Spatial perception of pain (IV) Empirical evidence

In this post I confront the propositions I previously described about where it hurts with experimental evidence. There is a recent review about spatial perception of pain, which contains a lot of relevant information (Haggard et al., 2013).

First of all, proposition A (spatial information is independently provided by tactile receptors) is quite clearly ruled out by empirical evidence. There are three types of nerve fibers innervating the skin. Aβ fibers mediate tactile sensations, while Aδ and C fibers mediate pain and temperature sensations. There is a lot of evidence that these are clearly separated, and the type of sensation does not depend on stimulation frequency (i.e., stimulating Aβ fibers never elicits pain). In addition, spatial localization does not seem to rely on Aβ fibers. For example, it is possible to block conduction in Aβ and Aδ fibers, leaving only C fibers. In this case, noxious stimuli can be localized on the skin with about the same resolution as when nerve conduction is normal (Koltzenburg et al., 1993). This implies that the pattern of activations of nociceptors conveys all the necessary information for spatial localization.

Now these patterns could be given a spatial meaning in different ways. One is learned association with tactile or visual stimuli (proposition C). In primary somatosensory cortex (S1), there are nociceptive somatotopic maps of single digits that are highly aligned with maps of responses to Aβ stimuli; there are also neurons that are sensitive to both mechanical stimulation and temperature. But this is only suggestive of a common spatial frame for both modalities. More specifically, if spatial information in pain-specific fibers were acquired from the tactile modality, then the spatial resolution of pain could never be better than that of touch – we would expect that it is similar and perhaps slightly worse. A systematic mapping on the whole body surface shows that this is the case in most locations, but not all (Mancini et al., 2014). Specifically, spatial resolution for pain is more accurate than for touch at the shoulder. In addition, the gradients of spatial resolution from shoulder to hand are opposite for touch and pain, in accordance with the gradients of innervation density of the corresponding fibers. Finally, there was one case of a subject completely lacking Aβ fibers that had normal spatial resolution. These observations rule out the proposition that spatial information in pain is acquired from the tactile modality (at least that it is entirely acquired from it).

It could be that spatial information in pain is acquired from vision. Although I have not seen such an experimental study, I would be very surprised to learn that blind people cannot localize pain. Finally, we could then postulate that spatial localization of pain is acquired from either touch or vision, whichever one is present. The best way to test it would be to map the spatial resolution of a blind subject lacking Aβ fibers. Without this test, the possibility is still quite implausible. Indeed, the subject lacking Aβ fibers had similar spatial resolution as other subjects. But the map includes the lower back, which cannot be seen directly (however the data is not shown for that specific location – I am assuming it follows the same pattern since the authors don't comment on it). Therefore, in that region there is neither vision nor touch for that subject. All these observations tend to reject proposition C.

There is an interesting observation about the relation between innervation density and spatial resolution. As I mentioned above, along the arm there are different gradients of spatial resolution between and touch and they agree with gradients of innervation density of the corresponding fibers. But in the fingertips, the relation does not hold: spatial resolution for pain is high but innervation density of pain-related fibers is low (Mancini et al., 2013). How is it possible? One possibility is cognitive factors, for example we use our hand a lot, perhaps attention is focused on the fingers or we have more experience grasping things and thus developing our spatial discrimination abilities. Another possibility (which I haven't seen mentioned in these studies) is that the patterns of activation may be intrinsically more discriminable, because of the shape, tissue composition or the presence of the fingerprints.

We are then left with proposition B (and variation B2): you feel pain at a particular location because specific movements that you make produce that pain. I noted earlier that this proposition raises a problem, which is that you cannot localize pain that you have not produced yourself in the past. It seems a bit implausible, when we think for example of tooth ache. I argued then that to solve this problem, one would need to postulate that nociceptors can be activated at a “subthreshold” level that does not produce pain. In this case, to feel pain at a particular location requires previously producing specific movements that produce a similar (but possibly less intense) pattern of activation of the pain receptors. The subthreshold activity of these fibers should reach the central nervous system and induce plastic changes supporting future localization of noxious stimuli, without producing any conscious sensation. Finally, I note that there is a potential problem in the fact that intensity and spatial information are carried through the same channel (pain-related fibers), and therefore intensity of pain changes the pattern of activation from which spatial information is extracted. If spatial localization is learned at subthreshold levels, then there is a potential issue of generalizing to pain-inducing levels, with possibilities for biases in pain localization.

