This list begins with those five senses defined by Aristotle and hence probably most familiar

to the original poster.

1. Seeing or vision describes the ability to detect light and interpret it as

"sight". There is disagreement as to whether or not 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 light) and brightness (the energy 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 derived from having stereoscopic vision (two eyes)

and is not a sensory perception as such.

2. Hearing or audition is the sense of sound perception and results from tiny

hair fibres in the inner ear detecting the motion of atmospheric particles

within (at best) a range of 20 to 20000 Hz. Sound can also be detected as

vibration by tactition. Lower and higher frequencies than can be heard are

detected this way only.

3. Taste or gustation is one of the two "chemical" senses. It is well-known

that there are at least four types of taste "bud" (receptor) 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. The umami receptor detects

the amino acid glutamate, a flavor commonly found in meat, and in artificial

flavourings such as monosodium glutamate.

4. Smell or olfaction is the other "chemical" sense. Olfactory neurons differ

from most other neurons in that they die and regenerate on a regular basis.

The remaining senses can be considered types of physical feeling.

5. Tactition is the sense of pressure perception.

6. Thermoception is the sense of heat and the absence of heat (cold). It is

also the first of the group of senses not identified explicitly by

Aristotle. Again 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. How warm or cold something feels does not only depend on

temperature, but also on specific heat capacity and heat conductance; e.g.,

warm metal feels warmer than warm wood, and cold metal feels colder than

cold wood, because metal has a higher thermal conductivity than wood. Wind

feels cold because of the heat withdrawn for evaporation of sweat or other

moisture, and because an isolating layer of warm air around the body blows

away; however, in the case of hot air, wind makes it feel hotter, for a

similar reason as the latter.

7. 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).

8. Equilibrioception is the perception of balance and is related to cavities

containing fluid in the inner ear. There is some disagreement as to whether

or not 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.

9. 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 closing their eyes and waving their 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).

Based on this outline and depending on the chosen method of classification,

somewhere between 9 and 21 human senses have been identified. Additionally

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.

This list concludes with known non-human senses.

10. Electroception (or "electroreception"), the most significant of the

non-human senses, is the ability to detect electric fields. Several species

of fish, sharks and rays have evolved the capacity to sense changes in

electric fields in their immediate vicinity. Some fish passively sense

changing nearby electric fields, some generate their own weak, electric

fields and sense the pattern of field potentials over their body surface,

and some use these generating and sensing capacities for social

communication. The mechanisms by which electroceptive fishes construct a

spatial representation from very small differences in field potentials

involve comparisons of spike latencies from different parts of the fish's body.

The only mammal which is known to demonstrate electroception is the platypus.

11. Magnetoception (or "magnetoreception") is the ability to detect fluctuations

in magnetic fields and is most commonly observed in birds. Although there is

no dispute that this sense exists in many avians (it is essential to the

navigational abilities of migratory birds) it is not a well understood

phenomenon.

12. Echolocation is the ability to determine orientation to other objects

through interpretation of reflected sound (like sonar). Bats and dolphins

are noted for this ability, though some other mammals and birds do as well.

It is most often used to navigate through poor lighting conditions or to

identify and track prey. There is presently an uncertainty as to whether

this is simply an extremely developed post-sensory interpretation of

auditory perceptions, or actually constitutes a separate sense. Resolution

of the issue will require brain scans of animals while they actually perform

echolocation, a task which has proved difficult in practice.

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