Introduction

Undoubtedly we all know how nasty it can be to enter a small, closed room occupied by someone not familiar with the concept of personal hygiene or, having a drunkard sit next to you in a crowded bus. The one thing to help endure it is the knowledge that in just a matter of minutes you won't even notice the unpleasant smell anymore. This is called sensory adaptation - your nose adapts to the odors in your present environment, negating the stimulating effect.

(Sudden) changes in sensory stimulation may be considered as neurological "noise" that the brain strives to balance - it is not willing to maintain prolonged states of sensory stimulation. This means that at first a sensory stimulation of constant intensity produces a sensation (perception of the stimulus) which declines with time. This decline may be neurologically explained by the alteration of the sensory output, via spike pattern modulation; during constant stimulation impulse frequency decreases with time. This decrease in the perception of a stimulus is partly caused by thalamus and partly the receptor site itself (depending on the sense) filtering information it deems irrelevant (such as the feeling of clothes touching the skin) and only letting important (such as the positions and angles of the body and limbs while moving) or critical (such as strong pain or extreme temperatures) information pass on to the cortex.

Naturally, sensory adaptation is not limited to the sense of smell - it occurs in all senses, with the sole exception of the sense of pain, which adapts very little if at all. Even the sense of pain can be reduced by sensory overload, though; if you accidentally hit your head, strongly rubbing the surroundings of the area hurt will reduce the sensation of pain (unless you managed to hurt yourself bad enough) but, there is no actual adaptation.

How senses adapt

Adaptation to darkness

When the intensity of light is reduced, eyes will begin to adapt to the changed environment. The first - immediately noticeable - effect is the dilation of the pupils, which allows more light to enter the eye and stimulate the retina. Also, the two different receptors (cones and rods, the name is adapted from the shapes of these cells) at the base of the eyes begin to sensitize. This is caused by an increase in the concentration of a chemical, that reacts to photons (light), in the cells.

Cones

Cones are the receptors normally used in daylight and they are very sensitive to different colors. It takes 5-10 minutes for the cones to become fully dark-adapted but, compared to the rods, they are very insensitive and unresponsive to light levels available during night time. Cones are far less sensitive to light intensity and they best respond to lively reds and yellows.

Rods

Rods are the receptors mainly used at low light intensity levels. They are far more sensitive to light intensity and much less sensitive to colors. Rods are most sensitive to "colder" colors, mainly greens and blues. While cones are somewhat sensitive to all colors, rods are completely blind to red. This is why red light will not disrupt the ability to see in the dark. This is also why the world seems to turn bluish - almost black and white - in the dark. It takes about 20-30 minutes for the rods to become fully dark-adapted.

Adaptation to light

In accordance to the laws of entropy, it is much easier to break than it is to build. This is why entering bright light from total darkness only takes seconds for the eyes to adapt. The pupils erode (become smaller) to admit less light in the eye and the high amount of photons entering the base of the eyes quickly destroys most of the light-sensitive chemical in the cells, thus destroying the ability to see in the dark. This explains the spots or, "tracers" of bright light, if you look at the Sun or get a flash of bright light in your eyes in the dark: the amount of the light-sensitive chemical directly opposite to the direction of the light is temporarily drastically reduced and you get "blind" (non-dark-adapted) spots in your eyes. These spots disappear in a short time, though, once the chemical levels balance again.

Adaptation to sound

In response to sustained loud sounds, muscle tension tightens the tympanic membrane and, acting through the tendon connecting the hammer and anvil, repositions the ossicles (the three tiniest bones in the body, that form a coupling between the vibration of the eardrum caused by a sound and the forces exerted on the oval window of the inner ear) to pull the stirrup back, thus lessening the effects of the sound in the ear and protecting it from being damaged. However, this repositioning is a relatively slow process which makes the ear unable to protect itself from sudden loud sounds (such as gunshot). This process also tends to weaken with age.

Adaptation to smell

The sense of smell is probably the quickest sense - as a whole - to adapt. To put it in a very simplified form, an odorant must reach certain receptors in the olfactory epithelium (the surface of the skin in a specific location), in the upper part of the nasal cavity and dissolve to cause a stimulus or, the perception of an odor. The actual process is far more complex and I will not cover it in this write-up.

