Beta oscillations reflect memory and motor aspects of spoken word production

Two major components form the basis of spoken word production: the access of conceptual and lexical/phonological information in long‐term memory, and motor preparation and execution of an articulatory program. Whereas the motor aspects of word production have been well characterized as reflected in alpha‐beta desynchronization, the memory aspects have remained poorly understood. Using magnetoencephalography, we investigated the neurophysiological signature of not only motor but also memory aspects of spoken‐word production. Participants named or judged pictures after reading sentences. To probe the involvement of the memory component, we manipulated sentence context. Sentence contexts were either constraining or nonconstraining toward the final word, presented as a picture. In the judgment task, participants indicated with a left‐hand button press whether the picture was expected given the sentence. In the naming task, they named the picture. Naming and judgment were faster with constraining than nonconstraining contexts. Alpha‐beta desynchronization was found for constraining relative to nonconstraining contexts pre‐picture presentation. For the judgment task, beta desynchronization was observed in left posterior brain areas associated with conceptual processing and in right motor cortex. For the naming task, in addition to the same left posterior brain areas, beta desynchronization was found in left anterior and posterior temporal cortex (associated with memory aspects), left inferior frontal cortex, and bilateral ventral premotor cortex (associated with motor aspects). These results suggest that memory and motor components of spoken word production are reflected in overlapping brain oscillations in the beta band. Hum Brain Mapp 36:2767–2780, 2015. © 2015 Wiley Periodicals, Inc.     

Independent distractor frequency and age-of-acquisition effects in picture-word interference: fMRI evidence for post-lexical and lexical accounts according to distractor type

In two fMRI experiments, participants named pictures with superimposed distractors that were high or low in frequency or varied in terms of age of acquisition. Pictures superimposed with low-frequency words were named more slowly than those superimposed with high-frequency words, and late-acquired words interfered with picture naming to a greater extent than early-acquired words. The distractor frequency effect (Experiment 1) was associated with increased activity in left premotor and posterior superior temporal cortices, consistent with the operation of an articulatory response buffer and verbal self-monitoring system. Conversely, the distractor age-of-acquisition effect (Experiment 2) was associated with increased activity in the left middle and posterior middle temporal cortex, consistent with the operation of lexical level processes such as lemma and phonological word form retrieval. The spatially dissociated patterns of activity across the two experiments indicate that distractor effects in picture-word interference may occur at lexical or postlexical levels of processing in speech production.     

Examining the neural and cognitive processes that underlie reading through naming speed tasks

We combined fMRI with eye tracking and speech recording to examine the neural and cognitive mechanisms that underlie reading. To simplify the study of the complex processes involved during reading, we used naming speed (NS) tasks (also known as rapid automatized naming or RAN) as a focus for this study, in which average reading right‐handed adults named sets of stimuli (letters or objects) as quickly and accurately as possible. Due to the possibility of spoken output during fMRI studies creating motion artifacts, we employed both an overt session and a covert session. When comparing the two sessions, there were no significant differences in behavioral performance, sensorimotor activation (except for regions involved in the motor aspects of speech production) or activation in regions within the left‐hemisphere‐dominant neural reading network. This established that differences found between the tasks within the reading network were not attributed to speech production motion artifacts or sensorimotor processes. Both behavioral and neuroimaging measures showed that letter naming was a more automatic and efficient task than object naming. Furthermore, specific manipulations to the NS tasks to make the stimuli more visually and/or phonologically similar differentially activated the reading network in the left hemisphere associated with phonological, orthographic and orthographic‐to‐phonological processing, but not articulatory/motor processing related to speech production. These findings further our understanding of the underlying neural processes that support reading by examining how activation within the reading network differs with both task performance and task characteristics.     

