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By Root 924 0

closure, similarity and continuity or continuation. Consider the following: DEMON DAY BREAK FAST. Looking at the groups of letters, you probably read the four words demon, day, break, and fast, rather than Monday, daybreak, or breakfast. Proximity, the nearness of objects to each other, is an organizing principle. We perceive objects that are close together as parts of the same pattern. Do you know someone who writes letters without quite closing the letter “o” or crossing the “t”? You probably still know what the letter is. The principle of closure states that we tend to fill in gaps in patterns. The closure principle is not limited to vision. For example, if someone started singing, “Happy Birthday to . . . ,” you might finish it in your mind. The principle of similarity states that like stimuli tend to be perceived as parts of the same pattern. The principle of continuity or continuation states that we tend to group stimuli into forms that follow continuous lines or patterns.

Optical or visual illusions are discrepancies between the appearance of a visual stimulus and its physical reality. Visual illusions, such as reversible figures, illustrate the mind’s tendency to separate figure and ground in the absence of sufficient cues for deciding which is which. Illusory contours illustrate the tendency of the perceptual system to fill in missing elements to perceive whole patterns.

Depth Perception

Survival in a three-dimensional world requires adaptations for determining the distances of objects around you. Depth perception is the ability to judge the distance of objects. You interpret visual cues that tell you how near or far away objects are. Cues are either monocular or binocular. Monocular cues are clues about distance based on the image of one eye, whereas binocular cues are clues about distance requiring two eyes.

Binocular cues include retinal disparity and convergence. Your principal binocular cue is retinal disparity, which is the slightly different view the two eyes have of the same object because the eyes are a few centimeters apart. You can experience retinal disparity by extending your arm directly in front of you with your thumb up. Close one eye while looking at your thumb with the other. Then close the open eye and open the closed eye. Your thumb appears to move with respect to the background. If you follow the same procedure with your thumb closer, you’ll notice that your thumb appears to move more. The degree of retinal disparity decreases with distance. With both eyes open, your brain fuses the two images, resulting in perception of depth. Convergence is the inward turning of your eyes that occurs when you look at an object that is close to you; the closer an object, the more convergence. You can experience convergence by looking at the tip of your nose with both eyes. Convergence is a less important distance cue than retinal disparity and cannot be used for objects beyond about 8 meters (about 25 feet).

Monocular cues include motion parallax, accommodation, interposition or overlap, relative size, relative clarity, texture gradient, relative height or elevation, linear perspective, and relative brightness. Motion parallax and accommodation require active use of your eye in viewing, whereas the other monocular cues are pictorial depth cues that can be given in a flat picture. Motion parallax involves images of objects at different distances moving across the retina at different rates. Closer objects appear to move more than distant objects when you move your head. When riding in a moving vehicle, you see very close objects move rapidly in the opposite direction; more distant objects move more slowly past you; extremely far away objects, such as the moon, seem to move with you. Accommodation of the lens increases as an object gets closer.

Look outside your window to notice all of the pictorial cues.

• Interposition or overlap can be seen when a closer object cuts off the view of part or all of a more distant one.

• Relative size of familiar objects provides a cue to their distance when the closer