Previously, I discussed how to conceptualize the experience of organisms with different dimensionalities of their color spaces, along with a few other effects like the varying color sensitivy across different parts of the retina as seen in rabbits. But, as hinted out by the mention of Mantis shrimp at the end, multichromatic visual systems can actually get a lot weirder than that.
Even humans, and in fact most vertebrate animals you are likely to be familiar with, actually have a more complex visual system than our 3-dimensional color space implies. After all, we have four different types of light sensing cells in our retinas, and yet we do not have tetrachromatic vision! What is that fourth type--the rods--doing? Most readers will probably already know that rod cells are what give us low-light vision. Most of the time there is very little, if any, interaction between rod cells and our three types of cones: either it is too bright, rods are completely bleached, and we only get visual information from cones (known as photopic vision); or, it's too dark for cones to respond at all, and we only get visual information from rods (known as scotopic vision). In those sorts of low-light situations, humans become monochromats--we physiologically cannot see color in dim light! Our brains, however, are very good at lying to us, and filling in the colors that we know things should be. Unless, perhaps, you are small child who does not have a whole lot of experience with what colors things should be yet--a situation which once led to an adorable experience with my oldest child when he was very small. Once, when he had woken up early in the morning, I found him playing in his bedroom with a pile of balls, sorting them into "black ball!" and "grey ball!"; and then, when I turned on the light in the bedroom he gasped and said "Oh! Color!"
Incidentally, it is possible for each of these parallel visual systems to fail independently, mostly due to genetic conditions that inactivate either rods or cones. Humans lacking cone cells are rod monochromats and experience day blindness; humans lacking functional rod cells are nyctalopic and experience night blindness. And while most mammals are at least dichromats, armadillos, anteaters, and tree sloths are all rod monochromats, as are 90% of deep-sea fish species.
In between these extremes, there is range of mesopic vision, where both rods and cones have significant activity, and color perception gradually shifts as light levels get progressively darker or lighter. At no point do we incorporate rod cell data into the opponent process to get tetrachromatic vision, though; it's essentially used to augment luminosity information when cone cells start to struggle, causing a shift in the spectral sensitivity peak and altering apparent saturations.
Not all vertebrates handle dark vision in the same way, though, or have the same rod or cone sensitivity limits. Birds transition into scotopic vision at much brighter illumination levels than we do, as their color vision is sharpened by oil droplets that cut out noise at the tails of cone cell receptivity--but that also means that they waste more light, and so need more light to see color. Meanwhile, although most nocturnal vertebrates rely heavily on rods and tend to reduce their color perception or lose it entirely (which is why so many mammals are dichromats, and as mentioned above some are even rod monochromats--we lost tetrachromacy when our ancestors were nocturnal, and occasionally re-evolved trichromacy in more recent eons) nocturnal geckos don't have rods at all--they rely entirely on cone cells, which have simply evolved to be more sensitive than ours, and so retain a constant sense of color perception across their entire perceptive range of luminosity. (This is probably because they evolved from diurnal ancestors who had already lost their rods, as most diurnal lizards and some snakes have). In fancy terms, they have simplex retinas (containing receptors for a sngle integrated visual system), while we have duplex retinas (containing receptors for two parallel visual systems). Hawkmoths and nocturnal bees have parallel adaptions, with altered ommatidium geometry that improves light concentration onto individual receptors, for monoplex trichromatic low-light vision. But that's less weird and complicated than humans--what about more weird?
Toads and frogs, it turns out, have multiple types of rod cells, which are sensitive to even lower levels of light than human rods are. Which means they have genuine dichromatic vision in situations that would seem to us pitch black! In theory, there could be creatures that integrate their visual experiences across different light levels, using multiple rod types so that the brain has to lie less about what colors things should be in the daylight--but amphibians don't do that! Neither do they have a single 5-dimensional color space--rather, they have two completely independent color spaces, one dichromatic and one trichromatic, overlapping the same frequency range, which could in the correct conditions be perceived at the same time, but generally show up in different environments and are used for different purposes. Frogs and toads use their cone-based vision to identify food and mates, but they use their rod-based vision exclusively for navigation, with dichromacy allowing them to better distinguish directions based on different colors of light sources (incidentally, they prefer to jump towards high-frequency sources in light conditions, and towards lower-frequency sources in dark conditions).
Now Mantis shrimp provide the most famous example of optical complexity, but plenty of arthropods have large numbers of opsin types. Even daphnia, or water fleas, which don't even have image-forming eyes, are tetrachromatic in their ability to respond to the colors of light sources! Does this mean that butterflies with 8 receptor types are octochromats, with a 6-dimensional hue space? Well, no, for the same reason that frogs aren't pentachromats. Like dichromatic rabbits, creatures with large numbers of photorecptor types tend to have them localized in different parts of the visual field, to serve different purposes, and the signals are not neurologically combined to form a single coherent color space. Papilio butterflies, for example, which do have 8 different photoreceptor types, behave like tetrachromats when identifying flowers as food sources, but behave like dichromats (despite using 3 receptor types to form the relevant dichromatic retinal signals!) when selecting leaves for egg-laying. This kind of behavior-specific segmentation of visual systems means that in some species, different sexes actually have completely different visual systems, because they need them for different reproductive tasks! Which suggests some interesting sci-fi possibilities. And while daphnia are individually tetrachromatic, they have genes for many more than just 4 opsin types. If different sets of opsin genes were expressed in different individuals, the philosophical question "is what I call red really the same as what you call red?" would have an objectively-verifiable answer, as every different morph of the species (whether segmented by sex or caste or random variation) would have different color perceptions.
