Painful Yarns

by G. Lorimer Moseley

  Although squares A and B are the same shade of grey, square A appears darker. I got Ted Adelson’s own explanation from his website (he said I could stick it in my book).

  This is the website:

  http://web.mit.edu/persci/people/adelson/checkershadow

  _illusion.html

  This is the explanation:

  The visual system needs to determine the color of objects in the world. In this case the problem is to determine the gray shade of the checks on the floor. Just measuring the light coming from a surface (the luminance) is not enough: a cast shadow will dim a surface, so that a white surface in shadow may be reflecting less light than a black surface in full light. The visual system uses several tricks to determine where the shadows are and how to compensate for them, in order to determine the shade of gray “paint” that belongs to the surface. The first trick is based on local contrast. In shadow or not, a check that is lighter than its neighboring checks is probably lighter than average, and vice versa. In the figure, the light check in shadow is surrounded by darker checks. Thus, even though the check is physically dark, it is light when compared to its neighbors. The dark checks outside the shadow, conversely, are surrounded by lighter checks, so they look dark by comparison. A second trick is based on the fact that shadows often have soft edges, while paint boundaries (like the checks) often have sharp edges. The visual system tends to ignore gradual changes in light level, so that it can determine the color of the surfaces without being misled by shadows. In this figure, the shadow looks like a shadow, both because it is fuzzy and because the shadow casting object is visible. The “paintness” of the checks is aided by the form of the “X-junctions” formed by 4 abutting checks. This type of junction is usually a signal that all the edges should be interpreted as changes in surface color rather than in terms of shadows or lighting. As with many so-called illusions, this effect really demonstrates the success rather than the failure of the visual system.

  The visual system is not very good at being a physical light meter, but that is not its purpose. The important task is to break the image information down into meaningful components, and thereby perceive the nature of the objects in view.

  It was me, not Ted, who made that bold. I did so because it is a beautiful conclusion and one that is so immediately applicable to pain. Watch this:

  The visual PAIN system is not very good at being a physical light DAMAGE meter, but that is not its purpose. The important task is to break the image information down into meaningful components, and thereby perceive the nature of the objects in view NEED FOR ACTION.

  The spectacular flop of clear cola

  This modulation of sensory input is not unique to vision. The worldwide flop of Clear Cola (Tab Clear© was Coca-Cola©’s particular version) demonstrated that taste is just as open to modulation as vision is. Clear colas were clear, like lemonade, but clear cola tasted like normal cola. Actually, clear cola tasted like normal cola if you couldn’t see it, but it didn’t if you could. This meant that people would drink clear cola if it came out of a can because it tasted just like normal cola that way. But they wouldn’t drink it if it came out of a bottle or a glass, because it didn’t taste like real cola then. Let’s face it, if you drink clear cola from a can, no one else can see how groovy you are because you’re drinking cola that is clear. What then, is the point? You might as well drink normal cola. So the whole thing flopped – the end of clear cola. And the creative team that thought of it I bet. Good work guys.

  The point is this: that clear cola was no good when you could see it shows that the brain uses visual information to mould, modify and modulate the sensory information coming from the taste buds. When you see that it looks like lemonade, the conscious experience that emerges is, it seems, not that good. Considering that normal cola is bad enough, it strikes me that clear cola may well have been very rank indeed.

  Still, it seems that there may be a few odd bods that are still fans of the clear stuff. Tab Clear hasn’t been made for more than a decade, but I just put £17.21 on an unopened bottle on E-bay…and got outbid!

  An alternative way to experience the modulation of taste by colour is by doing the following experiment. I have found it useful for convincing patients about the way the brain messes with sensory information, but it is most entertaining if you do it with your kids (or, if you don’t have any kids, borrow someone else’s).

  Get some lime soda. Pour two glasses of it and in one glass put some green food colouring. Ask the kiddies which drink has the more intense flavour (don’t tell them it is a trick because they will tell you the opposite of what they really think because they want to out-trick you! Actually, I might do that).

  This type of experiment has been undertaken in proper controlled environments by real-life scientists. In a range of studies, colour and viscosity have been shown to affect different aspects of taste and smell (e.g. sweetness, intensity, presumed calorie content, odour). The magnitude of the effect seems to relate to the age of the subject, which means that the older you get, the more you learn and the more your brain gets tricked by such things Ref List No. 3

  So, what has all that got to do with pain?

  The one sentence take home message: Pain, like vision, is a conscious experience that is based on many complex processes, not just the sensory information coming from your body (or, for vision, eyes).

  There are several things about vision that I think are particularly useful for understanding pain. More to the point, there are several principles of vision that can be applied to pain and help us to see that the brain is a very clever thing indeed. Here are what I consider to be the key points:

  Vision seems simple. More simple than pain at least. However, even vision is dependent on complex evaluative processes. What you see is not what you get, but rather the end result of many inputs. The conscious experience is based on all you know, what you know you know but more so on what you don’t know that you know. Because the brain is able to do this, what we actually see is sensible, and therefore biologically advantageous. Pain is like this too – the brain evaluates the sensory input from the tissues of the body and draws on complex evaluative processes. Pain then, can be considered a conscious experience based on the brain’s evaluation of how much danger the tissues are in.

