This article is about the empirical or natural senses of living organisms (vision, taste, etc.). For other uses, see Sense (disambiguation)
Senses are the physiological methods of perception. 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. The nervous system has a specific sensory system, or organ, dedicated to each sense.
There is no firm agreement among neurologists as to the number of senses because of differing definitions of what constitutes a sense. One definition states that an exteroceptive sense is a faculty by which outside stimuli are perceived. The traditional five senses are sight, hearing, touch, smell, taste: a classification attributed to Aristotle. Humans are considered to have at least five additional senses that include: nociception (pain), equilibrioception (balance), proprioception & kinaesthesia (joint motion and acceleration), sense of time, thermoception (temperature differences), with possibly an additional weak magnetoception (direction), and six more if interoceptive senses (see other internal senses below) are also considered.
One commonly recognized categorisation for human senses is as follows: chemoreception; photoreception; mechanoreception; and thermoception. This categorisation has been criticized as too restrictive, however, as it does not include categories for accepted senses such as the sense of time and sense of pain.
Different senses also exist in other animals, for example electroreception.
A broadly acceptable definition of a sense would be "A system that consists of a group of sensory cell types that responds to a specific physical phenomenon, and that corresponds to a particular group of regions within the brain where the signals are received and interpreted." Disputes about the number of senses typically arise around the classification of the various cell types and their mapping to regions of the brain.
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Sight or vision is the ability of the brain and eye to detect electromagnetic waves within the visible range (light) which is why people see interpreting the image as "sight." There is disagreement as to whether this constitutes one, two or three 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 stereopsis, the perception of depth, also constitutes a sense, but it is generally regarded as a cognitive (that is, post-sensory) function of brain to interpret sensory input and to derive new information. The inability to see is called blindness.
Hearing or audition is the sense of sound perception. Since sound is vibrations propagating through a medium such as air, the detection of these vibrations, that is the sense of the hearing, is a mechanical sense because these vibrations are mechanically conducted from the eardrum through a series of tiny bones to hair-like fibers in the inner ear which detect mechanical motion of the fibers within a range of about 20 to 20,000 hertz, with substantial variation between individuals. Hearing at high frequencies declines with age. Sound can also be detected as vibrations conducted through the body by tactition. Lower frequencies than that can be heard are detected this way. The inability to hear is called deafness.
Taste or gustation is one of the two main "chemical" senses. There are at least four types of tastes that "buds" (receptors) on the tongue detect, and hence there are anatomists who argue that these constitute five or more different senses, given that each receptor conveys information to a slightly different region of the brain. The inability to taste is called ageusia.
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 flavour commonly found in meat and in artificial flavourings such as monosodium glutamate.
Note: that taste is not the same as flavour; flavour includes the smell of a food as well as its taste.
Smell or olfaction is the other "chemical" sense. Unlike taste, there are hundreds of olfactory receptors, each binding to a particular molecular feature. Odor molecules possess a variety of features and thus excite specific receptors more or less strongly. This combination of excitatory signals from different receptors 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. The inability to smell is called anosmia. Some neurons in the nose are specialized to detect pheromones.
Touch, also called tactition or mechanoreception, is a perception resulting from activation of neural receptors, generally in the skin including hair follicles, but also in the tongue, throat, and mucosa. A variety of pressure receptors respond to variations in pressure (firm, brushing, sustained, etc). The touch sense of itching caused by insect bites or allergies involves special itch-specific neurons in the skin and spinal cord. The loss or impairment of the ability to feel anything touched is called tactile anesthesia. Paresthesia is a sensation of tingling, pricking, or numbness of the skin that may result from nerve damage and may be permanent or temporary.
Balance and acceleration
Balance, equilibrioception, or vestibular sense is the sense which allows an organism to sense body movement, direction, and acceleration, and to attain and maintain postural equilibrium and balance. The organ of equilibrioception is the vestibular labyrinthine system found in both of the inner ears. Technically this organ is responsible for two senses of angular momentum and linear acceleration (which also senses gravity), but they are known together as equilibrioception.
