sense sense a sense is a biological system used by an organism for sensation, the process of gathering information about the world through the detection of stimuli. (for example, in the human body, the brain which is part of the central nervous system receives signals from the senses which continuously receive information from the environment, interprets these signals, and causes the body to respond, either chemically or physically.) although traditionally five human senses were identified as such (namely sight, smell, touch, taste, and hearing), it is now recognized that there are many more. senses used by non-human organisms are even greater in variety and number. during sensation, sense organs collect various stimuli (such as a sound or smell) for transduction, meaning transformation into a form that can be understood by the brain. sensation and perception are fundamental to nearly every aspect of an organism's cognition, behavior and thought. sensation consists of signal collection and transduction. in organisms, a sensory organ consists of a group of interrelated sensory cells that respond to a specific type of physical stimulus. via cranial and spinal nerves (nerves of the central and peripheral nervous systems that relay sensory information to and from the brain and body), the different types of sensory receptor cells (such as mechanoreceptors, photoreceptors, chemoreceptors, thermoreceptors) in sensory organs transduct sensory information from these organs towards the central nervous system, finally arriving at the sensory cortices in the brain, where sensory signals are processed and interpreted (perceived). sensory systems, or senses, are often divided into external (exteroception) and internal (interoception) sensory systems. human external senses are based on the sensory organs of the eyes, ears, skin, nose, and mouth. internal sensation detects stimuli from internal organs and tissues. internal senses possessed by humans include the vestibular system (sense of balance) sensed by the inner ear, as well as others such as spatial orientation, proprioception (body position) and nociception (pain). further internal senses lead to signals such as hunger, thirst, suffocation, and nausea, or different involuntary behaviors, such as vomiting. some animals are able to detect electrical and magnetic fields, air moisture, or polarized light, while others sense and perceive through alternative systems, such as echolocation. sensory modalities or sub modalities are different ways sensory information is encoded or transduced. multimodality integrates different senses into one unified perceptual experience. for example, information from one sense has the potential to influence how information from another is perceived. sensation and perception are studied by a variety of related fields, most notably psychophysics, neurobiology, cognitive psychology, and cognitive science. definitions sensory organs main articles: human eye, human ear, human skin, human nose, and human mouth sensory organs are organs that sense and transduce stimuli. humans have various sensory organs (i.e. eyes, ears, skin, nose, and mouth) that correspond to a respective visual system (sense of vision), auditory system (sense of hearing), somatosensory system (sense of touch), olfactory system (sense of smell), and gustatory system (sense of taste). those systems, in turn, contribute to vision, hearing, touch, smell, and the ability to taste. internal sensation, or interoception, detects stimuli from internal organs and tissues. many internal sensory and perceptual systems exist in humans, including the vestibular system (sense of balance) sensed by the inner ear and providing the perception of spatial orientation; proprioception (body position); and nociception (pain). further internal chemoreception- and osmoreception-based sensory systems lead to various perceptions, such as hunger, thirst, suffocation, and nausea, or different involuntary behaviors, such as vomiting. nonhuman animals experience sensation and perception, with varying levels of similarity to and difference from humans and other animal species. for example, mammals in general have a stronger sense of smell than humans. some animal species lack one or more human sensory system analogues and some have sensory systems that are not found in humans, while others process and interpret the same sensory information in very different ways. for example, some animals are able to detect electrical fields and magnetic fields, air moisture, or polarized light. others sense and perceive through alternative systems such as echolocation. recent theory suggests that plants and artificial agents such as robots may be able to detect and interpret environmental information in an analogous manner to animals. sensory modalities main article: sensory modalities sensory modality refers to the way that information is encoded, which is similar to the idea of transduction. the main sensory modalities can be described on the basis of how each is transduced. listing all the different sensory modalities, which can number as many as 17, involves separating the major senses into more specific categories, or submodalities, of the larger sense. an individual sensory modality represents the sensation of a specific type of stimulus. for example, the general sensation and perception of touch, which is known as somatosensation, can be separated into light pressure, deep pressure, vibration, itch, pain, temperature, or hair movement, while the general sensation and perception of taste can be separated into submodalities of sweet, salty, sour, bitter, spicy, and umami, all of which are based on different chemicals binding to sensory neurons. receptors main article: sensory receptor sensory receptors are the cells or structures that detect sensations. stimuli in the environment activate specialized receptor cells in the peripheral nervous system. during transduction, physical stimulus is converted into action potential by receptors and transmitted towards the central nervous system for processing. different types of stimuli are sensed by different types of receptor cells. receptor cells can be classified into types on the basis of three different criteria: cell type, position, and function. receptors can be classified structurally on the basis of cell type and their position in relation to stimuli they sense. receptors can further be classified functionally on the basis of the transduction of stimuli, or how the mechanical stimulus, light, or chemical changed the cell membrane potential. location one way to classify receptors is based on their location relative to the stimuli. an exteroceptor is a receptor that is located near a stimulus of the external environment, such as the somatosensory receptors that are located in the skin. an interoceptor is one that interprets