Investigating language lateralization during phonological and semantic fluency tasks using functional transcranial Doppler sonography

Although there is consensus that the left hemisphere plays a critical role in language processing, some questions remain. Here we examine the influence of overt versus covert speech production on lateralization, the relationship between lateralization and behavioural measures of language performance and the strength of lateralization across the subcomponents of language. The present study used functional transcranial Doppler sonography (fTCD) to investigate lateralization of phonological and semantic fluency during both overt and covert word generation in right-handed adults. The laterality index (LI) was left lateralized in all conditions, and there was no difference in the strength of LI between overt and covert speech. This supports the validity of using overt speech in fTCD studies, another benefit of which is a reliable measure of speech production.     

Separating phonological and semantic processing in auditory sentence processing: a high-resolution event-related brain potential study

Phonological and semantic processing was studied using high-resolution event-related brain potentials (ERPs) during a sentence-matching task to investigate the spatial distribution of the phonological mismatch negativity (PMN) and the N400 response. It was hypothesized that the two components were spatially separable and that the activity matched prior localization knowledge. Participants examined visual-auditory sentence pairs that related within a semantic hierarchy (e.g., visual: "The man is teaching in the classroom"; Auditory: "The man is in the em leader school/barn"). Semantic congruency was varied for the final words of the spoken sentences. Incongruent words mismatched expectation in terms of both the initial phonological features (unexpected sound) and semantic features (unexpected meaning). In addition, the category-exemplar probability of the final words was either high or low, with low probability words being more difficult to anticipate. Low probability words were predicted to selectively affect PMN activity. We found that incongruent words elicited a PMN (287 msec) and a N400 (424 msec), for both the high and low probability words. As expected, low probability congruent words elicited a small PMN but no N400. In contrast, high probability congruent words elicited neither a detectible PMN nor a N400. The primary PMN sources were in left inferior frontal and inferior parietal lobes. The primary N400 source activation occurred along the left perisylvian cortex, consistent with prior N400 source localization work. From these results, it was concluded that the PMN and N400 were localized to separate cortical language (and memory) regions and had different source activation patterns.     

Independent access to phonological and orthographic lexical representations: a replication study

Traditionally, it has been assumed that written abilities are completely dependent on phonology (the so-called phonological mediation theory). In this view, the semantic system activates the phonological form of a word, which then activates the corresponding orthographic representation. This hypothesis predicts that in double naming tasks (two temporally close responses to the same picture in the two output modalities), the word selected for a written response will always correspond to the one selected for a spoken output. On the contrary,the orthographic autonomy theory hypothesises the independence of the written output lexicon, which, in the presence of severe damage to conversion procedures, could be directly activated from the semantic system. In this case, the theory predicts that double naming tasks give rise to lexically inconsistent responses.We describe a non-fluent aphasic patient, RA, with agrammatic speech and severe word- finding difficulties. He also presented with severe damage to the phonology-to-orthography and orthography-to-phonology conversion procedures. His performance on picture naming tasks was significantly worse in the spoken than in the written output modality. Errors were predominantly omissions. Few semantic paraphasias were present. Semantic errors did not arise from a deficit in the semantic system since the patient performed flawlessly on a semantic questionnaire and a word picture matching task and gave correct definitions of the items. Although not predicted by the phonological mediation theory, on double naming tasks the patient gave inconsistent responses (i.e. he correctly wrote the stimulus "z-e-b-r-a" but he orally produced "giraffe"). Taken together, these results further confirm the hypothesis that access to the orthographic form of words can be accomplished independently, without the mediation of phonology (the orthographic autonomy hypothesis).     

Prefrontal cortical response to conflict during semantic and phonological tasks

Debates about the function of the prefrontal cortex are as old as the field of neuropsychology--often dated to Paul Broca's seminal work. Theories of the functional organization of the prefrontal cortex can be roughly divided into those that describe organization by process and those that describe organization by material. Recent studies of the function of the posterior, left inferior frontal gyrus (pLIFG) have yielded two quite different interpretations: One hypothesis holds that the pLIFG plays a domain-specific role in phonological processing, whereas another hypothesis describes a more general function of the pLIFG in cognitive control. In the current study, we distinguish effects of increasing cognitive control demands from effects of phonological processing. The results support the hypothesized role for the pLIFG in cognitive control, and more task-specific roles for posterior areas in phonology and semantics. Thus, these results suggest an alternative explanation of previously reported phonology-specific effects in the pLIFG.     

