only on external perception processes, but also to "interoception", considered as the process of receiving, accessing and appraising internal bodily signals. maintaining desired physiological states is critical for an organism's well-being and survival. interoception is an iterative process, requiring the interplay between perception of body states and awareness of these states to generate proper self-regulation. afferent sensory signals continuously interact with higher order cognitive representations of goals, history, and environment, shaping emotional experience and motivating regulatory behavior. features constancy perceptual constancy is the ability of perceptual systems to recognize the same object from widely varying sensory inputs.: 118–120 for example, individual people can be recognized from views, such as frontal and profile, which form very different shapes on the retina. a coin looked at face-on makes a circular image on the retina, but when held at angle it makes an elliptical image. in normal perception these are recognized as a single three-dimensional object. without this correction process, an animal approaching from the distance would appear to gain in size. one kind of perceptual constancy is color constancy: for example, a white piece of paper can be recognized as such under different colors and intensities of light. another example is roughness constancy: when a hand is drawn quickly across a surface, the touch nerves are stimulated more intensely. the brain compensates for this, so the speed of contact does not affect the perceived roughness. other constancies include melody, odor, brightness and words. these constancies are not always total, but the variation in the percept is much less than the variation in the physical stimulus. the perceptual systems of the brain achieve perceptual constancy in a variety of ways, each specialized for the kind of information being processed, with phonemic restoration as a notable example from hearing.law of closure. the human brain tends to perceive complete shapes even if those forms are incomplete. grouping (gestalt) the principles of grouping (or gestalt laws of grouping) are a set of principles in psychology, first proposed by gestalt psychologists, to explain how humans naturally perceive objects with patterns and objects. gestalt psychologists argued that these principles exist because the mind has an innate disposition to perceive patterns in the stimulus based on certain rules. these principles are organized into six categories: proximity: the principle of proximity states that, all else being equal, perception tends to group stimuli that are close together as part of the same object, and stimuli that are far apart as two separate objects. similarity: the principle of similarity states that, all else being equal, perception lends itself to seeing stimuli that physically resemble each other as part of the same object and that are different as part of a separate object. this allows for people to distinguish between adjacent and overlapping objects based on their visual texture and resemblance. closure: the principle of closure refers to the mind's tendency to see complete figures or forms even if a picture is incomplete, partially hidden by other objects, or if part of the information needed to make a complete picture in our minds is missing. for example, if part of a shape's border is missing people still tend to see the shape as completely enclosed by the border and ignore the gaps. good continuation: the principle of good continuation makes sense of stimuli that overlap: when there is an intersection between two or more objects, people tend to perceive each as a single uninterrupted object. common fate: the principle of common fate groups stimuli together on the basis of their movement. when visual elements are seen moving in the same direction at the same rate, perception associates the movement as part of the same stimulus. this allows people to make out moving objects even when other details, such as color or outline, are obscured. the principle of good form refers to the tendency to group together forms of similar shape, pattern, color, etc. later research has identified additional grouping principles. contrast effects a common finding across many different kinds of perception is that the perceived qualities of an object can be affected by the qualities of context. if one object is extreme on some dimension, then neighboring objects are perceived as further away from that extreme. "simultaneous contrast effect" is the term used when stimuli are presented at the same time, whereas successive contrast applies when stimuli are presented one after another. the contrast effect was noted by the 17th century philosopher john locke, who observed that lukewarm water can feel hot or cold depending on whether the hand touching it was previously in hot or cold water. in the early 20th century, wilhelm wundt identified contrast as a fundamental principle of perception, and since then the effect has been confirmed in many different areas. these effects shape not only visual qualities like color and brightness, but other kinds of perception, including how heavy an object feels. one experiment found that thinking of the name "hitler" led to subjects rating a person as more hostile. whether a piece of music is perceived as good or bad can depend on whether the music heard before it was pleasant or unpleasant. for the effect to work, the objects being compared need to be similar to each other: a television reporter can seem smaller when interviewing a tall basketball player, but not when standing next to a tall building. in the brain, brightness contrast exerts effects on both neuronal firing rates and neuronal synchrony. theories perception as direct perception (gibson) cognitive theories of perception assume there is a poverty of stimulus. this is the claim that sensations, by themselves, are unable to provide a unique description of the world. sensations require 'enriching', which is the role of the mental model. the perceptual ecology approach was introduced by james j. gibson, who rejected the assumption of a poverty of stimulus and the idea that perception is based upon sensations. instead, gibson investigated what information is actually presented to the perceptual systems. his theory "assumes the existence of stable, unbounded, and permanent stimulus-information in the ambient optic array. and it supposes that the visual system can explore and detect this information. the theory is information-based, not sensation-based." he and the psychologists who work within this paradigm detailed how the world could be specified to a mobile, exploring organism via