Semantic context and visual feature effects in object naming: an fMRI study using arterial spin labeling

Previous behavioral studies reported a robust effect of increased naming latencies when objects to be named were blocked within semantic category, compared to items blocked between category. This semantic context effect has been attributed to various mechanisms including inhibition or excitation of lexico-semantic representations and incremental learning of associations between semantic features and names, and is hypothesized to increase demands on verbal self-monitoring during speech production. Objects within categories also share many visual structural features, introducing a potential confound when interpreting the level at which the context effect might occur. Consistent with previous findings, we report a significant increase in response latencies when naming categorically related objects within blocks, an effect associated with increased perfusion fMRI signal bilaterally in the hippocampus and in the left middle to posterior superior temporal cortex. No perfusion changes were observed in the middle section of the left middle temporal cortex, a region associated with retrieval of lexical-semantic information in previous object naming studies. Although a manipulation of visual feature similarity did not influence naming latencies, we observed perfusion increases in the perirhinal cortex for naming objects with similar visual features that interacted with the semantic context in which objects were named. These results provide support for the view that the semantic context effect in object naming occurs due to an incremental learning mechanism, and involves increased demands on verbal self-monitoring.     

Speech and song: The role of the cerebellum

An exploration into cerebellar activity during the perception and production of speech and song may elucidate general underlying cerebellar functions. Recently, the cerebellum has been hypothesized to be involved with sharpening sensory input, temporal coordination and processing of motor articulation and perception, as well as instantiation of internal models that simulate the input-output characteristics of a specific system. Sung language and spoken language share many common features (physiology for articulation and perception as well as phonology, phonotactics, syntax, and semantics of the underlying language), although they differ in certain vocal and prosodic aspects. A review of the literature on perception and production of singing and speech reveals considerable overlap in the lateral aspect of the VI lobule of the posterior cerebellum, a region known to somatotopically represent the lips and tongue. This region may instantiate internal models of vocal tract articulation that simulate well learned phonological and/or segmental articulatory—auditory/orosensory mappings utilized for both speech and singing. Recent results show tendencies for left cerebellar hemispheric specialization for processing of singing and right specialization for processing of speech, both in the VI lobule of the cerebellum, inferior to that found for representing both speech and singing. Given the crossed pattern of cerebellar-cortical anatomical connectivity the findings are consistent with the hypothesis that the right cerebellum differentially processes high pass filtered information (segmental properties) and the left cerebellum differentially processes low pass filtered information (prosodic, melodic properties). Further research is necessary to examine these hypotheses and their alternatives directly.     

The left posterior superior temporal gyrus participates specifically in accessing lexical phonology

Impairments in phonological processing have been associated with damage to the region of the left posterior superior temporal gyrus (pSTG), but the extent to which this area supports phonological processing, independent of semantic processing, is less clear. We used repetition priming and neural repetition suppression during functional magnetic resonance imaging (fMRI) in an auditory pseudoword repetition task as a semantics-free model of lexical (whole-word) phonological access. Across six repetitions, we observed repetition priming in terms of decreased reaction time and repetition suppression in terms of reduced neural activity. An additional analysis aimed at sublexical phonology did not show significant effects in the areas where repetition suppression was observed. To test if these areas were relevant to real word production, we performed a conjunction analysis with data from a separate fMRI experiment which manipulated word frequency (a putative index of lexical phonological access) in picture naming. The left pSTG demonstrated significant effects independently in both experiments, suggesting that this area participates specifically in accessing lexical phonology.     

Left superior temporal gyrus activation during sentence perception negatively correlates with auditory hallucination severity in schizophrenia patients

The left superior temporal cortex, which supports linguistic functions, has consistently been reported to activate during auditory-verbal hallucinations in schizophrenia patients. It has been suggested that auditory hallucinations and the processing of normal external speech compete for common neurophysiological resources. We tested the hypothesis of a negative relationship between the clinical severity of hallucinations and local brain activity in posterior linguistic regions while patients were listening to external speech. Fifteen right-handed patients with schizophrenia and daily auditory hallucinations for at least 3 months were studied with event-related fMRI while listening to sentences in French or to silence. Severity of hallucinations, assessed using the auditory hallucination subscales of the Psychotic Symptom Rating Scales (PSYRATS) and of the Scale for the Assessment of Positive Symptoms (SAPS-AH), negatively correlated with activation in the left temporal superior region in the French minus silence condition. This finding supports the hypothesis that auditory hallucinations compete with normal external speech for processing sites within the temporal cortex in schizophrenia.     

