121 Recognizing the Perceptual World Recognizing the Perceptual World 䉴 How do I recognize familiar people? In discussing how people organize the perceptual world, we have set the stage for addressing one of the most vital questions that perception researchers must answer: How do we recognize what objects are? If you are driving in search of Barney’s Diner, exactly what happens when your eyes ﬁnally locate the pattern of light that spells out its name? To know that you have ﬁnally found what you have been looking for, your brain must analyze incoming patterns of information and compare them with information about the target that you have stored in memory. If the brain ﬁnds a match, recognition takes place. Once you recognize a stimulus as belonging to a particular category, your perception of the stimulus may never be the same again. Look at Figure 3.25. Do you see anything familiar? If not, turn to Figure 3.26; then look at Figure 3.25 again. You should now see it in an entirely new light. The difference between your “before” and “after” experiences is the difference between the sensory world before and after a perceptual match occurs and recognition takes place. How does this matching process occur? Some aspects of recognition begin at the “top.” That is, they are guided by knowledge, expectations, and other psychological factors. This phenomenon is called top-down processing, because it involves high-level, knowledge-based information. Other aspects of recognition begin at the “bottom,” relying on speciﬁc, detailed information from the sensory receptors and assembling them into a whole. This phenomenon is called bottom-up processing, because it begins with basic information units that serve as a foundation for recognition. Bottom-Up Processing All along the path from the eye to the brain, certain cells respond to selected features of a stimulus. The stimulus is ﬁrst analyzed into these basic features, which are then recombined to create the perceptual experience. What are these features? As mentioned earlier, certain cells specialize in responding to lines, edges, corners, and stimuli having speciﬁc orientations in space (Hubel & Wiesel, 1979). For example, some cells in the cerebral cortex ﬁre only in response to a diagonal line, so they act as feature detectors for diagonal lines. The analysis by such feature detectors, early in the sensation-perception sequence, may contribute to recognition FIGURE 3.25 Categorizing Perceptions doing 2 learn by What do you see here? For the identity of this ﬁgure, turn to Figure 3.26. top-down processing Aspects of recognition guided by higher level cognitive processes and psychological factors such as expectations. bottom-up processing Aspects of recognition that depend ﬁrst on information about stimuli that come up to the brain from the sensory systems.