Biological and anatomical factors influencing interindividual variability to noninvasive brain stimulation of the primary motor cortex: a systematic review and meta-analysis

Abstract

<jats:title>Abstract</jats:title><jats:p>Noninvasive brain stimulation (NIBS) modifies corticospinal excitability (CSE) historically in a predictable manner dependent on stimulation parameters. Researchers, however, discuss high degrees of variability between individuals, either responding as expected or not responding as expected. The explanation for this interindividual variability remains unknown with suggested interplay between stimulation parameters and variations in biological, anatomical, and physiological factors. This systematic review and meta-analysis aimed to investigate the effect of variation in inherent factors within an individual (biological and anatomical factors) on CSE in response to NIBS of the primary motor cortex. Twenty-two studies were included investigating genetic variation (n=7), age variation (n=4), gender variation (n=7), and anatomical variation (n=5). The results indicate that variation in brain-derived neurotrophic factor genotypes may have an effect on CSE after NIBS. Variation between younger and older adults also affects CSE after NIBS. Variation between age-matched males and females does not affect CSE after NIBS, but variation across the menstrual cycle does. Variation between skull thickness and brain tissue morphology influences the electric field magnitude that ultimately reaches the primary motor cortex. These findings indicate that biological and anatomical variations may in part account for interindividual variability in CSE in response to NIBS of the primary motor cortex, categorizing individuals as responding as expected (responders) or not responding as expected (nonresponders).</jats:p>

The effects of transcranial direct current stimulation on corticospinal and cortico-cortical excitability and response variability: conventional versus high-definition montages

“…Inter-individual variability, as discussed above, is the variation in response to tDCS between participating individuals. Specific mechanisms to account for inter-individual variability have been discussed previously in recent reviews (Chew et al, 2015;Pellegrini et al, 2018a;Ridding & Ziemann, 2010). Intra-individual variability however refers to the variation in response to tDCS within an individual across different testing sessions.…”

Increased Excitability Induced in the Primary Motor Cortex by Transcranial Ultrasound Stimulation

“…Light stimulation, or photobiomodulation, is a promising but as yet little underexplored method of NIBS ( 9 ). Transcranial direct current stimulation (tDCS), while inexpensive and associated with minimal side-effects ( 10 ), can produce variable results across individuals and time points ( 11 , 12 ). TMS has a longer history of clinical and experimental application ( 13 ), but also comes with more contraindications ( 14 ), and is subject to variable effects across individuals ( 15 – 17 ).…”

Can genetic polymorphisms predict response variability to anodal transcranial direct current stimulation of the primary motor cortex?

“…Biological factors including differences in age Heise et al, 2014), gender (Inghilleri et al, 2004;Zoghi et al, 2015) and specific genetic variants (Antal et al, 2010;Hwang et al, 2015;Puri et al, 2015). With inter-individual variability to tDCS still remaining, it is likely that mechanisms behind inter-individual variability are multifactorial (Li et al, 2015;Pellegrini et al, 2018a;Ridding & Ziemann, 2010). Ongoing investigation into the contribution these intrinsic factors may play in tDCS inter-individual variability is crucial.…”