Double Vision – test of the rod and cone networks in the eye

Double Vision – test of the rod and cone networks in the eye – Brief Article

Eric Haseltine

A LARGE ORGANIZATION WORKS MOST EFFICIENTLY when a job gets divided up among specialists, each of whom does a limited set of things exceptionally well. The same holds true for the human nervous system. Such division of labor is especially prominent in the eye, home to two highly distinct networks of light processing neurons. In one network, visual nerve cells activated by rod-shaped receptors are exquisitely sensitive to dim light and motion, but hopeless at resolving colors and small contours. The other network, driven by receptors shaped like cones, is very proficient at discerning colors and fine details, but relatively inept at detecting faint light or subtle movement.

Despite their individual weaknesses, the rod and cone systems make a great team, working together like two army scouts who’ve agreed that one will scan the landscape for movement but hand off to his partner the job of determining what it is that’s moving. We don’t notice that we have two receptor systems because our brains stainlessly combine their outputs. But with these tests you can tease them apart.

EXPERIMENT 1 Compare the brightness of these two color patches. In strong light, the red should look a little punchier. But if you turn down the lights until the gap in the C above the colors is barely discernible, the red should be the dimmer of the two hues.

That’s because the cone system, which dominates vision in daytime, responds well to red light, whereas the rod system, which rules the night, can hardly see red at all. This change in relative color brightness from day to night is called the Purkinje shift, after Jan Evangelista Purkinje, the Czech scientist who first described it.

The proportion of rods to cones is greater away from the center of the retina where you fix your gaze, so the red splotch will look even darker if you move it into rod territory by staring at the C instead of directly at the colors.

EXPERIMENT 2 Make an artificial star by taping a piece of cardboard over the lens of a flashlight and then pricking a tiny hole in the cardboard. Ensuring that no light escapes except through the pinhole, put the flashlight on a flat surface in a dark room, and angle the light so you’re not looking straight into it. Move 10 feet away and you’ll notice that the star is brighter when you look away from it than it is when you look directly at it.

This provides more evidence that rods, which are most populous in the periphery of vision, have greater sensitivity than the cones that inhabit the center of the retina.

EXPERIMENT 3 Hold up your right index finger about 90 degrees from your center of gaze. Now wiggle the finger vigorously, and notice how it becomes much more visible while moving. But if you look right at the same finger, it’s just about as noticeable moving as it is stationary. The reason: Rod-based peripheral vision is vastly more sensitive to movement than it is to small details, while the cone-based central vision is less responsive to movement and more acute.

Remembering the physiological differences between your night and day eyes can come in handy at times. For example, now you’ll avoid wearing red if you want friends to find you while you’re scanning the night sky with your peripheral vision, seeking out both faint stars and the kind that you wish upon.


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