This article has been popping up a lot today, and no wonder: could it be the case that there are "tetrachromats" walking around among us, people who see not three primary colors but four? A particular combination of genetic anomalies might mean that certain women would have four distinct cone photopigments in their retinas: the usual red, blue and green ones plus a special bonus one attuned to a color somwhere between red and green: a nice vibrant yellow, perhaps.

Would it be yellow, though, or something else entirely? The subjective experience of color is one of the classic imponderable philosophical puzzlers, a sort of training-wheels stand-in for the puzzle of consciousness itself. Our terms for color can be assigned rigorous interpretations based on wavelengths of light, but the experience of the colorblind throws a monkey wrench into any such scheme: they tell you that "pink" and "blue" to them have always seemed like two words for the same color, but that "pink" and "red" are perfectly distinguishable. In face of such considerations, the mind starts to boggle. What on earth do we mean when we say that two colors are "close?" When you see the blue of the sky, is your experience of that "blue" anything like my experience of it, or have we managed to slap the same word on two totally different internal sensations? Does a tetrachromat see the familiar bands of the rainbow, or does their personal rainbow take a detour through octarine in the zone between "red" and "green?" Could the trichromat mind ever properly wrap itself around an "additional" color? Or is the human mind so hard-wired for tricolor vision that it sticks its fingers in its ears and says "nyah nyah nyah" when their octarine receptors fire? [Let me interject here a recommendation for Raymond Smullyan's sadly out-of-print 5000 B.C., the book that first made me realize philosophically how disturbing the very existence of a sense of color is.]

In a way, I'm disappointed that the extra receptor fits into an already-covered part of the visual spectrum. We can see roughly from red, up at 700nm down to violet, down at 400 nm. This is slightly less than one "octave," so that, in theory, the standard representation of a color wheel ought to have a gap between red and purple. In practice, our brain kinda sorta fills it in. But, my goodness, what if we could see colors an octave apart? All it would take would be another 100nm into the infared or the ultraviolet -- four equally spaced receptors instead of three, 400nm range instead of 300nm -- and we'd be able to do with our eyes what we do all the time with our ears: perceive overtones of light. The amazing harmonic richness of music, that whole unfathomable dance of consonance and dissonance, starts from our perceptions of overlap out beyond the first octave. Our visual cortex is wired up to perceive only melody, but if we could see an octave, would we say that a color harmonizes with its doubled-wavelength companion?

Human sensory input, in fact, gives rise to all sorts of astonishing what-if scenarios, not all of them entirely implausible. What if our sense of smell was as systematic as our sense of touch, sight, or hearing, so that we could smell a scent and know if you kept going from cumin towards cardamom you got allspice but with less pepper? What if our eyes weren't designed to be large water-filled sacs, so that our visual range wasn't required to be roughly the set of wavelengths neither wholly scattered nor wholly absorbed by water? What if taste and smell weren't linked so strongly to one another? What if we could echolocate? What if we didn't have proprioception (internal sensors that tell us whether our limbs are flexed or extended)? What if our sense of spatial orientation wasn't acceleration-sensitive?

From my perspective, the biggest scientific story of last year was the computer visualization, based on actual neural outputs, of what a cat sees. [Of course, we don't really know what the cat itself "sees," any more than we know what "blue" looks like to someone else, but still, it's quite an achievement.] This is a meaningful bio-electronic interface, and a pretty dramatic one. Combine this result with the one in which electrodes implanted in a monkey's brain allowed it to control a robotic arm 600 miles away, and you've got a pretty convincing case that the output half of the cyborg equation can be made to work. Which leaves only the input half: what would it take to let our brains "natively" process information not directly provided through one of the five traditional senses? Sure, you can project a heads-up display directly on the retina, but that's still overloading sensory role of vision, tapping into the same limited bandwidth along the optic nerve. The nervous system uses all sorts of cool but usually lossy compression techniques to send image data from retina to brain and I have a sense that there's only so much visual information anyone can process.

No (and, in case it's not clear already, I'm jumping off into one especially huge what-if right now) you're going to need to plug directly into the grey matter. But that gets you into plasticity trouble: if all sorts of seemingly random neural input started rolling into your brain, would you suddenly be able to piece it together into a smooth map of the electromagnetic fields in your area? I kind of doubt it: the sudden addition of a new modality of input into a portion of the brain already in use for some other purpose would either completely muck up your normal processing or be shrugged off as noise. I have this sense that if cybernetic input implants will ever work, they'll need to wired up to the brains of the very young. You go about your day-to-day life for five years getting constant GPS information and your brain will learn how to interpret that information, will have this strong but oddly inscruitable sense of position. Ask a kid with a GPS unit what it feels like to have one and he won't be able to explain: it'll have become native to him in the same way that touch is native to all of us. So in the end, this particular thought experiment doesn't really tell us much, I fear: it's one crazy might-have-been to imagine what life with certain bio-chips would be like, but the actual experience would nonetheless be so eerily different that such speculation is more a task for artists and poets than for scientists.

I had a couple of conversations last year, right after the news about the cat, following out the societal implications of this particular theory I've advanced about the limits of mental implant technology. For one thing, if you take the plasticity argument seriously, anyone with an implant would be stuck with the technology level current at the time the chip was implanted. Once it and the brain have agreed on a common language, there's no way to pop the chip and upgrade the API. Given little trends like Moore's Law, the implanted would be carrying around amazingly obsolete pieces of tech in their heads, which makes for something of an interesting twist on steampunk fiction. Most science fiction I've come across that takes the cyborg possibility seriously either assumes infinite adaptability -- pop out one chip and pop in another and poof, you know kung fu -- or a glacial rate of technological advance -- you don't see Lando trading in Lobot for a newer model every eighteen months. It would be interesting to to see some speculative sci-fi that follows this more neurobiologically-influenced way of looking at the prospect. The built-in limitations of the approach are rich with dramatic potential, I think: wars fought between modern-tech humans and electronically-enhanced but out-tech'ed cyborgs, societies where parents must make enormous gambles on picking the right electronic legs up to give their children, strange Y2K scenarios striking outmoded firmware sitting in people's brains.

And now for something completely different, as long as we're playing what-if. In the moon's motion around the earth, the same side always faces towards the Earth. Centered at 94 degrees west longitude and 12 degrees south latitude (in the obvious lunar coordinate system, given the preceding fact) is the Mare Orientale crater, about 700 miles across (the moon itself is about three times that in diameter), very nearly circular, and ringed by a huge bulls-eye from the impact. That is to say, one of the sides of the moon we never see (because it's rotated 90 degrees away from us) bears a very strong resemblance to an eyeball.

The mythology of the moon is peaceful and soothing. The Man in the Moon sprinkles moondust on nighttime dreamers: the moon is the calm protector of the natural and the feminine. Sure, the sun is an angry sky-god who burns with a fierce flame, but the moon brings healing and safety. Well, the friend who told this tidbit to me asked, how would human history have been different if every night the sun went down and the eyeball came up? How much worse would our fear of things that go bump in the night be if everything we did at night was seen by the great implacable eye in the sky? What paranoia would stalk our dreams, what fearsome offerings would we have given up to the unforgiving all-seeing eye, what ingrained terrors far above and beyond anything Jung ever imagined would be seared into the consciousness of humanity?