The vestibular nerve conducts information from the three semicircular canals corresponding to the three spatial planes, the utricle, and the saccule. The ampulla, or base, portion of the three semicircular canals each contain a structure called a crista. These bend in response to angular momentum or spinning. The saccule and utricle, also called the "otolith organs", sense linear acceleration and thus gravity. Otoliths are small crystals of calcium carbonate that provide the inertia needed to detect changes in acceleration or gravity.
Thermoception is the sense of heat and the absence of heat (cold) by the skin and including internal skin passages, or rather, the heat flux (the rate of heat flow) in these areas. There are specialized receptors for cold (declining temperature) and to heat. The cold receptors play an important part in the dogs sense of smell, telling wind direction, the heat receptors are sensitive to infrared radiation and can occur in specialized organs for instance in pit vipers. The thermoceptors in the skin are quite different from the homeostatic thermoceptors in the brain (hypothalamus) which provide feedback on internal body temperature.
Proprioception, the kinesthetic sense, provides the parietal cortex of the brain with information on the relative positions of the parts of the body. Neurologists test this sense by telling patients to close their eyes and touch the tip of a finger to their nose. 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. Proprioception and touch are related in subtle ways, and their impairment results in surprising and deep deficits in perception and action. 
Nociception (physiological pain) signals near-damage or damage to tissue. The three types of pain receptors are cutaneous (skin), somatic (joints and bones) and visceral (body organs). It was previously 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 of the other senses, including touch. Pain was once considered an entirely subjective experience, but recent studies show that pain is registered in the anterior cingulate gyrus of the brain.
Other internal senses
An internal sense or interoception is "any sense that is normally stimulated from within the body". These involve numerous sensory receptors in internal organs, such as stretch receptors that are neurologically linked to the brain.
- Pulmonary stretch receptors are found in the lungs and control the respiratory rate.
- Cutaneous receptors in the skin not only respond to touch, pressure, and temperature, but also respond to vasodilation in the skin such as blushing.
- Stretch receptors in the gastrointestinal tract sense gas distension that may result in colic pain.
- Stimulation of sensory receptors in the esophagus result in sensations felt in the throat when swallowing, vomiting, or during acid reflux.
- Sensory receptors in pharynx mucosa, similar to touch receptors in the skin, sense foreign objects such as food that may result in a gag reflex and corresponding gagging sensation.
- Stimulation of sensory receptors in the urinary bladder and rectum may result in sensations of fullness.
- Stimulation of stretch sensors that sense dilation of various blood vessels may result in pain, for example headache caused by vasodilation of brain arteries.
Analogous to human senses
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.
Certain animals, including bats and cetaceans, have the ability to determine orientation to other objects through interpretation of reflected sound (like sonar). They most often use this to navigate through poor lighting conditions or to identify and track prey. There is currently an uncertainty whether this is simply an extremely developed post-sensory interpretation of auditory perceptions or it actually constitutes a separate sense. Resolution of the issue will require brain scans of animals while they actually perform echolocation, a task that has proven difficult in practice. Blind people report they are able to navigate by interpreting reflected sounds (esp. their own footsteps), a phenomenon which is known as human echolocation.
Among non-human species, dogs have a much keener sense of smell than humans, although the mechanism is similar. Insects have olfactory receptors on their antennae. Some animals have a vomeronasal organ which is mainly used to detect pheromones; the organ is vestigial in humans and separate from the main olfactory system, analogous to the human sense of smell.
Cats have the ability to see in low light due to muscles surrounding their irises to contract and expand pupils as well as the tapetum lucidum, a reflective membrane that optimizes the image.Pitvipers, pythons 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. The common vampire bat may also have an infrared sensor on its nose. It has been found that birds and some other animals are tetrachromats and have the ability to see in the ultraviolet down to 300 nanometers. Bees and dragonflies are also able to see in the ultraviolet.
Ctenophora have a balance receptor (a statocyst) that works very differently from the mammalian's semi-circular canals.
Not analogous to human senses
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 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 electric field generating and sensing capacities for social communication. The mechanisms by which electroceptive fish 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 that is known to demonstrate electroception is the monotreme order. Among these mammals, the platypus has the most acute sense of electroception.
