Sensation and perception are intimately related and difficult to separate, but there is a distinct difference. Sensation generally refers to the process of detecting raw sensory information and converting the energy stimulating a receptor into neural impulse. Perception refers to the process of selecting, organizing, and interpreting sensory data into usable mental representations of the world. Whichever senses are involved, perception consists of three basic processes:
(1) selection in which we select which stimuli to focus on while disregarding the rest;
(2) organization, in which we assemble selected sensations into common patterns and shapes; and
(3) interpretation, in which we attempt to explain the selected and organized sensations and make reasonable judgments from them. We will examine all these concepts in detail in this lesson.
An illusion is a false impression of the environment. Our perceptions are normally in agreement with our actual sensations. Occasionally, sensations and perceptions do not match, resulting in an illusion. For instance, you might have observed the white highway guidelines on the road. When you look at them, they appear much shorter than they actually are. This is an example of an illusion.
Attention is the term given to the perceptual processes that select certain inputs for inclusion in our conscious experience, or awareness, at any given time. Imagine you are in a circus, with dancing bears, clowns and lions all performing at the same time. How do you know which act is important and how do you handle the overwhelming sensory stimuli? In almost every situation, there is an excess of sensory information, but the brain sorts important messages and discards the rest.
This process is known as selective attention. Another explanation uses the concept of filtering. Since we cannot process all the information in our sensory channels, we filter, or partially block out some inputs while letting others through. Three major factors involved in a selection decision are physiological factors, stimulus factors, and psychological factors.
The focus of attention is not just many disconnected sensations. We do not ordinarily perceive the world around us as patches of color, brightness or loud sounds. We see tables, buildings, trees; we hear voices, footsteps, music and automobile horns. The sensory inputs that are the focus of our attention have form and meaning. Having selected incoming information, we usually organize it into patterns and principles that will help us to understand the world. Raw sensory data must be assembled in a meaningful way before they are useful.
Gestalt psychologists proposed the laws of organization to explain how form is perceived. They highlighted the importance of organization and patterning in enabling us to perceive the whole stimulus rather than perceiving its parts as separate entities. The principles of perceptual organization are briefly discussed below.
- Figure and ground. The most fundamental law of organization is that we tend to distinguish between figure and ground. The most basic process in form perception is the recognition of a figure on the ground. We see objects and forms as standing out from a background. The ability to distinguish an object from its general background is basic to all form of perception.
- According to the principle of proximity, or nearness, elements that are physically close together will be grouped together and perceived as a single unit.
- According to the principle of continuity, we have a tendency to perceive patterns or objects that continue in one direction, as continuing in the same direction. For example, distant hills look continuous, even though buildings and trees interrupt.
- The principle of closure proposes that we have a tendency to perceive as finished or whole, even if the unit has gaps in it. We perceive a whole form, not disjointed parts.
- The law of contiguity states that when two events occur at the same time and place near each other, one is perceived as causing the other.
- Similar items tend to be grouped or organized together.
- Good figure. The law of good figure says that there is a tendency to organize things to make a balanced or symmetrical figure that includes all the parts.
You might have noticed in all these laws of organization, the principle of Gestalt, which states that the whole is more than the sum of its parts, is at work. In other words, the perceived organization has properties of its own that are not simply the result of adding together its parts.
Visual depth perception
Depth perception is the ability to accurately estimate the distance of perceived objects and thereby perceive the world in three dimensions. Scientists could not understand how we could see a three dimensional world with a flat, two-dimensional retina. There is sufficient evidence for the role of experience and learning in the perception of depth. The ability to accurately perceive distance and the ability to perceive to judge height and width of an object enable you to perceive the world in three dimensions. What are the cues which enable us to perceive depth? Monocular cues – cues received by one eye, and binocular cues – cues received by both eyes working together enable depth perception.
Monocular cues operate when only one eye is looking. Painters use these cues to give the impression of a three-dimensional picture on a flat canvas. Monocular cues include:
- Linear perspective. As two parallel lines recede from us, they appear to come together at the horizon. To put it simply, the distances separating the images of far objects appear to be smaller. The best example to illustrate the linear perspective is that of railway tracks which appear to meet at a single point at the horizon.