The constraints functional neuroimaging places on classical models of auditory word processing

Several previous functional imaging experiments have demonstrated that auditory presentation of speech, relative to tones or scrambled speech, activate the superior temporal sulci (STS) bilaterally. In this study, we attempted to segregate the neural responses to phonological, lexical, and semantic input by contrasting activation elicited by heard words, meaningless syllables, and environmental sounds. Inevitable differences between the duration and amplitude of each stimulus type were controlled with auditory noise bursts matched to each activation stimulus. Half the subjects were instructed to say "okay" in response to presentation of all stimuli. The other half repeated back the words and syllables, named the source of the sounds, and said "okay" to the control stimuli (noise bursts). We looked for stimulus effects that were consistent across task. The results revealed that central regions in the STS were equally responsive to speech (words and syllables) and familiar sounds, whereas the posterior and anterior regions of the left superior temporal gyrus were more active for speech. The effect of semantic input was small but revealed more activation in the inferior temporal cortex for words and familiar sounds than syllables and noise. In addition, words (relative to syllables, sounds, and noise) enhanced activation in the temporo-parietal areas that have previously been linked to modality independent semantic processing. Thus, in cognitive terms, we dissociate phonological (speech) and semantic responses and propose that word specificity arises from functional integration among shared phonological and semantic areas.     

Evidence for early specialized processing of speech formant information in anterior and posterior human auditory cortex

Many speech sounds, such as vowels, exhibit a characteristic pattern of spectral peaks, referred to as formants, the frequency positions of which depend both on the phonological identity of the sound (e.g. vowel type) and on the vocal‐tract length of the speaker. This study investigates the processing of formant information relating to vowel type and vocal‐tract length in human auditory cortex by measuring electroencephalographic (EEG) responses to synthetic unvoiced vowels and spectrally matched noises. The results revealed specific sensitivity to vowel formant information in both anterior (planum polare) and posterior (planum temporale) regions of auditory cortex. The vowel‐specific responses in these two areas appeared to have different temporal dynamics; the anterior source produced a sustained response for as long as the incoming sound was a vowel, whereas the posterior source responded transiently when the sound changed from a noise to a vowel, or when there was a change in vowel type. Moreover, the posterior source appeared to be largely invariant to changes in vocal‐tract length. The current findings indicate that the initial extraction of vowel type from formant information is complete by the level of non‐primary auditory cortex, suggesting that speech‐specific processing may involve primary auditory cortex, or even subcortical structures. This challenges the view that specific sensitivity to speech emerges only beyond unimodal auditory cortex.     

Out‐of‐synchrony speech entrainment in developmental dyslexia

Developmental dyslexia is a reading disorder often characterized by reduced awareness of speech units. Whether the neural source of this phonological disorder in dyslexic readers results from the malfunctioning of the primary auditory system or damaged feedback communication between higher‐order phonological regions (i.e., left inferior frontal regions) and the auditory cortex is still under dispute. Here we recorded magnetoencephalographic (MEG) signals from 20 dyslexic readers and 20 age‐matched controls while they were listening to ～10‐s‐long spoken sentences. Compared to controls, dyslexic readers had (1) an impaired neural entrainment to speech in the delta band (0.5–1 Hz); (2) a reduced delta synchronization in both the right auditory cortex and the left inferior frontal gyrus; and (3) an impaired feedforward functional coupling between neural oscillations in the right auditory cortex and the left inferior frontal regions. This shows that during speech listening, individuals with developmental dyslexia present reduced neural synchrony to low‐frequency speech oscillations in primary auditory regions that hinders higher‐order speech processing steps. The present findings, thus, strengthen proposals assuming that improper low‐frequency acoustic entrainment affects speech sampling. This low speech‐brain synchronization has the strong potential to cause severe consequences for both phonological and reading skills. Interestingly, the reduced speech‐brain synchronization in dyslexic readers compared to normal readers (and its higher‐order consequences across the speech processing network) appears preserved through the development from childhood to adulthood. Thus, the evaluation of speech‐brain synchronization could possibly serve as a diagnostic tool for early detection of children at risk of dyslexia. Hum Brain Mapp 37:2767–2783, 2016. © 2016 Wiley Periodicals, Inc .     

