Senses are the physiological methods of perception. The senses and their operation, classification, and theory are overlapping topics studied by a variety of fields, but most notably neuroscience, cognitive psychology (or cognitive science), and philosophy of perception.

Definition of "sense"

There is no firm agreement amongst neurologists as to exactly how many senses there are. The disagreements stem from a lack of consensus as to what the definition of a sense should be. Although school children are still routinely taught that there are five senses (sight, hearing, touch, smell, taste; a classification first devised by Aristotle), it is generally agreed that there are at least nine different senses in humans, and a minimum of two more observed in other organisms.

A broadly acceptable definition of a sense would be "a system that consists of a sensory cell type (or group of cell types) that respond to a specific kind of physical energy, and that correspond to a defined region (or group of regions) within the brain where the signals are received and interpreted". Where disputes arise is with regard to the exact classification of the various cell types and their mapping to regions of the brain.

List of senses

Using this definition several senses can be identified. This list begins with those five senses defined by Aristotle and probably most familiar to the reader.

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 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 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.

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).

The group of senses not identified explicitly by Aristotle (but generally agreed upon) are:

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 was a distinct phenomenon that intertwined with all other senses, including touch.

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.

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).

Non-human senses

All animals have receptors to sense the world around them, including many of the senses listed above for humans. However, the mechanisms and capabilities vary widely. Dogs have a much keener sense of smell than humans, although the mechanism is similar. Insects have olfactory receptors on their antennae. Ctenophores have a balance receptor (a statocyst) that works very differently from the mammalian semi-circular canals. In addition, some animals have senses that humans do not, including the following:

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 order of mammals which is known to demonstrate electroception is the monotreme order. Among these mammmals, the platypus (see [2]) has the most acute sense of electroception.

Humans (and probably other mammals) can detect electric fields indirectly by detecting the effect they have on hairs. An electrically charged balloon, for instance, will exert a force on human arm hairs, which can be felt through tactition and identified as coming from a static charge (and not from wind or the like).

Magnetoception (or "magnetoreception") is the ability to detect fluctuations in magnetic fields and is most commonly observed in birds, though it has also been observed in insects such as bees. 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 (see [3]).

Magnetotactic bacteria build miniature magnets inside themselves and use them to determine their orientation relative to the Earth's magnetic field.

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.

The lateral line is a pressure-sensing system of hairs found in fish and some aquatic amphibians. It is used primary for navigation, hunting, and schooling.

Pit vipers and some boas have organs that allow them to detect infrared light, such that these snakes are able to sense the body heat of their prey. This sense is in addition to and distinct from normal vision.