A PET investigation of lexicality and phonotactic frequency in oral language processing

Phonological Working Memory for Words and Nonwords in Cerebral Cortex

PURPOSE

The primary purpose of this study was to identify the brain bases of phonological working memory (the short-term maintenance of speech sounds) using behavioral tasks analogous to clinically sensitive assessments of nonword repetition. The secondary purpose of the study was to identify how individual differences in brain activation were related to participants' nonword repetition abilities.

METHOD

We used functional magnetic resonance imaging to measure neurophysiological response during a nonword discrimination task derived from standard clinical assessments of phonological working memory. Healthy adult control participants (N = 16) discriminated pairs of real words or nonwords under varying phonological working memory load, which we manipulated by parametrically varying the number of syllables in target (non)words. Participants' cognitive and phonological abilities were also measured using standardized assessments.

RESULTS

Neurophysiological responses in bilateral superior temporal gyrus, inferior frontal gyrus, and supplementary motor area increased with greater phonological working memory load. Activation in left superior temporal gyrus during nonword discrimination correlated with participants' performance on standard clinical nonword repetition tests.

CONCLUSION

These results suggest that phonological working memory is related to the function of cortical structures that canonically underlie speech perception and production.

The sensitivity of children with SLI to phonotactic probabilities during lexical access

The procedural deficit hypothesis (Ullman & Pierpont, 2005) has been proposed to account for the combination of linguistic and nonlinguistic deficits observed in specific language impairment (SLI). According to this proposal, SLI results from a deficit in procedural memory that prevents children from developing sensitivity to probabilistic sequences, amongst other deficits. We tested the ability of children with SLI to rely on a specific type of probabilities characterizing sequences that occur in a given language: phonotactic probabilities. Twenty French-speaking children with SLI (M=10;1), 20 typically developing children matched for chronological age (M=10;0) and 20 typically developing children matched for receptive vocabulary (M=7;4) performed an auditory lexical decision task. Pseudoword stimuli were built with combinations of either frequently associated phonemes (high phonotactic probability) or infrequently associated phonemes (low phonotactic probability). Phonotactic probabilities had a significant impact on the accuracy and speed of pseudoword rejection in children with SLI, but not in the two control groups. SLI children's greater reliance on phonotactic probabilities relative to typically developing children appears to contradict the PD hypothesis. Phonotactic probabilities may help them to partially overcome their difficulties in developing and accessing the phonological lexicon during spoken word recognition.

LEARNING OUTCOMES

After reading this article, readers will understand the importance of sensitivity to phonotactic probabilities in language processing. They will also learn that such sensitivity is preserved in children with SLI. Finally, readers will understand that children with SLI are more prone to use phonotactic information when accessing their lexicon than typically-developing children.

Messages from the brain connectivity regarding neural correlates of consciousness

Consciousness has become a legitimate theme of neuroscientific discourse over the last two decades. Neuroscientific investigation seeking neural correlates of consciousness (NCC) has ranged from the neuronal level to the system level. Regarding system level studies, there is a large body of evidence supporting the idea that functional connectivity studies can help in examining NCC. Functional connectivity studies have suggested the involvement of the thalamo-cortical, frontoparietal, and other cortico-cortical connectivity under anesthetic-induced unconsciousness and in disorders of consciousness. Likewise, effective connectivity has been used to investigate the causal interactions among elements of functional connectivity in various consciousness states, and provided a deeper understanding of NCC. Moreover, as an extended version of connectivity studies, complex network methods have also been used for studies on NCC. In this review, we focused on the aspect of the brain system level of NCC including functional and effective connectivity networks from methodological perspectives. In addition, as for states of consciousness, anesthetic-induced unconsciousness and disorders of consciousness are the main subjects. This review discusses what we have learned from recent studies about the exploration of human brain connectivity on consciousness and its neural correlates.