- Objects that are far away look “fuzzy” and blurred in comparison to near objects because of intervening particles of dust, haze, or smoke in the atmosphere. On a clear day, a distant mountain appears closer than on a hazy day.
- When one object obstructs our view of another, it appears to be closer. Thus, when one object is completely visible, while another is partly covered by it, the first object appears to be closer.
- Brighter objects are perceived as closer, whereas darker, dimmer objects are perceived as farther away. The variations in light produced by irregular surfaces also give cues for depth and distance as well as for shapes of objects.
- Gradients of texture. A gradient is a continuous change in something. Close objects appear to have a rough or detailed texture; as distance increases, the texture appears to become finer and finer. You may have noticed this effect when driving on a gravel road.
- Relative size. Objects that are far away look smaller than close objects of the same size.
- Motion parallax. Also known as relative motion it refers to the fact that when an observer is moving, objects at various distances move at different speeds across the retinal field. Close objects appear to whiz by, farther objects appear to move past slowly, while very distant objects appear to remain stationary. This effect can be seen while traveling by train.
- Accommodation refers to the changes in the shape of the lens of the eye in response to the distance of the object being focused. For near objects the lens bulges; for far objects it flattens.
- Retinal disparity. This is one of the most important cues to depth and distance perception. Because of the separation of the two eyes, each retina receives a slightly different view of the world. Different images fall on each retina, enabling depth perception.
- Stereoscopic vision. The brain fuses the different images received by the two eyes into one overall visual image so that an accurate sensation of depth occurs. This is known as stereoscopic vision.
- The amount of muscular strain created in the eyeball due to convergence is used as a cue by the brain to interpret distance. Convergence is the inward turning of the eyes as we move closer and closer to an object.
Although we are particularly alert to changes in our sensory input, we perceive a great deal of consistency in the environment due to the principle of perceptual constancy. Without perceptual constancies, our world would be chaotic. Things would seem to grow as we got closer to them, change shape as our viewing angle changed, and change color as light levels changed. Constancy is thus the tendency for the environment to be perceived as remaining the same even with changes in sensory input.
Most perceptual constancies are based on prior experience and learning. According to the principle of size constancy, the perceived size of an object remains the same even though the size of its retinal image changes. Children develop size constancy only when they are six or seven years old, after they have had sufficient experience with sizes.
There are other constancies which develop through individual experience. When you look at a chair from the front or the back, it has a rectangular shape, but when you look at it from the side, it has “h” shape. However, you still perceive the chair as having a single shape because your brain remembers past experiences with objects that only seemed to change shape as you moved, but remained constant. This is known as shape constancy. For example, divers wearing underwater masks experience several distortions of their normal visual fields, but they learn to adapt to these distortions.
Color and Brightness Constancy
A third form of constancy that adds stability to our world is color and brightness constancy. This enables us to perceive things as retaining the same color or brightness level even though the amount of light may vary. If you know an object from prior experience, you will know it to be the same color in bright or low light. You expect it to be its “right” color. Color constancy is the result of learning and experience and occurs primarily with familiar objects. The color of an unfamiliar object will be determined by the actual wavelength of reflected light; it will not be affected by prior experience.
Another factor in perceptual organization and everyday survival is the perception of movement. Each day we respond to movement to in our environment – we move out of the way of oncoming vehicles, avoid banging into people as they walk and so on. Occasionally, our perceptual processes are fooled by objects that look as if they’re moving when they really aren’t.
The perception of real movement is the result of an actual change in the object’s position in space. There are basically two ways in which we perceive motion: (1) an image moves across the retina, and (2) the eye moves in the head to follow the path of the moving object.
Hubel and Wiesel’s (1968) research has shown that there are motion detectors in the visual cortex which respond specifically to the movement of an object across the retina. Another explanation states that when the eyes move to follow an object, the brain sends signals to the eye muscles to keep the image on the fovea, at the centre of the retina. Due to motion perception, large objects are perceived as moving more slowly than small objects, and objects seem to move more slowly when the eyes track them, as opposed to when the head and eyes are kept still.
Unlike real motion, apparent motion is movement perceived in the absence of physical movement of an image across the retina. In other words, with the eyes, head, and body steady, and with no physical movement of an object, motion is still perceived. If you are seated in a darkened room and look at two adjacent lights being turned on one after the other, it will seem as if a single light is moving back and forth. This is known as stroboscopic motion, or the phi phenomenon. This principle explains the moving images in the movie – 24 pictures per second are projected, giving the illusion of motion.
