial consonant voicing and contained different vowels and final consonants (e.g., dip-ten). These speech conditions were compared to a tone-discrimination control condition. Behavioral data showed that subjects were highly accurate in both experiments, but revealed different patterns of reaction-time latencies between the two experiments. The imaging data indicated that whereas both speech conditions showed superior temporal activation when compared to tone discrimination, only the second experiment showed consistent evidence of frontal activity. Taken together, the results of Experiments 1 and 2 suggest that phonological processing per se does not necessarily recruit frontal areas. We postulate that frontal activation is a product of segmentation processes in speech perception, or alternatively, working memory demands required for such processing.

Acoustic and perceptual characteristics of voicing in fricatives and fricative clusters

Several types of measurements were made to determine the acoustic characteristics that distinguish between voiced and voiceless fricatives in various phonetic environments. The selection of measurements was based on a theoretical analysis that indicated the acoustic and aerodynamic attributes at the boundaries between fricatives and vowels. As expected, glottal vibration extended over a longer time in the obstruent interval for voiced fricatives than for voiceless fricatives, and there were more extensive transitions of the first formant adjacent to voiced fricatives than for the voiceless cognates. When two fricatives with different voicing were adjacent, there were substantial modifications of these acoustic attributes, particularly for the syllable-final fricative. In some cases, these modifications leads to complete assimilation of the voicing feature. Several perceptual studies with synthetic vowel-consonant-vowel stimuli and with edited natural stimuli examined the role of consonant duration, extent and location of glottal vibration, and extent of formant transitions on the identification of the voicing characteristics of fricatives. The perceptual results were in general consistent with the acoustic observations and with expectations based on the theoretical model. The results suggest that listeners base their voicing judgments of intervocalic fricatives on an assessment of the time interval in the fricative during which there is no glottal vibration. This time interval must exceed about 60 ms if the fricative is to be judged as voiceless, except that a small correction to this threshold is applied depending on the extent to which the first-formant transitions are truncated at the consonant boundaries.

Grammatical class and imageability in aphasic word production: their effects are independent

Abstract This paper addresses the recent claim that grammatical class differences found among aphasic patients may reflect semantic factors such as ease of imageability rather than lexical/syntactic class. Nouns and verbs equated for rated imageability, frequency and length were elicited as completions for spoken sentences. Five aphasic patients with significantly better production of nouns than verbs in picture naming continued to show a significant grammatical class effect in the completion task. Two patients with significant imageability effects in oral reading continued to show imageability effects in sentence completion, but only one of these patients showed any difficulty producing verbs. Inspection of the individual patient data indicated that either grammatical class, or imageability, or both variables may affect patient performance, but that their effects are independent of one another.

Effects of Speaking Rate on the Singleton/Geminate Consonant Contrast in Italian

A production and a perception experiment were conducted to investigate the effect of speaking rate on the singleton/geminate consonant contrast in Italian. Acoustic analyses were performed of labial and dental singleton and geminate consonants produced in words spoken in isolation and in a sentential context at both a slow and a fast rate of speech. Closure duration was found to discriminate between the two categories of consonants within a given speaking rate. For one speaker, at a fast speaking rate the distribution of closure duration of geminate consonants was identical to the distribution of closure duration of singleton consonants produced in isolation. However, a measure reflecting the relation between two durations, the ratio between consonant duration and preceding vowel duration, discriminated between singletons and geminates both within and across speaking rates. The perception experiment tested whether manipulation of the consonant to vowel ratio would result in perceptual shifts. Results indicated that changing the ratio did affect perception of the geminate/singleton distinction. However, these effects varied greatly across listeners. Taken together, these findings provide support for the view that there are stable acoustic properties corresponding to phonetic features in that even though one type of temporal information, closure duration, is greatly affected by changes in speaking rate, a higher-order relational measure remains constant.

Functional neuroanatomy of segmenting speech and nonspeech.

This fMRI study investigates the extent to which frontal brain activation observed during speech discrimination is due to processes specific to articulatory recoding of speech or is due to segmenting and comparing portions of any continuous acoustic stimuli. A set of ten participants performed same/different judgments on the first speech sound in pairs of consonant-vowel-consonant (CVC) syllables or the first tone in pairs of sequences of three tones. Comparison between speech and tone tasks demonstrated significant bilateral temporal activation, which was associated with differences in perceptual analysis of complex acoustic stimuli. Both speech and tone tasks also showed significant activation in the left inferior frontal gyrus (IFG) compared to baseline. These results suggest that portions of the left prefrontal cortex may be important for selecting and comparing auditory stimuli for decision, but may not be specifically related to speech.

Lexical effects on the phonetic categorization of speech: The role of acoustic structure.

