substantial variation between individuals. hearing at high frequencies declines with an increase in age. inability to hear is called deafness or hearing impairment. sound can also be detected as vibrations conducted through the body by tactition. lower frequencies that can be heard are detected this way. some deaf people are able to determine the direction and location of vibrations picked up through the feet. studies pertaining to audition started to increase in number towards the latter end of the nineteenth century. during this time, many laboratories in the united states began to create new models, diagrams, and instruments that all pertained to the ear. there is a branch of cognitive psychology dedicated strictly to audition. they call it auditory cognitive psychology. the main point is to understand why humans are able to use sound in thinking outside of actually saying it. relating to auditory cognitive psychology is psychoacoustics. psychoacoustics is more pointed to people interested in music. haptics, a word used to refer to both taction and kinesthesia, has many parallels with psychoacoustics. most research around these two are focused on the instrument, the listener, and the player of the instrument. somatosensory system (touch) main article: somatosensory system somatosensation is considered a general sense, as opposed to the special senses discussed in this section. somatosensation is the group of sensory modalities that are associated with touch and interoception. the modalities of somatosensation include pressure, vibration, light touch, tickle, itch, temperature, pain, kinesthesia. somatosensation, also called tactition (adjectival form: tactile) 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. two types of somatosensory signals that are transduced by free nerve endings are pain and temperature. these two modalities use thermoreceptors and nociceptors to transduce temperature and pain stimuli, respectively. temperature receptors are stimulated when local temperatures differ from body temperature. some thermoreceptors are sensitive to just cold and others to just heat. nociception is the sensation of potentially damaging stimuli. mechanical, chemical, or thermal stimuli beyond a set threshold will elicit painful sensations. stressed or damaged tissues release chemicals that activate receptor proteins in the nociceptors. for example, the sensation of heat associated with spicy foods involves capsaicin, the active molecule in hot peppers. low frequency vibrations are sensed by mechanoreceptors called merkel cells, also known as type i cutaneous mechanoreceptors. merkel cells are located in the stratum basale of the epidermis. deep pressure and vibration is transduced by lamellated (pacinian) corpuscles, which are receptors with encapsulated endings found deep in the dermis, or subcutaneous tissue. light touch is transduced by the encapsulated endings known as tactile (meissner) corpuscles. follicles are also wrapped in a plexus of nerve endings known as the hair follicle plexus. these nerve endings detect the movement of hair at the surface of the skin, such as when an insect may be walking along the skin. stretching of the skin is transduced by stretch receptors known as bulbous corpuscles. bulbous corpuscles are also known as ruffini corpuscles, or type ii cutaneous mechanoreceptors. 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. gustatory system (taste) main article: gustatory system the gustatory system or the sense of taste is the sensory system that is partially responsible for the perception of taste (flavor). a few recognized submodalities exist within taste: sweet, salty, sour, bitter, and umami. very recent research has suggested that there may also be a sixth taste submodality for fats, or lipids. the sense of taste is often confused with the perception of flavor, which is the results of the multimodal integration of gustatory (taste) and olfactory (smell) sensations. philippe mercier - the sense of taste - google art project within the structure of the lingual papillae are taste buds that contain specialized gustatory receptor cells for the transduction of taste stimuli. these receptor cells are sensitive to the chemicals contained within foods that are ingested, and they release neurotransmitters based on the amount of the chemical in the food. neurotransmitters from the gustatory cells can activate sensory neurons in the facial, glossopharyngeal, and vagus cranial nerves. salty and sour taste submodalities are triggered by the cations na+ and h+, respectively. the other taste modalities result from food molecules binding to a g protein–coupled receptor. a g protein signal transduction system ultimately leads to depolarization of the gustatory cell. the sweet taste is the sensitivity of gustatory cells to the presence of glucose (or sugar substitutes) dissolved in the saliva. bitter taste is similar to sweet in that food molecules bind to g protein–coupled receptors. the taste known as umami is often referred to as the savory taste. like sweet and bitter, it is based on the activation of g protein–coupled receptors by a specific molecule. once the gustatory cells are activated by the taste molecules, they release neurotransmitters onto the dendrites of sensory neurons. these neurons are part of the facial and glossopharyngeal cranial nerves, as well as a component within the vagus nerve dedicated to the gag reflex. the facial nerve connects to taste buds in the anterior third of the tongue. the glossopharyngeal nerve connects to taste buds in the posterior two thirds of the tongue. the vagus nerve connects to taste buds in the extreme posterior of the tongue, verging on the