Module 13: Perceptual Organization (pp. 171-179)
Use the player below to listen to the audio recording of this lecture.
Moving into Module 13, so far we have talked a lot about sensation, now we shift to perception. Our senses provide us with raw data about the external world but it must then be interpreted. Perception involves deciphering meaningful patterns in the jumble of sensory information and is the brain's process of organizing and making sense of sensory information. Perception begins with a real world object which activates our sensory system. We never experience the stimulus directly but our perception is usually very accurate.
In the early 20th century a group of German psychologists calling themselves Gestalt psychologists set out to discover the principles through which we interpret sensory information. Gestalt roughly translates to mean "whole" "form" or "pattern". They believed that our brains create a coherent perceptual experience that is more than simply the sum of the available sensory information and that it does so in predictable ways. One example of this is Figure-Ground. Sometimes when we look at an image, there aren't enough cues to allow us to distinguish the figure from the ground; they blend into each other in a sort of camouflage. We alternate back and forth between the two images because of the ambiguity of clues. Another example of Gestalt principles is the idea of Grouping or the perceptual tendency to organize stimuli into coherent groups. There are four principles of perceptual organization/grouping: 1) Proximity, 2) Similarity, 3) Closure/Connectedness, and 4) Continuity. These principles usually broaden our understanding of the world; our brain tries to fill in missing information rather than seeing things as random bits and pieces of raw data.
Perception of Distance and Depth
We are constantly judging the distance between ourselves and other objects along in a process known as depth perception. Depth perception can be accomplished with one eye (monocular cues) or two eyes (binocular cues). Binocular Cues which again are visual cues to gauge depth and distance requiring the use of both eyes. Two examples of these cues are:
1. Retinal Disparity: Our eyes are set roughly 2 ½ inches apart which means that each eye has a slightly different view of things. Binocular distance cues are based on the difference between the images cast on the two retinas when both eyes are focused on the same object.
2. Convergence: a visual depth cue that comes from muscles controlling eye movement as the eyes turn inward to view a nearby stimulus (no convergence if far away and can't converge if too close).
Monocular Cues again are visual cues requiring the use of one eye. There are several monocular cues that we use to gauge distance or depth.
1. Relative Size: If two objects are similar in size, we perceive the one that casts a smaller retinal image to be farther away.
2. Interposition: monocular distance cue in which one object, by partly blocking a second object, is perceived as being closer
3. Relative Height: We perceive objects that are higher in our field of vision to be farther away than those that are lower.
4. Relative Motion (Motion Parallax): monocular cue to distance in which objects closer than the point of visual focus seem to move in the direction opposite to the viewer's moving head, and objects beyond the focus point appear to move in the same direction as the viewer's head (example of being on a train)
5. Linear Perspective: monocular cue to distance and depth based on the fact that the two parallel lines seem to come together on the horizon
6. Light and Shadowing: monocular cue to distance and depth based on the fact that shadows often appear on the parts of objects that are more distant
Sometimes our visual cues can lead us astray and result in Visual Illusions (result from false and misleading depth cues). There are two main types of visual illusions; the first is Physical Illusions where the cause of the illusion is in the behavior of the light before it reaches the eye causing us to see something that isn't physically there. The second is Perceptual Illusions which occur because the stimulus contains misleading cues that give rise to inaccurate or impossible perceptions. An example of a physical illusion might be seeing water on the road such as a mirage and perceptual illusions can be seen in the images on the left. On a side note, many perceptual illusions were employed to help make the recent Lord of the Rings films! More examples of illusions can be seen if you click here!
We have a tendency to perceive objects as stable and unchanging despite changes in sensory stimulation which is called Perceptual Constancy. Once we have found a stable perception of an object, we can recognize it at almost any distance, from most any position, and under most any illumination. Without this ability, the world would be really confusing. There are a few different types of perceptual constancies that we possess. The first is Size Constancy or the perception of an object as the same size regardless of the distance from which it is viewed (wouldn't think that a woman 100's of feet away was only a few inches tall; experience tells us otherwise). The second type is Shape Constancy which is a tendency to see an object as the same shape no matter what angle it is viewed from (door remains a rectangle even though it is moved and viewed from other angles). Third is Color Constancy or an inclination to perceive familiar objects as retaining their color despite changes in sensory information (see our red car as red no matter if it is in bright light or in the dark). Finally we have Brightness Constancy which involves the perception of brightness being the same, even though the amount of light reaching the retina changes (compare objects with surrounding objects – know a white piece of paper is brighter than a piece of coal).