Sublexical properties of spoken words modulate activity in Broca's area but not superior temporal cortex: implications for models of speech recognition

Many models of spoken word recognition posit that the acoustic stream is parsed into phoneme level units, which in turn activate larger representations [McClelland, J. L., & Elman, J. L. The TRACE model of speech perception. Cognitive Psychology, 18, 1-86, 1986], whereas others suggest that larger units of analysis are activated without the need for segmental mediation [Greenberg, S. A multitier theoretical framework for understanding spoken language. In S. Greenberg & W. A. Ainsworth (Eds.), Listening to speech: An auditory perspective (pp. 411-433). Mahwah, NJ: Erlbaum, 2005; Klatt, D. H. Speech perception: A model of acoustic-phonetic analysis and lexical access. Journal of Phonetics, 7, 279-312, 1979; Massaro, D. W. Preperceptual images, processing time, and perceptual units in auditory perception. Psychological Review, 79, 124-145, 1972]. Identifying segmental effects in the brain's response to speech may speak to this question. For example, if such effects were localized to relatively early processing stages in auditory cortex, this would support a model of speech recognition in which segmental units are explicitly parsed out. In contrast, segmental processes that occur outside auditory cortex may indicate that alternative models should be considered. The current fMRI experiment manipulated the phonotactic frequency (PF) of words that were auditorily presented in short lists while participants performed a pseudoword detection task. PF is thought to modulate networks in which phoneme level units are represented. The present experiment identified activity in the left inferior frontal gyrus that was positively correlated with PF. No effects of PF were found in temporal lobe regions. We propose that the observed phonotactic effects during speech listening reflect the strength of the association between acoustic speech patterns and articulatory speech codes involving phoneme level units. On the basis of existing lesion evidence, we interpret the function of this auditory-motor association as playing a role primarily in production. These findings are consistent with the view that phoneme level units are not necessarily accessed during speech recognition.

The commonality of neural networks for verbal and visual short-term memory

Although many neuroimaging studies have considered verbal and visual short-term memory (STM) as relying on neurally segregated short-term buffer systems, the present study explored the existence of shared neural correlates supporting verbal and visual STM. We hypothesized that networks involved in attentional and executive processes, as well as networks involved in serial order processing, underlie STM for both verbal and visual list information, with neural specificity restricted to sensory areas involved in processing the specific items to be retained. Participants were presented sequences of nonwords or unfamiliar faces, and were instructed to maintain and recognize order or item information. For encoding and retrieval phases, null conjunction analysis revealed an identical fronto-parieto-cerebellar network comprising the left intraparietal sulcus, bilateral dorsolateral prefrontal cortex, and the bilateral cerebellum, irrespective of information type and modality. A network centered around the right intraparietal sulcus supported STM for order information, in both verbal and visual modalities. Modality-specific effects were observed in left superior temporal and mid-fusiform areas associated with phonological and orthographic processing during the verbal STM tasks, and in right hippocampal and fusiform face processing areas during the visual STM tasks, wherein these modality effects were most pronounced when storing item information. The present results suggest that STM emerges from the deployment of modality-independent attentional and serial ordering processes toward sensory networks underlying the processing and storage of modality-specific item information.

Neural systems for reading aloud: a multiparametric approach

Reading aloud involves computing the sound of a word from its visual form. This may be accomplished 1) by direct associations between spellings and phonology and 2) by computation from orthography to meaning to phonology. These components have been studied in behavioral experiments examining lexical properties such as word frequency; length in letters or phonemes; spelling-sound consistency; semantic factors such as imageability, measures of orthographic, or phonological complexity; and others. Effects of these lexical properties on specific neural systems, however, are poorly understood, partially because high intercorrelations among lexical factors make it difficult to determine if they have independent effects. We addressed this problem by decorrelating several important lexical properties through careful stimulus selection. Functional magnetic resonance imaging data revealed distributed neural systems for mapping orthography directly to phonology, involving left supramarginal, posterior middle temporal, and fusiform gyri. Distinct from these were areas reflecting semantic processing, including left middle temporal gyrus/inferior-temporal sulcus, bilateral angular gyrus, and precuneus/posterior cingulate. Left inferior frontal regions generally showed increased activation with greater task load, suggesting a more general role in attention, working memory, and executive processes. These data offer the first clear evidence, in a single study, for the separate neural correlates of orthography-phonology mapping and semantic access during reading aloud.

