Electrical stimulation of the cerebellum facilitates automatic but not controlled word retrieval

Recent research has indicated that the cerebellum is engaged in language functions, yet the role of the cerebellum in lexical-semantic memory is poorly understood. In a double-blind randomized controlled experiment, we therefore targeted the cerebellum by transcranial direct current stimulation (tDCS) to assess and compare the contribution of the cerebellar processing to automatic and controlled retrieval of words in healthy adults (n = 136). Anodal cerebellar tDCS facilitated retrieval of semantically related words in free-associative chains, which was not due to a non-specific acceleration of processing speed. The stimulation had no influence on controlled word retrieval that employed inhibition or switching. The effect of cathodal tDCS was opposite to the anodal stimulation, but statistically non-significant. Our data show that the cerebellum is engaged extracting associative information from the system of semantic representations, established and strengthened/automated by learning, and indicates a domain-general role of this structure in automation of behavior, cognition and language. Supplementary Information The online version contains supplementary material available at 10.1007/s00429-023-02712-0.


Sentence completion taskthe pilot study
Prior to the experiment, the set of 140 generated unfinished sentences was administered to subjects (not participating in the subsequent experiment) who were instructed to type the finishing words so the cloze probability of sentences could be checked.Evaluation criteria were in line with Block and Baldwin (Block & Baldwin, 2010) and were as follows: words with one missing/additional/transposed letter were included and counted as the intended word; synonyms and similar words were not counted toward the intended word (i.e., words "gun" and "rifle" were counted as separate responses).Based on the cloze probability scores 60 predictable and 60 unpredictable sentences were selected from the list to be used in this experiment.

Side effects of tDCS
The side effects of stimulation were measured with a modified and translated version of a sensation questionnaire used by Fertonani et al. (2010).In this questionnaire, various possible discomforting adverse effects were described (itchiness, pain, burning, heat, pinching, iron taste or fatigue) and participants were asked to score them on a scale from 1 (no sensation) to 10 (very strong sensation).Besides that, the questionnaire contained questions to determine when the sensations began, how long they lasted and also participant's assessment whether the active or sham stimulation was delivered.
The stimulation was well tolerated.The mean intensities of sensations reported are shown in Tab.10.Given that the data for each reported sensation were not normally distributed (Shapiro-Wilk tests yielded all p < .05), a nonparametric Kruskal-Wallis test was carried out to compare the groups.Vast majority of the participants reported the sensations only in the beginning of the stimulation (presumably during ramp-up).The guess of the applied stimulation (real vs. sham) was at chance level (55.1% correct).
Associations between the sensations and stimulation effects in ACT are shown in Tab.11.Notably, ratings of itching were unrelated to difference RT (post-tDCS minus baseline) and thus do not explain the effects of stimulation (group differences) in this condition.

Fig. 1 :
Fig. 1: ACT: the effect of tDCS in the Associative Alternating condition

Fig. 2 .
Fig. 2. ACT: the effect of tDCS in the Dissociative Fixed condition

Fig. 3 .
Fig. 3. ACT: the effect of tDCS in the Dissociative Alternating condition

Fig. 4 .
Fig. 4. SCT: the effect of tDCS on completion of predictable sentences

Fig. 5 .
Fig. 5. SCT: the effect of tDCS on completion of unpredictable sentences

10. Ratings of the sensations elicited by tCDS Stimulation
Data represent mean (±SD).The intensity was reported on a 10-point scale ranging from 1 to 10. Data were analyzed using Kruskal-Wallis test.* p < .05,** p < .