David W. Green

Neuroimaging of language control in bilinguals: neural adaptation and reserve

Speaking more than one language demands a language control system that allows bilinguals to correctly use the intended language adjusting for possible interference from the non-target language. Understanding how the brain orchestrates the control of language has been a major focus of neuroimaging research on bilingualism and was central to our original neurocognitive language control model (Abutalebi & Green, 2007). We updated the network of language control (Green & Abutalebi, 2013) and here review the many new exciting findings based on functional and structural data that substantiate its core components. We discuss the language control network within the framework of the adaptive control hypothesis (Green & Abutalebi, 2013) that predicts adaptive changes specific to the control demands of the interactional contexts of language use. Adapting to such demands leads, we propose, to a neural reserve in the human brain.

The relationship of bilingualism to cognitive decline: The Australian Longitudinal Study of Ageing

We wished to clarify the link between bilingualism and cognitive decline, and examine whether improved executive function due to bilingualism may be a factor in preventing cognitive decline.

Trajectories to third-language proficiency

Is there a single trajectory to third-language (L3) communicative proficiency in proficient, adult bilingual speakers? Parsimony favours such a possibility but in this theoretical paper we argue that multiple trajectories will be the norm. We focus on the processes of language control. These processes mediate the initial transfer of syntactic forms, entrain processes that change the language network and govern the selection of L3 syntactic structures and lexical items. Theoretical models of initial transfer differ in terms of their demands on top-down and bottom-up processes of language control. L3 learning, though, requires both types of process, yielding potential variability in the syntactic structures that populate the landscape of transfer. A language network can capture that landscape by tagging any existing structure (whether from the first language or from the second language) for use in the L3 by linking it to a L3 language node. Representational change incurs further processing costs because speakers must select L3 syntactic forms and lexical items in the face of competition. In line with earlier research, we propose that top-down control processes external to the language network help select outputs for speech production but these processes themselves must adapt to the demands of selecting amongst three rather than two languages. In a final section we review the nature of variability in language control processes and the processes they entrain. Such variability strongly predicts multiple trajectories to L3 proficiency. Exploring the nature of such variety, using converging methods in longitudinal designs, provides an opportunity for theoretical and practical advance.

Language control and the neuroanatomy of bilingualism: in praise of variety

ABSTRACT Structural data when allied to rich behavioural data offer an important resource for studying adaptive changes in the human brain contingent on the use of more than one language. In our commentary on the review paper by García-Pentón et al. (2015. The neuroanatomy of bilingualism: How to turn a hazy view into the full picture. Language, Cognition and Neuroscience.) we emphasise that data from a variety of methods already converge in identifying key regions subject to adaptive change. With richer characterisations of individual differences in language use more specific predictions can be tested (e.g. on fronto-cerebellar circuits) and such characterisations can in turn inform intervention studies to help specify the causal bases of adaptive change. In the meantime there is great research value in neuroanatomical studies of richly characterised cross-sectional samples of bilingual and multilingual speakers.

Code-switching and language control

Bilingual speakers can use one of their languages in a given interactional context or switch between them when addressing different speakers during the same conversation. Depending on community usage bilingual speakers may insert single lexical forms from one language into the morphosyntactic frame of another or alternate between languages at clause boundaries. They may also engage in dense code switching with rapid changes of language within a clause during a conversational turn (Green & Li, 2014). These varieties of language use configure the same speech production mechanism and so a theory of code-switching must be part of a theory that accounts for the range of bilingual speech. Does the proposal described in Goldrick, Putnam and Schwartz (2016) meet these criteria?

How distributed processing produces false negatives in voxel-based lesion-deficit analyses

ABSTRACT In this study, we hypothesized that if the same deficit can be caused by damage to one or another part of a distributed neural system, then voxel‐based analyses might miss critical lesion sites because preservation of each site will not be consistently associated with preserved function. The first part of our investigation used voxel‐based multiple regression analyses of data from 359 right‐handed stroke survivors to identify brain regions where lesion load is associated with picture naming abilities after factoring out variance related to object recognition, semantics and speech articulation so as to focus on deficits arising at the word retrieval level. A highly significant lesion‐deficit relationship was identified in left temporal and frontal/premotor regions. Post‐hoc analyses showed that damage to either of these sites caused the deficit of interest in less than half the affected patients (76/162=47%). After excluding all patients with damage to one or both of the identified regions, our second analysis revealed a new region, in the anterior part of the left putamen, which had not been previously detected because many patients had the deficit of interest after temporal or frontal damage that preserved the left putamen. The results illustrate how (i) false negative results arise when the same deficit can be caused by different lesion sites; (ii) some of the missed effects can be unveiled by adopting an iterative approach that systematically excludes patients with lesions to the areas identified in previous analyses, (iii) statistically significant voxel‐based lesion‐deficit mappings can be driven by a subset of patients; (iv) focal lesions to the identified regions are needed to determine whether the deficit of interest is the consequence of focal damage or much more extensive damage that includes the identified region; and, finally, (v) univariate voxel‐based lesion‐deficit mappings cannot, in isolation, be used to predict outcome in other patients. HIGHLIGHTSPost‐hoc analyses of results from voxel‐based lesion‐deficit analyses are reported.The effects of different lesion sites cancel each other out when all data are pooled.Removing patients with one lesion site can reveal other critical lesion sites.Our findings highlight the need for multivariate lesion‐deficit analyses.

