Broadly defined, a sense is a mechanism or faculty by which a living organism receives information about its external or internal environment. In other words, it is an inherent capability or power to receive and process stimuli from outside and inside the body, such as the sense of hearing or sense of sight.
As defined, this term includes both physiological methods of perception, involving reception of stimuli by sensory cells, and incorporeal methods of perception, as in spiritual senses (see spiritual senses below). It also allows sense by microorganisms, such as signal transduction pathways that allow detection of nutrients, including sugars, amino acids, and nitrogen sources. Geobacter metallireducens, a common soil bacterium, is reported to have a built-in sensor that allows it to locate iron and manganese oxides that serves as its food source.
The term sense is often more narrowly defined, as related to higher animals. In this case, a sense is considered a system involving sensory cells that respond to a specific kind of physical energy (both internal and external stimuli), which are converted into nerve impulses that travel to the brain (either a specialized area of the brain or specialized areas), where the signals are received and analyzed.
Because there are differing definitions of a "sense," there is no firm agreement among neurologists as to exactly how many senses there are. School children are routinely taught that there are five senses (sight, hearing, touch, smell, taste; a classification first devised by Aristotle), but it is generally agreed that there are at least nine different senses in humans, and a minimum of two more observed in other organisms.
The senses and their operation, classification, and theory are overlapping topics studied by a variety of fields, most notably neuroscience, cognitive psychology (or cognitive science), and philosophy of perception.
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 that may or may not fall within the above classification (for example the sensory awareness of hunger and thirst). In addition, many individuals report various spiritual senses. Finally, some individuals report synesthesia, the "crossing-over" of one sense to another, or even of senses associated with certain pure concepts. A synesthete may, for example, hear colors, see sounds, and taste shapes or tactile sensations.
Sight or vision describes the capability to detect electromagnetic energy within the visible range (light) by the eye, and the ability of 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 color (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 fibers 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 20,000 Hz (however this changes for each individual). Sound can also be detected as vibrations conducted through the body. Lower and higher frequencies that 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 theorized 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 flavorings 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.
Thermoception is the sense of heat and the absence of heat (cold), also detected 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 twentieth century indicated that pain is a distinct phenomenon that intertwines 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 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. (Anyone’s closing the eyes and waving the hand around can demonstrate this. 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).
Throughout history, there have been many published reports of people having "spiritual senses." In this case, a spiritual sense refers to a mechanism or faculty to receive and process internal or external stimuli of an incorporeal nature. For example, there are references in sacred scripture, as well as popular books and media, to individuals who see, hear or even touch relatives or others who have passed away.
Such a faculty can be postulated as the human being having a soul or spiritual self. The associated spiritual senses allow recognition of such essences in other individuals or receiving stimuli originating in an incorporeal world. Research into such spiritual senses is understandably problematic because it is not easily amenable to use of the logical-scientific method of inquiry, involving objective, verifiable observation, but tends toward the personal, subjective experience.
Extra-sensory perception, or ESP, is the name often given to an ability to acquire information by means other than the five canonical senses (taste, sight, touch, smell, and hearing), or any other physical sense well-known to science (balance, proprioception, etc). These include such as perception of events in other places (clairvoyance, clairaudience, clairsentience) and in other times (precognition, retrocognition, second sight); perception of aspects of others not perceivable by most people (aura reading); and the ability to sense communications from, and communicate with, people far away (telepathy), beyond the grave (medium-hood and séancing, spirit walking), or in other dimensions (astral projection). The term "sixth sense" is sometimes used to reference a vague spiritual sense.
It may be that the well-known phenomena of synesthesia (or "synaesthesia") sheds light on spiritual senses. Synesthesia involves a mixing of senses, such as an individual hearing colors or seeing sounds. For example, composer Franz Liszt claimed to see colors when hearing musical notes. A common form of synesthesia is seeing color assigned to particular letters, numbers, or days of weeks. Nobel Laureate physicist Richard Feynman claimed such for physics equations: "When I see equations, I see the letters in colors—I don't know why." Even a synaesthete who was born partially color blind saw certain "alien" colors in his synesthetic perceptions that he was incapable of seeing in the "real world." Such experiences could be attributed to use of a spiritual sense.
Other living organisms have receptors to sense the world around them, including many of the senses listed above for humans. However, the mechanisms and capabilities vary widely. Among non-human animals, dogs have a much keener sense of smell than humans, although the mechanism is similar. 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 is, however, also just sight extended to include more frequencies. Insects have olfactory receptors on their antennae. Ctenophores (comb-jellies) 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") 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.
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). This is, however, not electroception since there is no separate sense for it. The presence of an electrical field is merely concluded from a side effect on another sense.
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.
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 cetaceans are noted for this ability, though some other animals use it, as well. It is most often used to navigate through low lighting conditions or to identify and track prey. There is presently an uncertainty whether this is simply an extremely developed post-sensory interpretation of auditory perceptions or if it actually constitutes a separate sense. Resolution of the issue may require brain scans of animals while they actually perform echolocation, a task that has proven difficult in practice.
Pressure detection uses the lateral line, which is a pressure-sensing system of hairs found in fish and some aquatic Amphibians. It is used primary for navigation, hunting, and schooling.
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