Chiara Spironelli

Schizophrenia as Failure of Left Hemispheric Dominance for the Phonological Component of Language

Background T. J. Crow suggested that the genetic variance associated with the evolution in Homo sapiens of hemispheric dominance for language carries with it the hazard of the symptoms of schizophrenia. Individuals lacking the typical left hemisphere advantage for language, in particular for phonological components, would be at increased risk of the typical symptoms such as auditory hallucinations and delusions. Methodology/Principal Findings Twelve schizophrenic patients treated with low levels of neuroleptics and twelve matched healthy controls participated in an event-related potential experiment. Subjects matched word-pairs in three tasks: rhyming/phonological, semantic judgment and word recognition. Slow evoked potentials were recorded from 26 scalp electrodes, and a laterality index was computed for anterior and posterior regions during the inter stimulus interval. During phonological processing individuals with schizophrenia failed to achieve the left hemispheric dominance consistently observed in healthy controls. The effect involved anterior (fronto-temporal) brain regions and was specific for the Phonological task; group differences were small or absent when subjects processed the same stimulus material in a Semantic task or during Word Recognition, i.e. during tasks that typically activate more widespread areas in both hemispheres. Conclusions/Significance We show for the first time how the deficit of lateralization in the schizophrenic brain is specific for the phonological component of language. This loss of hemispheric dominance would explain typical symptoms, e.g. when an individuals own thoughts are perceived as an external intruding voice. The change can be interpreted as a consequence of “hemispheric indecision”, a failure to segregate phonological engrams in one hemisphere.

Delta EEG activity as a marker of dysfunctional linguistic processing in developmental dyslexia

The present study used delta EEG band to test the hypothesis of a cerebral maturational delay and a functional altered cerebral asymmetry for phonological processing in dyslexic children. A group of 14 children with dyslexia and 28 matched controls participated in a linguistic paradigm in which the same words were processed in three tasks: phonological, semantic, and orthographic. Delta amplitude was computed as an index of cortical inhibition in four different phases of word processing. In anterior sites, controls showed left activation (reduced delta) during the phonological task and bilateral activation in the other two tasks. Conversely, children with dyslexia showed greater overall delta amplitude, indexing a cerebral maturation delay and an altered language laterality pattern. In the phonological task they had larger left anterior delta (inhibition of left frontal linguistic locations) and smaller left posterior delta amplitude (activation of left posterior sites silent in controls). Results support the phonological deficit hypothesis of developmental dyslexia and the validity of EEG delta band as functional and clinical measure of language laterality.

Brain plasticity in developmental dyslexia after phonological treatment: A beta EEG band study

Linguistic EEG hemispheric reorganization was investigated in 14 dyslexic children after a 6-month phonological training (10 min/day through PC software). Error rates from three linguistic tasks significantly decreased and reading speed improved after the training. A significant positive correlation (r(12)=0.536) was found at posterior sites for the phonological task only, showing that those children who had the greatest reading speed enhancement showed the largest left posterior EEG beta power increase.

Language lateralization in phonological, semantic and orthographic tasks: A slow evoked potential study

Most of literature on language has shown how different word-classes activate distinct neural networks within linguistic cortical areas. The present investigation aimed to demonstrate that, by means of slow evoked potentials and using the same set of words in different tasks, it is possible to activate cortical networks that are spatially and temporally distinguished. Twenty healthy subjects had to evaluate, in a word pair matching session, whether two words rhymed (phonological task), were semantically related (semantic task) or were written in the same letter case (orthographic task). Slow wave amplitude was computed in three relevant time windows: the last 0.5 s of first word presentation (W1), the initial contingent negative variation (iCNV) and the terminal CNV (tCNV). During W1 and iCNV intervals, both the orthographic and the phonological tasks were left lateralized. Furthermore, the phonological task was more lateralized than the orthographic because of a greater inhibition of the right hemisphere, whereas the orthographic task was characterized by a greater bilateral posterior activation. During the tCNV, only the phonological task remained left lateralized while orthographic and semantic were bilaterally distributed. Although the use of the same set of words tends to activate widely overlapped networks, in the present research task manipulation was effective in demonstrating task dependent differences in brain lateralization. Thus, the present paradigm and the adopted tasks are especially suited for studying deficit and recovery of language in patients affected by linguistic disorders such as developmental dyslexia and aphasia.

