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Neurolinguistics Lab

Research

Current Research Projects

  1. Sentence processing and production in healthy adults and speakers with aphasia
  2. Gaming to support aphasia rehabilitation
  3. Phonological versus phonetic planning deficits in speech production
  4. Neural correlates of verb representation and retrieval
  5. Phonological encoding in fluency disorders
  6. Modulating vocal pitch motor control through neurostimulation
  7. Asymmetric Binarity as a Cognitive Universal: The Rhythm of Syntactic Structure

  

Sentence processing and production in healthy adults and speakers with aphasia

Many speakers who present with ‘agrammatic aphasia’ after left-hemisphere brain damage show problems with building complex sentence structures in production. At the same time, any impairment to their sentence comprehension may be much more subtle and masked by strategies that use extrasyntactic information to derive meaning, such as knowledge of the world and pragmatics, but also sentence intonation. Healthy speakers also use such information to derive meaning from sentences and do not rely exclusively on word order and the syntactic relations between words and phrases in natural language use. The Neurolinguistics Lab is interested in the interplay between sentence structure (syntax) and extrasyntactic factors in sentence parsing and particularly in how the brain supports this complex process, in healthy and impaired language systems. If we know which brain areas contribute to ambiguity resolution, and how these areas work together, we may use this knowledge to stimulate and enhance language comprehension in language-impaired speakers.

In collaboration with Mike Dickey, PhD (University of Pittsburgh), Kiel Christianson, PhD (University of Illinois at Urbana-Champaign) and Catherine Anderson, PhD (McMaster’s University), we have been studying the processing of ambiguous sentences in healthy adults and speakers with aphasia, in particular the use of prosodic and pragmatic information in aid of syntactic analysis. We have conducted functional MRI studies to establish the neural correlates of successful and unsuccessful comprehension of so-called garden-path sentences (e.g. While the man hunted the deer ran into the woods), focusing on the neural signatures associated with reliance on non-syntactic (prosodic, semantic) sources of information in garden-path sentence comprehension.

 

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Gaming to support aphasia rehabilitation

Most speakers with aphasia, i.e., language problems after stroke, have difficulty with confrontation naming, due to issues with lexical retrieval (accessing words from their mental lexicon) and/or articulatory planning (apraxia of speech). From earlier studies in the field, it is expected that repetitious training of the same lexical items, or ‘drilling’, may be beneficial to learning. However, this is often not a part of clinician-delivered language therapy, not in the least because of motivational problems associated with repetitious training. It is also known that providing a rhythm to which speech output can be timed benefits naming and fluency in many speakers with aphasia and/or apraxia of speech.  A gaming environment, in which lexical items form the targets, provides an incentive to score points through accurate and timely naming, crucially coupled with instantaneous and automated feedback on naming performance. To achieve this, the program assesses the level of accuracy of a player’s response to a given cue, which makes speech recognition an essential part of the game. One technical challenge, therefore, is to achieve adequate speech recognition accuracy for aphasic speech, which is by definition less clear and well-timed than unimpaired speech. The addition of a rhythm, to which the players’ utterances should be timed, enhances the speech recognition accuracy, in addition to being beneficial to speech output training itself. In collaboration with the lab of Jijun Tang, PhD (USC, Computer Science & Engineering) and Jeremiah Shepherd, PhD (USC, Computer Science & Engineering) we are developing a game program for the improvement of naming skills in speakers with aphasia, both in terms of lexical retrieval and speech fluency.


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Phonological versus phonetic planning deficits in speech production

This line of research goes back to Dirk den Ouden’s dissertation and aims to investigate the nature and source of word form errors in speakers with aphasia and/or apraxia of speech. To what extent is it possible to distinguish speech errors that are caused by impaired lexical access, phonological planning, or phonetic planning? One way in which to approach this question is to study how word form errors may be constrained by abstract phonological domains, such as syllables and phonemes, rather than by purely articulatory, or perhaps even auditory factors. Related to this is the question to what extent hypothesized abstract phonological domains and elements are constrained by articulatory, auditory or general cognitive factors that are peripheral to the language domain (or are they?). Progress in this field will benefit from the identification of the neural correlates of phonological processing at different stages of word form generation. How can formal linguistic and psycholinguistic models of speech production be mapped onto the neurophysiology of language production? The long-term goal of this research is to lead to the development of treatment methods that target specific phonological units, in combination with neurophysiological interventions that stimulate post-stroke brain plasticity.

