Anodal and cathodal electrical stimulation over V5 improves motion perception by signal enhancement and noise reduction

Background: The effect that transcranial direct current stimulation (tDCS) has on discrimination of coherent motion (CM) signals in a field of randomly moving dots (noise) can be accounted for by both noise reduction and signal enhancement. Objective: To distinguish between noise reduction and signal enhancement, we monitored the discrimination of the correct CM direction as a function of the coherence level (using the psychophysical method of constant stimuli). We then analyzed the threshold and slope parameters. Method: Thirty observers participated in the experiment; fifteen received cathodal stimulation, and fifteen received anodal stimulation, all over left V5. Results: The results showed that, rather than having opposite effects on CM discriminability, the positive- and negative-polarity tDCS over V5 affected the two parameters differently. When compared to a sham stimulation, anodal tDCS reduced the threshold, thus indicating signal enhancement. On the other hand, cathodal tDCS reduced the steepness of the slope (with better performance at low levels of coherence) compared to the sham stimulation, thus indicating noise reduction. Moreover, the results showed that late perceptual learning improved the participants' performance at medium/high CM similar to what anodal tDCS did. Conclusion: These results suggest a dissociation between the neural mechanisms responsible for enhanced CM discriminability: reduction of noisy or uncorrelated motion by cathodal tDCS versus increased activation of weakly correlated motion signals by anodal tDCS or perceptual learning.