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Sensory adaptation is a phenomenon in which sensory neurons change their level of sensitivity to a constant stimulus over time. This adaptation allows people to adapt to their environments while balancing the need to receive new sensory input. Neurons involved with smell, hearing, taste, touch, and sight can all exhibit this phenomenon. The only neurons which do not are nociceptors, the neurons involved in the sensation of pain. This is why the smell of a severe burn appears to dissipate quickly, while the pain lingers.
One of the best ways to illustrate sensory adaptation is by example. Many people are familiar with the adaptation of the eye to its environment. When someone emerges from a dark movie theater on a matinee day, the sunlight outside seems painfully bright. Within minutes, the eyes have adapted, and the light level feels comfortable and normal. The level of light has not changed. The receptors inside the eye have adjusted their sensitivity, recognizing that they need to be less sensitive to light to avoid damaging the retina. Conversely, someone walking into a movie theater will undergo the opposite, with the eyes increasing sensitivity to light to pick up all available visual information.
Likewise, many people have noticed that when they visit a stinky house, the first few minutes inside are often very uncomfortable. Over time, however, the smell seems to diminish. This is also a result of sensory adaptation. People may also note that if they pull the clothes worn in a strongly scented environment out of the hamper, the smell will still be evident, even if the clothes did not seem very smelly when they were thrown in the hamper. This is the result of the fact that while the clothes were worn, the receptors in the nose adapted so that the smell was not overwhelming, but when the clothes were taken off, the stimulus was no longer constant and as a result the nose is more sensitive to it.
Sensory adaptation can also be experienced with touch, in which hot water seems temperate after a few minutes, for example. Background noises are an excellent example of sensory adaptation in the case of hearing; these noises literally fade to the background because the ear is used to the constant stimulus. The tastebuds can also develop reduced sensitivity to intense stimuli, as people notice when strong flavors recede as they eat a dish.
Some people experience variances in sensory processing and perception, and sometimes this can result in a lack of sensory adaptation. For these individuals, for instance, a constant loud noise will always seem loud and it can result in high levels of stress. Likewise, people may have difficulty adapting to changes in light level, and sometimes this causes symptoms like headaches and eye strain.
I guess that's why, when I'm soaking in a hot tub, I prefer to be able to jump out and take a dip in a cold water pool for a while before getting back in.
After a while, just sitting in the hot water seems kind of boring, but when you've been in the cold it feels wonderful to get back in again.
The contrast to the adaptation is what makes it fun for me, otherwise it quickly gets old.
I wonder if it would be a good idea to try and make it so that pain receptors had sensory adaption the way that other receptors do. I mean, not for everyone, because that could be dangerous (you could end up with the frog in the slowly heating water metaphor being true). But, for people who suffer from chronic pain in a particular area and there's no way to really control it.
I really find that very sad, because I know how much pain can affect your quality of life and make you feel tired and defeated. If there was a way of making it so that people with chronic pain would feel it less and less all the time, that would be pretty cool.