Discourse is an integral part of everyday communication which allows humans to express their thoughts and feelings and effectively carry out daily activities (1, 2). Difficulties in discourse production (i.e., everyday use of language such as storytelling, giving instructions, conversing) are common in post-stroke aphasia and often impact psychosocial well-being and limit quality of life and social participation (3, 4). Speech and language therapy (SLT) has been recognized as the gold standard for treating aphasia including discourse impairments and intervention has been found to be effective (5, 6). However, intervention effect sizes are often small and naming remains the most regularly used outcome measure for aphasia recovery (3, 6–9). Treatments focused solely on word recovery can effectively target the essential components of communication (nouns, verbs etc.) but may fail to impact one's ability to produce sentences and longer utterances and therefore fail to address adequately the main objective of language intervention which is to improve everyday communication (7, 10, 11). Thus, there is a need for investigating new strategies which may enhance higher-level language skills important for every-day communication such as discourse production.

Non-invasive brain stimulation techniques like anodal tDCS which increases spontaneous cortical activity through subthreshold depolarization (12, 13) and promotes neuroplasticity (14, 15) may be effective in enhancing aphasia treatment effects in stroke survivors (16–19). People with post-stroke chronic aphasia have experienced improvement in naming accuracy and speed after application of anodal tDCS (16, 20, 21). Although, the majority of tDCS research has focused on word level recovery, a few studies have found that it may also improve language in the sentence and discourse context, particularly when applied over the left inferior frontal gyrus (LIFG). In a randomized cross-over study with neurotypical older adults the application of anodal tDCS on the LIFG resulted in significant improvements in recount, procedural and narrative discourse tasks compared to stimulation of the right IFG or sham (15). In post-stroke aphasia tDCS studies not working at the single word level have focused on conversation therapy. In a single case study, a participant with anomic type aphasia produced significantly more verbs in sentence production after application of anodal tDCS over the LIFG (Broca's area) when combined with sentence production training and conversational therapy (19), while in Marangolo et al. the same tDCS montage with conversational therapy in a group of eight and 12 participants, respectively resulted in greater language cohesion and increased informational content, verbs, and sentence production within a discourse context compared to stimulation of Wernicke's or sham stimulation (17, 22).

Despite the promising findings of these studies, the evidence-base for tDCS and discourse recovery remains limited. Presently there is a wide range of existing interventions for improving discourse production in chronic aphasia, which implement a variety of treatment methods (5). Some may have a word-level focus and aim to improve lexical retrieval, while others aim to improve aspects of sentence production such as syntactic structure within the discourse context (5). The effect of combining anodal tDCS with different discourse treatments is needed to better understand how best tDCS may be used to support discourse recovery, as currently studies have primarily been limited to the combination of tDCS with conversational therapy.

Therefore, the aim of the present study was to determine the feasibility of anodal tDCS as an adjunct to the Verb Network Strengthening Treatment (VNeST) to improve discourse production in people with post-stroke chronic aphasia. VNeST is a SLT treatment known to improve verb retrieval in discourse production in aphasia (23, 24). As the VNeST in combination with tDCS has not been previously examined, feasibility research is required before proceeding to a definitive randomized controlled trial to examine whether (a) it is feasible to combine VNeST with tDCS; and (b) the intervention shows potential in post-stroke chronic aphasia (25–27). Feasibility was assessed with screening, eligibility, retention, and completion rates, and adverse events (27, 28). Preliminary evaluation of response to intervention was also explored (27, 29) by investigating the impact of VNeST with and without tDCS on discourse production and functional communication, quality of life, psychological state, and cognitive function.

This double-blinded, randomized feasibility study aimed to examine the feasibility, including the preliminary participant response to intervention, of a combined anodal tDCS and SLT intervention on discourse production skills in post-stroke chronic aphasia. There were no adverse reactions, and all participants tolerated the use of tDCS as an adjunct to language therapy. To the best of our knowledge this is the first study to investigate the feasibility of VNeST combined with anodal tDCS on different types of discourse language, functional communication skills, quality of life, psychological symptoms, and cognition. Overall, study findings support the feasibility of using tDCS alongside VNeST. Exploratory analyses revealed promising changes (i.e., estimated large effect size) in discourse production measures across discourse language tasks and functional communication for the active tDCS group.