Recent evidence reveals a group of genes that code for specific receptor proteins associated with the receptors. This leads to the possibility of having over 1,000 specific odor receptors or, in plain english, making the sense of smell very sensitive to tiny variations in odorant(s). The mechanism to destroy the odorants from blocking the receptors is very effective. It is also analogous to the detoxification mechanisms in liver and it quickly eliminates the stimulus.

Adaptation to taste

Taste is an exceptional sense in the way that certain tastes may cause rather surprising (and unexpected) anomalies in other taste stimuli. For example, eating artichoke makes sour substances taste sweet briefly, while jujuba temporarily abolishes sweet sensitivity. The mechanisms behind this are unclear but modification or inhibition of receptor mechanisms is presumably involved.

(Personal note: just try and drink a cup of coffee immediately after eating any food that mainly consists of liver. *Awful*!)

As was the case with smell, taste buds rapidly adapt, i.e. rate of discharge falls off quickly even when the actual stimulus is maintained. Adaptation to one of the four generally agreed primary taste qualities (sweet, bitter, salt, sour) reduces the perceived intensity of the taste of other substances in the same category e.g. citric acid reduces the ability to taste other acids. In addition strong, spicy foods will also stimulate pain receptors. For those less experienced with spicy foods this sensation may be very unpleasant.

The mechanism by which the sense of taste works is somewhat similar with the way that sense of smell works, with the exception that sense of taste is suggested to work by "volume" while the sense of smell works by "quality". This means that all taste buds seem to respond to any of the primary taste qualities and that the quality of a perceived taste comes about by virtue of the overall pattern of discharge from a population of sensory neurons. In contrast, the sense of smell seems to work by distinctly identifying even very small changes in the odor quality with the use of highly tuned receptors, which favours the idea of "labelled line" odour reception mechanism.

Since the two senses are very closely related and the actual perceived taste is very much dependent on the sense of smell, it is no coincidence that food seems to be tasteless when having a flu or why bad-tasting medicines aren't that bad after all, if pinching your nose while taking them.

Adaptation to temperature

We quickly adapt to hot and cold stimulation, if the stimulus isn't too intense. Human skin is full of different receptors that sense heat, cold, pressure, pain etc. Possibly for evolutional reasons there are more cold receptors than there are heat receptors on the skin but, they both react to changes in temperature first and only after that to the actual "direction" of the change (either to colder or warmer). This is why sudden (strong) sensations of heat will at first cause a shiver and goosebumps (personal note: if you don't believe this, try Finnish sauna). Extreme temperatures also stimulate pain receptors in addition to stimulating the thermoreceptors. All adaptation related to senses that have the receptor sites on the skin (temperature, pressure, touch etc) mostly happens at the receptor sites already.

Adaptation to position

Adaptation to position is not the same as a limb going numb - which is caused by muscle tension and physical pressure on the nerve pathways - but refers to the loss of perception of the position or angle of part(s) of the body, caused by extended periods of stillness. In the long run this would lead to muscle atrophying and deterioration of nerve pathways but, that goes beyond the scope of this write-up. Adaptation to position is simply caused by the reducing intensity of nerve stimulation, as well as reduced blood circulation. It seems likely that the brains store information about the positions and angles of the body (sort of a reference when holding still), which is why this only occurs rather seldom in normal, healthy people. Mostly, it may happen when staying still (such as laying on a sofa) while being very tired or, more often in case of brain damage (such as a tumor or, after receiving a severe hit in the head).

Adaptation in hand-eye coordination

Eyeglasses or contact lenses that are meant to correct the errors in eyesight cause problems in hand-eye coordination, initially. Since the visual signal suddenly differs from the signal sent to the brains from the limbs, it is difficult to accurately position your hands and feet, or throw a ball but, with little time and practice you learn to readjust the relationship between what you see and what you feel. This adaptation is why those who have worn corrective lenses for extended periods of time, in addition to the adaptation of the touch of the earpieces (in the case of eyeglasses) on your skin, won't even notice their presence; at least not until they remove them. At that point they will be in the same position as when they first wore the glasses; only the error they make is in the exact opposite direction.


Sources

Robert M. Young: "Mind, Brain and Adaptation in the Nineteenth Century : Cerebral Localization and Its Biological Context from Gall to Ferrier", 1990
Linden RWA, Br. Dent J., "Taste", 1993
Axel R., "The Molecular Logic of Smell", 1995
+ various medical and .edu websites covering the topic


This write-up is a personal study made out of pure interest towards the subject.

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