Language lateralization in magnetoencephalography: two tasks to investigate hemispheric dominance

Hemispheric specialization for language has been the focus of many studies, mainly using functional magnetic resonance imaging. Here, we used magnetoencephalography to investigate hemispheric dominance and time-dependent aspects of cortical language processing. We implemented a verb generation task and a newly designed vowel identification task. Eleven healthy adults were investigated. By using oscillatory magnetoencephalography spectral analysis, significant hemispheric differences were found for both tasks in cerebral language areas. Robust left-lateralization in frontal brain regions was observed with the verb generation task, confirming previous functional magnetic resonance imaging and magnetoencephalography studies. Our new vowel identification task yields significant left-lateralization in posterior language regions, making this silent and child-friendly task a valuable alternative for non-invasive language assessment in difficult populations.     

Functional connectivity during orthographic,phonological, and semantic processing of Chinese characters identifies distinct visuospatial and phonosemantic networks

While neuroimaging studies have identified brain regions associated with single word reading, its three constituents, namely, orthography, phonology, and meaning, and the functional connectivity of their networks remain underexplored. This study examined the neurocognitive underpinnings of these neural activations and functional connectivity of the identified brain regions using a within‐subject design. Thirty‐one native Mandarin speakers performed orthographic, phonological, and semantic judgment tasks during functional magnetic resonance imaging. The results indicated that the three processes shared a core network consisting of a large region in the left prefrontal cortex, fusiform gyrus, and medial superior frontal gyrus but not the superior temporal gyrus. Orthographic processing more strongly recruited the left dorsolateral prefrontal cortex, left superior parietal lobule and bilateral fusiform gyri; semantic processing more strongly recruited the left inferior frontal gyrus and left middle temporal gyrus, whereas phonological processing more strongly activated the dorsal part of the precentral gyrus. Functional connectivity analysis identified a posterior visuospatial network and a frontal phonosemantic network interfaced by the left middle frontal gyrus. We conclude that reading Chinese recruits cognitive resources that correspond to basic task demands with unique features best explained in connection with the individual reading subprocesses.     

Language lateralization in schizophrenia, an fMRI study

Anatomical studies have shown that cerebral asymmetry is reduced in schizophrenia. Functional asymmetry appears to be reduced also, as was shown with dichotic listening studies. These studies, however, have not revealed whether reduced lateralization is the result of decreased language activity of the left hemisphere or whether it is the consequence of increased language-related activity in the right hemisphere. To elucidate this, we examined hemispheric dominance for language processing by means of functional MRI. Twelve schizophrenic patients and twelve healthy controls were scanned while they were engaged in a verb-generation and a semantic decision task. Activation was measured bilaterally in the frontal, temporal and temporo-parietal language areas, and a laterality index was derived from activity in these regions of interest in the left and the right hemispheres. Clinical symptoms were rated at the time of scanning.The results indicate that language processing is less lateralized in patients than in controls (a mean laterality index of 0.35 versus 0.63, respectively, difference p<0.01). Analysis of variance of the extent of activity, i.e. numbers of active voxels, revealed a significant hemisphere by group interaction (F(1,22)=11.2, p<0.001), which was due to increased activation in the right hemisphere of the patients (post hoc t-test p<0.05). We found no evidence of reduced activity in the left hemisphere. Further analysis of clinical symptoms rated prior to scanning revealed that decreased language lateralization was associated with more severe hallucinations (r=-0.54, p<0.05). We postulate that decreased language lateralization in schizophrenia may result from failure to inhibit the right hemisphere.     

