Hockenbury Discovering Psychology 5th txtbk

Perception121Binocular Disparity and the Perceptionof Depth in Stereograms This stereogram,Rustling Hares, was created by artistHiroshi Kunoh (Kunoh & Takaoki, 1994). Tosee the three-dimensional images, firsthold the picture close to your face. Focusyour eyes as though you are looking at anobject that is beyond the book and fartheraway. With out changing your focus, slowlyextend your arms and move the pictureaway from you. The image of the leaveswill initially be blurry, then details willcome into focus and you should see threerabbits. The three- dimensional images thatcan be perceived in stereograms occur becauseof binocular disparity—each eye ispresented with slightly different visualinformation.These images are quite different—that is, there is a great deal of binocular disparitybetween them. Thus you perceive the pencil as being very close. Now focus on anotherobject across the room and look at it first with one eye closed, then the other.These images are much more similar. Because there is less binocular disparity betweenthe two images, the object is perceived as being farther away. Finally, notice that withboth eyes open, the two images are fused into one.A stereogram is a picture that uses the principle of binocular disparity to createthe perception of a three-dimensional image (Kunoh & Takaoki, 1994). Look at thestereogram shown above. When you first look at it, you perceive a two-dimensionalpicture of leaves. Although the pictorial cues of overlap and texture gradient providesome sense of depth to the image, the elements in the picture appear to beroughly the same distance from you.However, a stereogram is actually composed of repeating columns of carefullyarranged visual information. If you focus as if you are looking at some object thatis farther away than the stereogram, the repeating columns of information will presenta slightly different image to each eye. This disparate visual information thenfuses into a single image, enabling you to perceive a three-dimensional image—three rabbits! To see the rabbits, follow the directions in the caption.The Perception of MotionWhere Is It Going?In addition to the ability to perceive the distance of stationary objects, we need theability to gauge the path of moving objects, whether it’s a baseball whizzingthrough the air, a falling tree branch, or an egg about to roll off the kitchen counter.How do we perceive movement?As we follow a moving object with our gaze, the image of the object movesacross the retina. Our eye muscles make microfine movements to keep the object infocus. We also compare the moving object to the background, which is usually stationary.When the retinal image of an object enlarges, we perceive theobject as moving toward us. Our perception of the speed of the object’s approachis based on our estimate of the object’s rate of enlargement (Schrater & others,2001). Neural pathways in the brain combine information about eye-muscleactivity, the changing retinal image, and the contrast of the moving object with itsstationary background. The end result? We perceive the object as moving.Neuroscientists do not completely understand how the brain’s visual systemprocesses movement. It’s known that some neurons are highly specialized todetect motion in one direction but not in the opposite direction. Other neurons are