Haggard P, Iannetti GD, Longo MR (2013) Spatial sensory organization and body representation in pain perception. Curr Biol CB 23:R164–176.
Koltzenburg M, Handwerker HO, Torebjörk HE (1993) The ability of humans to localise noxious stimuli. Neurosci Lett 150:219–222.
Mancini F, Bauleo A, Cole J, Lui F, Porro CA, Haggard P, Iannetti GD (2014) Whole-body mapping of spatial acuity for pain and touch. Ann Neurol 75:917–924.
Mancini F, Sambo CF, Ramirez JD, Bennett DLH, Haggard P, Iannetti GD (2013) A Fovea for Pain at the Fingertips. Curr Biol 23:496–500.

Spatial perception of pain (III) How can we feel pain inside the body?

I will first start with a summary of the different propositions I made in the previous post about where it hurts.

- Proposition A (independent channels): there are two independent channels, one that provides pain information (intensity or quality of pain, through pain receptors) and another one that provides spatial information (through tactile receptors or vision). The two channels are bound by co-occurrence.
- Proposition B (sensorimotor): you feel pain at a particular location because specific movements that you make produce that pain.
- Proposition B2 (sensorimotor): you feel pain at a particular location because whenever this particular activation pattern of pain receptors is present, you can manipulate this pattern or the intensity of pain by specific movements or actions.
- Proposition C (learned association): the localization of pain is inferred from the activation pattern of pain receptors (which must be spatially selective), by association with another channel that carries spatial information (e.g. tactile receptors).

Note that in A and C, I have moved the problem of spatial information to another modality, either touch or vision. We may consider that spatial information in touch and vision is constituted by sensorimotor contingencies, but it is not an important assumption here. The puzzle is the following: we can only touch our skin, the surface of our body, and we cannot see inside our body. If touch is central to the spatial perception of pain, then how is it possible that we can feel pain inside the body (say, in the stomach or in the head)?

I have discussed a similar example in spatial perception: when one hears music or speech through headphones, it usually feels like the sound comes from “inside the head”. First of all, there is a simple argument why sounds should feel as coming from your body in this case: when you move the head, the sound is unaffected, which means the source is part of your head – either on the surface (skin) or inside the head. The same argument applies to pain felt inside the body: rigid displacements of the body do not change the pain or any information associated with it. Therefore the pain is in you, not in the external world. However, this remark does not explain why pain feels inside the body and not on the skin.

I mentioned another possibility for sounds, inside as a default hypothesis: if you cannot identify the source as coming from somewhere outside, then the sound feels located inside. The default hypothesis raises a question: why does it feel located inside rather than not located at all? There is also another problem here: pain does not simply feel inside, it feels at a particular place inside the body (e.g. the stomach).

A first answer is proposition B2. Perhaps you feel a headache in the head and not in the stomach because the pain is only affected by movements of the head. In the same way, touching your stomach may alter the intensity of pain but not touching other parts. This explanation is a combination of default hypothesis (it's not on the skin so it's inside) and sensorimotor theory (B2). It is appealing but let's see how it applies to the perception of sounds inside the head. Here again, sounds do not simply feel inside the head, but at a particular place inside the head (say on the left or on the right). But no movement that you make has any impact on the sound, and so proposition B2 only explains why the sound is inside the head, but not where in the head it is.

Let us formalize the problem more precisely. Your stomach hurts. There is a pattern of activation of receptors that is characteristic of this condition, but no movement that you can make generates this pattern. In addition, in the case of auditory perception inside the head, no movement may alter this pattern. The default hypothesis is logical inference: since it is a new pattern, it must be located where I cannot produce it: in my body. But as we saw, this not sufficiently precise. To make some progress, I will start with an experiment of thought.

Imagine that in your life, you have touched only two points on your skin, points A and B. When something touches point A, you feel it located at A because you recognize the activation pattern of the tactile receptors. But what if something touches a point between A and B? One possibility would be that you don't feel it located at all, you just feel that something touches you. But it contradicts the fact that you feel sounds inside the head or pain inside the body. Another possibility is the default hypothesis: since you have never encountered the activation pattern, then you know it is neither A nor B, so you feel the touch somewhere outside of A and B. But this logical inference does not produce anything more precise. It seems to contradict the fact that we can hear sounds in our head on the left or on the right. To feel the touch somewhere between A and B requires some form of interpolation: if the new activation pattern resembles the pattern that is characteristic of A, then the touch was probably located somewhere near A; if it resembles both A and B, then it was probably located between A and B.

