SPACE PERCEPTION

process through which humans and other organisms become aware of the relative positions of their own bodies and objects around them. Space perception provides cues, such as depth and distance, that are important for locomotion and orientation to the environment. Human beings have been interested in the perception of objects in space at least since the question of how a picture or image arises in the human eye was first asked. It was popularly thought in ancient Greece that objects are seen because they emit what was imagined to be a continuous series of extremely thin membranes in their own image, which fall upon the eye and there merge into the picture that is perceived. Only after long experimental research was a more tenable conception reached, in which space was described in terms of three dimensions or planes: height (vertical plane), width (horizontal plane), and depth (sagittal plane). These planes all intersect at right angles, and their single axis of intersection is defined as being located within perceived three-dimensional spacethat is, in the eye of the perceiving individual. Humans do not ordinarily perceive a binocular space (a separate visual world from each eye), but sees a so-called Cyclopean space, as if the images from each eye fuse to produce a visual field akin to that of Cyclops, a one-eyed giant in Greek mythology. The horizontal, vertical, and sagittal planes divide space into various sectors: something is perceived as above or below (the horizontal plane), as in front of or behind (the vertical plane), or as to the right or to the left (of the sagittal plane). the process through which organisms become aware of the relative positions of their own bodies and objects around them. Space perception provides cues, such as depth and distance, that are important for locomotion and orientation to the environment. Detailed understanding of space perception has only come in the last hundred years. In the 18th century the British philosopher George Berkeley proposed that since the image on the retina is two-dimensional, an individual must learn to interpret visual data so as to deduce the spatial relations between objects of sight. Psychologists now believe, however, that there are certain innate elements in the ability to perceive space. The current study of space perception is concerned with a variety of complex issues, including the ability to judge the full shape of objects of which only one side is visible, and how pilots can compensate for unusual perceptual cues while navigating airplanes or spacecraft. Space perception is thought to serve the general purpose of orientation, i.e., of aiding an individual to grasp his position in relation to the objects around him. Perceiving one's spatial relation to a given object is more important the more relevant the object is to one's needs. For example, organisms are particularly sensitive to stimuli that warn them of dangers along their paths. Thus, space perception involves, in a central role, the selection of relevant sense data. Sense data that do not bear on one's needs are excluded, so that the incoming flow of information is not overwhelming. Another important feature of space perception is its ability to account for motion. When an object moves in relation to the perceiver, or vice versa, the stimuli presented to his senses are changed. However, he interprets the change in shape of an image on the retina, for example, not as a change in the shape of the given object, but as evidence that the object has changed its position in space, relative to the eye. That is, space perception relies on perceptual constancy (q.v.), without which the world would seem inconceivably chaotic. The perception of space makes use of data from all five outer senses, as well as the inner senses of balance and of the arrangement of the body. Using the stimuli to which it responds, each of the outer senses, in a manner of speaking, constructs its own version of spatial arrangement around the perceiver. Under normal circumstances one's perception of space results from the interweaving of all these sensory spaces with each other, and with the data of the inner senses. Nonetheless, the sense of sight plays a preeminent role in space perception because it is a so-called distance senseit can receive stimuli from over a vast range of space. There are a variety of cues by which data from sight are converted into perceptions of distance. Some of these cues are not, in fact, visualinstead they are produced by the muscles that control the movements of the eyes. One learns to judge distances in part by learning to interpret the slight contractions of these muscles as they adjust the eyes to see objects that are nearer or farther away. Purely visual cues to spatial relations are given by what is called the binocular disparity. Since a person's two eyes are in slightly different positions, the images of an object on the two retinas are not exactly the same. The brain combines the two retinal images and converts their difference into the experience of three-dimensionalityi.e., the scene is perceived to have depth. Visual movement parallax is a similar visual cue, but it acts even on one eye alone. If a person changes position, the retinal images of objects in different positions move by different amounts, indicating their distance from the viewer. When vision is impaired or totally lacking (e.g., in blindness), the sense of hearing commonly becomes a major means of space perception, and aural acuity is generally highly developed in the blind. The sense of balance is also important in orienting oneself in space. Closely related are the kinesthetic sensations of the arrangement of one's own body; combined with sensations of touch, these help one to determine the spatial relationships of nearby objects. In fact, the senses of touch and sight interact quite closely as young children begin to develop their capacities for space perception. Additional reading An excellent general text for undergraduates is Robert Sekuler and Randolph Blake, Perception, 3rd ed. (1994), focusing on perception through the senses. Irvin Rock, An Introduction to Perception (1975), is a standard introductory text for experimental psychology students. Also of interest are studies by Hermann von Helmholtz, Helmholtz's Treatise on Physiological Optics, ed. by James P. Southall, 3 vol. (1924, reissued 1962; originally published in German, 3rd ed., 190911), the classic work on visual perception and its physiological basis; Shimon Ullman, The Interpretation of Visual Motion (1979), an original and accessible account of how we connect successive views of a moving object; and David Marr, Vision: A Computational Investigation into the Human Representation and Processing of Visual Information (1982), the book that triggered the computer revolution in vision science. More advanced reading on the perception of objects and space is presented in James E. Cutting, Perception with an Eye for Motion (1986). Louis Jolyon West The Editors of the Encyclopdia Britannica