Free Idiot Brain by Dean Burnett

Pain, aka nociception, has its own dedicated receptors and nerve fibers throughout the body.
    Pretty much the only organ that doesn’t have pain receptors is the brain itself, and that’s because it’s responsible for receiving and processing the signals. You could argue that the brain feeling pain would be confusing, like trying to call your own number from your own phone and expecting someone to pick up.
    What is interesting is that touch sensitivity isn’t uniform; different parts of the body respond differently to the same contact. Like the motor cortex discussed in a previous chapter, the somatosensory cortex is laid out like a map of the body corresponding to the areas it’s receiving information from, with the foot region processing stimuli from feet, the arm region for the arm, and so on.
    However, it doesn’t use the same dimensions as the actual body. This means that the sensory information received doesn’t necessarily correspond with the size of the region the sensations are coming from. The chest and back areas take up quite a small amount of space in the somatosensory cortex, whereas the hands and lips take up a very large area. Some parts of the body are far more sensitive to touch than others; the soles of the feet aren’t especially sensitive, which makes sense as it wouldn’t be practical to feel exquisite pain whenever you step on a pebble or a twig. But the hands and lips occupy disproportionately large areas of the somatosensory cortex because we use them for very fine manipulation and sensations. Consequently, they are very sensitive. As are the genitals, but let’s not go into that.
    Scientists measure this sensitivity by simply prodding someone with a two-pronged instrument and seeing how close together these prongs can be and still be recognized as separate pressure points. 6 The fingertips are especially sensitive, which is why braille was developed. However, there are some limitations: braille is a series of separate specific bumps because the fingertips aren’t sensitive enough to recognize the letters of the alphabet when they’re text sized. 7
    Like hearing, the sense of touch can also be “fooled.” Part of our ability to identify things with touch is via the brain being aware of the arrangement of your fingers, so if you touch something small (for instance, a marble) with your index and middle finger, you’ll feel just the one object. But if you cross your fingers and close your eyes, it feels more like two separate objects. There’s been no direct communication between the touch-processing somatosensory cortex and the finger-moving motor cortex to flag up this point, and the eyes are closed so aren’t able to provide any information to override the inaccurate conclusion of the brain. This is the Aristotle illusion.
    So there are more overlaps between touch and hearing than is immediately apparent, and recent studies have found evidence that the link between the two may be far more fundamental than previously thought. While we’ve always understood that certain genes were strongly linked to hearing abilities and increased risk of deafness, a 2012 study by Henning Frenzel and his team 8 discovered that genes also influenced touch sensitivity, and interestingly that those with very sensitive hearing also showed a finer sense of touch too. Similarly, those with genes that resulted in poor hearing also had a much higher likelihood of showing poor touchsensitivity. A mutated gene was also discovered that resulted in both impaired hearing and touch.
    While there is still more work to be done on this area, this does strongly suggest that the human brain uses similar mechanisms to process both hearing and touch, so deep-seated issues that affect one can end up affecting the other. This is perhaps not the most logical arrangement, but it’s reasonably consistent with the taste–smell interaction we saw in the previous