Abstract
Understanding processes performed by an intact visual cortex as the basis for developing methods that enhance or restore visual perception is of great interest to both researchers and medical practitioners. Here, we explore whether contrast sensitivity, a main function of the primary visual cortex (V1), can be improved in healthy subjects by repetitive, noninvasive anodal transcranial direct current stimulation (tDCS). Contrast perception was measured via threshold perimetry directly before and after intervention (tDCS or sham stimulation) on each day over 5 consecutive days (24 subjects, double-blind study). tDCS improved contrast sensitivity from the second day onwards, with significant effects lasting 24 h. After the last stimulation on day 5, the anodal group showed a significantly greater improvement in contrast perception than the sham group (23 vs. 5%). We found significant long-term effects in only the central 2–4° of the visual field 4 weeks after the last stimulation. We suspect a combination of two factors contributes to these lasting effects. First, the V1 area that represents the central retina was located closer to the polarization electrode, resulting in higher current density. Second, the central visual field is represented by a larger cortical area relative to the peripheral visual field (cortical magnification). This is the first study showing that tDCS over V1 enhances contrast perception in healthy subjects for several weeks. This study contributes to the investigation of the causal relationship between the external modulation of neuronal membrane potential and behavior (in our case, visual perception). Because the vast majority of human studies only show temporary effects after single tDCS sessions targeting the visual system, our study underpins the potential for lasting effects of repetitive tDCS-induced modulation of neuronal excitability.
Highlights
Sensitivity to contrast is crucial, for vision at dusk and nighttime (Brabyn et al, 2005), and in daylight, e.g., while reading
Because skull thickness determines the flow of current through the brain and the strength of transcranial direct current stimulation (tDCS) effects (Datta et al, 2011; Giordano et al, 2017), one plausible reason for smaller tDCS effects over V1 compared to M1 can be seen in the relatively greater skull thickness and density of the occipital bone compared to the parietal bone (Voie et al, 2014; Zarghooni et al, 2016)
In contrast to the majority of previous studies, we used a more precise method to determine the location of the polarization electrode on the surface using individual MRI data and navigation software
Summary
Sensitivity to contrast is crucial, for vision at dusk and nighttime (Brabyn et al, 2005), and in daylight, e.g., while reading. The processing of visual contrast is one of the main functions of the primary visual cortex (V1; Foster et al, 1985; Mullen et al, 2010), which plays a key role in humans’ faculty to visually process their environment. While the functions of V1 are relatively well understood, the degree of plasticity of the human visual cortex is still largely unclear. Given the increasing average life expectancy of large parts of the world’s population and the fact that stroke occurs more frequently in the elderly (Feigin et al, 2014), often affecting the visual cortex, potential treatments with the aim of rehabilitating the visual system are needed
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