Inverted EEG theta lateralization in dyslexic children during phonological processing.

The phonological deficit hypothesis of dyslexia has been investigated in the present research by analysing language-related lateralization of the EEG theta band in a sample of dyslexic children. To this aim, a paradigm based on word-pair visual presentation was used in which the same words were processed in Semantic and Phonological tasks. Theta band amplitude, a cortical index that has been related to working memory processing, was analysed during four different phases of word elaboration, thus allowing to measure also the temporal dynamics of word reading/encoding in the verbal working memory. Control subjects showed a specific (and therefore efficient) task-related and time-dependent cortical activation: a peak of theta activity during word reading was found that decayed during the next inter stimulus interval. Furthermore, during word presentation in the Phonological task, theta amplitude was greater on the left hemisphere. Dyslexics evidenced an altered pattern of theta activation both in the temporal dimension and in the cortical space: their peak of activity was delayed to the first inter stimulus interval after word offset and was shifted to the right hemisphere throughout the whole epoch of Phonological task and in two phases of the Semantic task. Analysis of alpha band failed to replicate the complex pattern of lateralization found for theta band in the two groups, a result that suggests a specific functional role of theta band, which cannot be interpreted as a simple marker of cortical inhibition. Results point to a deficit, in dyslexic children, to recruit left hemisphere structures for the elaboration of the phonological component of the verbal working memory. This deficit was marked by a different, unspecific and dysfunctional hemispherical asymmetry of theta activation to language, a deficit that involved also the time course of phonological linguistic elaboration.

Mass and Count nouns activate different brain regions : An ERP study on early components

In the present study, event related brain potentials (ERPs) showed that, in an implicit Lexical decision task in which participants had to decide whether a word or a pseudoword was presented, a very early distinction between Mass and Count nouns was found at 160 ms after word onset (N150). Mass nouns elicited greater left-lateralization over frontal locations while Count nouns were more lateralized in the left occipito-parietal sites. In the 430-490 ms interval activity and lateralization shifted to anterior sites and a different distribution was found between Mass nouns, Count nouns and Pseudowords. Mass nouns showed greater left-lateralization both in anterior and posterior regions, whereas Count nouns showed relatively less left-lateralization especially over frontal cortex. Results point to a functional distinction between Mass and Count nouns as indicated by the very early automatic N150 difference between the two categories. Count nouns involved left visual associative regions that are typically relevant for object recognition and categorization. Mass nouns, instead, required the activation of more widely spread out linguistic networks that included also left frontal sites, a result that indicates a more difficult and engaging automatic retrieval and an extended cortical representation of these nouns.

Language plasticity in aphasics after recovery: Evidence from slow evoked potentials

With the present experiment we sought to investigate brain plasticity underlying language recovery in a group of seventeen patients with non-fluent aphasia mainly caused by stroke. Patients were screened along three domains of measures: analysis of linguistic components by the Aachener Aphasie Test, combined mapping of their lesion from CT/MRI scans, and functional measure of the reorganized linguistic processes by means of mapping of slow evoked potentials. The spatial dimension and temporal dynamics of word processing were measured in three tasks, Phonological, Semantic and Orthographic. Compared with the matched control group, patients showed relative inhibition (decreased negativity) of left central regions in perisylvian areas, which were damaged in most subjects. In addition, reorganization of linguistic functions occurred within the left hemisphere both at frontal and posterior sites corresponding to spared brain regions. Correlations between linguistic lateralization in the three tasks and AAT subtests point to functional reorganization of phonological processes over left frontal sites and dysfunctional reorganization of semantic processing over left posterior regions.