Recruitment to stroke trials has been reported as challenging and everyday issues (i.e., commuting to study sessions) can hinder the process of recruiting people with aphasia (71). Despite these challenges, study retention and completion rates for all outcome measures including at 6-month follow-up were 100% which supports the feasibility of tDCS combined with VNeST as an intervention. The screening rate was high (86%) and greater than 50% of screened individuals met inclusion criteria, indicating the study was representative of the target population (39). However, many screened individuals (47%) were not suitable mainly due to tDCS contraindications. These findings indicate that tDCS contraindications may be common in stroke survivors, thus this should be taken into account when planning future studies as recruitment may be hindered. Overall, feasibility data provides impetus for a future large-scale study.

This feasibility study did not have sufficient power to test the effectiveness of tDCS combined with VNeST. Nevertheless, our preliminary findings suggest that it is feasible to apply anodal tDCS over the LIFG as an adjunct to VNeST and tDCS may have the potential to boost VNeST treatment outcomes. These findings provide further support for the exploration of tDCS in post-stroke chronic aphasia (72). Our early findings demonstrate that anodal tDCS combined with VNeST may possibly result in improvements in verb retrieval, and other measures of language quantity (total number of words, total number of utterances) at discourse level. Both groups showed improvement in the richness of verbs produced (VTTR) as expected as the VNeST intervention is known to improve verb retrieval at discourse level for trained and untrained items (24). However, only the active tDCS group demonstrated a significant improvement (large effect size) in verb retrieval (verb token total, verb type total) across all three discourse production tasks. This novel finding supports the potential of this combined tDCS treatment in improving verb production, a major difficulty for many with aphasia (42), across discourse genres.

Our results further support previous findings of combined SLT and tDCS studies for improving language quantity at the discourse level. Marangolo et al. combined conversational therapy with anodal tDCS over the LIFG which resulted in a significant improvement in the number of verbs and sentences produced during videoclip descriptions compared to sham condition (22). Similarly, Campana et al. combined anodal tDCS over LIFG with conversational therapy which resulted in a significant improvement in verb naming during picture description (18). Although improvements in language quantity were evident in the active tDCS group compared to sham in this study, effect size calculations showed insignificant differences between groups for language complexity, suggesting that anodal tDCS may have a greater impact on language quantity. However, the small number of participants in this study, although similar to previously published studies in aphasia research (7) and tDCS feasibility studies in stroke (73), limits the interpretation of these results. As this was a feasibility study, a formal sample size calculation was not appropriate. A future pilot study with at least 12 participants per group (74, 75) would allow for a formal sample size calculation for a full-scale study and clearer understanding of the effect of tDCS on language quantity and complexity in discourse production.

Previously the exploration of tDCS effects on everyday communication has been very limited (72), however our preliminary results indicate that anodal tDCS may improve functional communication in post-stroke chronic aphasia. Unlike a recent review which found no evidence of tDCS improving everyday communication (76), our exploratory analyses revealed a significant difference post-treatment and at 6-month follow-up between- and within-groups in CETI scores, indicating a potential for anodal tDCS application to result in improvements in functional communication. Furthermore, two of the three participants in the active tDCS group showed clinically meaningful change in functional communication at 6-month follow-up. These findings are in agreement with a previous RCT assessing the effect of a combined intensive naming therapy with anodal tDCS which also resulted in significant improvements in everyday communication as measured by the CETI (9). Contrarily, Guillouët et al. found that application of bi-hemispheric tDCS (anode on LIFG, cathode on contralesional IFG) did not significantly improve spontaneous speech, an important functional communication skill (7). The assessment of spontaneous speech however only involved assessing change within a single response to an open-ended question, whereas the CETI, a validated functional communication measure, covers a wide range of everyday communication scenarios (57). Although bi-hemispheric stimulation may have a positive effect on naming (7, 77), the difference in our results suggests that it may not have the same impact on higher level language skills and uni-hemispheric stimulation may be the optimal choice for targeting discourse production.

