Sense: Encyclopedia II - Sense - List of Human senses

Sense - List of Human senses

Using this definition several senses can be identified.

Based on this outline and depending on the chosen method of classification, somewhere between 9 and 21 human senses have been identified. In addition, there are some other candidate physiological experiences which may or may not fall within the above classification (for example the sensory awareness of hunger and thirst). Finally, some individuals report synesthesia, the "crossing-over" of one sense to another, or even of senses associated with certain pure concepts. For example, the letter "A" may appear "black," certain notes may seem "orange" or "blue," and so forth.

Sense - Special senses

Sight or vision describes the ability to detect electromagnetic energy within the visible range (light) by the eye and the brain to interpret the image as "sight." There is disagreement as to whether this constitutes one, two or even three distinct senses. Neuroanatomists generally regard it as two senses, given that different receptors are responsible for the perception of colour (the frequency of photons of light) and brightness (amplitude/intensity - number of photons of light). Some argue that the perception of depth also constitutes a sense, but it is generally regarded that this is really a cognitive (that is, post-sensory) function of brain to interpret sensory input to derive new information.

Hearing or audition is the sense of sound perception and results from tiny hair fibres in the inner ear detecting the motion of a membrane which vibrates in response to changes in the pressure exerted by atmospheric particles within (at best) a range of 9 to 20000 Hz, however this changes for each individual. Sound can also be detected as vibrations conducted through the body by tactition. Lower and higher frequencies than can be heard are detected this way only.

Taste or gustation is one of the two main "chemical" senses. It is well-known that there are at least four types of taste "bud" (receptor) on the tongue and hence, as should now be expected, there are anatomists who argue that these in fact constitute four or more different senses, given that each receptor conveys information to a slightly different region of the brain.

The four well-known receptors detect sweet, salt, sour, and bitter, although the receptors for sweet and bitter have not been conclusively identified. A fifth receptor, for a sensation called umami, was first theorised in 1908 and its existence confirmed in 2000 (see [1]). The umami receptor detects the amino acid glutamate, a flavor commonly found in meat, and in artificial flavourings such as monosodium glutamate.

Smell or olfaction is the other "chemical" sense. Unlike taste, there are hundreds of olfactory receptors, each binding to a particular molecular feature, according to current theory. The combination of features of the odor molecule makes up what we perceive as the molecule's smell. In the brain, olfaction is processed by the olfactory system. Olfactory receptor neurons in the nose differ from most other neurons in that they die and regenerate on a regular basis.

If the different taste-senses are not regarded as separate senses one may argue that Taste and Smell should likewise be grouped together as one sense.

Sense - Somatic senses

Touch or tactition is the sense of pressure perception, generally in the skin. There are a variety of pressure receptors that respond to variations in pressure (firm, brushing, sustained, etc).

Thermoception is the sense of heat and the absence of heat (cold), also by the skin and including internal skin passages. There is some disagreement about how many senses this actually represents - the thermoceptors in the skin are quite different from the homeostatic thermoceptors which provide feedback on internal body temperature.

Nociception is the perception of pain. It can be classified as from one to three senses, depending on the classification method. The three types of pain receptors are cutaneous (skin), somatic (joints and bones) and visceral (body organs). For a considerable time, it was believed that pain was simply the overloading of pressure receptors, but research in the first half of the 20th century indicated that pain is a distinct phenomenon that intertwines with all other senses, including touch.

Sense - Other

Equilibrioception is the perception of balance and is related to cavities containing fluid in the inner ear. There is some disagreement as to whether this also includes the sense of "direction" or orientation. However, as with depth perception earlier, it is generally regarded that "direction" is a post-sensory cognitive awareness.

Proprioception is the perception of body awareness and is a sense that people rely on enormously, yet are frequently not aware of. More easily demonstrated than explained, proprioception is the "unconscious" awareness of where the various regions of the body are located at any one time. (This can be demonstrated by anyone's closing the eyes and waving the hand around. Assuming proper proprioceptive function, at no time will the person lose awareness of where the hand actually is, even though it is not being detected by any of the other senses).

Extra-sensory perception (also called "ESP" and the "sixth sense") is a "sense" that supposedly can pick up information that cannot be obtained through the other senses. It eludes scientific confirmation and has not been proved or disproved to exist. It is often associated with fortune-telling and "gut feelings."

Adapted from the Wikipedia article "List of Human senses", under the G.N U Free Docmentation License. Please also see http://en.wikipedia.org/wiki