Blog Archives

Evidence for right hemisphere phonology in a backward masking task

The extent to which orthographic and phonological processes are available during the initial moments of word recognition within each hemisphere is under specified, particularly for the right hemisphere. Few studies have investigated whether each hemisphere uses orthography and phonology under constraints that restrict the viewing time of words and reduce overt phonological demands. The current study used backward masking in the divided visual field paradigm to explore hemisphere differences in the availability of orthographic and phonological word recognition processes. A 20 ms and 60 ms SOA were used to track the time course of how these processes develop during pre-lexical moments of word recognition. Nonword masks varied in similarity to the target words such that there were four types: orthographically and phonologically similar, orthographically but not phonologically similar, phonologically but not orthographically similar and unrelated. The results showed the left hemisphere has access to both orthography and phonology early in the word recognition process. With more time to process the stimulus, the left hemisphere is able to use phonology which benefits word recognition to a larger extent than orthography. The right hemisphere also demonstrates access to both orthography and phonology in the initial moments of word recognition, however, orthographic similarity improves word recognition to a greater extent than phonological similarity.

from Brain and Language

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Hemispheric asymmetries depend on the phonetic feature: A dichotic study of place of articulation and voicing in French stops

Dichotic listening experiments show a right-ear advantage (REA), reflecting a left-hemisphere (LH) dominance. However, we found a decrease in REA when the initial stop consonants of two simultaneous French CVC words differed in voicing rather than place of articulation (Experiment 1). This result suggests that the right hemisphere (RH) is more involved in voicing than in place processing. The voiceless–voiced contrast is realised as short positive vs. long negative VOT in French stop consonants, but as long vs. short positive VOT in English. We tested whether the relative involvement of the LH and RH is governed by their respective putative specialisation for short and long events. As expected, in French, the REA decreased when a voiced stop was presented to the left ear and a voiceless stop to the right ear (+V −V), whereas the REA had been shown to decrease for (−V +V) pairs in English. Additionally, voiced stops were more frequently reported among blend responses when a voiced consonant was presented to the left ear. In Experiment 2, VCV pairs were used to reduce the stimulus dominance effect for voiced consonants, which probably contributed to the low REA for (+V –V) pairs in Experiment 1. The reduction of the REA due to a voicing difference was maintained, which provides evidence for the relative independence of the mechanisms responsible for stimulus dominance and perceptual asymmetries in dichotic listening. The results are discussed in the light of the Asymmetric Sampling in Time (AST) model.

from Brain and Language

Interhemispheric asymmetry in EEG photic driving coherence in childhood autism

Spectral and coherence characteristics of the EEG photic driving show different aspects of latent abnormal interhemispheric asymmetry in autistics: the right hemisphere “hyporeactivity” and potential “hyperconectivity” of likely compensatory nature in the left hemisphere.

from Clinical Neurophysiology

Language Control and Lexical Competition in Bilinguals: An Event-Related fMRI Study

from Cerebral Cortex

Language selection (or control) refers to the cognitive mechanism that controls which language to use at a given moment and context. It allows bilinguals to selectively communicate in one target language while minimizing the interferences from the nontarget language. Previous studies have suggested the participation in language control of different brain areas. However, the question remains whether the selection of one language among others relies on a language-specific neural module or general executive regions that also allow switching between different competing behavioral responses including the switching between various linguistic registers. In this functional magnetic resonance imaging study, we investigated the neural correlates of language selection processes in German–French bilingual subjects during picture naming in different monolingual and bilingual selection contexts. We show that naming in the first language in the bilingual context (compared with monolingual contexts) increased activation in the left caudate and anterior cingulate cortex. Furthermore, the activation of these areas is even more extended when the subjects are using a second weaker language. These findings show that language control processes engaged in contexts during which both languages must remain active recruit the left caudate and the anterior cingulate cortex (ACC) in a manner that can be distinguished from areas engaged in intralanguage task switching.

