Hockenbury Discovering Psychology 5th txtbk

Perception123surface of the retina. Somehow the brain’s visual system combines this rapidsequence of visual information to arrive at the perceptual conclusion of motion,even though no movement has occurred. The perception of smooth motion in amovie is also due to stroboscopic motion.Perceptual ConstanciesConsider this scenario. As you’re driving on a flat stretch of highway, a red SUV zipspast you and speeds far ahead. As the distance between you and the SUV grows, itsimage becomes progressively smaller until it is no more than a dot on the horizon.Yet, even though the image of the SUV on your retinas has become progressivelysmaller, you don’t perceive the vehicle as shrinking. Instead, you perceive its shape,size, and brightness as unchanged.This tendency to perceive objects, especially familiar objects, as constant andunchanging despite changes in sensory input is called perceptual constancy. Withoutthis perceptual ability, our perception of reality would be in a continual state offlux. If we simply responded to retinal images, our perceptions of objects wouldchange as lighting, viewing angle, and distance from the object changed from onemoment to the next. Instead, the various forms of perceptual constancy promote astable view of the world.Size and Shape ConstancySize constancy is the perception that an objectremains the same size despite its changing imageon the retina. When our distance from anobject changes, the image of the object that iscast on the retinas of our eyes also changes, yetwe still perceive it to be the same size. The exampleof the red SUV illustrates the perceptionof size constancy. As the distance between youand the red SUV increased, you could eventuallyblock out the retinal image of the vehiclewith your hand, but you don’t believe that yourhand has suddenly become larger than theSUV. Instead, your brain automatically adjustsyour perception of the vehicle’s size by combininginformation about retinal image size anddistance.An important aspect of size constancy is thatif the retinal image of an object does not changebut the perception of its distance increases, the object is perceived as larger. To illustrate,try this: Stare at a 75-watt lightbulb for about 10 seconds. Then focus on abright, distant wall. You should see an afterimage of the lightbulb on the wall thatwill look several times larger than the original lightbulb. Why? When you looked atthe wall, the lingering afterimage of the lightbulb on your retina remained constant,but your perception of distance increased. When your brain combined and interpretedthis information, your perception of the lightbulb’s size increased. Rememberthis demonstration. We’ll mention it again when we explain how some perceptualillusions occur.Shape constancy is the tendency to perceive familiar objects as having a fixedshape regardless of the image they cast on our retinas. Try looking at a familiarobject, such as a door, from different angles, as in the photograph above. Your perceptionof the door’s rectangular shape remains constant despite changes in its retinalimage. Shape constancy has a greater influence on your perceptions than youprobably realize (see Figure 3.17).perceptual constancyThe tendency to perceive objects, especiallyfamiliar objects, as constant and unchangingdespite changes in sensory input.size constancyThe perception of an object as main tainingthe same size despite changing images onthe retina.shape constancyThe perception of a familiar object as maintainingthe same shape regardless of theimage produced on the retina.The Doors of Perception Each doorin the photograph is positioned ata different angle and thus producesa differently shaped image on yourretinas. Nevertheless, because ofthe perceptual principle of shapeconstancy, you easily identify allfive shapes as rectangular doors.Figure 3.17 How many right angles doyou see? Most people find 12 right anglesin this drawing of a slightly tilted cube.But look again. There are no right anglesin the drawing. Shape constancy leads youto perceive an image of a cube with rightangles, despite the lack of sensory data tosupport that perception.