The lasting effects of tDCS has recently been a topic of interest. The aftereffects of single session tDCS are transient, lasting up to an hour, whereas effects of multiple stimulations can last for days and months (9, 78). The findings of this study, the first to examine the long-term effect (6 months) of anodal tDCS on discourse production, provide further support that the impact of multiple tDCS sessions may be long lasting. The majority of large gains in the active tDCS group were maintained at 6-months and for some measures further enhancements were noted at this time. These findings suggest that neuroplastic aftereffects of anodal tDCS may potentially lead to optimum improvement months after stimulation, highlighting the importance of longer follow-up periods. The mechanism underlying the effects of repeated tDCS sessions applied concurrently with behavioral training is believed to be similar to long term potentiation (LTP), a critical process for neuroplasticity (9, 79). Imaging studies with healthy speakers have also found that decreased brain activity was associated with improvements in naming ability during application of anodal tDCS over the LIFG (13, 15, 80, 81), suggesting that tDCS may indirectly modulate behavior by increasing processing efficiency (15, 81, 82). Although these mechanisms may rationalize our findings, further research is required to understand the underlying mechanisms of tDCS which allow it to modulate language in persons with aphasia.

Higher anxiety levels have been linked with deficits in communication (15, 83). The preliminary results for the active tDCS group show a reduction in HADS scores for anxiety in only one out of the three participants. Due to these limited findings and the small sample size no conclusions can be drawn regarding whether SLT in combination with anodal tDCS may potentially contribute to lowering anxiety symptoms in post-stroke aphasia. There were no evident improvements in quality of life or cognitive skills in either intervention group. As VNeST is a language therapy aimed at improving lexical retrieval and not typically expected to impact cognitive skills, the lack of change in the MOCA supports that observed post-treatment improvements may possibly be due to VNeST treatment effects. However, larger full-scale evaluation studies are required to establish the effect of tDCS on psychological symptoms, quality of life and cognition in both the short and long term.

Although early exploratory findings were promising, this study had a number of limitations. Due to difficulties with recruitment which were further exacerbated by the COVID-19 pandemic, the sample size was small. As a feasibility study with a small sample size comparative analysis could only be exploratory and thus no valid conclusions could be made from differences between the groups for all of the outcomes in the study (29). Although the outcome assessor was blinded, the lack of an additional assessor did not allow us to establish inter-rater reliability and increased evaluation bias (7). The study also lacked multiple baseline evaluations which would have controlled for possible intraindividual variability commonly seen in post-stroke aphasia (7, 84).

This study demonstrates that combing anodal tDCS over the LIFG with VNeST for improving discourse production in post-stroke chronic aphasia is feasible and well-accepted by participants. Preliminary findings indicate a potential for the use of tDCS as a safe and inexpensive tool for boosting aphasia rehabilitation in chronic aphasia. As a feasibility study, our findings provide motivation for future large-scale trials in this field to establish the effect of tDCS on discourse production and its potential to be used routinely as a rehabilitative tool in clinical practice.

The original contributions presented in the study are included in the article/Supplementary Material, further inquiries can be directed to the corresponding author/s.

The studies involving human participants were reviewed and approved by King's College London Biomedical & Health Sciences, Dentistry, Medicine and Natural & Mathematical Sciences Research Ethics Review Board. The patients/participants provided their written informed consent to participate in this study.

SM, CN, IS, and MP: study concept. SM, CN, and MP: development of methodology and manuscript draft revisions and editing. SM: data collection and original draft preparation. SM and MP: data analysis. MP: supervision. All authors read and approved the final manuscript.

The authors declare that the research was conducted in the absence of any commercial or financial relationships that could be construed as a potential conflict of interest.

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