There is some research from Japan which I read more than a decade ago (haven't been able to find since). While it was specifically on color blindness, I wonder if it's related. The research confirms that rod/cone ratios isn't the sole source of color blindness. In fact a large minority of color blind people have normal rod/cone ratios but suffer from a brain/nerve affliction which prevents full processing of the color spectrum. In other words, eyes work correctly but the brain refuses to process all available information. Accordingly, light and electric stimulus therapy results in full color vision restoration. But only so long as the therapy is maintained. It falls off after 4-6 weeks without it. This makes me wonder if the improvement is related.