The interaction between orthographic and phonological information in children: an fMRI study

We examined the neural representations of orthographic and phonological processing in children, while manipulating the consistency between orthographic and phonological information. Participants, aged 9-15, were scanned while performing rhyming and spelling judgments on pairs of visually presented words. The orthographic and phonological similarity between words in the pair was independently manipulated, resulting in four conditions. In the nonconflicting conditions, both orthography and phonology of the words were either (1) similar (lime-dime) or (2) different (staff-gain); in conflicting conditions, words had (3) similar phonology and different orthography (jazz-has) or (4) different phonology and similar orthography (pint-mint). The comparison between tasks resulted in greater activation for the rhyming task in bilateral inferior frontal gyri (BA 45/47), and greater activation for the spelling task in bilateral inferior/superior parietal lobules (BA 40/7), suggesting greater involvement of phonological and semantic processing in the rhyming task, and nonlinguistic spatial processing in the spelling task. Conflicting conditions were more difficult in both tasks and resulted in greater activation in the above regions. The results suggest that when children encounter inconsistency between orthographic and phonological information they show greater engagement of both orthographic and phonological processing.     

EEG brain mapping of phonological and semantic tasks in Italian and German languages.

OBJECTIVES: Event-related potential correlates of phonological encoding - as compared with lexical access and semantic categorization - were measured in two studies involving two groups of 14 German and 14 Italian subjects.METHODS: A two stimulus reaction time paradigm was used. Stimulus pairs presented one-by-one with 2 s inter-stimulus intervals (ISI) had to be matched with respect to lexical identity (word-picture) in a word comprehension task or with respect to the phonological representative of objects in a rhyming task. A semantic categorization task was added for the Italian sample. In both studies, the EEG was recorded from 26 scalp electrodes according to the 10-20 system. The slow negative potential during the ISI (CNV) was determined as the electrocortical correlate of preparation for and activation of the specific language-related task.RESULTS: In both samples, phonological encoding (rhyming) evoked a more pronounced CNV over the left- compared with the right-frontal area, while less lateralized central dominance of the CNV was found in the word comprehension task. Semantic categorization was accompanied by the least asymmetry of activity.CONCLUSIONS: Results indicate that the different degree of asymmetry induced by phonological and semantic processing may be determined from the scalp distribution of slow cortical potentials with cross-lingual reliability.     

Disrupted neural responses to phonological and orthographic processing in dyslexic children: an fMRI study

Developmental dyslexia, characterized by difficulty in reading, has been associated with phonological and orthographic processing deficits. fMRI was performed on dyslexic and normal-reading children (8-12 years old) during phonological and orthographic tasks of rhyming and matching visually presented letter pairs. During letter rhyming, both normal and dyslexic reading children had activity in left frontal brain regions, whereas only normal-reading children had activity in left temporo-parietal cortex. During letter matching, normal-reading children showed activity throughout extrastriate cortex, especially in occipito-parietal regions, whereas dyslexic children had little activity in extrastriate cortex during this task. These results indicate dyslexia may be characterized in childhood by disruptions in the neural bases of both phonological and orthographic processes important for reading.     

Proactive interference in a semantic short-term memory deficit: role of semantic and phonological relatedness

Previous research has indicated that patients with semantic short-term memory (STM) deficits demonstrate unusual intrusions of previously presented material during serial recall tasks (Martin and Lesch, 1996). These intrusions suggest excessive proactive interference (PI) from previous lists. Here, we explore one such patient's susceptibility to PI. Experiment 1 demonstrated patient M.L.'s extreme susceptibility to PI using a probe recognition task that manipulates the recency of negative probes (the recent negatives task). When stimuli consisted of letters, M.L. showed greatly exaggerated effects of PI, well outside of the range of healthy control participants. Experiment 2 used a variation of the recent negatives task to examine the relative contribution of semantic and phonological relatedness in PI. This task manipulated semantic and phonological relatedness of probes and recently presented list items. Relative to healthy control participants, patient M.L. showed exaggerated interference effects for both phonological and semantically related probes, both for probes related to the current list and for probes related to the previous list. These data have important implications for theories of semantic STM deficits. Specifically, these data suggest that it is not the rapid decay of semantic representations that is responsible for difficulties in short-term recall, but rather the abnormal persistence of previously presented material. We propose that this susceptibility to PI is the result of a deficit in control processes acting on STM.     