Interhemispheric compensation: A hypothesis of TMS-induced effects on language-related areas

from European Psychiatry

Repetitive transcranial magnetic stimulation (rTMS) applied over brain regions responsible for language processing is used to curtail potentially auditory hallucinations in schizophrenia patients and to investigate the functional organisation of language-related areas. Variability of effects is, however, marked across studies and between subjects. Furthermore, the mechanisms of action of rTMS are poorly understood. Here, we reviewed different factors related to the structural and functional organisation of the brain that might influence rTMS-induced effects. Then, by analogy with aphasia studies, and the plastic-adaptive changes in both the left and right hemispheres following aphasia recovery, a hypothesis is proposed about rTMS mechanisms over language-related areas (e.g. Wernicke, Broca). We proposed that the local interference induced by rTMS in language-related areas might be analogous to aphasic stroke and might lead to a functional reorganisation in areas connected to the virtual lesion for language recovery.

Language Dysfunction After Stroke and Damage to White Matter Tracts Evaluated Using Diffusion Tensor Imaging

from the AJNR – American Journal of Neuroradiology

BACKGROUND AND PURPOSE: Knowledge of the anatomic basis of aphasia after stroke has both theoretic and clinical implications by informing models of cortical connectivity and providing data for diagnosis and prognosis. In this study we use diffusion tensor imaging to address the relationship between damage to specific white matter tracts and linguistic deficits after left hemisphere stroke.

MATERIALS AND METHODS: Twenty patients aged 38–77 years with a history of stroke in the left hemisphere underwent diffusion tensor imaging, structural MR imaging, and language testing. All of the patients were premorbidly right handed and underwent imaging and language testing at least 1 month after stroke.

RESULTS: Lower fractional anisotropy (FA) values in the superior longitudinal and arcuate fasciculi of the left hemisphere, an indication of greater damage to these tracts, were correlated with decreased ability to repeat spoken language. Comprehension deficits after stroke were associated with lower FA values in the arcuate fasciculus of the left hemisphere. The findings for repetition were independent of MR imaging ratings of the degree of damage to cortical areas of the left hemisphere involved in language function. There were no findings for homotopic tracts in the right hemisphere.

CONCLUSION: This study provides support for a specific role for damage to the superior longitudinal and arcuate fasciculi in the left hemisphere in patients with deficits in repetition of speech in aphasia after stroke.

Right hemisphere activation in recovery from aphasia

from Neurology

Background: Some neuroimaging studies have suggested that specific right hemispheric regions can compensate deficits induced by left hemispheric lesions in vascular aphasia. In particular, the right inferior frontal cortex might take part in lexical retrieval in patients presenting left-sided lesions involving the homologous area.

Objective: To address whether the involvement of the right inferior frontal cortex is either unique to recovering aphasic patients or present also in other circumstances of enrichment of lexical abilities, i.e., in non–brain-damaged subjects over learning of new vocabulary.

Methods: Ten post-stroke aphasic patients experiencing word finding difficulties were intensively trained to retrieve object names in French over a 4-week period. Twenty healthy subjects were similarly trained to name these items in either Spanish or English, i.e., foreign languages that they learned at school but did not master. By analogy to aphasic patients, healthy subjects had to work out the phonetic/phonologic representations of long-acquired but forgotten words. Brain activity changes were assessed in two H215O PET sessions involving picture naming tasks that were performed before and after training.

Results: Comparable post-training performance and changes in regional cerebral blood flow including mainly the right insular and inferior frontal regions were found in both groups.

Conclusion: Our results suggest that enhanced activities in right-sided areas observed in recovering aphasia is not the mere consequence of damage to left-sided homologous areas and could reflect the neural correlates of lexical learning also observed in control subjects.

Reorganization of the cerebro-cerebellar network of language production in patients with congenital left-hemispheric brain lesions

from Brain and Language

Patients with congenital lesions of the left cerebral hemisphere may reorganize language functions into the right hemisphere. In these patients, language production is represented homotopically to the left-hemispheric language areas. We studied cerebellar activation in five patients with congenital lesions of the left cerebral hemisphere to assess if the language network is reorganized completely in these patients, i.e. including also cerebellar language functions. As compared to a group of controls matched for age, sex, and verbal IQ, the patients recruited an area not in the right but in the left cerebellar hemisphere. The extent of laterality of the cerebellar activation correlated significantly with the laterality of the frontal activation. We suggest that the developing brain reacts to early focal lesions in the left hemisphere with a mirror-image organization of the entire cerebro-cerebellar network engaged in speech production.

