18-5 What is the function of sensory adaptation?

Entering your neighbors’ living room, you smell a musty odor. You wonder how they endure it, but within minutes you no longer notice it. Sensory adaptation has come to your rescue. When we are constantly exposed to an unchanging stimulus, we become less aware of it because our nerve cells fire less frequently. (To experience sensory adaptation, move your watch up your wrist an inch: You will feel it—but only for a few moments.)

Why, then, if we stare at an object without flinching, does it not vanish from sight? Because, unnoticed by us, our eyes are always moving. This continual flitting from one spot to another ensures that stimulation on the eyes’ receptors continually changes (FIGURE 18.4).

What if we actually could stop our eyes from moving? Would sights seem to vanish, as odors do? To find out, psychologists have devised ingenious instruments that maintain a constant image on the eye’s inner surface. Imagine that we have fitted a volunteer, Mary, with one of these instruments—a miniature projector mounted on a contact lens (FIGURE 18.5a). When Mary’s eye moves, the image from the projector moves as well. So everywhere that Mary looks, the scene is sure to go.

If we project images through this instrument, what will Mary see? At first, she will see the complete image. But within a few seconds, as her sensory system begins to fatigue, things get weird. Bit by bit, the image vanishes, only to reappear and then disappear—often in fragments (FIGURE 18.5a).

Although sensory adaptation reduces our sensitivity, it offers an important benefit: freedom to focus on informative changes in our environment without being distracted by background chatter. Stinky or heavily perfumed people don’t notice their odor because, like you and me, they adapt to what’s constant and detect only change. Our sensory receptors are alert to novelty; bore them with repetition and they free our attention for more important things. The point to remember: We perceive the world not exactly as it is, but as it is useful for us to perceive it.

Our sensitivity to changing stimulation helps explain television’s attention-grabbing power. Cuts, edits, zooms, pans, sudden noises—all demand attention. The phenomenon is irresistible even to TV researchers. One noted that during conversations, “I cannot for the life of me stop from periodically glancing over to the screen” (Tannenbaum, 2002).

Sensory adaptation even influences how we perceive emotions. By creating a 50-50 morphed blend of an angry face and a scared face, researchers showed that our visual system adapts to a static facial expression by becoming less responsive to it (Butler et al., 2008; FIGURE 18.6). The effect is created by our brain, not our retinas. We know this because the illusion also works when we view either side image with one eye, and the center image with the other eye.

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Sensory adaptation and sensory thresholds are important ingredients in our perceptions of the world around us. Much of what we perceive comes not just from what’s “out there” but also from what’s behind our eyes and between our ears.