More generally, we can only have a finite number of experiences, and so t is unlikely that the exact activation pattern of receptors is encountered twice. Even if physical stimuli were identical, the body changes over time. Thus, it appears that we could not have any perceptual experience at all unless there is some form of interpolation. A natural proposition is then that detailed perception inside our body results from perceptual interpolation. This is not the same as logical inference, as in the case of the default hypothesis, because it necessary involves some arbitrariness: there is no way you can logically know where exactly between A and B your skin was touched if you have never encountered the activation pattern before, so the perceived location is a guess.

Now let us go back to our specific problem. How can pain be located inside our body? The idea of interpolation seems to imply that the pattern of receptor activation induced by such pains should resemble that of pains induced on the skin at opposite locations on the body. For example, pain in a joint, say the knee, should produce activation patterns resembling those of pains induced at the skin all around the knee.

There are two interesting points to note about the interpolation idea:
1) Sounds and pains located inside the body tend be less precisely localized, the location is “vague”. This means that the concept of interpolation as in picking a particular point between two points is incorrect: somehow the process of perceptual interpolation also affects the uncertainty of the location, or perhaps the perceived size.
2) How specifically are perceptual locations interpolated? In other words, what is the topology of spatial perception?

Spatial perception of pain (II)

Where does it hurt? A common answer to this question is: it hurts at the location of the pain receptors in the body. I will discuss three counter-arguments to this proposition, starting with the simple one. The simple argument is that there is a discrete set of pain receptors on the skin, but the spatial perception of pain is not discrete (the same argument applies to touch): we do not feel pain at discrete locations on our skin. It seems that the phenomenal space of pain is continuous, and this observation does not match anatomy. Then there are the anecdotical arguments: we have headaches but there is no pain receptor in the head; when you lose a limb (say a hand), the non-existent limb can hurt (phantom limb pain); there are systematic mislocalizations of the causes of pain, for example heart pains are felt in the arm. Finally there is a more sophisticated argument. Let us assume that we do feel pain at the location of our pain receptors. But then how do you know where your pain receptors are? One answer would be: we know it because it is somehow encoded in our genes. The “somehow” would deserve some precise explanation, but this is not necessary for this argument. This proposition requires that there is a systematic mapping between our genes and our body at a fine level of detail. That is, the precise location of pain receptors should depend only on our genes. But we know that this is not true. For example the size of our body depends on the diet we had when we were kids, and therefore so does the location of nerves. Therefore, even if we felt pain at the location of the receptors, we would still need to find out where these receptors are.

Another common answer to the question “where does it hurt?” is the objectivist answer: it hurts where the cause of the pain is, or where the injury is. An important difference with the previous one is that it does not imply discrete spatial perception. From our experience, this proposition seems to be correct most of the time but a simple objection is that there are cases of mislocalizations of pain (e.g. heart pains). The same argument as above leads us to the same question: how do you know where the injury or cause of pain is?

If genes are not sufficient, then it must be based on experience. Let us imagine that you hit your knee against a wall. You can see the knee hitting the wall; you also have a tactile experience; you feel an intense pain at the moment of contact, which perhaps gradually fades out. I start with proposition A: there are two independent channels, one that provides pain information (intensity of pain, through pain receptors) and another one that provides spatial information (through tactile receptors or vision). The same question now applies to the spatial channel: how do you know where something touches you? This is simpler to answer because you can touch yourself: you can associate your movements with activation patterns of tactile receptors when you touch your skin. You know where a tactile stimulus is in the sense that you know how to make movements to touch it. An objection to proposition A is: what if there is no external stimulus that activates the tactile receptors? For example, your stomach could hurt because of acidity or a tooth could hurt because of bacteria. There is nothing you can see, and all the tactile receptors are on the skin, so there is no independent source of spatial information, and yet the pain feels precisely localized. The only way to save proposition A is to assume that there actually is another source of spatial information. For example, in the case of tooth pain, maybe tactile receptors (or the nerves) are actually activated. In the case of stomach ache, it is harder to imagine that these receptors on the skin are activated (but I leave it as an open question), and in this case you would need to hypothesize that there are other types of receptors, perhaps acidity receptors, that carry spatial information. But then we are back to the same problem as before: how do these receptors get to carry any spatial information at all? (how do you know where these neurons are?) You would then need to assume that these receptors inside your body can also be activated with your own movements. I leave this as an open possibility. There is still one difficulty, which I will address later because it is shared with other propositions: how can pain be localized inside the body?

