The Unusual Suspects: Linguistic Deficits in Non-Language-Dominant Neurodegenerative Diseases

Discussions on linguistic deficits in neurodegenerative diseases (NDs) are often circumscribed to language-dominant conditions, such as the variants of primary progressive aphasia. However, verbal dysfunctions are also pervasive across non-language-dominant NDs (nldNDs), mainly typified by mnesic, motoric, socio-cognitive, behavioral, perceptual, or otherwise cognitive alterations. Indeed, different dimensions of language are subserved by perisylvian, extra-sylvian, and subcortical regions which become affected in multiple forms of neurodegeneration, yielding dimensional impairments beyond categorical diagnostic delimitations. This is true, for example, of the three most prevalent NDs worldwide (Alzheimer’s disease, Parkinson’s disease, and behavioral variant frontotemporal dementia), which often entail systematic verbal disruptions irrespective of the extent of cardinal symptoms and, on occasion, even before these become manifest. Indeed, in these and other nldNDs, specific aspects of language become dysfunctional in roughly 50% of mild cases and over 90% of severe cases.

Yet, initiatives to characterize these dysfunctions have proven isolated and preliminary at best, arguably because leading works on (and standard diagnostic criteria for) nldNDs underestimate the ubiquity, functional impact, and informativeness of verbal impairments. This scenario is variously counterproductive, as it hinders valuable avenues to boost fine-grained characterizations of each disease, inform theory-building, and even nurture the quest for transdiagnostic and disease-specific neurocognitive markers –a most pressing task given the escalating growth of NDs and associated forms of dementia. To bridge these gaps, synergistic efforts are required from relevant teams across the globe. The present Research Topic offers a strategic platform to this end. We aim to provide a cross-dimensional view of language disorders (cutting across phonetic, phonological, lexico-semantic, morpho-syntactic, and discursive-pragmatic levels) in any and all types of nldNDs (e.g., Alzheimer’s disease, Parkinson’s disease, behavioral variant frontotemporal dementia, multiple sclerosis, Lewy body disease, Huntington’s disease, corticobasal degeneration, amyoptrophic lateral sclerosis, spinocerebellar ataxia, posterior cortical atrophy, Lyme disease), integrating theoretical, methodological, and/or translational innovations.

In particular, we target five outstanding questions:
(i) which linguistic dimensions are mainly affected and spared in different nldNDs?;
(ii) which are the neural signatures of those impairments?;
(iii) are such deficits related to, predictive of, or as discriminatory as non-verbal symptoms?;
(iv) is it possible to identify transnosological and disease-specific linguistic deficits across nldNDs?;
(v) how consistent are these disruptions across different languages?

We welcome submissions rooted in cognitive neuroscience, neuropsychology, language therapy, psycholinguistics, linguistics, and other pertinent fields. Topics of interest include, but are not limited to, behavioral manifestations, neuroanatomical and neurofunctional correlates, relations among linguistic and non-linguistic deficits, transnosological comparisons, cross-cultural assessments, and therapeutic developments. Original research may be based on standardized language batteries, experimental paradigms or ecological discourse tasks, complemented with behavioral (e.g., accuracy, reaction time), neuroanatomical (e.g., VBM, DTI), hemodynamic (e.g., fMRI, PET, fNIRS), electrophysiological (e.g., EEG, MEG), brain stimulation (e.g., tDCS, TMS), or biomarker approaches. We are especially interested in multi-participant experiments, but we also welcome single-case studies as well as reviews, commentaries, and position papers. Overall, we expect this initiative to give impactful visibility to a promising yet underexploited line of research on NDs.