Abstract
Studies on noninvasive motor cortex stimulation and motor learning demonstrated cortical excitability as a marker for a learning effect. Transcranial direct current stimulation (tDCS) is a non-invasive tool to modulate cortical excitability. It is as yet unknown how tDCS-induced excitability changes and perceptual learning in visual cortex correlate. Our study aimed to examine the influence of tDCS on visual perceptual learning in healthy humans. Additionally, we measured excitability in primary visual cortex (V1). We hypothesized that anodal tDCS would improve and cathodal tDCS would have minor or no effects on visual learning. Anodal, cathodal or sham tDCS were applied over V1 in a randomized, double-blinded design over four consecutive days (n = 30). During 20 min of tDCS, subjects had to learn a visual orientation-discrimination task (ODT). Excitability parameters were measured by analyzing paired-stimulation behavior of visual-evoked potentials (ps-VEP) and by measuring phosphene thresholds (PTs) before and after the stimulation period of 4 days. Compared with sham-tDCS, anodal tDCS led to an improvement of visual discrimination learning (p < 0.003). We found reduced PTs and increased ps-VEP ratios indicating increased cortical excitability after anodal tDCS (PT: p = 0.002, ps-VEP: p = 0.003). Correlation analysis within the anodal tDCS group revealed no significant correlation between PTs and learning effect. For cathodal tDCS, no significant effects on learning or on excitability could be seen. Our results showed that anodal tDCS over V1 resulted in improved visual perceptual learning and increased cortical excitability. tDCS is a promising tool to alter V1 excitability and, hence, perceptual visual learning.
Highlights
Transcranial direct current stimulation is a noninvasive tool to modulate cortical excitability in a polarity dependent manner
The aim of our present study was to investigate the impact of anodal and cathodal Transcranial direct current stimulation (tDCS) applied over primary visual cortex (V1) for four consecutive days upon visual perceptual learning; tDCS Effects on Visual Cortex as well as its influence on cortical excitability, measured by phosphene threshold (PT) and pairedstimulation visually evoked potentials (psVEP)
Paired two-tailed t-tests revealed a significant decrease between day 5 (D5) and day 1 (D1) for anodal tDCS group
Summary
Transcranial direct current stimulation (tDCS) is a noninvasive tool to modulate cortical excitability in a polarity dependent manner. Complex homeostatic plastic mechanisms limit uncontrolled increase in synaptic effectiveness and prevent potential destabilization of the neuronal system (Bienenstock et al, 1982; Fricke et al, 2011) In this concept, the activation history of the postsynaptic neuron decides whether a facilitating tDCS protocol leads to further facitilation or even inhibition (Lang et al, 2004; Fricke et al, 2011). The activation history of the postsynaptic neuron decides whether a facilitating tDCS protocol leads to further facitilation or even inhibition (Lang et al, 2004; Fricke et al, 2011) Another factor that influences the duration of tDCS-induced offline effects is a repetition of stimulation over several consecutive days, which can activate LTP-like molecular mechanisms (Fritsch et al, 2010)
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