There is no firm agreement among neurologists as to exactly how many senses there are, because of differing definitions of a sense. In general, one can say that a "sense" is a faculty by which outside stimuli are perceived. School children are routinely taught that there are five senses (sight, hearing, touch, smell, taste; a classification devised by ancient peoples). It is generally agreed that there are at least seven 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.
SIGHT**
Sight or vision describes the ability to detect electromagnetic waves within the visible range (light) by the eye and 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 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 that this is really a cognitive (that is, post-sensory) function of brain to interpret sensory input to derive new information. The inability to see is called blindness.
HEARING***
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 can be heard are detected this way only. The inability to hear is called deafness.
TASTE***
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 in fact constitute four or more different senses, given that each receptor conveys information to a slightly different region of the brain. The inabilty 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**
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. The inability to smell is called anosmia.
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.
TOUCH***
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 inability to feel anything or almost anything is called Paresthesia.
OTHER SENSES***
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 (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). 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 is a distinct phenomenon that intertwines with all other senses, including touch. At present pain is defined scientifically as a wholly subjective experience.
Equilibrioception, the vestibular sense, 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, 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 ). In contrast, an octopus has no or limited proprioception due to the complicated shapes their tentacles can form.
THE SENSES AND INTELLIGENCE***
Out of research into how creativity manifests in different individuals, Howard Gardner described multiple kinds of intelligence: visual, musical, logical/mathematical, linguistic, movement, naturalistic, kinesthetic, intrapersonal and interpersonal intelligences. Most of these correspond to particular senses - in fact, all of them may if Rudolf Steiner's higher senses of language, thought and ego are included. Other senses (taste, smell) may also have their own particular intelligences (Gardner has said that his search for additional intelligences continues). The relationship between intelligence and sensory perception thus appears to be a close one. Guy Murchie proposed more than 30 senses exist including the sense of time and the sense of fear.
