Cerebellar tDCS in bilingual post-stroke aphasia: a case-study

Introduction and aim
DCS is a promising neuromodulation tool that has been found to increase efficiency of speech-and language therapy in patients suffering from post-stroke aphasia (Biou et al.,2019). However, while an increasing number of patients speak multiple languages, past research has mainly focused on monolinguals. In addition, the search for (an) optimal stimulation site(s) is ongoing: targets of tDCS stimulation have mainly been limited to one cortical area, usually the left hemisphere language areas, and right hemisphere language homologues. The efficiency of tDCS on cortical regions differs between individuals, and speech-and language involve multiple complex neural networks. Limiting stimulation to cortical language areas might conceal the importance of other valuable network components (Sebastian et al., 2020).This case study illustrates the potential usefulness of targeting the right cerebellum in a bilingual chronic post-stroke patient. The cerebellum is a strong candidate location for tDCSin bilingual patients, because of its anatomical connections with left hemisphere language areas, and the role it plays in language function, (bilingual) language control, executive functions, and cognition (Mariën et al., 2017). Anodal stimulation of the inhibitory Purkinje cells of the cerebellum leads to increased inhibitory effects on the basal ganglia and contralateral cortical prefrontal regions, additional regions included in the language control network, specifically important for bilinguals. This leads to disinhibition of basal ganglia output, increasing stimulation of the array of cortical regions reached by the basal ganglia and cerebellum: the prefrontal, temporal, posterior parietal, oculomotor and premotor cortex (Booth et al., 2007). In monolinguals, three studies so far have found cerebellar tDCS to lead to positive results on language improvement (Marongolo et al., 2018, Sebastian et al., 2017, Sebastian et al., 2020).Our aim is to investigate whether cerebellar tDCS stimulation applied to the right cerebellum in right-handed bilingual aphasic patients, and hence contralateral to the left cerebral hemisphere, has a positive impact on language outcome in the treated language(language of therapy) and non-treated language.

Methods
Participant: The individual with aphasia was a right-handed 73-year-old male who experienced a left-hemisphere stroke 27 months before entering the study. The participant was bilingual in French (L1) and Dutch (L2), with aphasia most severe in L2. He presented with non-fluent aphasia, with weakest test performance on the naming of nouns, verbal fluency and oral and written sentence comprehension subtests of the Dutch version of the Comprenhensive Aphasia Test.

Language outcomes: The main outcome measure was change in oral naming accuracy for trained and untrained picture exemplars, tested prior to the start of the treatment, end of treatment, and 2 months post-treatment. Stimuli of both measures were matched for lexical frequency, concreteness and number of syllables. Secondary language outcomes measures were performance on subtests of the Bilingual Aphasia Test, picture description tasks and verbal fluency. Language training was provided in L2, upon request of the participant.

tDCS: We used a double-blind, within-subject crossover trial design, with two experimental conditions: “right cerebellar tDCS + L2 oral anomia treatment” and “sham tDCS + L2 oral anomia treatment”. Each condition consisted of 15 consecutive training sessions, 3 per week, separated by 2 months. tDCS was delivered at a constant current of 2mA, administered for the first 20 min of the 30-min treatment session. The anode was centered on the right cerebellum, 1 cm under, and 4 cm lateral to the inion, and the cathode was placed on the righted deltoid muscle (Sebastian et al., 2020). Sham tDCS was applied using the same electrode configuration, but current intensity was ramped down to zero after 30 s.ResultsResults will be added on language outcomes in a bilingual non-fluent aphasia patient after cerebellar tDCS. A comparison is made in changes from baseline after real and sham tDCS.For each treatment condition (sham and tDCS), we compared the performance 1) pre-treatment and immediately after treatment, (2) pre-treatment and 2 months post-treatment on each stimulus type. The patient is evaluated on oral naming accuracy of trained items(Dutch), oral naming accuracy of untrained items (Boston Naming Test, in Dutch andFrench), and secondary outcome measures in Dutch and French.

Discussion
tDCS is a promising tool for neuromodulation to enhance effects of speech and language therapy of post-stroke aphasia, however, an optimal stimulation site has yet to be identified.Further, tDCS research in bilingual aphasia is scarce, but necessary, as bilingualism is more and more common in today’s world. In this case-study, we expect to show whether stimulation of the right posterolateral cerebellum combined with oral anomia treatment can improve picture naming in post-stroke aphasia, and take results in both patient’s languages under investigation. Targeting the intact right cerebellum, an area involved in language and(bilingual) language control, allows for the possibility of targeting a single region that may possibly be used across different types of aphasic patients, with varying lesion sites and sizes, often occurring in the left hemisphere.