Interpreting Eye Signals

Our brains receive massive amounts of information from our eyes. While that information has many distortions, alterations, and anomalies that the various components of our visual process creates, the brain has a marvelous way of massaging all those confusing and contradictory signals, to provide us a very workable representation of our world. Our eyes have several shortcomings that stem from either the fact that their capabilities are limited (for example, we lack Superman's X-ray vision or the ability to sense the infrared part of the visual spectrum, as some animals can), but mostly because of out-and-out “design” defects.

Evolution has done a magnificent job of gradually “designing” and developing our bodies over millions of years, acquiring some pretty amazing attributes along the way. But the evolutionary process is a sequential one that must add each tiny developmental step to those that went before. It is not a conscious process that has a goal, but a trial-and-error unfolding that must build upon what is already there. If an earlier incremental step worked in the past (that is, it previously was a useful addition to the organism), it gets locked in. There is no backing up and correcting an earlier innovation that does not make sense later on.

Our spine, for example, evolved to work well when we were moving on all four legs (and similar spines still do, for our horizontal mammal cousins), but when we came to stand up on two feet, the angle and curve of our spine created lots of aches and pains that a dog never experiences. But it's too late; evolution can't back up and start over, improving the basic design in the process. It's stuck with what is at hand.

Here are a few of the eccentricities of the human eye that got built in: anomalies that cause the brain to work hard to compensate:

1. Inverted retinal image—Light enters the eye through a lens. It is a physical property of lenses to invert the image, so the brain must correct for an upside-down signal. In fact, the brain must deal with two slightly different inverted images—one from each eye.
2. Eye movement—Our eyes constantly jitter about—several times a second. They send an unstable string of signals to the brain that must be accounted for by some sort of smoothing. The situation gets very complicated when we need to track a moving object across a stationary background. Somehow the brain compensates and gives us a stable visual world.
3. Blind spot—At the center of our retina a tiny signal transmission cable (that transmits visual information to the brain) does not allow the retina to register light falling on that point. This blind spot occurs at dead center of our visual field, but the brain compensates and provides us continuous coverage, even when the signal is absent at that point.
4. Obstructions—Between the lens and the retina is a jelly-like blob of fluid that is filled with various kinds of hindrances to the path of light. Nerve fibers, blood vessels, and those infamous floaters all get in the way and distort light waves, yet the brain manages to compensate for them and give us a clear image of what's out there... well, except for those pesky floaters, which flit around like little bugs in our field of vision.
5. Variable resolving power—Each eye has about 6 million cones clustered at its center. They are good at resolving detail and discerning color. Some 120 million rods encircle the cones. They are poor at resolution and are essentially color blind, but they do an excellent job at seeing in low-light conditions and noticing movement (which helped our ancestors to spot fast-moving threats like snakes and lions at the peripheral regions of their vision). The brain's job is to account for this varying resolving power and color sensitivity, to provide us an in-focus image, in all its color, across our whole visual field.
6. Depth vision—Each eye registers a two-dimensional image that is unable to distinguish between a close, small object and a farther-away, large object. The brain receives both (slightly different) two-dimensional images and creates a 3D version, giving us depth perception.
7. Time—Finally, the brain must account for the fact that light travels to our eyes at an incredibly fast speed (nearly 200,000 miles per second), but then the signal creeps along nerve pathways from our eyes to the brain (at about 50 feet per second). The result: our brain must adjust for the fact that the signal it receives is already history... it's nearly a half-second old. How does a batter in baseball manage to connect, when the pitched ball takes about that same amount of time to reach him? That wonderfully compensating brain again.

It is amazing that our sense of vision (which we rely upon more than any other sense) serves us as well as it does. In doing so, the brain must process conflicting signals from each eye that are distorted and incomplete, and give us a steady, flowing experience. This raises a final question: After all this processing by the brain, is the world we perceive in our head an accurate “picture” of what's really out there? Or do we experience an illusion—not at all like what the real world is?

These and similar questions are at the center of ongoing research into human consciousness. No, the world we perceive is not fully real—our senses cannot give us complete accuracy; just, it seems, the accuracy we need to survive. The world we perceive is a construct, but the system works for us. We manage to get along quite well with the visual world that our brain gives us—despite the conflicting information it receives from those odd sensing organs, the eyes. The brain essentially must make up a story, and it does a pretty good job of it.