- Autokinetic effect. This is another form of apparent movement – perceived motion of a single stationary light or object. You can experience this illusion by looking at a small dim light at the far end of a completely dark room. If you stare at the light for a few seconds, it will appear to wander around erratically. This apparent movement occurs because there are no cues to tell you that the light is really stationary, and the slight constant movement of the eye makes the light appear to move.
- Induced movement. Induced movement occurs if a stationary spot or object is perceived as moving when its frame or background moves. For example, the moon is often perceived as racing through a thin layer of clouds. Movement of the framework of clouds “induces” movement in the relatively stationary moon.
Plasticity refers to the modifiability of perception. Special situations, such as prolonged changes in sensory input can modify the ways information is processed in generating perceptions of the world around us.
Visual deprivation is the restriction of the visual input in some manner. This is especially effective during what is known as the sensitive period for visual development. A sensitive period is the time in the early development of a person or animal during which time, the environment has the greatest effect on behavior, or on the brain processes underlying behavior.
In one study with chimpanzees, it was found that early visual deprivation can lead to permanent degeneration of the retina. Another study demonstrated that kittens raised in a cylinder with vertically or horizontally striped walls develop severe behavioral and neurological impairments. Research on visual deprivation has established that infants need a certain amount of sensory stimulation even in the earliest days of life. Other research suggests that certain childhood visual defects such as squinting, must be corrected early in life.
Nature and nurture
Nature refers to the innate or inborn processes that influence behavior and perception, while nurture refers to learning and the effects of the environment on behavior and perception.
Nativists argue for the importance of nature in perception, and say that brain organization is patterned by genetic codes. The perception that depends on this organization is also innate. Plastic changes are explained as due to loss or attrition of inborn connection during the sensitive periods. In simpler terms, sensory deprivation during sensitive periods results in a loss of the brain connections that were genetically determined.
Empiricists argue for nurture stating that while genetic codes provide the basic structure, nurture interacts with the genetic endowment during sensitive periods to guide growth and to cause the proper brain connections to be made. Alterations to the environment during sensitive periods may change the way the brain grows and the manner in which it makes connections.
People differ in the way they process sensory inputs which lead to what they experience. Two people may have very different perceptions of the same television programme, lecture, or interpersonal exchange. Individual differences in learning, sets, motives, and perceptual styles explain why one person’s perceptions differ from those of another.
Perceptual learning has been defined as an “increase in the ability to extract information from the environment as a result of experience or practice with the stimulation coming out of it”. Perceptual learning can be considered as a variation of cognitive learning. For example, people trained in a particular occupation can make minute perceptual distinctions which untrained people cannot. Perceptual skills thus, can be learned from experience and not from books. In another instance, blind people even learn to distinguish among various shapes and textures of surfaces by perceiving the differences in their sound echoes.
Set refers to the idea that we may be ready and primed for certain kinds of sensory input. Such expectancies or sets vary from person to person and are a factor in both the selection of sensory inputs for focus of attention and in the organization of inputs. For example, if a person is expecting a phone call in the night, he or she will hear it ring, while the rest of the family continues to sleep. The new mother, for instance, will hear the baby cry while others are unaware of its cries.
Motives and needs
The idea that motives and needs influence perception was called the “new look” in perception. This approach to understanding perception stresses the fact that individual differences in motives and needs affect perception. That is, we may attend to and organize sensory inputs in ways that match our needs. For example, people who are hungry or thirsty are likely to pay attention to events in the environment which will satisfy these needs. Projective tests such as Rorschach are based on the principle that motivation influences perception.
People differ in the ways they typically process information. Perceptual-cognitive styles are the general processing strategies that distinguish different people. People vary in (1) the degree to which their perceptions are flexible or constricted, and (2) their field dependence or field independence.
People whose perceptions are flexible are said to have a wider focus of attention, to be less affected by interfering influences and to be less dominated by internal needs and motives than people who have a constricted perceptual-cognitive style. A field dependent person unifies and organizes sensory inputs, emphasizing the whole over the parts. Field-independent people emphasize the parts in perception.