  Evaluation of all the other stuff happens really quickly and outside of your awareness and control. No one would expect you to consciously decide what to see on the basis of previous experience, calculating light vectors (in your head as you go!), blurred edges etc. No one would imply that you are seeing an illusion because you are consciously deciding to see an illusion and that you are just after some attention or someone to compensate you for seeing that illusion. Pain is like this too – the evaluation of how much danger the tissues are actually in happens really quickly and outside of your awareness and control. Pain then, depends on the unconscious evaluation of threat to body tissue.

  the thirsty idiots

  Or: Pain is the conscious correlate of perceived threat to tissues that motivates us to get our tissues out of danger.

  There’s a road in Australia called the Gunbarrel Highway. It might be a bit generous to call it a road – in some places it has not been graded since it was originally constructed, several decades ago. It is about 1400 km long. It is called the Gunbarrel Highway because the lead surveyor had a thing for neat straight lines on maps, so he did his best to make the highway as straight as possible. Because of the long straight stretches, his construction team got the nickname the ‘gunbarrel team’ and the name stuck. The highway links Wiluna in the west with Giles in the east. Wiluna is about 500 km nor-noreast of Kalgoorlie. Kalgoorlie is about 600 km east of Perth. Perth is officially the most isolated city in the world. I don’t know what that makes Wiluna, but it sure doesn’t have a Tube station. Giles is not far from Uluru15, that mighty monolith right at the heart of the Australian continent and its people.

  Why one might want to link Wiluna and Giles is not completely obv
ious. Wiluna consists of a service station, to service vehicles as they prepare for, or recover from, the Highway; a general store; a pub and a camping ground. Giles consists of a remote meteorology station, known as Giles. The initial reason for building the road was to service a weapons research facility called Woomera. This part of Australia was considered by the British to be the best place in the world for a rocket range, presumably because it was a long way from Britain, and from where British constituents lived. It is certainly a part of the world not well suited to Europeans or their frigid descendents. Of course, people have been living happily in that part of the world for about 60 thousand years, but those people know things that most of us don’t. At each end of the Gunbarrel Highway, there are strong warnings to take sufficient water for two days and enough fuel to get between stops, the longest gap being 600 km.

  One pair of clever fellows, Adam and Tony, decided to drive the Gunbarrel Highway in their Lada Niva, as fast as they could. They were, apparently, experienced outback adventurers. They were also New Zealanders, which casts some doubt over the ‘outbackness’ of their adventuring, but that is a bit by the by. Adam and Tony had well designated roles and followed all the normal procedures.

  That didn’t make up for the fact that they were in a Lada Niva, a vehicle notorious for being crap. The Lada lived up to its reputation and broke down smack bang in the middle of the longest unbroken stretch on the highway. All the electronics were out, which meant the car wouldn’t work. More importantly, it meant the two-way radio wouldn’t work.

  These two fellows – Adam and Tony, were a thousand kilometres from anywhere worth being. It would have been 55°C in the shade (~ 130°F), if there was any shade. The point is, it was hot. Damn hot. This is obviously a potentially dangerous situation. Any experienced rally team would be prepared for such an event and indeed, Adam and Tony were prepared, although they didn’t realise it.

  One designated role that Adam had was to pack the water. After an hour or so in desert sun he went to get the water out of the back of the Lada and saw that it was missing. Adam told Tony that somehow he must have forgotten to pack the water. They resigned themselves to having to sit it out – the ranger at Giles would expect them in about two days time and would give them a few hours grace. Adam and Tony hoped that help would arrive before death did and if not, that death would arrive via sleep and not via the dingo’s and eagles that would already be aware of the lame kiwis lying underneath their Lada Niva.

  Now in this situation, one is sweating, as they say, ‘like a pig’ (which is a daft saying because pigs don’t sweat). Sweating leads to dehydration, which makes one thirsty. We all know that thirst is pretty much an upside down measure of hydration (or a right-way up measure of dehydration). Don’t we? Read on my fellow hydroheathens! As time went on, these two lads were getting very, very dehydrated. They were also getting very, very thirsty. Just less than 48 hours later, they heard the distinctive drizzly drone of the Royal Flying Doctor Service Cessna and scrambled, with what little energy they had, to get something to wave. There on the back seat was a silver thermal blanket, which would contrast beautifully with the red sands of the desert. On ripping the blanket out, Tony saw, and immediately remembered, that when he replaced the spare wheel, he had moved the water from the trunk to underneath the thermal blanket on the back seat. They were so thirsty that the sight of the water sent them into a frenzy. They waved the sheet, noted the change in trajectory of the Cessna, which indicated that they had been seen, and started drinking. Adam and Tony drank like they were on their last legs, which they were.

