I'm the author of the article, and I'm quite happy to see a response from a neuroscientist who studied this topic in the past. I wanted to ask you about point 3, because obviously I could have made a mistake. From what I understand (and what I have studied) retinal ganglion cells computing the S vs M and S vs M+L dimensions are the critical ones underlying coding of colour information along the blue-yellow and red-green axis. The additive axis, featuring amplitude information, from what I understand primarily concerns luminosity rather than colour discrimination. Without any of the ratio information, colour discrimination would not exist. Am I wrong? Would genuinely love to hear from you on this.