That brings us to the Mantis shrimp. With 12 different spectral receptor types, they could be doing a multiple-parallel-colorspace thing, like frogs and butterflies do. But... they aren't. As mentioned in that previous post, Mantis shrimp don't actually have particularly high spectral resolution, and they don't have the neural architecture to construct decorrelated opponent channels to produce a single perceptual color space. Instead, their large number of receptor types seems to exist to avoid the need for that kind of complex neural architecture! Instead, the Mantis shrimp visual system is built for speed and efficiency. Because of the spatial distribution of different receptor types into bands across their compound eyes, getting a full spectral profile on any given object requires mechanical scanning, which is relativey slow, but metabolically cheap; and wherever a given object falls in the visual field, determining whether or not it matches the spectral sensitivity of that region is instantaneous.
If Mantis shrimp were conscious, we might imagine their experience of color as being more analogous to our own perceptions of sound or taste. Mantis shrimp don't recognize abstract colors--they recognize specific fuzzy spectral patterns. Similarly, we have thousands of auditory hair cells that each respond to a specific frequency, but we don't uniformly group them into a kilodimensional "sonic color" space--we can selectively identify individual frequencies overlayed, or recognize particular spectral patterns of timbres and specific known source types. Taste and smell are similar; we have more than 400 types of olfactory receptors and at least 5 taste receptors, but we don't have a 405-dimensional experience of taste and smell (in fact, we don't know what the neurological dimensionality of human chemoreception is; to date, there is no model that can predict olfactory sensation from receptor activations). Instead, we can pick out individual receptor channels that are useful for specific purposes (sour helps us identify acids; bitter helps us identify poisons; salty helps us identify, well... salt; sweet helps us identify carbohydrates; and umami helps us identify proteins), and we can recognize specific fuzzy patterns that form the chemical signature of specific source types. For a good long time, western philosophy held that smell was "ineffable", and impossible to describe in language through any means other than "smells like a specific thing"; that turns out to be a symptom of western philosophers just not being bothered to try, though, and in fact there are many languages around the world which have generic olfactory terms disconnected from a specific source just as we have generic color terms. Statistical analysis of those language's vocabularies suggests that humans actually conceive of smells arranged in a two-or-maybe-three-dimensional space, where the major axes are "edible vs. non-edible" and "pleasant vs. unpleasant" (or "dangerous vs. safe"). Thus, durian is unpleasant but edible, ammonia is unpleasant and inedible (and dangerous), flowers are (generally) pleasant but inedible, and fruits are pleasant and edible. Languages which have generic olfactory terms generally have 12-16 of them--similar to the maximum number of basic color terms found in human languages.
So, a conscious alien species with Mantis-shrimp-like vision, or even a large number of parallel multidimensional color systems like butterflies have, might experience their spectral perceptions not in an analogous manner to our experience of color, but collapsed down into a small number of behaviorally-relevant dimensions. Is this the spectral pattern of something I can eat? Is this the spectral pattern of something dangerous? Is this the spectral pattern of something useful to me? Is this the spectral pattern of a potential mate? Etc. And depending on how important vision is to their culture (vision doesn't have to be an alien's primary sense just because it's ours!), they may consider the categorization and naming of generic colors to be completely ineffable, or totally normal--just disconnected from the raw physiological inputs which exist below the level of conscious awareness.
But could there be creatures with extremely high dimensional color vision? Aside from the lack of evidence that they exist on Earth implying that high-dimensional vision probably wouldn't evolve elsewhere either, there are some practical arguments for why they shouldn't exist. Dichromatic vision permits distinguishing between objects and areas exhibiting predominantly higher-frequency vs. predominantly lower-frequency light, which is useful for picking out objects and against a background and general navigation, as seen in amphibians; however, because dichromatic vision conflates hue and saturation, it is not reliable for picking out specific wavelengths. While trichromatic vision can still be fooled by pairs of inputs that are indistinguishable from monochromatic light, it at least provides the possibility of identifying a unique spectral peak, giving us perception of the spectral colors. A lot of animal behaviors rely on this ability, such as the aforementioned Papilio butterflies which use a g-(r+b) opponent color signal to identify green leaves for egg laying, excluding objects which are too red or too blue; or apes and humans, whose trichromatic vision allows us to distinguish ripe, unripe, and overripe fruit (among other things!) as the peak reflectance shifts across the spectrum. Separating out the hue and saturation dimensions also gives us more information about the material properties of reflecting objects. So if trichromacy is alreadys so much better, why are there so many tetrachromats in the world? Well... we don't know. There are probably multiple contributing factors; trichromacy is mostly-adequate for disinguishing most ecologically-significant variation in most natural spectra, but tetratchromacy does reduce further reduce the possibility of spectral confusion. It may assist with color constancy--the ability to calculate what the color of a reflecting object "should" be under varying light conditions (although even dichromats can do that to some extent). Having more receptor types may provide better spectral resolution when covering a wider visual range--note that most tetrachromats can see further into the infrared and ultraviolet than we can. So perhaps pentachromacy or hexachromacy would be more useful to creatures that evolved in an environment with a different atmosphere that transmitted a wider band of potentially-visible light!
References:
Thresholds and noise limitations of colour vision in dim lightFrom spectral information to animal colour vision: experiments and concepts
No comments:
Post a Comment