Are the same phoneme and lexical layers used in speech production and comprehension? A case-series test of model of aphasic speech production

In this paper, we investigate the claim that although the same lexical units are involved in speech production and comprehension, there are separate input and output phoneme layers (Foygel and Dell, 2000). Data from a case series of aphasic patients are used to test this claim by examining the relationship between performance on a test of picture naming and performance on tests of phonological input. Estimates of each patient's semantic-lexical and phonological impairments in speech production were derived from Foygel and Dell's computational model of picture naming. It was found that the strength of the semantic-lexical impairments in speech production was significantly correlated with performance on auditory comprehension tests. This finding is consistent with the claim that the same lexical units are involved in speech comprehension and production. Conversely, the correlations between the strength of the phonological lesions in speech production and performance on tests of phonological input were non-significant, consistent with Foygel and Dell's claim that there are distinct input and output phoneme layers.     

Abnormal functional activation during a simple word repetition task: A PET study of adult dyslexics

Eight dyslexic subjects, impaired on a range of tasks requiring phonological processing, were matched for age and general ability with six control subjects. Participants were scanned using positron emission tomography (PET) during three conditions: repeating real words, repeating pseudowords, and rest. In both groups, speech repetition relative to rest elicited widespread bilateral activation in areas associated with auditory processing of speech; there were no significant differences between words and pseudowords. However, irrespective of word type, the dyslexic group showed less activation than the control group in the right superior temporal and right post-central gyri and also in the left cerebellum. Notably, the right anterior superior temporal cortex (Brodmann's area 22 [BA 22]) was less activated in each of the eight dyslexic subjects, compared to each of the six control subjects. This deficit appears to be specific to auditory repetition as it was not detected in a previous study of reading which used the same sets of stimuli (Brunswick, N., McCrory, E., Price, C., Frith, C.D., & Frith, U. [1999]. Explicit and implicit processing of words and pseudowords by adult developmental dyslexics: A search for Wernicke's Wortschatz? Brain, 122, 1901-1917). This implies that the observed neural manifestation of developmental dyslexia is task-specific (i.e., functional rather than structural). Other studies of normal subjects indicate that attending to the phonetic structure of speech leads to a decrease in right-hemisphere processing. Lower right hemisphere activation in the dyslexic group may therefore indicate less processing of non-phonetic aspects of speech, allowing greater salience to be accorded to phonological aspects of attended speech.     

The insula (Island of Reil) and its role in auditory processing: Literature review

The insular cortex is a complex structure which contains areas that subserve visceral sensory, motor, vestibular, and somatosensory functions. The role of the insular cortex in auditory processing was poorly understood until recently. However, recent case studies indicate that bilateral damage to the insulae may result in total auditory agnosia. Functional imaging studies demonstrate that the insulae participate in several key auditory processes, such as allocating auditory attention and tuning in to novel auditory stimuli, temporal processing, phonological processing and visual–auditory integration. These studies do not clarify the issue of further specialisation within the insular cortex, e.g. whether the posterior insulae are primarily sensory areas, while the anterior insulae serve mainly as integration/association auditory areas, two hypotheses that would be compatible with the cytoarchitectonic structure and connectivity of the insulae. The functional characterisation of the insulae remains incomplete, underlining the need for further studies.     

