Dynamics of oral language and speech production through neuromodulation: A systematic review of non-invasive brain stimulation in neurodegeneration

Oral language and speech production analyses are emerging as innovative indicators of neurodegeneration. These components of expressive discourse permit dementia identification at early stages, even in neurodegenerative pathologies with non-dominant linguistic symptomatology. Simultaneously, non-invasive brain stimulation (NIBS) has been described as an effective neuromodulation technique to counter neurodegenerative symptoms. This systematic review assesses whether neuromodulation via NIBS can impact oral expressive discourse through oral language and speech production parameters in neurodegenerative contexts. We conducted a systematic review of NIBS studies reporting longitudinal changes of oral expressive output tasks in patients with a neurodegenerative condition. A systematic search of studies published between 2011 and 2024 was performed from four databases (Scopus, PubMed, PsycINFO, Embase). Data analysis yielded 27 relevant studies with quality assessed using the Cochrane RoB2 and JBI tools This review suggests that neuromodulation via NIBS can positively influence oral expression capacities. Nonetheless, neuromodulation impact varies based on clinical diagnosis, intervention parameters, and patient demographics. Speech production measures appear more sensitive to neuromodulation than oral production capability scores. Neuroimaging data further reveals that speech production longitudinal changes appear linked to NIBS-induced changes in brain activity. In that sense, speech and oral language production parameters emerge as potential sensitive indicators of neuromodulation effects and neurodegenerative symptomatology fluctuations. This review highlights the potential of NIBS in enhancing speech and oral language production in neurodegenerative conditions, highlighting the importance of personalized interventions and multidimensional assessment approaches. Future research should explore the integration of speech markers as progressive indicators of both symptomatic and neural developments.