Anomia is simply a reflection of semantic and phonological impairments: Evidence from a case-series study

A number of recent studies have attempted to explain patterns of normal and impaired performance in a variety of different language tasks with respect to the same set of ''primary'' systems rather than resorting to explanations in terms of dedicated processes, specific to each and every language activity. In this study we consider whether the same approach can be taken to patterns of impaired single-word speech production. Specifically, using cross-sectional data from 21 aphasic patients we tested the hypothesis that the degree and nature of anomia can be explained using independently derived measures of the integrity of the patients' phonological and semantic/conceptual representations, without postulating a role for an abstract lexical level of representation. At a global level, we found that these two measures explained 55-80%of the variance in the patients' naming accuracy, a figure which approaches that found for test reliability. There was also a close fit between observed and expected naming accuracy for all individual patients. The same two measures also predicted the rate of different types of anomic error across individuals. Measures intended to assess lexical integrity did not explain any additional, unique variance in naming accuracy. We discuss these results and the theoretical approach with respect to existing theories of speech production, and evaluate the case-series methodology itself, both as a tool to reveal the underpinnings of speech production and as a neuropsychological technique in general.     

Language lateralization in a bimanual language

Unlike spoken languages, sign languages of the deaf make use of two primary articulators, the right and left hands, to produce signs. This situation has no obvious parallel in spoken languages, in which speech articulation is carried out by symmetrical unitary midline vocal structures. This arrangement affords a unique opportunity to examine the robustness of linguistic systems that underlie language production in the face of contrasting articulatory demands and to chart the differential effects of handedness for highly skilled movements. Positron emission tomography (PET) technique was used to examine brain activation in 16 deaf users of American Sign Language (ASL) while subjects generated verb signs independently with their right dominant and left nondominant hands (compared to the repetition of noun signs). Nearly identical patterns of left inferior frontal and right cerebellum activity were observed. This pattern of activation during signing is consistent with patterns that have been reported for spoken languages including evidence for specializations of inferior frontal regions related to lexical-semantic processing, search and retrieval, and phonological encoding. These results indicate that lexical-semantic processing in production relies upon left-hemisphere regions regardless of the modality in which a language is realized, and that this left-hemisphere activation is stable, even in the face of conflicting articulatory demands. In addition, these data provide evidence for the role of the right posterolateral cerebellum in linguistic-cognitive processing and evidence of a left ventral fusiform contribution to sign language processing.     

Neural correlates of orthographic and phonological consistency effects in children

The objective of this study was to examine the neural correlates of phonological inconsistency (relationship of spelling to sound) and orthographic inconsistency (relationship of sound to spelling) in visual word processing using functional magnetic resonance imaging (fMRI). Children (9- to 15-year-old) performed a rhyming and spelling task in which two words were presented sequentially in the visual modality. Consistent with previous studies in adults, higher phonological inconsistency was associated with greater activation in several regions including left inferior frontal gyrus and medial frontal gyrus/anterior cingulate cortex. We additionally demonstrated an effect of orthographic inconsistency in these same areas, suggesting that these regions are involved in the integration of orthographic and phonological information and, with respect to the medial frontal/anterior cingulate, greater demands on executive function. Higher phonological and orthographic consistency was associated with greater activation in precuneus/posterior cingulate cortex, the putative steady state system active during resting, suggesting lower demands on cognitive resources for consistent items. Both consistency effects were larger for the rhyming compared with the spelling task suggesting greater demands of integrating spelling and sound in the former task. Finally, accuracy on the rhyming task was negatively correlated with the consistency effect in left fusiform gyrus. In particular, this region showed insensitivity to consistency in low performers, sensitivity to inconsistency (higher activity) in moderate performers, and sensitivity to inconsistency (high activation) and to consistency (deactivation). In general, these results show that the influence of spelling-sound (and sound-spelling) correspondences on processing in fusiform gyrus develops as a function of skill.     

Language lateralization in female patients with schizophrenia: an fMRI study

Gender differences in schizophrenia are among the most consistently reported findings in schizophrenia research. However, the biological substrate underlying these gender differences is still largely unknown. Differences in language lateralization between men and women may underlie some gender differences in schizophrenia.

In previous functional imaging studies, language lateralization was found to be decreased in male schizophrenia patients as compared to healthy males, which was due to enhanced language activation of the right hemisphere as compared to the healthy males. It could be hypothesized that decreased language lateralization in schizophrenia is gender specific, i.e. decreased lateralization in male patients and normal lateralization in female patients.

