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18-1 What are sensation and perception? What do we mean by bottom-up processing and top-down processing?

HEATHER SELLERS CURIOUS MIX OF “perfect vision” and face blindness illustrates the distinction between sensation and perception. When she looks at a friend, her sensation is normal: Her sensory receptors detect the same information yours would, and her nervous system transmits that information to her brain. Her perception—the processes by which her brain organizes and interprets sensory input—is almost normal. Thus, she may recognize people from their hair, gait, voice, or particular physique, just not their face. Her experience is much like the struggle you or I would have trying to recognize a specific penguin.

In our everyday experiences, sensation and perception blend into one continuous process.

As our brain absorbs the information in FIGURE 18.1, bottom-up processing enables our sensory systems to detect the lines, angles, and colors that form the flower and leaves. Using top-down processing we interpret what our senses detect.

But how do we do it? How do we create meaning from the blizzard of sensory stimuli that bombards our bodies 24 hours a day? Meanwhile, in a silent, cushioned, inner world, our brain floats in utter darkness. By itself, it sees nothing. It hears nothing. It feels nothing. So, how does the world out there get in? To phrase the question scientifically: How do we construct our representations of the external world? How do a camp-fire’s flicker, crackle, and smoky scent activate neural connections? And how, from this living neurochemistry, do we create our conscious experience of the fire’s motion and temperature, its aroma and beauty? In search of answers, let’s look at some processes that cut across all our sensory systems.