What Crosses Our Minds When Danger's Afoot

An artist who was persuaded by his father to become a physician, Cajal spent much of his career looking through a microscope at slides of brain and spinal cord tissue. His ink drawings of what he saw were as exquisite as they were scientifically informative. In 1906, he was awarded the Nobel Prize for Physiology or Medicine.

Trying to come up with an explanation for why one side of the brain controls the opposite side of the body, Cajal noted that when an optical lens forms an image of an object, it inverts and reverses it. Bottom becomes top, right becomes left. When the lens of the eye projects an image on the retina, it depicts a world 180-degrees reversed from reality. The brain must compensate if it wants to get an image of the world as it exists.

It could do that by "higher-order" processing -- something akin to mental software -- that unscrambles the message. But that is not the strategy evolution favored. Instead, it favored reconstructing something close to a literal version of the perceived world in the dark, solid, wet brain.

Objects that are next to each other in the physical world and in the image projected onto the retina are also next to each other in the brain cells that process the impulses from the retina and "see" the objects. To make this happen, information about the right side of the world is sent to the left side of the brain, and information about the left side of the world is sent to the right. This undoes the side-to-side reversal of the image created by the lens.

This crossover of visual information occurs in its purest form in animals that have eyes on opposite sides of their heads, the better to scan the world for danger. (Fish are a good example.) When one eye sees a threatening object, it sends an image of it across the midline of the brain to the side opposite where the threat is.

If the way to escape the danger is to move muscles on the side of the body where the "image" now resides, the impulses sent out by nerve cells controlling those movements do not have to cross the midline. This is the case with fish and other limbless vertebrates that move by bending the entire body.

However, if the means of escape involves moving limbs on the side of the body closest to the threat, things are not so simple. In that case, the motor nerve cells in the part of the brain where the "image" of the threat is held have to send fibers across the midline -- back toward the threat and to the muscles that will help escape it.

The existence of limbs that push away from threats -- combined with the immutable properties of optical lenses -- "is the engine that is driving the evolution of crossed motor pathways," Jabaudon said in an interview.

This sets the stage for the crossing of sensory impulses, as well.

For example, an animal moving its left legs to escape a threat on its left side will want to tell the brain how the job is going. The brain needs to know: How hard are the feet pushing? Is there something painful underfoot? Furthermore, it makes sense to send the answers to the part of the brain where the threat was "seen" and the order to move the legs was given.

But that is on the opposite side of the body from the leg that is being told to move. Consequently, that foot-to-brain sensory information has to cross the midline, too.

The pattern of crossed motor nerve tracts is found in nearly all limbed creatures. It even exists in some organisms with "pseudo-limbs," such as rays, whose wings function like arms.

Things get more complicated in animals with stereoscopic vision, where both eyes see an object. In that case, only half the nerve impulses from each eye are sent across the midline.

But the architecture of crossed motor and sensory pathways -- established much earlier by natural selection -- is maintained all the way up the evolutionary tree to human beings.