Perception

Human perception is the active reception and coordination of information received through our sensory systems in order to make sense of the environment and to behave effectively within it. In contrast with the direct and immediate sensations actually received and transmitted, perception is the transformation of that information into nerve cell activity that is transferred to the brain where further processing occurs. Our perceptual systems do not passively receive stimuli from the world, instead they actively select, organize, interpret, and sometimes distort sensory information. The real world then may not be the same as the one we perceive. Broadly, perception can be said to be the study of the human organism's relation to the physical world.

Many aspects of perception are, quite simply, amazing, often denying easy explanation based on stimulation of the sensory systems alone. The perceptual phenomena discussed below have historically been the subject of much research and they pose a continuing challenge to researchers and theoreticians in perception.

Constancy

One of the most striking aspects of perception is constancy. Constancy refers to how our perception of objects remains the same despite changes in their image on the retina. Constancy is seen in the perception of a number of different properties of objects such as size, shape, color, and orientation. We will discuss only size and shape constancies.

Size constancy refers to perceiving familiar objects as approximately the same size regardless of their distance from the observer. Thus, for example, a person's size does not appear to expand or contract as they come toward you or move away, even though their image on the retina does become larger and smaller based on their distance.

Shape constancy refers to an object's shape being perceived as remaining the same despite being viewed from different perspectives with different shapes being projected onto the retina. A circular shape such as a pie on a table for example, is still perceived as circular even when you sit down at the table and perceive it from the side. This is despite the fact that circles viewed from the side produce not circular but elliptical images on the retina.

Perceptual constancy is one of the hallmarks of the field of perception, for it strongly indicates that visual perception involves more than the direct registration of the retinal image in the brain. Without perceptual constancy the world would be perceived as a booming blur of chaotic confusion in which the sizes, shapes, and colors of objects would be constantly shifting. Thus it can be seen that perceptual constancy serves an important adaptive function.

Despite constancy's great importance and prevalence across much of visual perception, there still no widely agreed upon explanation of it. There is, however, great agreement that constancy is based in part on the observer using appropriate contextual cues in the environment. For example, in size constancy it seems that in most instances we use estimates of detected distance based on various cues (for instance, haze, and a smoothing of textured surfaces such as grass are indicators of distance) to estimate an object's true size. And it seems that without cues enabling an accurate estimation of distance from an object, the perceived constancy of size ceases. For example, if one is extremely distant from objects without many intermediate visual cues enabling an accurate assessment of that distance, their perceived size would decrease. This is evident when looking at the world from a very high mountain top, or from a very tall building. From this distance, houses, cars, trees, and people below look very small, as if they were in miniature.

Perception of motion

The perception of motion has been the subject of much research. The mystery lies in how perceived movement cannot be accounted for by the movement of an object's image across the retina. If that were so, movement of the observer, or eye movement would lead to perceived object movement. For example, when riding a bike the rest of the world would be perceived as moving. Another phenomenon of motion perception that cannot be fully explained by sensory processes involves saccades, which are rapid directed eye movements. Because the eye sees detail only in a small area in the center of the eye called the fovea, in order to obtain detailed information from any object or scene, the eye must perform saccades so that the fovea receives enough information. Yet the images of stationary objects do not appear to move even when their retinal image moves due to saccades.

Evidence suggests motion perception can be partially explained by our apparently automatic use of numerous specific spatial and sequential relations between stimuli. Perceived motion then depends on such factors as the change in angular direction of the object from the observer, and the relation of the object in motion to the field in which it is perceived. For example, as an object moves through space it systematically covers and uncovers the background through which it is moving. Thus if a lion is running toward you across an open grassy plain, the grass will appear to be blocked out at the lion's leading edges as he moves toward you, and the grass that was not visible behind him will become visible as he gets closer.

In addition to contextual environmental factors, specific visual receptor cells that detect different types of movement such as up and down have been discovered. Despite this knowledge, many questions about the exact mechanisms of motion perception remain unanswered.

Form perception

Form perception is what enables us to identify objects and distinguish them from each other. Rather than a loose grouping of apparently separate stimuli, we see the world as organized with interrelated objects having definite shapes and forms. And as with many other perceptual phenomena, the light projected onto the retina from objects cannot account for our visual perception of the world. It seems perceiving form involves certain organizational principles, many of which were discovered by the Gestalt school of psychology. These rules or principles illustrate our tendency to organize and group separate elements of the visual world.

In the figure-ground rule, Gestalt psychologists found that when looking at unfamiliar scenes, familiar or consistent shapes tend to stand out as figures, and unfamiliar or undifferentiated shapes are perceived as the background. So, when looking at an abstract painting in which there are very few clearly defined forms, those forms that appear familiar, or that are repeated, will tend to be perceived as standing out from the rest of the painting which is then perceived as the background for those forms.

Gestalt psychologists also described a number of perceptual grouping principles. They found that when we perceive various stimuli we tend to group them according to their similarity, or according to their closeness to one another. Another principle, that of good configuration, is a very general organizing tendency that incorporates a number of figural characteristics. These include a tendency toward closure or perceiving a whole figure when there are actually gaps in its contour, and continuation in form where smooth continuous contours tend to be perceived over uneven or irregular contours.

The perception of form can be said to result, in part, from characteristics of the nervous system, as well as learning and experience. Yet there is no single theory of form perception that can fully account for the ability to perceive form. Nor is there a general principle that can pull together the many different types of form perception.

Depth perception

This entails perceiving the three-dimensionality of the world and objects. This clearly involves more than the nature of images sent to the retina since the retina has a two dimensional surface and images projected onto it are two-dimensional.

