Inconsistent language lateralisation - Testing the dissociable language laterality hypothesis using behaviour and lateralised cerebral blood flow.

BACKGROUND: Most people have strong left-brain lateralisation for language, with a minority showing right- or bilateral language representation. On some receptive language tasks, however, lateralisation appears to be reduced or absent. This contrasting pattern raises the question of whether and how language laterality may fractionate within individuals. Building on our prior work, we postulated (a) that there can be dissociations in lateralisation of different components of language, and (b) these would be more common in left-handers. A subsidiary hypothesis was that laterality indices will cluster according to two underlying factors corresponding to whether they involve generation of words or sentences, versus receptive language. METHODS: We tested these predictions in two stages: At Step 1 an online laterality battery (Dichotic listening, Rhyme Decision and Word Comprehension) was given to 621 individuals (56% left-handers); At Step 2, functional transcranial Doppler ultrasound (fTCD) was used with 230 of these individuals (51% left-handers). 108 left-handers and 101 right-handers gave useable data on a battery of three language generation and three receptive language tasks. RESULTS: Neither the online nor fTCD measures supported the notion of a single language laterality factor. In general, for both online and fTCD measures, tests of language generation were left-lateralised. In contrast, the receptive tasks were at best weakly left-lateralised or, in the case of Word Comprehension, slightly right-lateralised. The online measures were only weakly correlated, if at all, with fTCD measures. Most of the fTCD measures had split-half reliabilities of at least .7, and showed a distinctive pattern of intercorrelation, supporting a modified two-factor model in which Phonological Decision (generation) and Sentence Decision (reception) loaded on both factors. The same factor structure fitted data from left- and right-handers, but mean scores on the two factors were lower (less left-lateralised) in left-handers. CONCLUSIONS: There are at least two factors influencing language lateralization in individuals, but they do not correspond neatly to language generation and comprehension. Future fMRI studies could help clarify how far they reflect activity in specific brain regions.

The role of orthographic neighbourhood effects in lateralized lexical decision: a replication study and meta-analysis.

The effect of orthographic neighbourhood size (N) on lexical decision reaction time differs when words are presented in the left or right visual fields. Evidence suggests a facilitatory N effect (i.e., faster reaction times for words with larger neighbourhoods) in the left visual field. However, the N effect in the right visual field remains controversial: it may have a weaker facilitative role or it may even be inhibitory. In a pre-registered online experiment, we replicated the interaction between N and visual field and provided support for an inhibitory N effect in the right visual field. We subsequently conducted a pre-registered systematic review and meta-analysis to synthesise the available evidence and determine the direction of N effects across visual fields. Based on the evidence, it would seem the effect is inhibitory in the right visual field. Furthermore, the size of the N effect is considerably smaller in the right visual field. Both studies revealed considerable heterogeneity between participants and studies, and we consider the implications of this for future work.

Cerebral lateralisation of first and second languages in bilinguals assessed using functional transcranial Doppler ultrasound.

Background: Lateralised representation of language in monolinguals is a well-established finding, but the situation is much less clear when there is more than one language. Studies to date have identified a number of factors that might influence the brain organisation of language in bilinguals. These include proficiency, age of acquisition and exposure to the second language. The question as to whether the cerebral lateralisation of first and second languages are the same or different is as yet unresolved. Methods: We used functional transcranial Doppler sonography (FTCD) to measure cerebral lateralisation in the first and second languages in 26 high proficiency bilinguals with German or French as their first language (L1) and English as their second language (L2). FTCD was used to measure task-dependent blood flow velocity changes in the left and right middle cerebral arteries during word generation cued by single letters. Language history measures and handedness were assessed through self-report questionnaires. Results:The majority of participants were significantly left lateralised for both L1 and L2, with no significant difference in the size of asymmetry indices between L1 and L2. Asymmetry indices for L1 and L2 were not related to language history, such as proficiency of the L2. Conclusion:  In highly proficient bilinguals, there is strong concordance for cerebral lateralisation of first and second languages.