Consciousness and the Eye

Consciousness and the Eye

If you look into the eye of an animal (say, a bird) you gain a strong impression of consciousness: the eye seems to brim with consciousness. The same is not true of other sense organs: you don’t get a comparable impression from looking at the ears or nose or skin. This suggests that there is particularly close connection between vision and consciousness—vision is par excellence an exemplar of consciousness. The eye is a receptor that responds to light, a transducer that converts an impinging light stimulus into nervous activity, and ultimately into a conscious impression of the world. Thus there is a close connection between light and consciousness. Elsewhere I have suggested that light is the reason that consciousness exists on planet Earth: because of light from the Sun evolution designed an organ that could respond consciously to this rich source of information, and no other stimulus could trigger such a radical biological innovation.  [1] I call this the “optical theory of consciousness”. The optical theory says, crudely, that light causes consciousness; less crudely, that light-sensitive tissue is the kind that uniquely gives rise to consciousness. No light, no consciousness; with light, consciousness predictably develops. I will make some further remarks in defense of this theory, particularly with respect to the uniqueness claim.

            Light has some claim to be the most remarkable physical phenomenon in the universe (in some sense of “remarkable”). The eye has some claim to be the most remarkable biological adaptation, and consciousness the most remarkable psychological phenomenon. Clearly, there must be a reason that the properties of light lead so reliably to the existence of eyes (the convergent evolution of eyes is well known): there is something about light that makes it worth responding to—and worth being conscious of. It is a stimulus that craves a response—that demands to be recognized. You would be a fool to take no notice of light, biologically speaking. Hence animals that live in light tend to be sensitive to it; and there are eyes everywhere, beady and acute. But what exactly is so special about light? We know its speed is special, being both constant and maximal. That sets it apart from everything else—the character of its motion. But it is hard to see how this property of light would be biologically significant: even if light were slower or its velocity variable, it would still be the stimulus that it is from the organism’s point of view. Light also travels in a vacuum, divides into a spectrum of wavelengths, has particle-wave duality, and has zero mass—but again, these properties have little to do with light as a biological stimulus. The feature of light that is most relevant to its stimulus profile is surely its informational density: there is just so much information present in the light that impinges on the typical retina. This is because of its physical structure—all the billions of photons reflected off surfaces bearing fine-grained information about those surfaces. This is a powerful resource for organisms to tap into—a detailed map of what is going on around the organism. The claim of the optical theory is that consciousness arises when a threshold of informational complexity is reached, but not before, and that light alone reaches that threshold. I won’t repeat the arguments for this view now, but I do want to address some potential objections that cry out for answers.

            The most obvious objection is that vision is not the only conscious sense. How then could light uniquelycause consciousness to evolve? Don’t sound and physical contact also cause hearing and touch to evolve? Aren’t these also associated with states of consciousness? The optical theory must reply that consciousness originallyarose in connection with eyes and only subsequently spread to the other senses. That may sound ad hoc, but actually it is not so far-fetched. It is quite commonplace for an adaptation to arise in a certain way and then become repurposed for other functions—as when feathers arose for thermal regulation and then were co-opted for flight. Maybe consciousness came with eyes alone but then the other senses recruited it as an enrichment—before that they operated without consciousness. Given that there can be subconscious perception, this is perfectly possible. Similarly, other properties common to the senses might have arisen originally in connection with vision, but were then distributed more broadly: that might be true of perceptual segmentation or perceptual computation or perceptual constancy. Segmentation is useful in visual perception and it may have started life in that domain—indeed, it may not have been possible in a different domain as an initial adaptation. We have trait transfer following trait origination. Evidence for this would be telltale remnants of the origin of the trait—say, visual-auditory synesthesia. Maybe such synesthesia was more common soon after hearing borrowed consciousness from vision, later phased out as pointless. Or there might be abstract structural features of visual consciousness that show up in auditory consciousness, bearing witness to their origins (“quality spaces”). Or there may be metaphors drawn from vision and applied to the other senses. So the objection is not as decisive as it may sound—the optical theory has potential resources with which to explain the spread of consciousness to non-visual senses. If we grant that the non-visual senses don’t command stimuli that inherently cross the threshold of informational density required to create consciousness, then the optical theory might be the only way to explain why they are nevertheless accompanied by consciousness.

            A sharper test of the optical theory is supplied by the physiology of actual animals on Earth. The theory would be decisively refuted by the existence of two sorts of animal: those with eyes but no consciousness, and those with consciousness but no eyes. Such animals would show that eyes are neither sufficient nor necessary for consciousness—seeing automata and blind sentience. A survey of the animal kingdom fails to turn up any animals of the former kind: no animals both have eyes and are clearly not conscious. You might point to the compound eyes of insects, but (a) it is not clear that insects are devoid of consciousness and (b) we could always restrict the optical theory to more sophisticated eyes (such as the camera-like variety). It is a striking fact how well correlated eyes and sentience are—we don’t find animals with developed eyes going around like zombies! In fact, the animals with the best eyes always seem particularly alert and conscious—birds, for example. In contrast, the dull and sluggish eye tends to be correlated with manifest somnolence, a drowsy mode of being. But the second kind of animal is more challenging for the theory: what about animals that see nothing but feel at lot? We talk about being “as blind as a bat”, but we don’t hesitate to call bats conscious. Why can’t there be an animal with acutely conscious hearing but no seeing eyes? Isn’t that the case for the human blind?