Endogenous Cortical Rhythms Determine Cerebral Specialization for Speech Perception and Production

from Neuron

Across multiple timescales, acoustic regularities of speech match rhythmic properties of both the auditory and motor systems. Syllabic rate corresponds to natural jaw-associated oscillatory rhythms, and phonemic length could reflect endogenous oscillatory auditory cortical properties. Hemispheric lateralization for speech could result from an asymmetry of cortical tuning, with left and right auditory areas differentially sensitive to spectro-temporal features of speech. Using simultaneous electroencephalographic (EEG) and functional magnetic resonance imaging (fMRI) recordings from humans, we show that spontaneous EEG power variations within the gamma range (phonemic rate) correlate best with left auditory cortical synaptic activity, while fluctuations within the theta range correlate best with that in the right. Power fluctuations in both ranges correlate with activity in the mouth premotor region, indicating coupling between temporal properties of speech perception and production. These data show that endogenous cortical rhythms provide temporal and spatial constraints on the neuronal mechanisms underlying speech perception and production.

Abstract Coding of Audiovisual Speech: Beyond Sensory Representation

from Neuron

Is there a neural representation of speech that transcends its sensory properties? Using fMRI, we investigated whether there are brain areas where neural activity during observation of sublexical audiovisual input corresponds to a listener’s speech percept (what is “heard”) independent of the sensory properties of the input. A target audiovisual stimulus was preceded by stimuli that (1) shared the target’s auditory features (auditory overlap), (2) shared the target’s visual features (visual overlap), or (3) shared neither the target’s auditory or visual features but were perceived as the target (perceptual overlap). In two left-hemisphere regions (pars opercularis, planum polare), the target invoked less activity when it was preceded by the perceptually overlapping stimulus than when preceded by stimuli that shared one of its sensory components. This pattern of neural facilitation indicates that these regions code sublexical speech at an abstract level corresponding to that of the speech percept.

Language Dysfunction After Stroke and Damage to White Matter Tracts Evaluated Using Diffusion Tensor Imaging

from the AJNR – American Journal of Neuroradiology

BACKGROUND AND PURPOSE: Knowledge of the anatomic basis of aphasia after stroke has both theoretic and clinical implications by informing models of cortical connectivity and providing data for diagnosis and prognosis. In this study we use diffusion tensor imaging to address the relationship between damage to specific white matter tracts and linguistic deficits after left hemisphere stroke. Materials and METHODS: Twenty patients aged 38-77 years with a history of stroke in the left hemisphere underwent diffusion tensor imaging, structural MR imaging, and language testing. All of the patients were premorbidly right handed and underwent imaging and language testing at least 1 month after stroke. RESULTS: Lower fractional anisotropy (FA) values in the superior longitudinal and arcuate fasciculi of the left hemisphere, an indication of greater damage to these tracts, were correlated with decreased ability to repeat spoken language. Comprehension deficits after stroke were associated with lower FA values in the arcuate fasciculus of the left hemisphere. The findings for repetition were independent of MR imaging ratings of the degree of damage to cortical areas of the left hemisphere involved in language function. There were no findings for homotopic tracts in the right hemisphere. CONCLUSION: This study provides support for a specific role for damage to the superior longitudinal and arcuate fasciculi in the left hemisphere in patients with deficits in repetition of speech in aphasia after stroke.

Brain’s left hemisphere picks up peripheral noise

from News-Medical.net

Our brain is very good at picking up speech even in a noisy room, an adaptation essential for holding a conversation at a cocktail party, and now we are beginning to understand the neural interactions that underlie this ability.
An international research team reports, in the online open access journal BMC Biology, how investigations using neuroimaging have revealed that the brain’s left hemisphere helps discern the signal from the noise.

Sign language aphasia due to left occipital lesion in a deaf signer

from Neurology

Localization of sign language production and comprehension in deaf people has been described as similar to that of spoken language aphasia. However, sign language employs a visuospatial modality through visual information. We present the first report of a deaf signer who showed substantial sign language aphasia with severe impairment in word production due to a left occipital lesion. This case may indicate the possibility of other localizations of plasticity.

Clarifying further the ambiguity advantage effect in word recognition: Effects of aging and left-hemisphere damage on the processing of homonymy and polysemy

from Brain and Language

Abstract of a paper to be presented at the 45th Annual Meeting of the Academy of Aphasia, Washington D.C., October 21-23, 2007