P 155. Serial anodal tDCS over V1 induces long-term effects on colour discrimination in V4 measured in the unimpaired hemifield of patients with occipital stroke

Abstract

Question Little is known about the impact of tDCS on higher level visual functions such as colour perception. Previous studies have primarily investigated the effects of anodal and cathodal direct current stimulation over the occipital cortex on basic visual functions (Antal and Paulus, 2008). In a recent single-session tDCS study in healthy young subjects, anodal stimulation was ineffective in transiently improving red/green discrimination (Costa, 2012). One possible explanation for this may be a ceiling effect of red/green discrimination in the healthy visual system. Given the high degree of plasticity in the sensory cortices of adults (Fahle and Poggie, 2002) we examined whether serial anodal tDCS can induce long-lasting improvements in colour discrimination in the unimpaired visual hemifield of patients with occipital stroke. Materials and methods Twelve chronic stroke patients with unilateral visual cortex lesions (mean age 54.0years, 5 male) participated in a within-subject, sham-controlled, double-blind study. MRI-registered anodal(1.5mA, 20min.)or sham tDCS was applied on 5 consecutive days above the ipsilesional calcarine sulcus. Campimetric testing of age matched colour discrimination was performed in the unaffected hemifield before and after each stimulation session and at 14-and 28-day follow-up. After a 14-day interval, an identical stimulation block with the other stimulation parameter was performed. The visual task was a forced choice task involving binocular detection of red circular stimuli, presented for 200ms in one visual field quadrant at 5° offset from fixation against a green background. Results The age matched overall deviation of colour discrimination was entered in a 2-way ANOVA. In order to control for assumable learning effects, a covariate was added to the ANOVA model. The repeated measures ANOVA attained significance for the factor stimulation ( p =0.004) (stimulation∗learning: p =0.006). The factor time reached significance by trend ( p =0.065) (time∗learning: p =0.181).No significant interaction was revealed for the factor stimulation∗time (stimulation∗time∗learning).Due to a positive learning effect post hoc tests were performed within the first block. No significant difference was seen between the baselines of anodal and sham conditions ( p =0.21).In the anodal condition colour discrimination improved compared to sham on day 5 ( p =0.045) and at two ( p =0.066) and four week ( p =0.041) follow-up. Conclusion Long-term modulation in colour perception following serial anodal tDCS may represent evidence of inducible long-term plasticity in distantly connected components of the visual system (V1 and V4) in patients with occipital stroke measured in the unimpaired hemifield. This builds on studies describing the immediate effects of tDCS in the visual system in healthy volunteers. The temporal dynamics of serial anodal tDCS seem to interact with learning processes, and may yield potential support for neuroplastic adaptation processes following cerebral lesions. Future studies should examine the long-term outcomes and dynamics of tDCS-induced neuromodulation.