- Body modification enthusiasts have experimented with magnetic implants to attempt to replicate this sense, however in general humans (and probably other mammals) can detect electric fields only 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 as it is a post-sensory cognitive action.
- 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. One study has found that cattle make use of magnetoception, as they tend to align themselves in a north-south direction. Magnetotactic bacteria build miniature magnets inside themselves and use them to determine their orientation relative to the Earth's magnetic field.
- Pressure detection uses the organ of Weber, a system consisting of three appendages of vertebrae transferring changes in shape of the gas bladder to the middle ear. It can be used to regulate the buoyancy of the fish. Fish like the weather fish and other loaches are also known to respond to low pressure areas but they lack a swim bladder.
- Current detection The lateral line in fish and aquatic forms of amphibians is a detection system of water currents, mostly consisting of vortices. The lateral line is also sensitive to low frequency vibrations. The mechanoreceptors are hair cells, the same mechanoreceptors for vestibular sense and hearing. It is used primarily for navigation, hunting, and schooling. The receptors of the electrical sense are modified hair cells of the lateral line system.
- Polarized light direction/detection is used by bees to orient themselves, especially on cloudy days. Cuttlefish can also perceive the polarization of light. Most sighted humans can in fact learn to roughly detect large areas of polarization by an effect called Haidinger's brush, however this is considered an entoptic phenomenon rather than a separate sense.
- Slit sensillae of spiders detect mechanical strain in the exoskeleton, providing information on force and vibrations.
Some plants have sensory organs, for example the Venus fly trap, that respond to vibration, light, water, scents, or other specific chemicals. Some plants sense the location of other plants and attack and eat part of them.
The five senses are enumerated as the "five material faculties" (pañcannaṃ indriyānaṃ avakanti) in Buddhist literature. They appear in allegorical representation as early as in the Katha Upanishad (roughly 6th century BC), as five horses drawing the "chariot" of the body, guided by the mind as "chariot driver".
Depictions of the five senses as allegory became a popular subject for seventeenth-century artists, especially among Dutch and Flemish Baroque painters. A typical example is Gérard de Lairesse's Allegory of the Five Senses (1668), in which each of the figures in the main group allude to a sense: sight is the reclining boy with a convex mirror, hearing is the cupid-like boy with a triangle, smell is represented by the girl with flowers, taste by the woman with the fruit and touch by the woman holding the bird.
- ↑ Senses
- ↑ JewishEncyclopedia.com - Senses, the five
- ↑ Magnetic fields and the central nervous system, Clinical Neurophysiology, Volume 111, Issue 11, Pages 1934 - 1935, A . Voustianiouk
- ↑ Frequency Range of Human Hearing, Physics Factbook by Glenn Elert (ed)
- ↑ The Sense of Taste
- ↑ Press Releases - Nature Neuroscience
- ↑ Science, August 6, 2009, Zhou-Feng Chen, et al.
- ↑ Robles-De-La-Torre 2006
- ↑ Functional MR Imaging of Regional Brain Activation Associated with the Affective Experience of Pain -- Robert K. Fulbright et al., American Journal of Roentgenology, 2001; vol. 177, pages 1205-1210
- ↑ Dorland's Medical Dictionary 26th edition, under sense
- ↑ "The illustrated story of the Vampire bat". http://www.pitt.edu/AFShome/s/l/slavic/public/html/courses/vampires/images/bats/vambat.html. Retrieved 2007-05-25.
- ↑ "Directional Selectivity in the Simple Eye of an Insect". http://www.jneurosci.org/cgi/content/abstract/28/11/2845. Retrieved 2009-05-20.
- ↑ Electroreception in the Platypus
- ↑ "Implant gives artist the sense of "magnetic vision"". http://www.boingboing.net/2005/05/05/implant_gives_artist.html. Retrieved 2007-05-25.
- ↑ The Magnetic Sense of Animals
- ↑ BBC science news article
- ↑ "No brainer Behavior", Susan Milius, Science News, June 20, 2009, vol 175, no 13, pages 16-19. (http://www.sciencenews.org/view/feature/id/44327/title/No_brainer_behavior)