Failure of tDCS to modulate motor excitability and speech motor learning

Abstract

Transcranial direct current stimulation (tDCS) modulates cortical excitability in a polarity-specific way and, when used in combination with a behavioural task, it can alter performance. TDCS has the potential, therefore, for use as an adjunct to therapies designed to treat disorders affecting speech, including, but not limited to acquired aphasias and developmental stuttering. For this reason, it is important to conduct studies evaluating its effectiveness and the parameters optimal for stimulation. Here, we aimed to evaluate the effects of bi-hemispheric tDCS over speech motor cortex on performance of a complex speech motor learning task, namely the repetition of tongue twisters. A previous study in older participants showed that tDCS could modulate performance on a similar task. To further understand the effects of tDCS, we also measured the excitability of the speech motor cortex before and after stimulation. Three groups of 20 healthy young controls received: (i) anodal tDCS to the left IFG/LipM1 and cathodal tDCS to the right hemisphere homologue; or (ii) cathodal tDCS over the left and anodal over the right; or (iii) sham stimulation. Participants heard and repeated novel tongue twisters and matched simple sentences before, during and 10 min after the stimulation. One mA tDCS was delivered concurrent with task performance for 13 min. Motor excitability was measured using transcranial magnetic stimulation to elicit motor-evoked potentials in the lip before and immediately after tDCS. The study was double-blind, randomized, and sham-controlled; the design and analysis were pre-registered. Performance on the task improved from baseline to after stimulation but was not significantly modulated by tDCS. Similarly, a small decrease in motor excitability was seen in all three stimulation groups but did not differ among them and was unrelated to task performance. Bayesian analyses provide substantial evidence in support of the null hypotheses in both cases, namely that tongue twister performance and motor excitability were not affected by tDCS. We discuss our findings in the context of the previous positive results for a similar task. We conclude that tDCS may be most effective when brain function is sub-optimal due to age-related declines or pathology. Further study is required to determine why tDCS failed to modulate excitability in the speech motor cortex in the expected ways.

Timing is everything: event-related transcranial direct current stimulation improves motor adaptation

“…These results suggest that brief periods of TDCS applied in synchrony with movement can selectively and specifically improve motor adaptation of that movement, while leaving adaptation of interleaved movements unaffected. Although many studies have reported positive effects of TDCS on motor learning and rehabilitation, when applied continuously for 10-20 minutes (Hummel et al, 2005; Fregni et al, 2006; Galea, Vazquez, Pasricha, Orban de Xivry, & Celnik, 2010; Hardwick & Celnik, 2014; Allman et al, 2016; Benussi et al, 2017; Chiou, Morris, Gou, Alexander, & Gay, 2020; Weightman et al, 2020), there are a growing number of studies reporting null or mixed effects (Jalali, Miall, & Galea, 2017; Hulst et al, 2017; Mamlins, Hulst, Donchin, Timmann, & Claassen, 2019; Wiltshire & Watkins, 2020), leading to uncertainty around the effectiveness of TDCS (Horvath, Forte, & Carter, 2015). During continuous stimulation, for 10-20 minutes, any number of different behaviours may be performed alongside the specific task that is the ‘target’ of stimulation.…”

“…These initial results suggest that brief periods of TDCS applied over the cerebellum and in synchrony with movement can selectively and specifically improve motor adaptation of that movement, while leaving adaptation of interleaved movements unaffected. Although many studies have reported positive effects of TDCS on motor learning and rehabilitation, when applied continuously for 10-20 minutes [21,[28][29][30][31][32][33][34], there are a growing number of studies reporting null or mixed effects [35][36][37][38], leading to uncertainty around the effectiveness of TDCS [39]. During continuous stimulation, for 10-20 minutes, any number of different behaviours may be performed alongside the specific task that is the 'target' of stimulation.…”

Speech Fluency Improvement in Developmental Stuttering Using Non-invasive Brain Stimulation: Insights From Available Evidence

“…Non-invasive brain stimulation techniques are currently and extensively employed in the functional improvement of various motor/cognitive functions in healthy participants (e.g., Moret et al, 2019 ; Masina et al, 2021 ) as well as in experimental rehabilitation trials (see Hamilton et al, 2011 ; Campana et al, 2014 ; Moret et al, 2018 ). However, while protocols may be sometimes ineffective in the healthy population (e.g., Wiltshire and Watkins, 2020 ), the involvement of clinical (or subclinical) participants may result in an effective advantage for their conditions. In this context, a wide range of neural impairments may be considered, such as stroke (e.g., aphasia) (see Hamilton et al, 2011 ; Marangolo et al, 2013a , b ; Khedr et al, 2014 ), neurodegenerative diseases (e.g., Alzheimer's disease and Parkinson's disease) (e.g., Cotelli et al, 2011 ; Goodwill et al, 2017 ), and psychiatric disorders (e.g., schizophrenia, depression, obsessive-compulsive disorder, attention deficit, hyperactivity disorder, etc.)…”

Central nervous system physiology

“…However, a number of papers have drawn attention to the large variability in response to tDCS both between and within individuals, with several studies failing to observe any net effects ( Abdelmoula et al, 2019 ; Lopez-Alonso et al, 2014 ; Lopez-Alonso et al, 2015 ; Vannorsdall et al, 2016; Wiethoff et al, 2014 ; Wiltshire and Watkins, 2020 ; Wrightson et al, 2020 ). The discrepancies probably arise because, as noted above, the results of tDCS depend on so many factors, including the precise orientation neuronal/synaptic populations and current flow, the intensity of current in each individual, and the effects of ongoing (or past) brain states, or individual genetic susceptibility.…”