Why do lemons taste sour? The puzzle of innate qualia.
A really nice recent paper reported the identification of a family of proteins that seem to act as sour taste receptors. They are expressed in our taste buds and allow us to detect the positively charged hydrogen ions produced by acidic substances. This is important because it lets us identify foods that are unripe or spoiled by bacterial growth, like sour milk. The discovery of these sour receptors is a big step forward – it adds to our understanding of how different kinds of chemicals are detected in the sensory neurons of the tongue and processed in the brain. But it leaves one really big question unanswered – why do sour things taste like that?
Image credit: http://www.funcage.com/blog/babies-tasting-lemons-for-the-first-time/
Why does eating a lemon produce that specific reaction – the scrunched up face, puckered lips, eyes squinting, head drawn back, eyebrows raised in surprise? This is an incredibly universal and apparently innate reaction – you can see it in unsuspecting babies eating lemons for the first time (to great comic effect). It’s not just humans, either – you can see something roughly similar in the way dogs react when they taste lemons (again, worth a look just for kicks and giggles).
That very specific response to that very specific stimulus is clearly wired into our nervous systems. Now, maybe that’s not that amazing – we have lots of reflexive responses to various stimuli that are pre-wired into our neural circuitry, like withdrawing your hand from a hot stimulus, for example. You could imagine programming that kind of thing into a robot.
But I think we can say something much more profound – that the qualitative nature of the experience of eating lemons is somehow wired into our nervous systems. In fact, you might even say that the qualia associated with that experience are in effect encoded in our genomes, as this is where the instructions are to wire the nervous system in such a way that entails that response.
I’m on shaky ground, here, I know, making inferences about subjective states. However, I think we can say, first off, that even babies and dogs are having an experience when they eat a lemon and react that way. Given that the outward signs of that experience are so universal, I see no good reason to think that the subjective experience is likely to differ between individuals – it certainly seems more parsimonious to expect that it wouldn't. The experience that babies and dogs have will not be exactly like the one adult humans would have, of course, but it seems likely that there must be some shared perceptual primitive that is the basis for this experience.
How could this possibly be established? How is the system wired to drive this kind of perceptual experience?
Wired for taste
The anatomical system for detecting and discriminating tastes is now quite well understood. Taste and smell are our two chemical senses and they have quite different jobs to do. Our sense of smell is all about detecting and discriminating between a huge range of different chemicals, each of which smells different to us. We have thousands of different odorant receptor proteins that do that job – each one specialised to bind to a different chemical. The taste or gustatory system is quite different – rather than discriminating, it instead lumps things together into just six or seven broad categories (that we know of): sweet, bitter, sour, salty, fatty, and savoury (and maybe carbonated).
These categories refer in one sense to the chemical properties of the substances being tasted (the “tastants”) and, in another, to the nature of the perceptual experience they induce. Sugars, salts and fats are all types of molecules with specific chemical properties, which are detected by specialised proteins expressed in the taste buds. The taste of savouriness (or “umami”) is induced by the chemical monosodium glutamate, which is present in things like meats and cheeses. And things that taste sour are chemically acidic – they produce positively charged hydrogen ions, which is what sour receptors detect. (Since hydrogen atoms are made up of one proton and one electron, a positively charged hydrogen ion is simply a proton).
Image credit: http://science.sciencemag.org/content/359/6379/991.full
Compounds that taste bitter are an exception – they do not necessarily share a specific chemical property or structure. They are, in fact, extremely chemically diverse – the one thing they have in common is that they may be toxic to us and should be avoided if we don’t want to poison ourselves. Animals have therefore evolved a large family of bitter taste receptor proteins, capable of detecting a wide range of such chemicals, but the taste system does not discriminate between them – that’s the job of the olfactory system. The taste system simply codes them all as “bitter”, sounding a general alarm that they should be avoided.
The thing that links the chemical properties of these tastants to the perceptual experiences of sweetness, bitterness, etc., is the way the taste receptor neurons are wired into the brain. Each of our taste buds contains a dozen or more taste receptor neurons. Each one of these neurons expresses exclusively just one of the types of taste receptor proteins – sweet receptors, or bitter receptors, or sour receptors and so on.
This exclusivity is the key, because it means each taste receptor neuron responds to only one class of tastants. So the brain just has to figure out which neurons were activated to know what kind of chemical was detected. That is accomplished by specifically wiring the different kinds of taste receptor neurons into different regions of the gustatory cortex in the brain, creating labelled lines for each taste.
A set of primary sensory neurons associated with the facial and glossopharyngeal cranial nerves innervate the taste receptor cells in the tongue and send another projection into the brainstem. Cells from that area of the brainstem project onwards to a specific part of the thalamus (a central subcortical structure which acts as a relay station for lots of types of sensory information). And cells from this ventral posterior nucleus of the thalamus project in turn to the primary gustatory cortex, which is located near the front of the brain.
The important thing in all this wiring is that the different types of taste receptor neurons get selectively wired into distinct subregions of the primary gustatory cortex. Despite being intermingled and distributed across the surface of the tongue, the nerves carrying information for each taste get segregated as they project into the brain, so that, ultimately, the different tastes are mapped across the gustatory cortex.
This is what that looks like in mouse cortex:
Image credit: http://science.sciencemag.org/content/333/6047/1213.full
And in humans:
Image credit: https://www.ncbi.nlm.nih.gov/pubmed/?term=29281722
The mechanisms that direct this selective wiring are not fully understood but some of the molecules responsible for the very first wiring "decision" - which sensory neurons innervate which taste receptor neurons - have recently been discovered. They are members of the semaphorin gene family (my favourite!) that are used as connectivity labels in many areas of the developing nervous system.