stimuli from internal organs and tissues, such as the receptors that sense the increase in blood pressure in the aorta or carotid sinus. cell type the cells that interpret information about the environment can be either (1) a neuron that has a free nerve ending, with dendrites embedded in tissue that would receive a sensation; (2) a neuron that has an encapsulated ending in which the sensory nerve endings are encapsulated in connective tissue that enhances their sensitivity; or (3) a specialized receptor cell, which has distinct structural components that interpret a specific type of stimulus. the pain and temperature receptors in the dermis of the skin are examples of neurons that have free nerve endings (1). also located in the dermis of the skin are lamellated corpuscles, neurons with encapsulated nerve endings that respond to pressure and touch (2). the cells in the retina that respond to light stimuli are an example of a specialized receptor (3), a photoreceptor. a transmembrane protein receptor is a protein in the cell membrane that mediates a physiological change in a neuron, most often through the opening of ion channels or changes in the cell signaling processes. transmembrane receptors are activated by chemicals called ligands. for example, a molecule in food can serve as a ligand for taste receptors. other transmembrane proteins, which are not accurately called receptors, are sensitive to mechanical or thermal changes. physical changes in these proteins increase ion flow across the membrane, and can generate an action potential or a graded potential in the sensory neurons. functional receptor types a third classification of receptors is by how the receptor transduces stimuli into membrane potential changes. stimuli are of three general types. some stimuli are ions and macromolecules that affect transmembrane receptor proteins when these chemicals diffuse across the cell membrane. some stimuli are physical variations in the environment that affect receptor cell membrane potentials. other stimuli include the electromagnetic radiation from visible light. for humans, the only electromagnetic energy that is perceived by our eyes is visible light. some other organisms have receptors that humans lack, such as the heat sensors of snakes, the ultraviolet light sensors of bees, or magnetic receptors in migratory birds. receptor cells can be further categorized on the basis of the type of stimuli they transduce. the different types of functional receptor cell types are mechanoreceptors, photoreceptors, chemoreceptors (osmoreceptor), thermoreceptors, electroreceptors (in certain mammals and fish), and nociceptors. physical stimuli, such as pressure and vibration, as well as the sensation of sound and body position (balance), are interpreted through a mechanoreceptor. photoreceptors convert light (visible electromagnetic radiation) into signals. chemical stimuli can be interpreted by a chemoreceptor that interprets chemical stimuli, such as an object's taste or smell, while osmoreceptors respond to a chemical solute concentrations of body fluids. nociception (pain) interprets the presence of tissue damage, from sensory information from mechano-, chemo-, and thermoreceptors. another physical stimulus that has its own type of receptor is temperature, which is sensed through a thermoreceptor that is either sensitive to temperatures above (heat) or below (cold) normal body temperature. absolute threshold main article: absolute threshold each sense organ (eyes or nose, for instance) requires a minimal amount of stimulation in order to detect a stimulus. this minimum amount of stimulus is called the absolute threshold. the absolute threshold is defined as the minimum amount of stimulation necessary for the detection of a stimulus 50% of the time. absolute threshold is measured by using a method called signal detection. this process involves presenting stimuli of varying intensities to a subject in order to determine the level at which the subject can reliably detect stimulation in a given sense. differential threshold differential threshold or just noticeable difference (jds) is the smallest detectable difference between two stimuli, or the smallest difference in stimuli that can be judged to be different from each other. weber's law is an empirical law that states that the difference threshold is a constant fraction of the comparison stimulus. according to weber's law, bigger stimuli require larger differences to be noticed. human power exponents and steven's power law magnitude estimation is a psychophysical method in which subjects assign perceived values of given stimuli. the relationship between stimulus intensity and perceptive intensity is described by steven's power law. signal detection theory signal detection theory quantifies the experience of the subject to the presentation of a stimulus in the presence of noise. there is internal noise and there is external noise when it comes to signal detection. the internal noise originates from static in the nervous system. for example, an individual with closed eyes in a dark room still sees something—a blotchy pattern of grey with intermittent brighter flashes—this is internal noise. external noise is the result of noise in the environment that can interfere with the detection of the stimulus of interest. noise is only a problem if the magnitude of the noise is large enough to interfere with signal collection. the nervous system calculates a criterion, or an internal threshold, for the detection of a signal in the presence of noise. if a signal is judged to be above the criterion, thus the signal is differentiated from the noise, the signal is sensed and perceived. errors in signal detection can potentially lead to false positives and false negatives. the sensory criterion might be shifted based on the importance of the detecting the signal. shifting of the criterion may influence the likelihood of false positives and false negatives. private perceptive experience subjective visual and auditory experiences appear to be similar across humans subjects. the same cannot be said about taste. for example, there is a molecule called propylthiouracil (prop) that some humans experience as bitter, some as almost tasteless, while others experience it as somewhere between tasteless and bitter. there is a genetic basis for this difference between perception given the same sensory stimulus. this subjective difference in taste perception has implications for individuals' food preferences, and consequently, health. sensory adaptation main article: sensory adaptation when a stimulus is constant and unchanging, perceptual sensory adaptation occurs. during this process, the subject becomes less sensitive to the stimulus. fourier analysis biological auditory (hearing), vestibular and spatial, and visual systems (vision) appear to