Topographical gradients of semantics and phonology revealed by temporal lobe stimulation

Word retrieval is a fundamental component of oral communication, and it is well established that this function is supported by left temporal cortex. Nevertheless, the specific temporal areas mediating word retrieval and the particular linguistic processes these regions support have not been well delineated. Toward this end, we analyzed over 1000 naming errors induced by left temporal cortical stimulation in epilepsy surgery patients. Errors were primarily semantic (lemon → “pear”), phonological (horn → “corn”), non‐responses, and delayed responses (correct responses after a delay), and each error type appeared predominantly in a specific region: semantic errors in mid‐middle temporal gyrus (TG), phonological errors and delayed responses in middle and posterior superior TG, and non‐responses in anterior inferior TG. To the extent that semantic errors, phonological errors and delayed responses reflect disruptions in different processes, our results imply topographical specialization of semantic and phonological processing. Specifically, results revealed an inferior‐to‐superior gradient, with more superior regions associated with phonological processing. Further, errors were increasingly semantically related to targets toward posterior temporal cortex. We speculate that detailed semantic input is needed to support phonological retrieval, and thus, the specificity of semantic input increases progressively toward posterior temporal regions implicated in phonological processing. Hum Brain Mapp 38:688–703, 2017. © 2016 Wiley Periodicals, Inc.     

Neural correlates of word production stages delineated by parametric modulation of psycholinguistic variables

Word production is a complex multistage process linking conceptual representations, lexical entries, phonological forms and articulation. Previous studies have revealed a network of predominantly left‐lateralized brain regions supporting this process, but many details regarding the precise functions of different nodes in this network remain unclear. To better delineate the functions of regions involved in word production, we used event‐related functional magnetic resonance imaging (fMRI) to identify brain areas where blood oxygen level‐dependent (BOLD) responses to overt picture naming were modulated by three psycholinguistic variables: concept familiarity, word frequency, and word length, and one behavioral variable: reaction time. Each of these variables has been suggested by prior studies to be associated with different aspects of word production. Processing of less familiar concepts was associated with greater BOLD responses in bilateral occipitotemporal regions, reflecting visual processing and conceptual preparation. Lower frequency words produced greater BOLD signal in left inferior temporal cortex and the left temporoparietal junction, suggesting involvement of these regions in lexical selection and retrieval and encoding of phonological codes. Word length was positively correlated with signal intensity in Heschl's gyrus bilaterally, extending into the mid‐superior temporal gyrus (STG) and sulcus (STS) in the left hemisphere. The left mid‐STS site was also modulated by reaction time, suggesting a role in the storage of lexical phonological codes. Hum Brain Mapp, 2009. © 2009 Wiley‐Liss, Inc.     

Auditory-motor interaction revealed by fMRI: speech,music, and working memory in area Spt

The concept of auditory-motor interaction pervades speech science research, yet the cortical systems supporting this interface have not been elucidated. Drawing on experimental designs used in recent work in sensory-motor integration in the cortical visual system, we used fMRI in an effort to identify human auditory regions with both sensory and motor response properties, analogous to single-unit responses in known visuomotor integration areas. The sensory phase of the task involved listening to speech (nonsense sentences) or music (novel piano melodies); the "motor" phase of the task involved covert rehearsal/humming of the auditory stimuli. A small set of areas in the superior temporal and temporal-parietal cortex responded both during the listening phase and the rehearsal/humming phase. A left lateralized region in the posterior Sylvian fissure at the parietal-temporal boundary, area Spt, showed particularly robust responses to both phases of the task. Frontal areas also showed combined auditory + rehearsal responsivity consistent with the claim that the posterior activations are part of a larger auditory-motor integration circuit. We hypothesize that this circuit plays an important role in speech development as part of the network that enables acoustic-phonetic input to guide the acquisition of language-specific articulatory-phonetic gestures; this circuit may play a role in analogous musical abilities. In the adult, this system continues to support aspects of speech production, and, we suggest, supports verbal working memory.     