A complementary systems account of word learning: neural and behavioural evidence

In this paper we present a novel theory of the cognitive and neural processes by which adults learn new spoken words. This proposal builds on neurocomputational accounts of lexical processing and spoken word recognition and complementary learning systems (CLS) models of memory. We review evidence from behavioural studies of word learning that, consistent with the CLS account, show two stages of lexical acquisition: rapid initial familiarization followed by slow lexical consolidation. These stages map broadly onto two systems involved in different aspects of word learning: (i) rapid, initial acquisition supported by medial temporal and hippocampal learning, (ii) slower neocortical learning achieved by offline consolidation of previously acquired information. We review behavioural and neuroscientific evidence consistent with this account, including a meta-analysis of PET and functional Magnetic Resonance Imaging (fMRI) studies that contrast responses to spoken words and pseudowords. From this meta-analysis we derive predictions for the location and direction of cortical response changes following familiarization with pseudowords. This allows us to assess evidence for learning-induced changes that convert pseudoword responses into real word responses. Results provide unique support for the CLS account since hippocampal responses change during initial learning, whereas cortical responses to pseudowords only become word-like if overnight consolidation follows initial learning.

Educational Neuroscience: New Discoveries from Bilingual Brains, Scientific Brains, and the Educated Mind

We discuss the fruits of educational neuroscience research from our laboratory and show how the typical maturational timing milestones in bilingual language acquisition provide educators with a tool for differentiating a bilingual child experiencing language and reading delay versus deviance. Further, early schooling in two languages simultaneously affords young bilingual children a reading advantage and may also ameliorate the negative effect of low socioeconomic status on literacy. Using powerful brain imaging technology, functional Near Infrared Spectroscopy, we provide a first‐time look into the developing brains of bilingual as comapred to monolingual children. We show unequivocally that the age of first bilingual exposure is a vital predictor of bilingual language and reading mastery. Accounts that promote later dual language and reading instruction, or those that assert human brain development is unrelated to bilingual language mastery, are not supported by the present findings. We discuss the implications for education, teachers, and developmental brain sciences.

The neural correlates of verbal short-term memory in Alzheimer's disease: an fMRI study

Although many studies have shown diminished performance in verbal short-term memory tasks in Alzheimer's disease, few studies have explored the neural correlates of impaired verbal short-term memory in Alzheimer's disease patients. In this fMRI study, we examined alterations in brain activation patterns during a verbal short-term memory recognition task, by differentiating encoding and retrieval phases. Sixteen mild Alzheimer's disease patients and 16 elderly controls were presented with lists of four words followed, after a few seconds, by a probe word. Participants had to judge whether the probe matched one of the items of the memory list. In both groups, the short-term memory task elicited a distributed fronto-parieto-temporal activation that encompassed bilateral inferior frontal, insular, supplementary motor, precentral and postcentral areas, consistent with previous studies of verbal short-term memory in young subjects. Most notably, Alzheimer's disease patients showed reduced activation in several regions during the encoding phase, including the bilateral middle frontal and the left inferior frontal gyri (associated with executive control processes) as well as the transverse temporal gyri (associated with phonological processing). During the recognition phase, we found decreased activation in the left supramarginal gyrus and the right middle frontal gyrus in Alzheimer's disease patients compared with healthy seniors, possibly related to deficits in manipulation and decision processes for phonological information. At the same time, Alzheimer's disease patients showed increased activation in several brain areas, including the left parahippocampus and hippocampus, suggesting that Alzheimer's disease patients may recruit alternative recognition mechanisms when performing a short-term memory task. Overall, our results indicate that Alzheimer's disease patients show differences in the functional networks underlying memory over short delays, mostly in brain areas known to support phonological processing or executive functioning.

Learning and consolidation of novel spoken words

Two experiments explored the neural mechanisms underlying the learning and consolidation of novel spoken words. In Experiment 1, participants learned two sets of novel words on successive days. A subsequent recognition test revealed high levels of familiarity for both sets. However, a lexical decision task showed that only novel words learned on the previous day engaged in lexical competition with similar-sounding existing words. Additionally, only novel words learned on the previous day exhibited faster repetition latencies relative to unfamiliar controls. This overnight consolidation effect was further examined using fMRI to compare neural responses to existing and novel words learned on different days prior to scanning (Experiment 2). This revealed an elevated response for novel compared with existing words in left superior temporal gyrus (STG), inferior frontal and premotor regions, and right cerebellum. Cortical activation was of equivalent magnitude for unfamiliar novel words and items learned on the day of scanning but significantly reduced for novel words learned on the previous day. In contrast, hippocampal responses were elevated for novel words that were entirely unfamiliar, and this elevated response correlated with postscanning behavioral measures of word learning. These findings are consistent with a dual-learning system account in which there is a division of labor between medial-temporal systems that are involved in initial acquisition and neocortical systems in which representations of novel spoken words are subject to overnight consolidation.