the lawful projection of information about the world into energy arrays. "specification" would be a 1:1 mapping of some aspect of the world into a perceptual array. given such a mapping, no enrichment is required and perception is direct. perception-in-action from gibson's early work derived an ecological understanding of perception known as perception-in-action, which argues that perception is a requisite property of animate action. it posits that, without perception, action would be unguided, and without action, perception would serve no purpose. animate actions require both perception and motion, which can be described as "two sides of the same coin, the coin is action." gibson works from the assumption that singular entities, which he calls invariants, already exist in the real world and that all that the perception process does is home in upon them. the constructivist view, held by such philosophers as ernst von glasersfeld, regards the continual adjustment of perception and action to the external input as precisely what constitutes the "entity," which is therefore far from being invariant. glasersfeld considers an invariant as a target to be homed in upon, and a pragmatic necessity to allow an initial measure of understanding to be established prior to the updating that a statement aims to achieve. the invariant does not, and need not, represent an actuality. glasersfeld describes it as extremely unlikely that what is desired or feared by an organism will never suffer change as time goes on. this social constructionist theory thus allows for a needful evolutionary adjustment. a mathematical theory of perception-in-action has been devised and investigated in many forms of controlled movement, and has been described in many different species of organism using the general tau theory. according to this theory, "tau information", or time-to-goal information is the fundamental percept in perception. evolutionary psychology many philosophers, such as jerry fodor, write that the purpose of perception is knowledge. however, evolutionary psychologists hold that the primary purpose of perception is to guide action. they give the example of depth perception, which seems to have evolved not to aid in knowing the distances to other objects but rather to aid movement. evolutionary psychologists argue that animals ranging from fiddler crabs to humans use eyesight for collision avoidance, suggesting that vision is basically for directing action, not providing knowledge. neuropsychologists showed that perception systems evolved along the specifics of animals' activities. this explains why bats and worms can perceive different frequency of auditory and visual systems than, for example, humans. building and maintaining sense organs is metabolically expensive. more than half the brain is devoted to processing sensory information, and the brain itself consumes roughly one-fourth of one's metabolic resources. thus, such organs evolve only when they provide exceptional benefits to an organism's fitness. scientists who study perception and sensation have long understood the human senses as adaptations. depth perception consists of processing over half a dozen visual cues, each of which is based on a regularity of the physical world. vision evolved to respond to the narrow range of electromagnetic energy that is plentiful and that does not pass through objects. sound waves provide useful information about the sources of and distances to objects, with larger animals making and hearing lower-frequency sounds and smaller animals making and hearing higher-frequency sounds. taste and smell respond to chemicals in the environment that were significant for fitness in the environment of evolutionary adaptedness. the sense of touch is actually many senses, including pressure, heat, cold, tickle, and pain. pain, while unpleasant, is adaptive. an important adaptation for senses is range shifting, by which the organism becomes temporarily more or less sensitive to sensation. for example, one's eyes automatically adjust to dim or bright ambient light. sensory abilities of different organisms often co-evolve, as is the case with the hearing of echolocating bats and that of the moths that have evolved to respond to the sounds that the bats make. evolutionary psychologists claim that perception demonstrates the principle of modularity, with specialized mechanisms handling particular perception tasks. for example, people with damage to a particular part of the brain are not able to recognize faces (prosopagnosia). evolutionary psychology suggests that this indicates a so-called face-reading module. closed-loop perception the theory of closed-loop perception proposes dynamic motor-sensory closed-loop process in which information flows through the environment and the brain in continuous loops. feature integration theory anne treisman's feature integration theory (fit) attempts to explain how characteristics of a stimulus such as physical location in space, motion, color, and shape are merged to form one percept despite each of these characteristics activating separate areas of the cortex. fit explains this through a two part system of perception involving the preattentive and focused attention stages. the preattentive stage of perception is largely unconscious, and analyzes an object by breaking it down into its basic features, such as the specific color, geometric shape, motion, depth, individual lines, and many others. studies have shown that, when small groups of objects with different features (e.g., red triangle, blue circle) are briefly flashed in front of human participants, many individuals later report seeing shapes made up of the combined features of two different stimuli, thereby referred to as illusory conjunctions. the unconnected features described in the preattentive stage are combined into the objects one normally sees during the focused attention stage. the focused attention stage is based heavily around the idea of attention in perception and 'binds' the features together onto specific objects at specific spatial locations (see the binding problem). other theories of perception enactivism the interactive activation and competition model recognition-by-components theory (irving biederman) effects on perception effect of experience with experience, organisms can learn to make finer perceptual distinctions, and learn new kinds of categorization. wine-tasting, the reading of x-ray images and music appreciation are applications of this process in the human sphere. research has focused on the relation of this to other kinds of learning, and whether it takes place in peripheral sensory systems or in the brain's processing of sense information.