Transcranial Direct Current Stimulation Modulates Human Color Discrimination in a Pathway-Specific Manner

Abstract

Previous research showed that transcranial direct current stimulation (tDCS) can modulate visual cortex excitability. However, there is no experiment on the effects of tDCS on color perception to date. The present study aimed to investigate the effects of tDCS on color discrimination tasks. Fifteen healthy subjects (mean age of 25.6 ± 4.4 years) were tested with Cambridge Color Test 2.0 (Trivector and ellipses protocols) and a Forced-choice Spatial Color Contrast Sensitivity task (vertical red-green sinusoidal grating) while receiving tDCS. Anodal, cathodal, and sham tDCS were delivered at Oz for 22 min using two square electrodes (25 cm2 with a current of 1.5 mA) in sessions separated by 7 days. Anodal tDCS significantly increased tritan sensitivity (p < 0.01) and had no significant effect on protan, deutan, or red-green grating discrimination. The effects on the tritan discrimination returned to baseline after 15 min (p < 0.01). Cathodal tDCS reduced the sensitivity in the deutan axis and increased sensitivity in the tritan axis (p < 0.05). The lack of anodal tDCS effects in the protan, deutan, and red-green grating sensitivities could be explained by a “ceiling effect” since adults in this age range tend to have optimal color discrimination performance for these hues. The differential effects of cathodal tDCS on tritan and deutan sensitivities and the absence of the proposed ceiling effects for the tritan axes might be explained by Parvocellular (P) and Koniocellular (K) systems with regard to their functional, physiological, and anatomical differences. The results also support the existence of a systematic segregation of P and K color-coding cells in V1. Future research and possible clinical implications are discussed.

Transcranial direct current stimulation as a tool in the study of sensory-perceptual processing

“…As the neural correlates of many visual functions are better known than the ones underlying higher order cognitive functions or the ones underlying most (or all) other senses, visual perception research has also helped us to understand how specific tDCS effects are. This can be seen in the early investigations by Antal et al (2001Antal et al ( , 2003bAntal et al ( , 2004a and in the more recent works showing a high functional specificity of tDCS (Costa et al, 2012(Costa et al, , 2015a. In what concerns clinical research, works like Plow et al (2011Plow et al ( , 2012a and Sunwoo et al (2013), have shown that tDCS can be effectively used in post-stroke rehabilitation of a sensory function (as until recently we only knew cases of tDCS use in post-stroke rehabilitation of motor functions).…”

“…In fact, there is some psychophysical evidence supporting this interpretation. First, many studies show changes in thresholds (e.g., Antal et al, 2003a;Grundmann et al, 2011;Costa et al, 2012Costa et al, , 2015a. Some studies have also analyzed tDCS outcomes using a signal detection theory approach.…”

“…Costa et al (2012) explored this issue by applying tDCS during a color discrimination task known to measure thresholds indicative of the parvocellular and koniocellular pathways functions (known to mediate the red-green and blue-yellow discriminations, respectively). For anodal tDCS the authors found a highly significant improvement of thresholds in the blueyellow range and no effect in the red-green range.…”

“…There is some evidence that the function of different cell groups within the early visual cortex (e.g., different spatial frequency channels, different visual field eccentricities) were distinctively affected by the same tDCS stimulation polarity on a number of occasions (Costa et al, 2012(Costa et al, , 2015a. There is also evidence for some selectivity in the auditory cortex, as supported by specific tDCS effects on one temporal frequency band (Ladeira et al, 2011).…”

“…Costa et al (2012) investigated how different color processing pathways are integrated in the early visual cortex. tDCS was shown to affect the function of different pathways in different ways, suggesting some level of functional or structural segregation of these pathways in the stimulated area.…”