            The answer to this worry is that the optical theory does not claim (absurdly) that consciousness is only possible in the sighted; it claims that consciousness arises in evolution specifically from the visual sense. So it is entirely possible for sight to be reduced or eliminated in a species as a result of changing environmental conditions, despite the fact that it arose in a visual setting. A genuine counterexample would have to be an animal with no sighted ancestors that is nevertheless fully conscious. Bats have vision and may well have evolved from animals with better vision (not being nocturnal), so their consciousness could easily have arisen from vision, and only from vision. Similarly for blind snakes and moles: their evolutionary history might well have included sighted ancestors. I don’t know of any animals that are clearly conscious but have nothing visual in their present make-up or evolutionary history; so vision could be the sole source of the brain structures that make for consciousness. Those brain structures may persist in a biological line while eyes themselves are phased out. There are certainly organisms that have blindness written deep in their genes—such as bacteria, viruses, plants, plankton, jellyfish, and eyeless worms—but they are all strong candidates for zombie status. The correlation between consciousness and eyes is strong, and not coincidental according to the optical theory. It is also strong between other sense organs and consciousness, but the optical theory claims that consciousness arises (originally) only in the context of stimuli that reach a certain threshold of informational density, which light alone reaches.

The kind of density found in a typical visual experience is never matched by the density found in an auditory experience—simply because packets of light are physically much richer than sound waves. The stimulus causing your eardrum to vibrate has nothing like the complexity of the stimulus falling on your retina, and similarly for tactile and olfactory stimuli. The brain will process a stimulus unconsciously if it can (consciousness being biologically costly); it will only invoke consciousness if the stimulus is so complex as not to be susceptible to unconscious analysis. Natural selection has apparently decided that the costs of consciousness are worth it in order to extract encyclopedias of information from the visual stimulus. If the planet were perpetually dark, there would be no need for consciousness: eyes would be pointless and the other sorts of physical stimulus don’t call for anything beyond unconscious processing. Imagine that those stimuli were even more impoverished than they are now, with precious little information contained in them: the atmosphere might be thin and unresponsive, so that sound waves possess little structure or energy; or the chemicals emitted by objects might reach the nostrils in small numbers. A relatively simple sensory mechanism will suffice for maximum information extraction from these limited stimuli; there is no need for the elaborate and metabolically expensive machinery of sensory consciousness. Well, compared to light stimuli, actual-world non-visual stimuli are similarly impoverished—as a matter of the simple physics of the stimuli.

Let me drive the point home by distinguishing synchronic and diachronic informational density. We have not only the photons currently striking the retina; we also have the photons striking it a moment later, updating the perceiver’s image of the surrounding world. This happens with inconceivable rapidity as battalions of photons continuously bombard the retina; the information conveyed concerning changes in the environment is virtually incalculable (just think of catching a ball). This is a colossal task of perceptual analysis, orders of magnitude beyond what can be achieved with the ears (not that this is unimpressive). Echolocation is remarkable, but it is not to be compared to vision, which is presumably why so few animals substitute the one for the other. The auditory stimulus is simply not rich enough, no matter how adept the animal may be at echo detection. Sounds are mediated by perturbations of molecules in the atmosphere, a fairly crude way to transfer information, but light has limitless bandwidth and extreme fineness of structure. The retina has to absorb what this stimulus imparts, then have it instantly replaced by a new stimulus, keeping track of the changes. The optical theory says that it is this challenge that brings sensory consciousness into existence. We don’t know how this happens, or even why it needs to happen, but evidently that is the way things work. It just seems to be a natural fact about light and consciousness that the former triggers the latter (given the right evolutionary context). If we subscribe to panpsychism, we could say that the consciousness-creating properties of matter would remain latent if it were not for vision.  [2] The other senses don’t need to go beyond the non-conscious aspects of matter to perform their work, but vision needs more advanced machinery to perform its work (why, we don’t know). If the world were completely dark, the psychic properties would not coalesce into conscious experience, but light operates to elicit them to produce such experience. However consciousness may be generated, the optical theory says that light-sensitivity is the original trigger that causes it to emerge. We know that light provides the energy that drives the entire biological world (via photosynthesis) but it also turns out that it drives the emergence of consciousness in the psychological world (what we might call “photo-sentience”).

If the optical theory is on the right lines, a good place to look to gain understanding of consciousness would be the psychophysics of light. This has been extensively studied, particularly with respect to receptor cells in the retina (rods and cones etc). But a more self-conscious study of light as it relates to consciousness might prove helpful: particularly the kind of internal structure both have. Is it a granular or a continuous structure, or possibly a mixture of both? Light seems to have both sorts of structure depending on the context (behaving sometimes like a wave, sometimes like a particle), and there is some reason to find the same sort of duality in visual consciousness. If consciousness evolved as an adaptive response to light, it might inherit some of the features of light, structurally if not in substance (consciousness is surely not made of light). Response mirrors stimulus; effect reflects cause. At least this looks like an interesting research program. The nature of the biological world is shaped by the fact that photosynthesis lies at the root of everything organic, so it is reasonable to expect that the psychological world would reflect the centrality of light in shaping the mind. Light is the aspect of the environment that has exercised the most profound influence on the nature of sensory experience (according to the optical theory). Consciousness is the progeny of light—what happens to organisms when light-sensitive receptors come into the world.

No doubt the optical theory is highly speculative, also counterintuitive in some respects, but it provides a novel way to think about a problem sorely in need of fresh avenues of inquiry. It also has a poetically attractive quality: light and consciousness turn out to be made for each other, in virtue of the brute physics of light. There is nothing in the universe quite like light, and there is nothing in the universe quite like consciousness: these two remarkable things turn out to be deeply connected.

Colin McGinn         

  [1] See “Consciousness and Light”, in Philosophical Provocations: 55 Essays (MIT Press, 2017). I will not here repeat the arguments for this position.

  [2] I put it this way for expository purposes not because I subscribe to panpsychism. I also hope it is clear that the optical theory is in no way incompatible with the idea that consciousness is mysterious, quite the contrary.