I will now discuss two other related propositions. Proposition B is purely sensorimotor: you feel pain at a particular location because specific movements that you make produce that pain. This explanation only requires pain receptors, but these receptors must be activated in a way that is spatially specific (i.e., which varies systematically with the location of the pain stimulus). For example, by pinching yourself, you associate the pattern of pain receptor activation with the location of the skin where you pinched yourself. This proposition implies that you cannot feel any localized pain unless you have previously produced it yourself. But what about when a tooth aches? It seems that you could feel tooth ache without having hurt your tooth yourself before. To save proposition B, it seems necessary to assume that the pain receptors can be activated at a “subthreshold” level that does not produce pain. In this case, to feel pain at a particular location requires previously producing specific movements that produce a similar (but possibly less intense) pattern of activation of the pain receptors.

There is a variation of B, which I will call proposition B2, which goes as follows. You feel the pain at a particular location because whenever this particular activation pattern of pain receptors is present, you can manipulate this pattern or the intensity of pain by specific movements or actions. For example, you hurt your knee and then you know you will feel a bit better if you put your hand on it, maybe because of the heat. Proposition B2 is slightly different from B by the fact that it is how you can manipulate pain, rather than how you can cause pain, that provides spatial information. The example of the tooth would then be: your tooth aches, and you know where it aches because by moving your tongue on your teeth you alter the intensity of pain.

Proposition C is learned association: the localization of pain is inferred from the activation pattern of pain receptors (which must be spatially selective), by association with another channel that carries spatial information (e.g. tactile receptors). For example: you hit your knee against the wall, the tactile receptors carry information about the location of pain, which you associate with the activation pattern of pain receptors. Later, you knee hurts but there is no mechanical contact with anything: you still feel the pain in the knee because it is the same activation pattern as when it was hit by the wall. In proposition C, you could not experience the location of a pain unless you have previously experienced the same pain in conjunction with an independent cue of location. So we have the same problem as in proposition A: what if a tooth aches for the first time? The proposition can be saved in the same way by assuming that pain receptors can be activated at a subthreshold level that does not induce pain.

There are now two questions I want to address: 1) How can we feel pain inside the body? 2) Why do we make systematic errors in localizing pain?

Spatial perception of pain (I)

Pain is a great example of many important themes in philosophy of perception. Here I want to focus on spatial perception, but I will start with a few general comments.

First of all, why do we feel pain in the first place? Scientists tend to offer two types of explanations. One is expressed in terms of efficient causes: you hit your knee against a wall, it activates receptors in your skin, these receptors make some neurons in a particular region of the brain fire, and then these neurons produce a substance (molecules) that is characteristic of pain experience (material cause). Such an explanation has some value, for example it might suggest pharmacological targets for pain relief. However, it does not explain the experience of pain at all. Why is it that a particular molecule induces the experience of pain? This does not seem to be a much better explanation than to say that the contact of the wall induces pain (it is somewhat better because it is more universally associated with pain – i.e., pain caused by other events). This problem is what philosophers call the problem of “qualia”: to explain how pain feels like, why pain hurts rather than just being some information about the state of your body. It is notoriously difficult to explain qualia in terms of efficient causes (see Thomas Nagel's famous paper, “What is it like to be a bat?”). It is much easier to explain the informative content of pain (what is going on in your body), than to explain the phenomenal content of pain (how it feels like).

A second type of explanation is in terms of final cause: it hurts because it is an experience you should avoid. It is useful for you to feel pain because then you will learn to avoid dangerous stimuli: pain has a survival value, which is why it has been selected by evolution. But again this type of explanation fails to address the phenomenal content of pain, because what it requires is a modification of behavior with dangerous stimuli, not necessarily an emotional experience. You could well imagine that when your knee is hit, you get the information that this is something that you should avoid in the future, without carrying an unpleasant feeling. You could also imagine that the event triggers a series of cognitive responses (e.g. negative conditioning) without producing any feeling at all. You could imagine that you hit your knee while sleeping, without being conscious of the event, and that your body reacts to it, perhaps even with negative conditioning (e.g. avoiding to turn in the same direction again), without you actually experiencing pain. So why does it hurt?

I do not know why it hurts. So in this series I want to address another question: where does it hurt? This is also quite an interesting question, because although it sounds obvious to us what is meant by the location of pain, we are really asking about the perceived location in the world of a feeling. What kind of weird question is this?