19-1 What are the characteristics of the energy that we see as visible light? What structures in the eye help focus that energy?

Our eyes receive light energy and transduce (transform) it into neural messages that our brain then processes into what we consciously see. How does such a taken-for-granted yet extraordinary thing happen?

When you look at a bright red tulip, the stimuli striking your eyes are not particles of the color red but pulses of electromagnetic energy that your visual system perceives as red. What we see as visible light is but a thin slice of the whole spectrum of electromagnetic energy, ranging from imperceptibly short gamma waves to the long waves of radio transmission (FIGURE 19.1). Other organisms are sensitive to differing portions of the spectrum. Bees, for instance, cannot see what we perceive as red but can see ultraviolet light.

Two physical characteristics of light help determine our sensory experience. Light’s wavelength—the distance from one wave peak to the next (FIGURE 19.2a)—determines its hue (the color we experience, such as the tulip’s red petals or green leaves). Intensity—the amount of energy in light waves (determined by a wave’s amplitude, or height)—influences brightness (FIGURE 19.2b). To understand how we transform physical energy into color and meaning, consider the eye.

Light enters the eye through the cornea, which bends light to help provide focus (FIGURE 19.3). The light then passes through the pupil, a small adjustable opening. Surrounding the pupil and controlling its size is the iris, a colored muscle that dilates or constricts in response to light intensity—or even to imagining a sunny sky or a dark room (Laeng & Sulutvedt, 2014). The iris also responds to our cognitive and emotional states. When you feel disgust or you are about to answer No to a question, your pupils constrict (de Gee et al., 2014; Goldinger & Papesh, 2012). When you’re feeling amorous, your telltale dilated pupils and dark eyes subtly signal your interest. Each iris is so distinctive that an iris-scanning machine can confirm your identity.

Behind the pupil is a transparent lens that focuses incoming light rays into an image on the retina, a multilayered tissue on the eyeball’s sensitive inner surface. The lens focuses the rays by changing its curvature and thickness in a process called accommodation.

For centuries, scientists knew that when an image of a candle passes through a small opening, it casts an inverted mirror image on a dark wall behind. If the image passing through the pupil casts this sort of upside-down image on the retina, as in Figure 19.3, how can we see the world right side up? The ever-curious Leonardo da Vinci had an idea: Perhaps the eye’s watery fluids bend the light rays, reinverting the image to an upright position as it reaches the retina. Unfortunately for da Vinci, that idea was disproved in 1604, when the astronomer and optics expert Johannes Kepler showed that the retina does receive upside-down images of the world (Crombie, 1964). So how could we understand such a world? “I leave it,” said the befuddled Kepler, “to natural philosophers.”

Today’s answer: The retina doesn’t “see” a whole image. Rather, its millions of receptor cells convert particles of light energy into neural impulses and forward those to the brain. There, the impulses are reassembled into a perceived, upright-seeming image. And along the way, visual information processing percolates through progressively more abstract levels. All this happens with astonishing speed. As a baseball pitcher’s fastball approaches home plate, the light signals work their way from the batter’s retina to the visual cortex, which then informs the motor cortex, which then sends out orders to contract the muscles—all in the 4/10ths of a second that the ball is in flight.