Hockenbury Discovering Psychology 5th txtbk

122 CHAPTER 3 Sensation and PerceptionMike and Motion Perception Catching aball involves calculating an array of rapidlychanging bits of visual information, includingthe ball’s location, speed, and trajectory.Mike was especially appreciative ofhis newly regained motion perception. AsMike wrote in his journal, “Top on my listis being able to catch a ball in the air. Thisis pretty hard to do if you are totally blind,and now I can play ball with my boys andcatch the ball 80 percent of the time it isthrown to me. I have spent half my lifechasing a ball around in one way oranother, so this is a big deal.”specialized to detect motion at one particular speed. Research also shows thatdifferent neural pathways in the cerebral cortex process information about thedepth of objects, movement, form, and color (Zeki, 2001).Psychologically, we tend to make certain assumptions when we perceivemovement. For example, we typically assume that the object, or figure, moveswhile the background, or frame, remains stationary (Rock, 1995). Thus, as youvisually follow a bowling ball down the alley, you perceive the bowling ball asmoving and not the alley, which serves as the background.Because we have a strong tendency to assume that the background is stationary,we sometimes experience an illusion of motion called induced motion.Induced motion was first studied by Gestalt psychologist Karl Duncker in the1920s (King & others, 1998). Duncker (1929) had subjects sit in a darkenedroom and look at a luminous dot that was surrounded by a larger luminous rectangularframe. When the frame slowly moved to the right, the subjects perceivedthe dot as moving to the left.Why did subjects perceive the dot as moving? Part of the explanation has to dowith top-down processing. Perceptually, Duncker’s subjects expected to see thesmaller dot move within the larger rectangular frame, not the other way around. Ifyou’ve ever looked up at a full moon on a windy night when the clouds were movingquickly across its face, you’ve probably experienced the induced motion effect.The combination of these environmental elements makes the moon appear to beracing across the sky.Another illusion of apparent motion is called stroboscopic motion. First studied byGestalt psychologist Max Wertheimer in the early 1900s, stroboscopic motion createsan illusion of movement with two carefully timed flashing lights (Wertheimer,1912). A light briefly flashes at one location, followed about a tenth of a secondlater by another light briefly flashing at a second location. If the time interval anddistance between the two flashing lights are just right, a very compelling illusion ofmovement is created.What causes the perception of stroboscopic motion? Although different theorieshave been proposed, researchers aren’t completely sure. The perception of motiontypically involves the movement of an image across the retina. However, duringstroboscopic motion the image does not move across the surface of the retina.Rather, the two different flashing lights are detected at two different points on theStroboscopic Motion andMovies The perception ofsmooth movements in amovie is due to stroboscopicmotion. Much likethis series of still photographsof an athlete performinga long jump, amotion picture is actuallya series of static photographsthat are projectedonto the screen at therate of 24 frames per second,producing theillusion of smooth motion.