  Here is the groovy bit – by guzzling down a couple of litres or so of water, their thirst was quenched. The plane landed, the paramedics arrived, Adam and Tony indicated that they were not thirsty because they had just had plenty of water to drink. In actual fact, they were still so dangerously dehydrated that both lost consciousness before the plane had swung around to head for Kalgoorlie Base Hospital.

  Aside from narrowly missing the Darwin Awards16, Adam and Tony’s experience demonstrates a critical aspect of thirst. After their big drink they were then no longer thirsty, but they were still severely dehydrated. So, thirst does not tell us about hydration. Rather, it makes us drink. It works like this:

  As you become dehydrated, blood volume starts to drop and receptors in your cardiovascular system respond to that drop. These receptors sit on the end of small diameter myelinated neurons. When the receptors are activated, they cause those neurones to send action potentials into the central nervous system and thence to the brain. The brain, outside of consciousness, evaluates this information in light of every other piece of information available, and evokes a response. In the first instance, the response may be to constrict blood vessels, reduce blood flow to non-critical areas, reduce respiratory rate. If the brain evaluates the situation as requiring a behavioural response from the organism, then a conscious experience will emerge – thirst. That’s the thing about thirst – it is the single best way to get someone to drink. As the proverb goes – ‘You can take a horse to water but you can’t make it drink. Unless it is thirsty’17. So, it is thirst that motivates us to do what is required to get a drink. When you look at it this way, it is clear to see that thirst is a conscious experience that makes us do something. The Wiggles, a children’s music band and Australia’s most successful entertainers, put it so eloquently in their absolute classic: “Drink drink, drink some water, it’s so good for you”.

  The other critical thing about this system is that when the brain is satisfied that enough has been done, then it will stop creating the experience of thirst. This is how we can make sense of what happened to Adam and Tony.

  So, what has the thirsty idiots got to do with pain?

  The one sentence take home message: Pain, like thirst, is a conscious experience that motivates you to do something to protect your body.

  These are the points that I like to get out of this story:

  Thirst, like vision, is an experience we are all pretty comfortable with, not stigmatised, and so-on and so- forth. Thirst, like vision (and pain), is dependent on unconscious evaluative brain processes, such that it does not accurately reflect the world, but our place in it.

  Thirst is a conscious experience that motivates us to do something to survive. The reason that it is effective is that it is sufficiently unpleasant to make us want to stop it. That is, thirst does not provide a measure of dehydration, thirst makes us drink. Pain is like this. Pain is a conscious experience that motivates us to do something to protect the tissues that the brain perceives to be under threat. Explain Pain Ref List No. 1 and Patrick Wall’s superb book, Pain: The science of suffering Ref List No. 4 make this point really clear, by drawing on experimental and anecdotal evidence. That is, pain does not provide a measure of the state of the tissues, pain makes us do something to protect tissues that are perceived to be under threat.

  When the brain is satisfied that enough has been done, it will stop creating the experience of thirst. This confirms that thirst is not a measure of hydration. If it was, Tony and Adam would have been thirsty until their hydration was back to normal. I reckon that the 25 minutes or so between drinking the water and blood volume etc returning to normal is the most poignant aspect of the story because there was severe dehydration and no thirst. This proves that thirst is not a measure of hydration. Pain is like this. If the brain is satisfied that enough has been done to get the tissues out of danger, then it stops making the body part painful. This is a really nice way of understanding why inert pills and injections can still reduce pain – the brain has every good reason to conclude that what you have just done should reduce the danger level, so it reduces the pain. There are good studies that look at how it does this and we know it involves opiate-related and non-opiate related bits18. If you take this principle further, you can say that if pain reduces, it shows that the perceived threat to body tissues has reduced. It is very tricky to know what it was that changed that perception. This p
rinciple is defended in the following papers and chapters:

  Melzack, R. Gate control theory. On the evolution of pain concepts. Pain Forum 5, 128-38 (1996).

  Wall, P. in Ciba foundation symposium 174, experimental and theoretical studies of consciousness 187-216 (Wiley, New York, 1993).

  Wall, P. Pain. The science of suffering (Orion Publishing, London, 1999).

  Moseley, G. L. A pain neuromatrix approach to patients with chronic pain. Man Ther 8, 130-140 (2003).

  Moseley, G. L. Reconceptualising pain according to its underlying biology. Physical Therapy Reviews. In Press (2007).

  Jones, L. & Moseley, G. L. in Tidy’s physiotherapy (ed. Porter, S.) In press (Elsevier, Oxford, 2007).

  Thirst is not the only thing that happens when hydration reduces. Lots of things happen outside of consciousness. For example, changes in blood flow, respiration, motor output, renal flow etc. All of that happens outside of consciousness. Thirst is a sign that the brain perceives that those things alone are not enough to maintain sufficient hydration. Pain is like this: When tissues are perceived to be under threat, a whole lot of stuff happens outside of your consciousness. For example, blood flow changes, motor output changes, immune system activates, autonomic system activates. Pain is a sign that the brain perceives that those things alone are not enough.

  Twonames & the magic button.