Abstract
Measuring phosphene thresholds (PTs) is often used to investigate changes in the excitability of the human visual cortex through different brain stimulation methods like repetitive transcranial magnetic stimulation (rTMS) or transcranial direct current stimulation (tDCS). In several studies, PT increase or decrease has been shown after rTMS or tDCS application. Recently, using PT measurements we showed that the state of the neurons in the visual cortex after rTMS might have an influence on the modulatory effects of stimulation. In the present study we aimed to investigate whether visual cortex activity following stimulation influences the modulatory effects of tDCS as well. In a between-group design, anodal or cathodal tDCS was applied to the visual cortex twice per subject, with either high or low visual demand following stimulation. We observed no modulation of PT neither directly following both anodal and cathodal tDCS nor following the visual demand periods. We rather found high inter-individual variability in the response to tDCS, and intra-individual reliability in the direction of modulation was observed for cathodal tDCS only. Thus, our results do not confirm the modulatory effects of tDCS on visual cortex excitability published previously. Moreover, they support the confirmation that tDCS effects have little reliability on varied TMS outcome measurements.
Highlights
An essential method for exploring the modulatory effects of different brain stimulation techniques is measuring cortical excitability by single-pulse transcranial magnetic stimulation (TMS)
Our hypotheses were: (a) anodal transcranial direct current stimulation (tDCS) will decrease phosphene thresholds (PTs) and cathodal tDCS will increase PT [16, 17]; (b) if subjects are exposed to low visual demand following tDCS, the modulatory effect will last for at least 10minutes [16]; and (c) if subjects are exposed to high visual demand following tDCS, the modulatory effect will be modified, i.e., there will be a decrease in PT independent from tDCS polarity [23]
The aim of the study was to investigate the effects of anodal and cathodal tDCS on visual cortex excitability in dependency of whether there is low or high visual demand following stimulation
Summary
An essential method for exploring the modulatory effects of different brain stimulation techniques is measuring cortical excitability by single-pulse transcranial magnetic stimulation (TMS). This is often realized by comparing the amplitude of motor evoked potentials (MEPs) before and after repetitive TMS (rTMS) or transcranial direct current stimulation (tDCS). For theta burst stimulation (TBS), continuous application (cTBS) decreases and intermittent application (iTBS) increases MEP amplitudes [5]. Applying tDCS, a decrease of MEP amplitude was observed with cathodal and an increase with anodal polarity [6]. There is evidence for interindividual differences in the response to all of those methods in the motor system, i.e. rTMS [7, 8], TBS [9] and tDCS [10]. Phosphenes are commonly defined as PLOS ONE | DOI:10.1371/journal.pone.0167697 December 9, 2016
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