Does repetitive anodal tDCS applied on the visual cortex induce long-term improvements in visual contrast perception?

Introduction: Anodal transcranial direct current stimulation (tDCS) can induce upward excitability shifts in the primary visual cortex, e.g. as measured by an altered TMS phosphene threshold in V1 (Nitsche et al. 2008). However the effect of a singular stimulation is limited to 10–20 minutes (Antal et al. 2006). In the motor system it was demonstrated that anodal tDCS over multiple days facilitated motor skill acquisition through an effect of consolidation. The skill acquisition measure remained greater for anodal tDCS even after three months (Reis et al. 2009). In analogy to the motor system we investigate in the present study if anodal tDCS applied to the visual cortex over multiple days can induce additive and long lasting improvements in visual contrast perception.

Methods: Using a double-blind and sham controlled between subject design, MRI-guided transcranial current stimulation (1.5 mA) was applied on the visual cortex for 20 minutes over 5 successive days in 4 healthy subjects (planned 24 subjects). Before and after anodal or sham stimulation, subjects performed threshold perimetry using a 10–2 strategy including 68 visual field positions within the central 10 degree of the visual field.

Results: Preliminary data show that anodal stimulation of the visual cortex over 5 successive days yielded an improvement of visual contrast sensitivity after the fifth stimulation day relative to pre-stimulation baseline (mean=0.43 dB, N=2). Sham stimulation of the visual cortex did not improve performance relative to pre-stimulation baseline (mean=0.08 dB, N=2). Up to now we conducted follow-up measurements 2 and 4 weeks after the fifth stimulation day in 2 subjects (1 anodal, 1 sham), which showed an improvement of visual contrast sensitivity even 4 weeks after anodal stimulation (0.36 dB).

Discussion: Previous results of our group showed that single-session tDCS over the visual cortex could improve visual contrast sensitivity (Kraft et al. 2010). According to our preliminary data, multi-session tDCS might improve visual contrast sensitivity more effectively. This is in line with the reported results of Reis and colleagues (2009) in the motor system. Establishing a stimulation protocol to induce prolonged improvements of visual perceptual functions provides a promising tool for any therapeutic driven approach, i.e. for patients with impaired visual perceptual functions.