Abstract
It has been proposed that the effectiveness of non-invasive brain stimulation (NIBS) as a cognitive enhancement technique may be enhanced by combining the stimulation with concurrent cognitive activity. However, the benefits of such a combination in comparison to protocols without ongoing cognitive activity have not yet been studied. In the present study, we investigate the effects of fMRI-guided high-frequency repetitive transcranial magnetic stimulation (HF rTMS) over the left dorsolateral prefrontal cortex (DLPFC) on working memory (WM) in healthy volunteers, using an n-back task with spatial and verbal stimuli and a spatial span task. In two combined protocols (TMS + WM + (maintenance) and TMS + WM + (rest)) trains of stimuli were applied in the maintenance and rest periods of the modified Sternberg task, respectively. We compared them to HF rTMS without a cognitive load (TMS + WM −) and control stimulation (TMS − WM + (maintenance)). No serious adverse effects appeared in this study. Among all protocols, significant effects on WM were shown only for the TMS + WM − with oppositely directed influences of this protocol on storage and manipulation in spatial WM. Moreover, there was a significant difference between the effects of TMS + WM − and TMS + WM + (maintenance), suggesting that simultaneous cognitive activity does not necessarily lead to an increase in TMS effects.
Highlights
Working memory (WM) can be defined as a combination of temporary storage of task-relevant information and processing upon it
In another case of headache after the vertex stimulation, the headache was diffuse, had moderate intensity (6 points, with maximal intensity within 1–2 h after the stimulation) and was accompanied by lacrimation, a dull sensation in the left orbita, neck stiffness, and had a total duration of more than 24 h after the stimulation. Concentration difficulties appeared both after the dorsolateral prefrontal cortex (DLPFC) and the vertex stimulation, and in one case, accompanied the headache. In this proof-of-concept cross-over study, we investigated if different combinations of functional magnetic resonance imaging (fMRI)-guided navigated high-frequency repetitive transcranial magnetic stimulation (HF rTMS) over the left DLPFC with a cognitive task might influence the effects of a single rTMS session on different components of WM compared to stimulation alone, as well as to control stimulation
In contrast to other studies, assessing online-effects of the stimulation over DLPFC on the test performance immediately during a stimulation session, we investigated if combination of high frequency (HF) rTMS with a cognitive task had an influence on its offline-effects lasting beyond a stimulation session
Summary
Working memory (WM) can be defined as a combination of temporary storage of task-relevant information and processing upon it. WM performance has been shown to be associated with complex cognitive functions such as learning ability and fluid reasoning [1,2]. Brain Sci. 2020, 10, 83; doi:10.3390/brainsci10020083 www.mdpi.com/journal/brainsci. Dorsolateral prefrontal cortex (DLPFC) is one of the brain areas playing a key role in WM processes [5,6], which was initially determined in invasive neurophysiologic studies performed on non-human primates [7]. Further human studies using functional magnetic resonance imaging (fMRI). Have confirmed the activation of PFC as a part of WM-related neural networks during WM tasks [8,9,10]. The activation of DLPFC during the WM task can be load-dependent, providing additional evidence for its role in WM processes [11]
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