To test this hypothesis, language activation was measured in 12 right-handed female patients with schizophrenia and 12 healthy females, and compared to findings in 12 male patients and 12 male controls of an earlier study.

Language lateralization was significantly lower in the female patients (0.44) as compared to the female controls (0.75), which was due to increased activation of the right-sided language areas (patients: 19 voxels; controls: 8 voxels), while left hemisphere activation was similar in patients and controls. When these data are compared to the male patients and controls, both patient groups had lower lateralization than their healthy counterparts, but there was no difference between male and female patients. In both sexes, decreased lateralization resulted from increased right hemispheric language activation, which suggests a failure to inhibit nondominant language areas in schizophrenia. These findings indicate that lower language lateralization in women is not likely to underlie gender differences in schizophrenia.     

On-line orthographic influences on spoken language in a semantic task

Literacy changes the way the brain processes spoken language. Most psycholinguists believe that orthographic effects on spoken language are either strategic or restricted to meta-phonological tasks. We used event-related brain potentials (ERPs) to investigate the locus and the time course of orthographic effects on spoken word recognition in a semantic task. Participants were asked to decide whether a given word belonged to a semantic category (body parts). On no-go trials, words were presented that were either orthographically consistent or inconsistent. Orthographic inconsistency (i.e., multiple spellings of the same phonology) could occur either in the first or the second syllable. The ERP data showed a clear orthographic consistency effect that preceded lexical access and semantic effects. Moreover, the onset of the orthographic consistency effect was time-locked to the arrival of the inconsistency in a spoken word, which suggests that orthography influences spoken language in a time-dependent manner. The present data join recent evidence from brain imaging showing orthographic activation in spoken language tasks. Our results extend those findings by showing that orthographic activation occurs early and affects spoken word recognition in a semantic task that does not require the explicit processing of orthographic or phonological structure.     

Language lateralization in individuals with callosal agenesis: an fMRI study

Since the seminal work of Broca in 1861, it is well established that language is essentially processed in the left hemisphere. However, the origin of hemispheric specialization remains controversial. Some authors posit that language lateralization is genetically determined, while others have suggested that hemispheric specialization develops with age. Tenants of the latter view have further suggested that the adult pattern of left hemispheric specialization is achieved by means of callosal inhibition of homologous speech areas in the right hemisphere during ontogeny. According to this hypothesis, one would expect language to develop bilaterally in the acallosal brain. A recent functional magnetic resonance imaging (fMRI) study in one patient with agenesis of the corpus callosum suggests that this might indeed be the case (Riecker et al., 2007). However, given the large anatomic and functional variability in the population of subjects with agenesis of the corpus callosum, this finding needs to be more extensively replicated. In the present study, we explored language lateralization in six individuals with agenesis of the corpus callosum using an fMRI protocol which included a syntactic decision task and a sub-vocal verbal fluency task. Two neurologically intact control groups, one comparable to the acallosals in terms of IQ, age and education (n = 6) and one group with a high IQ (n = 5), performed the same tasks. No differences were found between language lateralization of the subjects with agenesis of the corpus callosum and the control groups in the receptive speech task. However, for expressive speech, the groups differed with respect to frontal activations, with the acallosal participants showing a more bilateral pattern of activation than the high-IQ participants only. No differences were found for temporal regions. Overall, these results indicate that the corpus callosum is not essential for the establishment of lateralized language functions.     

Spinal evoked potential in man: a maturational study.

Summated evoked potentials to peroneal nerve stimulation which arise in the cauda equina and spinal cord afferent pathways were recorded from surface electrodes placed over the spine of 95 infants, children and adults. The conduction velocity of these potentials from lumbar to cervical recording sites was calculated. Additionally, segmental conduction velocities over peroneal nerve, caudal and rostral spinal cord were determined. In all age groups the speed of conduction up the spine was non-linear. It was slower over caudal spinal cord segments than over peripheral nerve or rostral cord. All these velocities were about half adult values in the newborn and progressively increased with age. The conduction velocity over peripheral nerve was within the adult range by 3 years of age. The speed of conduction over the spinal cord did not reach adult values until the 5th year. This suggests that spinal cord afferent pathways mature at a slower rate than peripheral nerve fibers.