This study examines the extent to which acoustic parameters contribute to lexical effects on the phonetic categorization of speech. Experiment 1 was designed to replicate previous findings. Two test continua were created varying in voice onset time. Results of both identification and reaction time (RT) range data showed an effect of lexical status at the phonetic boundary, but only in the slowest RT ranges, suggesting that lexical effects on phonetic categorization are postperceptual. Experiment 2 explored whether the lexical effect would emerge when the stimulus continua more nearly approximated the parameter values of natural speech. Both identification and RT range data indicated that the lexical effect disappeared. These results suggest that without attention to the acoustic structure of the stimuli, the role of top-down processing in phonetic categorization may be overemphasized.

An fMRI Investigation of Speech and Tone Segmentation

Recent research strongly indicates that phonological tasks activate a subregion of the inferior frontal gyrus. The purpose of the present fMRI study was to investigate the extent to which activation of this region during phonological processing is due to speech processes per se such as articulatory recoding or to other cognitive task demands such as working memory. Thus, we compared activation patterns during segmentation of speech and tone sequences to a tone discrimination task. In particular, participants performed same/different judgments on pairs of words, pseudowords, and tone sequences that required segmentation of a continuous acoustic signal as well as tone pairs that did not require segmentation. Accuracy and reaction time data showed that speech and tone sequence segmentation conditions patterned more similarly to each other than to tone discrimination pairs. Analyses of group data revealed strong activation of the region at the border of the left inferior and middle frontal gyrus for all three segmentation conditions compared to tone discrimination, but no consistent differences were observed when word and pseudoword segmentation were directly contrasted. Analyses of individual subjects indicated that a large number of participants activated a small area of the middle frontal gyrus during the speech conditions compared to the sequences. These results suggest that a significant portion of active frontal areas is recruited for extracting acoustic information and maintaining it in memory for decision. However, some regions at the border of the inferior/middle frontal gyrus may be unique to speech segmentation.

Effects of subphonetic and syllable structure variation on word recognition

This study explored whether natural acoustic variations as exemplified by either subphonetic changes or syllable structure changes affect word recognition processes. Subphonetic variations were realized by differences in the voice-onset time (VOT) value of initial voiceless stop consonants, and syllable structure variations were realized by vowel deletion in initial unstressed syllables in multisyllable words. An auditory identity priming paradigm was used to determine whether the amount of facilitation obtained to a target stimulus in a lexical decision task was affected by the presence of these acoustic variations in a prime stimulus. Results revealed different patterns for the two types of variability as a function of lexical status. In the case of subphonetic variations, shortening of VOT resulted in reduced facilitation for words but not for nonwords, whereas in the case of syllable structure variation, vowel deletion in an unstressed syllable resulted in reduced facilitation for nonwords and increased facilitation for words. These findings indicate that subphonetic variability interferes with word recognition, whereas syllable structure variability does not, and that this effect is independent of the magnitude of the acoustic difference between a citation form and its variant. Furthermore, the results suggest that the lexical status of the target item plays a crucial role in the processing of both types of variability. Results are considered in relation to current models of word recognition.

Production of nouns and verbs in aphasia: Effects of elicitation context

This study investigated the ability of 10 aphasic speakers and 9 normal controls to produce unambiguous, frequency-matched nouns and verbs in four elicitation conditions. Two auditory conditions included naming to definition and sentence completion; two picture conditions preceded the presentation of the picture with an auditory cue consisting of a question (what is the action shown here?) or a sentence completion (this is a picture of the action to...). Patients were grouped in terms of whether they demonstrated only word retrieval problems (anomia), or also showed difficulty with sentence comprehension and production. Contrary to expectations, there were no reliable effects of elicitation condition on performance. Although both groups of aphasic speakers found verbs more difficult than nouns to retrieve across conditions, the sentence production-impaired group showed a more severe impairment of verb production that was reliable for individual subjects. Results reinforce the importance of grammatical class as a factor in the word retrieval impairments found in aphasia.

Understanding the Role of the Prefrontal Cortex in Phonological Processing.

Lesion studies have demonstrated impairments of specific types of phonological processes. However, results from neuropsychological studies of speech sound processing have been inconclusive as to the role of specific brain regions because of a lack of a one‐to‐one correspondence between behavioural patterns and lesion location. Functional neuroimaging studies have contributed more detailed information about the involvement of specific brain regions in a wide range of phonological tasks. A framework developed by Hickok and Poeppel to account for these neuropsychological and neuroimaging results is evaluated in light of a series of phonological studies in which cognitive load is manipulated by changing the acoustic properties and lexical status of stimuli, as well as the type of phonological judgement. Overall, the findings for speech stimuli are consistent with the view that tasks that require increased articulatory recoding result in increased activation of the posterior aspect of the inferior frontal gyrus (BA 44). However, similar activation patterns for tone sequences as compared to speech may challenge whether the recoding is speech‐specific. Implications of these investigations for future neuroimaging studies of individuals with aphasia are discussed.