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 sensory system dedicated to each sense.
Definition of 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 a sense is a faculty by which outside stimuli are perceived. The conventional five senses are sight, hearing, touch, smell, taste: a classification traditionally attributed to Aristotle. However, humans have at least nine different senses, and there are at least two other senses in other organisms.
A broadly acceptable definition of a sense would be "a system that consists of a group 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 arise typically regarding the classification of the various cell types and their mapping to regions of the brain.
Five classical senses
Sight or vision is the ability of the brain and eye to detect electromagnetic waves within the visible range (light) 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 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 22000 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 that can be heard are detected this way only. The inability to hear is called deafness.
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 there are anatomists who argue that these constitute four 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 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. 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.
Touch, also called tactition or mechanoreception, is the sense of pressure perception, generally in the skin. There are a variety of pressure receptors that respond to variations in pressure (e.g., firm, brushing, and sustained). The inability to feel anything or almost anything is called anesthesia. Paresthesia is a sensation of tingling, pricking, or numbness of a person's skin with no apparent long term physical effect.
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 in the brain (hypothalamus) which provide feedback on internal body temperature.
Nociception (physiological pain) is the nonconscious perception of near-damage or damage to tissue. 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). 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 of the other senses, including touch. Pain was once considered a wholly subjective experience, but recent studies show that pain is registered in the anterior cingulate gyrus of the brain.
Balance and acceleration
Equilibrioception, the vestibular sense, is the perception of balance or acceleration and is mainly 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, the kinesthetic sense, is the perception of body awareness and is a sense that people are frequently not aware of, but rely on enormously. 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). It can be used in reaction time. Proprioception and touch are related in subtle ways, and their impairment results in surprising and deep deficits in perception and action (Robles-De-La-Torre 2006).
Other internal senses
An internal sense 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 gagging 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.
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. The common vampire bat may also have an infrared sensor on its nose. Infrared senses are, however, just sight in a different light frequency range. 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 are also able to see in the ultraviolet.
Not analogous to human senses
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 electric field 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 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.
- 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 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.
- 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. There is experimental and physical evidence to suggest this sense exists in a weak form in humans.
- 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 lateral line, which is a pressure-sensing system of hairs found in fish and some aquatic amphibians. It is used primarily for navigation, hunting, and schooling. Humans have a basic relative-pressure detection ability when eustachian tube(s) are blocked, as demonstrated in the ear's response to changes in altitude.
- Polarized light direction / detection is used by bees to orient themselves, especially on cloudy days.
- Auditory illusion, Optical illusion, Touch illusion
- Basic tastes
- Multimodal integration
- Sensitivity (human)
- Sense of time
- Dorland's Medical Dictionary 26th edition, under sense
- "www.pitt.edu/AFShome/s/l/slavic/public/html/courses/vampires/images/bats/vambat.html". Retrieved 2007-05-25.
- "Implant gives artist the sense of "magnetic vision"". Retrieved 2007-05-25.
- The 2004 Nobel Prize in Physiology or Medicine (announced 4 October 2004) was won by Richard Axel and Linda Buck for their work explaining olfaction, published first in a joint paper in 1991 that described the very large family of about one thousand genes for odorant receptors and how the receptors link to the brain.
- HHMI - World renowned centre of medical excellence
- IASE - senses.info - A research center that focuses on the basis of senses.
- Answers to several questions related to senses and human feeling from curious kids
- The Physiology of the Senses tutorial 12 animated chapters on vision, hearing, touch, balance and memory.