In this way, the coding of tastes gets transformed from a distributed set of different cell types in the periphery into a segregated spatial map in the brain. Now the rest of the brain just has to know which part of the gustatory cortex is active to know which type of chemical was detected. That’s a nice, even elegant, system for coding chemical tastants in the brain. An observer looking at patterns of brain activity could probably even infer what taste someone was detecting.
The problem is, there is no such observer inside the brain. The logic of the anatomy doesn’t really tell us anything about how specific patterns of neural activity in those areas give rise to conscious, subjective percepts. And it certainly doesn’t explain the qualitative nature of these percepts.
Where is the sourness of lemons perceived?
If we go back to our lemons, here’s what we know so far: the protons produced by citric acid are detected by these newly discovered specialised proteins, which are expressed by dedicated taste receptor neurons, which eventually send information, through converging connections, via several relays, to the “sour” domain of the gustatory cortex. That’s all important information, but we might ask: at what point along this pathway does perception actually occur?
Clearly, if we just stimulated the tongue, but it wasn’t connected to the brain, you would not perceive anything (in the same way that you wouldn’t see anything if your optic nerves were destroyed, even though your retina might still be electrically responding to light). And if the gustatory regions of the brainstem or of the thalamus were activated, but not connected to the cortex, my guess is you wouldn't perceive anything either.
Now, what about the primary gustatory cortex? If you stuck an electrode in there and gave it a zap, you might well induce a taste percept. Indeed, cross-activation of gustatory cortex might underlie certain forms of synaesthesia, where various stimuli in other sensory or conceptual modalities – like words or musical notes, for example – induce strong, involuntary taste percepts.
But, again, if primary gustatory cortex were activated and not telling any other part of the brain about it, you probably wouldn’t perceive anything. In fact, you can see the problem that arises with this kind of thinking – you keep on passing the signal from one station to the next, but you never reach any area that could possibly do the job of perception all by itself. The mistake is to think that perception just entails feedforward propagation and processing of sensory stimuli from the periphery. This leads to an infinite regress. There is no final station in the brain that “does conscious perception”.
Instead, we should think of perception as a comparison between incoming sensory stimuli and an internal model of the world, which is instantiated in widespread activity patterns across the brain. This comparison of bottom-up signals with top-down expectations can lead to an updating of the model to accommodate new information, which, (in some way that remains completely mysterious) may constitute the act of perception. This kind of process necessarily involves information flowing in both directions and neuronal activity reverberating through multiple cortical areas and subcortical regions, such as the thalamus.
The point of all this is to enable the organism to infer what it is out in the world that is the source and explanation of the sensory stimuli it is receiving. In the case of taste, the inference is that there is something sour, or sweet, or bitter in your mouth and there are certain appropriate responses to those different stimuli.
So, all that (vague as it is) gets us a little further in understanding how perception may happen. It probably requires all kinds of additional weird recursiveness (in the vein of Douglas Hofstadter’s Strange Loops) to get conscious awareness out of the physical system of the brain. But let’s say, for argument’s sake, that those kinds of structures and system dynamics exist. Now, one of our specialised sour receptor proteins has bound a proton and, through the magic of all this distributed and recursive circuitry, we have perceived “sour” and can infer there is an acidic thing in our mouth.
But why are sour things sour?
But we still haven’t explained why sour things taste like that. This goes beyond the fact that mildly sour things are pleasant and attractive, while extremely sour things are unpleasant or aversive. It is not just a matter of attaching a positive or negative valence for the organism to the sensations or the casual stimuli. Again, that kind of thing can be implemented in a robot, without it involving any qualitative experience.
If we assume that tasting lemons really does involve a very particular, common, or even universal, qualitative experience, then there is something else (a very big something!) that we still have to explain. (And, actually, if we assume the opposite – that the quality of the experience may vary across individuals – that only gives us more to explain). Where does the quality of this experience come from?
The chemical senses are unlike vision or touch or hearing. For those other senses, the stimuli have some properties that can be actively explored and that can be compared across the senses. They have statistical and physical properties and sensorimotor contingencies that in some way can inform the nature of the attendant visual or tactile or auditory percepts. Information from each sense can also be used to calibrate responses in the others, especially as babies and infants grow and explore their world – calibrating their visual system based first on things they can touch and only later on things at a distance. You can even think of vision as a skill – one that we get better at with experience.
Some people hold that because that kind of “ecological information” is always available in the environment, we don’t even really have to have internal representations of these stimuli. I wouldn’t go that far myself, but it is certainly true that many aspects of the phenomenal experience of visual or tactile or auditory stimuli are experience-dependent, integrative, and responsive to active exploration (enactive and embodied).
This really isn’t the case for the chemical senses. The phenomenology of smell or taste doesn’t map in any sensible way to the properties of the stimulus (unlike for audition or vision). There is no physical property of a proton that in any way relates to the properties of the sour percept it induces. And it’s also not experience-dependent or learned or calibrated versus the other senses. When you smell or taste something – especially for the first time – there is no reference point, no perceptual anchor, no sensorimotor contingencies – it just is like that.
Somehow, the quality of that experience is entailed in the wiring of the gustatory system, and the way it is linked to other areas of the brain. Which means, ultimately, that it is entailed in the program in the genome that directs that wiring. In a brain that is wired that way, detecting protons with your taste buds just will lead to that kind of subjective experience that feels like that. Those qualia are somehow innate and nothing we know (maybe nothing we can know) about the anatomy and physiology of the system can even approach providing an explanation for that.
And that is doing my head in.