Abstract
Motor cortex activation observed during body-related verb processing hints at simulation accompanying linguistic understanding. By exploiting the up- and down-regulation that anodal and cathodal transcranial direct current stimulation (tDCS) exert on motor cortical excitability, we aimed at further characterizing the functional contribution of the motor system to linguistic processing. In a double-blind sham-controlled within-subjects design, online stimulation was applied to the left hemispheric hand-related motor cortex of 20 healthy subjects. A dual, double-dissociation task required participants to semantically discriminate concrete (hand/foot) from abstract verb primes as well as to respond with the hand or with the foot to verb-unrelated geometric targets. Analyses were conducted with linear mixed models. Semantic priming was confirmed by faster and more accurate reactions when the response effector was congruent with the verb’s body part. Cathodal stimulation induced faster responses for hand verb primes thus indicating a somatotopical distribution of cortical activation as induced by body-related verbs. Importantly, this effect depended on performance in semantic discrimination. The current results point to verb processing being selectively modifiable by neuromodulation and at the same time to a dependence of tDCS effects on enhanced simulation. We discuss putative mechanisms operating in this reciprocal dependence of neuromodulation and motor resonance.
Highlights
The assumption that cognition is grounded in simulation processes[1,2] implies a cross-talk between action-related language and neurophysiological motor mechanisms
Significant reduction in gamma-aminobutyric acid (GABA) concentration corroborates the excitatory effect of anodal stimulation of the hand-related cortical motor area[29]
Anodal Transcranial direct current stimulation (tDCS) induces firing of neurons that are near threshold: if neurons are not influenced by the task, they will be far from the threshold and will less likely discharge[37]
Summary
The assumption that cognition is grounded in simulation processes[1,2] implies a cross-talk between action-related language and neurophysiological motor mechanisms. Anodal tDCS induces firing of neurons that are near threshold: if neurons are not influenced by the task, they will be far from the threshold and will less likely discharge[37] This makes tDCS suited to address the role of semantic processing for cortical motor activation. A dual, double-dissociation task was applied in which a) a verb had to be semantically categorized as concrete versus abstract and b) in case of a concrete verb, hand/foot responses had to be given to a verb-unrelated prompt (a shape with rounded or pointed corners) This priming paradigm has the advantage of disentangling hand from foot-related verbal and motor contributions: responses from both effectors follow the same stimuli allowing the attribution of possible differences to priming. Planned comparisons between verum and sham stimulation were expected to result in opposite effects of anodal and cathodal tDCS on behavioural measures
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