Task load modulates tDCS effects on language performance

Abstract

Transcranial direct current stimulation (tDCS) effects in cognition are inconsistent across studies. This study aimed to discuss why typical models might be insufficient to explain these effects, and to investigate a brain state factor, task load, with behavioral experiments on phonological processing. The motor theory of speech perception states that motor codes for articulation take part in speech perception, a view sharpened by neuroimaging findings, which show that the motor role in phonological processing is weighted by the nature of the tasks. Three groups of 20 participants, each under a different tDCS condition (anodal, cathodal, or sham), performed a categorical perception (CP), a lexical decision (LD), and a word naming (WN) task while stimulated on the pars opercularis of the left inferior frontal gyrus, a language area typically involved with the motor role. These tasks were assumed to be subserved by a network of nodes which included the target, believed to be increasingly relevant for performance from speech perception to speech production. A-tDCS facilitation and C-tDCS downregulation should directly increase with the relevance of the target for the task. Downregulation of a low relevance node could result in facilitation by compensation from other nodes. Overall, our brain stimulation findings support the neuroimaging literature in that motor participation in phonological processing depends on task nature and show that tDCS effects are modulated by task load relative to the target. Outcomes such as the improved performance following cathodal tDCS in CP and WN suggest that compensatory mechanisms may take place when the tasks involve more complex neuronal networks. K E Y W O R D Scognition, language, pars opercularis of the left inferior frontal gyrus (LIFGop), phonological processing, task load, transcranial direct current stimulation (tDCS) | 1431 RODRIGUES DE ALMEIDA Et AL.

Task load modulates tDCS effects on brain network for phonological processing

“…However, it still yields inconsistent results across studies, especially in those studying cognition (Jacobson et al 2012 ). Rodrigues de Almeida et al ( 2019 ) pointed out that this could occur because aspects that are crucial for tDCS to have an effect in cognition have been largely overlooked. We previously discussed, under what we call the “multi-node framework”, the relevance of two aspects in particular, namely task load and the network structure underlying cognitive functions.…”

“…Regarding task load (see Rodrigues de Almeida et al 2019 for a more comprehensive account), the effects of tDCS positively relate to the relevance of the task for the target of stimulation (which we call “task load”). Functional targeting (Bikson and Rahman 2013 ) needs to be satisfactory for tDCS to be effective.…”

“…Network nodes may have different weightings for a task (e.g. Hartwigsen et al 2012 , 2016 ; Rodrigues de Almeida et al 2019 ), and target responses to tDCS are expected to be more noticeable when these targets are nodes of higher weighting (Nozari et al 2014 ; Pope et al 2015 ). However, network responses may also arise when the target is a node of lower weighting.…”

“…However, network responses may also arise when the target is a node of lower weighting. This is expected when a low weighting node is downregulated, as in this case there is enough room for compensation by other nodes (Rodrigues de Almeida et al 2019 ). Evidence for network compensation in cognitive functions has been observed in the brain stimulation literature through behavioural and neuroimaging findings.…”

“…For example, compensation can be inferred from behavioural findings where a satisfactory level of performance has been achieved despite the brain stimulation downregulating neuronal activity (e.g. Hartwigsen et al 2013 ; Nozari et al 2014 ; Rodrigues de Almeida et al 2019 ). Compensation can also be inferred from neuroimaging findings, when increased activation of nontarget regions is reported (e.g.…”