In the 1800s researchers discovered that our binocular vision greatly aids depth perception. Binocular vision refers to having two eyes that are at slight distances from one another, so that each receives a slightly different perspective of the object or scene being focussed upon. It seems that these small differences in perspective greatly aid depth perception. The ears also use the slight differences in time between stimulation received to locate the source of sounds.

In addition, most environments have common patterns corresponding with varying distances that provide visual clues about space and depth. Such clues include blocking of a far object by a near one, increasing haze with increasing distance, perspective, and shadow.

In sum, binocular vision and environmental cues can account for many aspects of depth perception. In addition, based on research with animals and human infants too young to have had experience with depth perception, it appears that humans and various species of animals are born with some innate visual mechanism to perceive depth.

Illusions

Illusions are misperceptions of stimuli, where what is perceived does not correspond to the actual dimensions or qualities of the physical stimulus. Geometrical illusions usually involve the misperception of the direction or size of parts of figures.

The mechanisms that produce many types of illusions are as yet not understood, but they seem to involve the misapplication of perceptual phenomena like constancy. Illusions are natural, occurring regularly and following regular rules. Illusions should not be confused with hallucinations which are responses in the absence of any external stimulus, or with delusions which are basically mistaken beliefs.

Over the last century a number of theories have been proposed to account for perception. Each theory, however, has encountered difficulties in accounting for some of the above-discussed phenomena of perception. And most perception researchers today do not adhere to one theory, instead they believe those aspects of the theories that have some experimental support, or that seem most logical and sound.

Classical theory

What is sometimes referred to as classical theory is usually associated with Hermann von Helmholtz who believed perception results from a process of unconscious inference about what the stimulus affecting the sense organs is most likely to be. He thought these unconscious inferences are formed by past experiences and learning, and they are unconscious because people are clearly not aware of making them.

Gestalt theory

Probably the most well-known theory of perception, Gestalt theory, developed partly as a reaction against the view that perception could be broken into simpler elements and that it was the result of learned mental associations between simple sensations. This view, the basis of Helmholtz's theory, was also put forth without the process of unconscious inference by such famous psychologists as W. Wundt and E.B. Titchener. Gestalt theory, founded by K. Koffka, W. Köhler, and M. Wertheimer, argued that while simple sensations could be seen as making up organized perceptions, our nervous system is primed to perceive the organization of sensory stimuli over the individual sensory elements themselves. The process of organization is basic to perception, and the common saying, "the whole is greater than the sum of its parts," illustrates this important concept.

Moreover, while Gestalt theorists believe learning may play a role in perception, perceptual organization results from innate organizing processes in the brain itself. To Gestaltists then, studying perception was in effect studying the brain.

Psychophysical or direct theory

This theory as put forth by J. J. Gibson holds that perception may be fully explained by the properties of the stimulation we receive from the world interacting with our sensory capabilities. Characteristics of scenes and events in the physical world may give sufficient information for the nervous system to be able to specify them. Thus, there is no need to posit unconscious mechanisms of inference as put forth by the Helmholtzian theory, or higher order organizations of stimuli as proposed by Gestalt theory.

Modern sensory physiology

This theory proposed by E. Hering and E. Mach believes the structure of the nervous system may fully explain at least some perceptual constancies as well as depth perception. E. Hering also proposed that there may be visual receptor cells organized into certain functional patterns that provide color sensation. And in fact these functionally patterned receptor cells have been found. These findings have strongly influenced current views of color perception, and the study of perception and sensation in general. It is still unknown, however, whether similarly organized receptor cells may exist for, or contribute to, perceptual phenomena such as the constancies and illusions. But their demonstrated existence may indicate that many perceived qualities of the physical world are based on such specific sensory mechanisms.

Some of the more recent theoretical and research developments fall within the areas of emotion, neuropsychology, ecological psychology, and artificial intelligence.

Emotion

Reflecting a trend across psychology as a whole, there has been a renewed and increased interest in how emotion influences perception and attention. This research investigates such questions as how emotion influences the focus and duration of attention, how quickly the emotional meaning of various stimuli can be processed, and whether individuals attend to positive and negative stimuli in different ways. Unlike most past research on perception this evolving area often researches socially meaningful perceptual stimuli, such as the perception of emotion in facial expressions and in vocal tones.

Neuropsychology

Neuropsychologists study changes in thinking due to brain injury, and use brain imaging techniques such as magnetic resonance imaging (MRI) and positron emission tomography (PET) scans, to examine the activity of the brain while performing high-level mental tasks such as problem-solving. A number of their findings have challenged explanations of perception based on behavioral studies. And improvements in brain imaging techniques hold the promise of shedding even more light on the neural basis of perception.

Ecological psychology

Ecological psychology attempts to specify the unchanging and limiting aspects of perceptual stimuli in the environment. They also stress how the nature of perceptual stimuli supports perception. This approach is most closely associated with the psychologist J.J. Gibson.

Artificial intelligence

This is an interdisciplinary field combining research and theory from cognitive psychology and computer sciences. It focuses on the development of artificial systems, such as computers, that show thinking processes similar to humans. This approach believes that for a complete explanation of perception it is necessary to divide it into three levels of analysis: 1) hardware, or its physiological aspects, 2) algorithms for operation, or what the processes of perceiving are, and 3) the theory of the task to be performed, or what are the qualities of our environment that enable perception. It is hoped that these divisions will serve as an important intellectual tool and aid our understanding of perception.

Perception is a field ripe with unanswered questions that continues to fascinate researchers who may greatly benefit from new technologies and new perspectives. Indeed, recent technological advances in the measurement of eye movements (saccades) have made their study much easier for researchers interested in changes in the focus of visual attention.