Neural basis of bilingual language control

Acquiring and speaking a second language increases demand on the processes of language control for bilingual as compared to monolingual speakers. Language control for bilingual speakers involves the ability to keep the two languages separated to avoid interference and to select one language or the other in a given conversational context. This ability is what we refer with the term “bilingual language control” (BLC). It is now well established that the architecture of this complex system of language control encompasses brain networks involving cortical and subcortical structures, each responsible for different cognitive processes such as goal maintenance, conflict monitoring, interference suppression, and selective response inhibition. Furthermore, advances have been made in determining the overlap between the BLC and the nonlinguistic executive control networks, under the hypothesis that the BLC processes are just an instantiation of a more domain‐general control system. Here, we review the current knowledge about the neural basis of these control systems. Results from brain imaging studies of healthy adults and on the performance of bilingual individuals with brain damage are discussed.

The impact of sample size on the reproducibility of voxel-based lesion-deficit mappings

ABSTRACT This study investigated how sample size affects the reproducibility of findings from univariate voxel‐based lesion‐deficit analyses (e.g., voxel‐based lesion‐symptom mapping and voxel‐based morphometry). Our effect of interest was the strength of the mapping between brain damage and speech articulation difficulties, as measured in terms of the proportion of variance explained. First, we identified a region of interest by searching on a voxel‐by‐voxel basis for brain areas where greater lesion load was associated with poorer speech articulation using a large sample of 360 right‐handed English‐speaking stroke survivors. We then randomly drew thousands of bootstrap samples from this data set that included either 30, 60, 90, 120, 180, or 360 patients. For each resample, we recorded effect size estimates and p values after conducting exactly the same lesion‐deficit analysis within the previously identified region of interest and holding all procedures constant. The results show (1) how often small effect sizes in a heterogeneous population fail to be detected; (2) how effect size and its statistical significance varies with sample size; (3) how low‐powered studies (due to small sample sizes) can greatly over‐estimate as well as under‐estimate effect sizes; and (4) how large sample sizes (N≥90) can yield highly significant p values even when effect sizes are so small that they become trivial in practical terms. The implications of these findings for interpreting the results from univariate voxel‐based lesion‐deficit analyses are discussed. HIGHLIGHTSThe same lesion‐deficit analysis was repeated on thousands of bootstrap samples.Replicability of the original effect was contingent upon the size of the sample.With smaller samples, only inflated effect size estimates reached significance.With larger samples, even trivial effect sizes yielded significant p values.

Response to commentary on “The relationship of bilingualism to cognitive decline: The Australian Longitudinal Study of Ageing”

We thank Wei Xing NoahToh and colleagues for their interest in our paper on bilingualism and cognitive decline. We agree with the authors who say verbal measures of executive function, which we used in our paper, may bias results against those for whom English is not a first language. We have discussed this as a limitation in the paper. The authors also point out that MMSE score is likely to be affected by education, which is why we adjusted for education in our linear regression model investigating the effect of bilingualism on MMSE score and found no significant difference of bilingualism once education was taken into account. This is not surprising as our analysis was of decline and therefore baseline function on the MMSE was taken into account. We also checked for collinearity between MMSE and education‐related variables before fitting our models. The MMSE does not, as the authors point out, measure Executive Function (EF), but it is still a well‐validated measure of cognitive decline which is why it was suited for our primary analysis and why we included tests of EF that were available in this cohort, in a separate analysis. We agree that the measure of bilingualism was coarse‐grained in this cohort, but it is rare for cohort studies to measure bilingual status at all. Our circumspect conclusion takes this into account and states that “bilingualism is complex and that simply speaking two languages does not protect from cognitive decline or enhance executive function. The precise pattern of language use in bilingual speakers may be critical and certainly such information is necessary to more fully disentangle the longer‐term neuroprotective effect of bilingualism.” Mukadam et al further investigated this topic in their systematic review of bilingualism and cognitive decline and again concluded that bilingualism itself is not protective against cognitive decline when analyses from cohort studies are appropriately adjusted for education and socioeconomic status. We would welcome future research that could measure the more fine‐grained effects of bilingualism, comparing, for example, bilinguals who frequently switch between different languages and those who use their languages in separate environments.