Neural substrates of phonological and lexicosemantic representations in Alzheimer's disease

The language profile of patients suffering from Alzheimer's disease (AD) is characterized not only by lexicosemantic impairments but also by phonological deficits, as shown by an increasing number of neuropsychological studies. This study explored the functional neural correlates underlying phonological and lexicosemantic processing in AD. Using H(215)O PET functional brain imaging, a group of mild to moderate AD patients and a group of age-matched controls were asked to repeat four types of verbal stimuli: words, wordlike nonwords (WL+), non-wordlike nonwords (WL-) and simple vowels. The comparison between the different conditions allowed us to determine brain activation preferentially associated with lexicosemantic or phonological levels of language representations. When repeating words, AD patients showed decreased activity in the left temporo-parietal and inferior frontal regions relative to controls, consistent with distorted lexicosemantic representations. Brain activity was abnormally increased in the right superior temporal area during word repetition, a region more commonly associated with perceptual-phonological processing. During repetition of WL+ and WL- nonwords, AD patients showed decreased activity in the middle part of the superior temporal gyrus, presumably associated with sublexical phonological information; at the same time, AD patients showed larger activation than controls in the inferior temporal gyrus, typically associated with lexicosemantic levels of representation. Overall, the results suggest that AD patients use altered pathways to process phonological and lexicosemantic information, possibly related to a progressive loss of specialization of phonological and lexicosemantic neural networks.

The problem of aphasia in the assessment of consciousness in brain-damaged patients

The assessment of the level and content of consciousness in brain-damaged patients relies to a large extent on behavioral assessment techniques. The limited behavioral repertoire displayed by vegetative and minimally conscious states requires the use of highly sensitive and reliable behavioral assessment methods, allowing the detection of subtle changes in behavior and associated level of consciousness. This situation is further complicated when patients with such disorders of consciousness have underlying deficits in the domain of communication functions, such as aphasia. The present paper examines the consequences of receptive and/or productive aphasia on the already limited behavioral repertoire presented in these patients and discusses a number of behavioral and neuroimaging assessment procedures designed to: (1) detect the presence of aphasia in patients with disorders of consciousness, and (2) reliably assess the level of consciousness of brain-damaged patients while taking into account the existence of receptive and/or expressive language deficits. The combined use of behavioral and neuroimaging assessment techniques appears to be particularly promising for disentangling impaired consciousness and aphasia.

Auditory processing of different types of pseudo-words: an event-related fMRI study

Imaging results on real word and pseudo-word processing have been heterogeneous, allowing only cautious claims about the neuroanatomical loci of lexico-semantic processing. In order to shed more light on this issue, we examined the impact of different structures of non-lexical stimuli on the outcome of comparisons between such items and matched real words. We anticipated that the degree to which a pseudo-word still resembles a particular real word template determines how word-like it is processed. To verify this idea, we tested different types of pseudo-words (either phonotactically legal and transparently or opaquely derived from real words or phonotactically illegal) in an event-related fMRI paradigm utilizing a lexical decision task. All types of pseudo-words elicited a stronger hemodynamic brain response than real words in the bilateral superior temporal gyri. Real words produced stronger brain activations than pseudo-words in the left posterior middle temporal and angular gyri, the rostral and caudal cingulate gyrus, the precuneus and the right inferior temporal gyrus. When contrasted to opaque pseudo-words transparent pseudo-words elicited a stronger brain response in a temporo-parietal region adjacent to the one observed for real words. Our results provide further support for the involvement of the left posterior middle temporal and angular gyri in lexical-semantic processing. The data also indicate that transparently derived pseudo-words are processed similarly to real words. In contrast, semantic operations are blocked when opaquely derived pseudo-words are processed.

The Unnoticed Contributions of the Cerebellum to Language

BACKGROUND

In addition to its well-known role in motor processing, the cerebellum has been shown to contribute to a number of nonmotor cognitive abilities. However, despite (1) the acknowledged demonstration of the motor, perceptual and cognitive contributions of the cerebellum and (2) the growing number of neuroimaging studies allowing for the exploration of the neurobiological bases of language abilities, only a small number of neuroimaging studies focus on the cerebellar contribution to language.