Relations between the neural bases of dynamic auditory processing and phonological processing: evidence from fMRI

Functional magnetic resonance imaging (fMRI) was used to examine how the brain responds to temporal compression of speech and to determine whether the same regions are also involved in phonological processes associated with reading. Recorded speech was temporally compressed to varying degrees and presented in a sentence verification task. Regions involved in phonological processing were identified in a separate scan using a rhyming judgment task with pseudowords compared to a lettercase judgment task. The left inferior frontal and left superior temporal regions (Broca's and Wernicke's areas), along with the right inferior frontal cortex, demonstrated a convex response to speech compression; their activity increased as compression increased, but then decreased when speech became incomprehensible. Other regions exhibited linear increases in activity as compression increased, including the middle frontal gyri bilaterally. The auditory cortices exhibited compression-related decreases bilaterally, primarily reflecting a decrease in activity when speech became incomprehensible. Rhyme judgments engaged two left inferior frontal gyrus regions (pars triangularis and pars opercularis), of which only the pars triangularis region exhibited significant compression-related activity. These results directly demonstrate that a subset of the left inferior frontal regions involved in phonological processing is also sensitive to transient acoustic features within the range of comprehensible speech.     

Letter fluency in 7-8-year-old children is related to the anterior,but not posterior,ventral occipito-temporal cortex during an auditory phonological task

Previous studies have shown that reading skill in 3- to 6-year-old children is related to the automatic activation of the posterior left ventral occipitotemporal cortex (vOT) during spoken language processing, whereas 8- to 15-year-old children and adult readers activate the anterior vOT. However, it is unknown how children who are between these two age groups automatically activate orthographic representations in vOT for spoken language. In the current study, we recruited 153 7- to 8-year-old children to fill the age gap from previous studies. Using functional magnetic resonance imaging (fMRI), we measured children’s reading-related skills and brain activity during an auditory phonological task with both a small (i.e. onset) and a large (i.e. rhyme) grain size condition. We found that letter fluency, but not reading accuracy, was correlated with activation in the anterior vOT for the rhyme condition. There were no reading-related skill correlations for the posterior vOT or for activation during the onset condition in this age group. Our findings reveal that automatic activation in the anterior vOT during spoken language processing already occurs in higher skilled 7- to 8-year-old children. In addition, increases in naming automaticity is the primary determinant of the engagement of vOT during phonological awareness tasks.     

Functional anatomy of auditory verbal imagery in schizophrenic patients with auditory hallucinations

OBJECTIVE: This study investigated the functional neuroanatomy of inner speech and auditory verbal imagery in schizophrenic patients predisposed to auditory hallucinations. METHOD: Eight patients with schizophrenia with a history of prominent auditory hallucinations and six comparison subjects underwent functional magnetic resonance imaging while generating inner speech or imagining external speech. RESULTS: Patients showed no differences while generating inner speech but experienced a relatively attenuated response in the posterior cerebellar cortex, hippocampi, and lenticular nuclei bilaterally and the right thalamus, middle and superior temporal cortex, and left nucleus accumbens during auditory verbal imagery. CONCLUSIONS: Patients with schizophrenia who were prone to auditory hallucinations show attenuated activation when processing inner speech in areas implicated in verbal self-monitoring.     

Top-down influences on lexical selection during spoken word production: A 4T fMRI investigation of refractory effects in picture naming

Spoken word production is assumed to involve stages of processing in which activation spreads through layers of units comprising lexical-conceptual knowledge and their corresponding phonological word forms. Using high-field (4T) functional magnetic resonance imaging (fMRI), we assessed whether the relationship between these stages is strictly serial or involves cascaded-interactive processing, and whether central (decision/control) processing mechanisms are involved in lexical selection. Participants performed the competitor priming paradigm in which distractor words, named from a definition and semantically related to a subsequently presented target picture, slow picture-naming latency compared to that with unrelated words. The paradigm intersperses two trials between the definition and the picture to be named, temporally separating activation in the word perception and production networks. Priming semantic competitors of target picture names significantly increased activation in the left posterior temporal cortex, and to a lesser extent the left middle temporal cortex, consistent with the predictions of cascaded-interactive models of lexical access. In addition, extensive activation was detected in the anterior cingulate and pars orbitalis of the inferior frontal gyrus. The findings indicate that lexical selection during competitor priming is biased by top-down mechanisms to reverse associations between primed distractor words and target pictures to select words that meet the current goal of speech.