 

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Neural correlates of verb representations and retrieval

Verbs form the core of sentence frames and problems with verb retrieval may underlie deficits in sentence processing and production. The Neurolinguistics Lab is interested in the types of information that are accessed during verb retrieval, automatically, as an integral part of verb representation, or by association with verb semantics. For example, a verb’s subcategorization frames, in terms of its argument structure and the types of complements that a verb can take, affect the processing load that is associated with its retrieval from the mental lexicon, as evident from reaction-time and functional imaging experiments, including our own. Also, there is a question about the extent to which a verb’s mental representation includes information on semantic associations and pragmatic aspects of how such a verb is used in daily language. Does the retrieval of verbs that are typically ‘hand actions’ (to grab) automatically activate motor cortex areas that subserve the execution of such hand actions? And what if they do? Does that mean this co-activation is a necessary part of the verb’s mental representation?

 

In collaboration with Cindy Thompson, PhD (Northwestern University), our own neuroimaging research in this field suggests that somatotopic (body-part association) co-activation may not reflect an obligatory part of a verb’s representation, but rather that it is task-related. However, even associative patterns of co-activation may have the potential to improve access to verbs, for example in speakers who have verb retrieval problems.

 


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Phonological encoding in fluency disorders

Our research on phonological encoding in fluency disorders focuses on the ways in which people who stutter (PWS) and people who clutter (PWC) retrieve and assemble the sounds and words used to communicate. Cluttering and stuttering are hypothesized to be related disorders of fluency with different speech output characteristics. PWC exhibit a rapid, irregular rate of speech that is typically filled with speech errors such as collapsed syllables, misarticulations, and abnormal pausing, resulting in reduced intelligibility. PWS present with repetitions, prolongations and blocks. The cause of cluttering, specifically the functional level of deficit, has not been established. It has been suggested that the errors present in the speech of clutterers arise because of a lack of planning or formulation time, which is possibly related to phonological encoding (Van Zaalen, Wijnen, & Dejonckere, 2009). Cluttering is closely linked to stuttering, and PWS have been shown to be slower during phoneme monitoring tasks, which suggests deficits at the level of phonological encoding (Sasisekaran, De Nil, Smyth, & Johnson, 2006; Postma & Kolk, 1993). Therefore, it makes sense to study cluttering using some of the same methodologies previously applied to stuttering. The purpose of this ongoing work is to compare the performance of PWS and PWC on various tasks designed to tap into the planning stages of speech production.

 

 

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Modulating Vocal Pitch Motor Control through Neurostimulation

Brain damage, such as that due to neurological diseases or stroke, often impairs the functioning of speech-related areas of the brain. In some cases, notably that of Parkinson’s disease (PD), this causes difficulties in controlling voice pitch (intonation) during speech and is manifested as voice tremors. This difficulty in communicating can have a strong negative impact on quality of life, emphasizing the importance of understanding how vocal pitch is controlled by speech-related areas in the brain.   

High-Definition transcranial Direct Current Stimulation (HD-tDCS) is a technique which stimulates neurons to increase or decrease their excitability. Findings in previous studies have shown that HD-tDCS affects functional behavior and neural plasticity (Kuo et al., 2013; Monti et al., 2013; Malyutina & Den Ouden, 2014), which suggests that it may be effective for improving vocal pitch motor control. Evidence from earlier functional magnetic resonance imaging (fMRI) studies has shown that ventral motor cortex, an area of the brain that controls the movement of speech production muscles, is strongly activated when humans control their voice pitch (Parkinson et al., 2012). Therefore, in this project we aim to study how HD-tDCS of ventral motor cortex can modulate pitch motor control during vocal production.

In the current study, the Neurolinguistics Lab is working in conjunction with the lab of Dr. Roozbeh Behroozmand to investigate the effects of neurostimulation on vocal pitch control.  We are analyzing subjects’ behavioral and neural responses to a pitch shift task before and after administering High-Definition transcranial Direct Current Stimulation (HD-tDCS) to the ventral motor cortex.

 

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Asymmetric Binarity as a Cognitive Universal: The Rhythm of Syntactic Structure

This project investigates the overlap in neural support for two basic processes of structure assignment: combining words into phrases and beats into rhythmic units.  What rhythm and grammatical structure have in common is an asymmetry in the hierarchical structure of their elements, known as headedness. In linguistics, this asymmetric hierarchy is represented in syntax as the operation Merge (Chomsky, 1995), in which two elements are combined in a phrasal structure, one element governing the other. In musical theory, asymmetric binarity can be understood through the Generative Theory of Tonal Music (GTTM; Lerdahl & Jackendoff, 1983), in which musical elements are hierarchically ranked, one element having more strength than another. The human mind assigns such rhythmic patterns to strings of beats, a process named ‘beat induction’ (Honing, 2012). If Merge and beat induction have psychological realities that are controlled by the same brain region, this has implications for the autonomy of language as a cognitive domain. Although there is clear evidence of theoretical structural overlap in language and rhythm, the shared cognitive process underlying the two domains is not evident. This study will address this issue through a functional MRI study of participants' brains while processing either rhythmic or linguistic stimuli.

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