AIMS

To look for unreported cerebellar activations in the neuroimaging literature for language, in order to systematically describe the unreported or otherwise unnoticed cerebellar activations associated with language tasks.

METHODS

A recent review paper by Démonet et al. [Physiol Rev 2005;85:49-95] was used as a base in order to investigate the literature on the neuroimaging of language abilities.

RESULTS

Of the 450 papers cited in this review, 100 articles were directly related to single-word processing, of which only 34 reported cerebellum activations.

CONCLUSION

The full integration of the cerebellum in the network allowing for language and communication is still to come, as very few neuroimaging studies do report cerebellar activations underlying the processing of words.

An event-related fMRI investigation of phonological-lexical competition

This study explored the neural correlates of phonological-lexical competition and frequency on word recognition. An event-related fMRI experiment was conducted using an auditory lexical decision task in which word and nonword stimuli varied in terms of neighborhood density (high and low). Word stimuli also varied in terms of frequency (high and low). Behavioral results were similar to those of Luce and Pisoni [Luce, P. A., & Pisoni, D. B. (1998). Recognizing spoken words: The neighborhood activation model. Ear and Hearing, 19, 1-36], with the reaction time data showing a main effect of word frequency and density as well as a significant interaction effect between these two factors. fMRI results revealed an overall greater neural response for high-density compared to low-density words in the left supramarginal gyrus, consistent with the view that there are greater demands on phonological processing under conditions of increased phonological-lexical competition. The comparison between high and low frequency words revealed greater activation for high frequency words in both anterior and posterior left middle temporal gyrus. A significant interaction between density and frequency was found in lateral and medial frontal structures. This frontal activation may reflect the greater computational resources required in integrating frequency and density information in order to access a word. Overall, these findings demonstrate the sensitivity of neural structures to different properties of the lexicon.

The left intraparietal sulcus and verbal short-term memory: focus of attention or serial order?

One of the most consistently activated regions during verbal short-term memory (STM) tasks is the left intraparietal sulcus (IPS). However, its precise role remains a matter of debate. While some authors consider the IPS to be a specific store for serial order information, other data suggest that it serves a more general function of attentional focalization. In the current fMRI experiment, we investigated these two hypotheses by presenting different verbal STM conditions that probed recognition for word identity or word order and by assessing functional connectivity of the left IPS with distant brain areas. If the IPS has a role of attentional focalization, then it should be involved in both order and item conditions, but it should be connected to different brain regions, depending on the neural substrates involved in processing the different types of information (order versus phonological/orthographic) to be remembered in the item and order STM conditions. We observed that the left IPS was activated in both order and item STM conditions but for different reasons: during order STM, the left IPS was functionally connected to serial/temporal order processing areas in the right IPS, premotor and cerebellar cortices, while during item STM, the left IPS was connected to phonological and orthographic processing areas in the superior temporal and fusiform gyri. Our data support a position considering that the left IPS acts as an attentional modulator of distant neural networks which themselves are specialized in processing order or language representations. More generally, they strengthen attention-based accounts of verbal STM.

Renewal of the neurophysiology of language: functional neuroimaging

Functional neuroimaging methods have reached maturity. It is now possible to start to build the foundations of a physiology of language. The remarkable number of neuroimaging studies performed so far illustrates the potential of this approach, which complements the classical knowledge accumulated on aphasia. Here we attempt to characterize the impact of the functional neuroimaging revolution on our understanding of language. Although today considered as neuroimaging techniques, we refer less to electroencephalography and magnetoencephalography studies than to positron emission tomography and functional magnetic resonance imaging studies, which deal more directly with the question of localization and functional neuroanatomy. This review is structured in three parts. 1) Because of their rapid evolution, we address technical and methodological issues to provide an overview of current procedures and sketch out future perspectives. 2) We review a set of significant results acquired in normal adults (the core of functional imaging studies) to provide an overview of language mechanisms in the "standard" brain. Single-word processing is considered in relation to input modalities (visual and auditory input), output modalities (speech and written output), and the involvement of "central" semantic processes before sentence processing and nonstandard language (illiteracy, multilingualism, and sensory deficits) are addressed. 3) We address the influence of plasticity on physiological functions in relation to its main contexts of appearance, i.e., development and brain lesions, to show how functional imaging can allow fine-grained approaches to adaptation, the fundamental property of the brain. In closing, we consider future developments for language research using functional imaging.