Functional plasticity in Alzheimer's disease: effect of cognitive training on language-related ERP components.

Starting from the observation of a reduced gray matter in the inferior temporal regions of Alzheimers disease (AD) patients, the present study hypothesized an altered language-related functional activity in left occipito-temporal areas in AD, and the possibility of a plastic change of these regions induced by an intensive cognitive training. To this aim, eleven mild/moderate AD underwent to a 5-week cognitive training (40 h). Before and after the training, evoked potentials were recorded from 26 scalp electrodes during a lexical decision task which required word/no-word discrimination. Stimuli included high- and low-frequency words and non-words, and the recognition potential (RP) together with the N400 have been analyzed and compared with those collected from a matched healthy control group. Results comparing controls and patients before training showed a normal RP in AD patients with a clear peak over left occipito-temporal sites. In addition, controls exhibited a left anterior lateralization of N400 component to words and an inverted pattern for non-words, whereas an altered N400 with bilateral distribution at both word and non-word conditions was found in AD patients. After the cognitive training, AD patients did not show changes in the N400, but revealed a significant enhanced amplitude of RP to high-frequency words. Behavioral responses to the lexical decision task and scores from neuropsychological tests did not evidence improvements nor worsening after training. These data point to an intact functionality of left posterior linguistic networks in mild/moderate AD, and the possibility to increase plastically their activity after a cognitive training.

Multi-tasking uncovers right spatial neglect and extinction in chronic left-hemisphere stroke patients

Unilateral Spatial Neglect, the most dramatic manifestation of contralesional space unawareness, is a highly heterogeneous syndrome. The presence of neglect is related to core spatially lateralized deficits, but its severity is also modulated by several domain-general factors (such as alertness or sustained attention) and by task demands. We previously showed that a computer-based dual-task paradigm exploiting both lateralized and non-lateralized factors (i.e., attentional load/multitasking) better captures this complex scenario and exacerbates deficits for the contralesional space after right hemisphere damage. Here we asked whether multitasking would reveal contralesional spatial disorders in chronic left-hemisphere damaged (LHD) stroke patients, a population in which impaired spatial processing is thought to be uncommon. Ten consecutive LHD patients with no signs of right-sided neglect at standard neuropsychological testing performed a computerized spatial monitoring task with and without concurrent secondary tasks (i.e., multitasking). Severe contralesional (right) space unawareness emerged in most patients under attentional load in both the visual and auditory modalities. Multitasking affected the detection of contralesional stimuli both when presented concurrently with an ipsilesional one (i.e., extinction for bilateral targets) and when presented in isolation (i.e., left neglect for right-sided targets). No spatial bias emerged in a control group of healthy elderly participants, who performed at ceiling, as well as in a second control group composed of patients with Mild Cognitive Impairment. We conclude that the pathological spatial asymmetry in LHD patients cannot be attributed to a global reduction of cognitive resources but it is the consequence of unilateral brain damage. Clinical and theoretical implications of the load-dependent lack of awareness for contralesional hemispace following LHD are discussed.

Response selection and attention orienting: a computational model of Simon effect asymmetries.

Recently, there has been a redirection of research efforts toward the exploration of the role of hemispheric lateralization in determining Simon effect asymmetries. The present study aimed at implementing a connectionist model that simulates the cognitive mechanisms implied by such asymmetries, focusing on the underlying neural structure. A left-lateralized response-selection mechanism was implemented alone (Experiment 1) or along with a right-lateralized automatic attention-orienting mechanism (Experiment 2). It was found that both models yielded Simon effect asymmetries. However, whereas the first model showed a reversed pattern of asymmetry compared with human, real data, the second models performance strongly resembled human Simon effect asymmetries, with a significantly greater right than left Simon effect. Thus, a left-side bias in the response-selection mechanism produced a left-side biased Simon effect, whereas a right-side bias in the attention system produced a right-side biased Simon effect. In conclusion, results showed that the bias of the attention system had a larger impact than the bias of the response-selection mechanism in producing Simon effect asymmetries.