health and medicine question #303
Ashley Robertson, a 24 year old male from the Internet asks on December 13, 1997,Q:
Why is the sound of fingernails scratching against a chalkboard so irritating to listen to?
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This has to do with the wonderful sensory apparatus at our fingertips.
- The skin of the nail beds is exquisitely supplied with densely packed nerve endings, including many pain endings (as torturers of every era have always discovered). This is partly to give us the very high degree of haptic (touch) sensitivity we enjoy in our finger tips and partly to help ensure that we take extra special care of these wonderful, survival-enhancing manipulative instruments.
- Researchers have shown that there are some high-threshold nociceptors among these intertwined nerve endings in the skin; i.e., receptors that respond almost exclusively to tissue-damaging stimuli. Many of the rest, however, are "polymodal" receptors and nerves that signal other haptic modalities by combined (cross line) *patterns* of responses in many adjacent receptors and their connecting neural pathways. That is presumably why you can get funny effects like a strong burning sensation by placing a rack on the skin that has slightly-spaced alternating cold (but not painful) and warm (not hot) bars. The brain interprets this pattern of stimulation as intense burning, even though it's a totally innocuous stimulus.
- Running your fingernail down the board causes a pattern of high frequency microvibrations in the nail, much like blowing on a clarinet reed causes sound vibrations. (Think about the ugly sounds the chalk makes when a careless lecturer hits the board at a bad angle). I suspect these vibrations of the nail are transferred to the dense net of nerve endings beneath the nail where they do some mischief within the touch system--it's quite an unphysiological stimulus, i.e., unlike the sort of stimuli the system usually encounters in everyday life. What we have learned from phenomena such as the "phantom limb" is that any unphysiological, random pattern of impulses in parts of this polymodal network is likely to be interpreted by the brain as noxious and that is what is happening in this instance.
- An interesting experiment would be to try to recreate this pattern of stimulation to the nailbed in the laboratory using microvibrators instead of a blackboard, to see how people would react. It might be hard to get volunteer subjects, though, and one can only imagine what the ethics review panel would say!
The question asks why listening to the sound is irritating. The answer explains why making the sound is irritating. It seems likely that the response would be a result of preventing physical damage. If it was a response in the nerves of the fingertips, it would affect only the person making the noise, but as most everyone has experienced, the sound of fingernails scraping a chalkboard is painful to everyone within earshot. The response probably comes because the frequency of the sound is similar to the sound created by damage to the human body. Perhaps the chalkboard sound is like the sound of bones breaking, specifically the type of break that would come from jumping off of something too high to safely jump off of. Moreover, the slip and stick motion is similar to the feeling of teeth scraping together, or teeth being scraped by something too hard to chew. Perhaps the reaction comes from the need to preserve one's teeth for eating meat.
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