pharynx, which are more sensitive to noxious stimuli such as bitterness. flavor depends on odor, texture, and temperature as well as on taste. humans receive tastes through sensory organs called taste buds, or gustatory calyculi, concentrated on the upper surface of the tongue. other tastes such as calcium and free fatty acids may also be basic tastes but have yet to receive widespread acceptance. the inability to taste is called ageusia. there is a rare phenomenon when it comes to the gustatory sense. it is called lexical-gustatory synesthesia. lexical-gustatory synesthesia is when people can "taste" words. they have reported having flavor sensations they are not actually eating. when they read words, hear words, or even imagine words. they have reported not only simple flavors, but textures, complex flavors, and temperatures as well. olfactory system (smell) main article: olfactory system like the sense of taste, the sense of smell, or the olfactory system, is also responsive to chemical stimuli. unlike taste, there are hundreds of olfactory receptors (388 functional ones according to one 2003 study), 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 humans perceive as the molecule's smell. the olfactory receptor neurons are located in a small region within the superior nasal cavity. this region is referred to as the olfactory epithelium and contains bipolar sensory neurons. each olfactory sensory neuron has dendrites that extend from the apical surface of the epithelium into the mucus lining the cavity. as airborne molecules are inhaled through the nose, they pass over the olfactory epithelial region and dissolve into the mucus. these odorant molecules bind to proteins that keep them dissolved in the mucus and help transport them to the olfactory dendrites. the odorant–protein complex binds to a receptor protein within the cell membrane of an olfactory dendrite. these receptors are g protein–coupled, and will produce a graded membrane potential in the olfactory neurons. the sense of smell. bequest of mrs e.g. elgar, 1945 museum of new zealand te papa tongarewa. in the brain, olfaction is processed by the olfactory cortex. 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. loss of the sense of smell can result in food tasting bland. a person with an impaired sense of smell may require additional spice and seasoning levels for food to be tasted. anosmia may also be related to some presentations of mild depression, because the loss of enjoyment of food may lead to a general sense of despair. the ability of olfactory neurons to replace themselves decreases with age, leading to age-related anosmia. this explains why some elderly people salt their food more than younger people do. internal main article: interoception vestibular system (balance) main article: vestibular system the vestibular sense, or sense of balance (equilibrium), is the sense that contributes to the perception of balance (equilibrium), spatial orientation, direction, or acceleration (equilibrioception). along with audition, the inner ear is responsible for encoding information about equilibrium. a similar mechanoreceptor—a hair cell with stereocilia—senses head position, head movement, and whether our bodies are in motion. these cells are located within the vestibule of the inner ear. head position is sensed by the utricle and saccule, whereas head movement is sensed by the semicircular canals. the neural signals generated in the vestibular ganglion are transmitted through the vestibulocochlear nerve to the brain stem and cerebellum. the semicircular canals are three ring-like extensions of the vestibule. one is oriented in the horizontal plane, whereas the other two are oriented in the vertical plane. the anterior and posterior vertical canals are oriented at approximately 45 degrees relative to the sagittal plane. the base of each semicircular canal, where it meets with the vestibule, connects to an enlarged region known as the ampulla. the ampulla contains the hair cells that respond to rotational movement, such as turning the head while saying "no". the stereocilia of these hair cells extend into the cupula, a membrane that attaches to the top of the ampulla. as the head rotates in a plane parallel to the semicircular canal, the fluid lags, deflecting the cupula in the direction opposite to the head movement. the semicircular canals contain several ampullae, with some oriented horizontally and others oriented vertically. by comparing the relative movements of both the horizontal and vertical ampullae, the vestibular system can detect the direction of most head movements within three-dimensional (3d) space. the vestibular nerve conducts information from sensory receptors in three ampulla that sense motion of fluid in three semicircular canals caused by three-dimensional rotation of the head. the vestibular nerve also conducts information from the utricle and the saccule, which contain hair-like sensory receptors that bend under the weight of otoliths (which are small crystals of calcium carbonate) that provide the inertia needed to detect head rotation, linear acceleration, and the direction of gravitational force. proprioception proprioception, the kinesthetic sense, provides the parietal cortex of the brain with information on the movement and relative positions of the parts of the body. neurologists test this sense by telling patients to close their eyes and touch their own nose with the tip of a finger. 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. pain nociception (physiological pain) signals nerve-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. the main function of pain is to attract our attention to dangers and motivate us to