Effects of transcranial direct current stimulation on threshold perimetry

Introduction: Several studies documented the effect of anodal and cathodal direct current stimulation over the occipital cortex (see Antal & Paulus 2008 for an overview). The measurement of contrast sensitivity changes after transcranial direct current stimulation (tDCS) using a standard clinical tool like threshold perimetry may provide an interesting perspective in assessing therapeutic effects of tDCS in neurological or ophthalmological patients. In this study, we aim to characterize the effect of anodal and cathodal direct current stimulation on contrast sensitivity in the central ten degree of the visual field in healthy test subjects. Distinct eccentricities should be investigated separately, since contrast sensitivity decreases with eccentricity and on the cortical level, more central regions of the visual field are represented closer to the occipital pole, i.e. represented closer to the polarizing electrodes than more peripheral regions of the visual field.

Methods: In 12 healthy subjects, Humphrey perimetry was performed using a 10–2 strategy (Swedish interactive threshold algorithm „SITA“), including 68 visual field positions within the central 10° of the visual field. Using a counterbalanced double-blind and sham controlled within subject design, the effect of stimulation can be measured independent of a potential learning effect across testing days.

Results: Anodal stimulation of the visual cortex compared to sham stimulation yielded a significant increase in contrast sensitivity at the eccentricities 0–2°, >4–6° and >6–8° (p<0.05). A significant increase in contrast sensitivity between the conditions „pre“ and „post“ anodal stimulation can only be obtained for the central positions at eccentricities smaller than 2 degrees (p<0.05). In contradistinction, cathodal stimulation of the visual cortex did not affect contrast sensitivity at neither eccentricity. All post-hoc comparisons between cathodal and sham, as well as between cathodal and pre stimulation were not significant (p>0.05).

Conclusion: We could show that anodal tDCS applied on the visual cortex can transiently increase contrast sensitivity within 8° of the visual field. Our data agree well with a reduced TMS phosphene threshold in V1 after anodal tDCS (Antal et al. 2003). Interestingly, in a tDCS study from Antal et al. (2001), 7min of anodal tDCS of the visual cortex on had no effect on the sensitivity for temporal and spatial frequencies. Only cathodal stimulation had a short-lasting negative effect during and immediately after stimulation. Most probably, this might be due to the shorter stimulation duration or due to specificities of the visual stimuli used. The results of our current study imply that improved visual sensitivity might have a positive impact on visual function. Further studies are now motivated to explore the therapeutic potential of tDCS in patients with visual field defects measured by threshold perimetry.