Mind & Brain

Using a fast code helps speed up thought, but to a large extent the brain—like a telegraph network—really depends on efficient pathways. Impulses from the retinas, for instance, have to travel up the optic nerve to the thalamus, which relays the signals to the visual cortex in the back of the brain. Then they ripple forward to other brain centers, where we use the visual information to make decisions and take actions. One way to hasten that journey is to use fast wiring. In 1854 physicist William Thomson showed that the wider a telegraph wire, the faster its signal and the farther the signal could travel. That same principle applies to nerves. The fattest axons, such as Betz cells in the brain, are 200 times thicker than the thinnest ones.

In principle, our thoughts could race far more efficiently if all the axons in our brains were thick. But the human brain has at least a quarter of a million miles of wiring—more than enough to reach from Earth to the moon—and is already packed tight. Sam Wang, a Princeton University neuroscientist, calculated how big our brain would be if it were built with thick axons. “Making an entire brain out of them would create heads so large that we couldn’t fit through doorways,” he concluded. Such a brain would also consume a tremendous amount of energy.

Given the constraints of biology and physics, our brains appear to have evolved to run very efficiently. For instance, neurons in the brain tend to be joined together into small networks, which are then linked to one another by relatively few long-range connections. This kind of network needs less wiring than other arrangements, and therefore shortens the distance signals need to travel.

In some ways evolution has fine-tuned our brains to run like a digital superhighway. In other ways it has left us with a Pony Express.

Our brains also speed up through practice. Rene Marois, a neuro­scientist at Vanderbilt University, measured this effect by having people perform a basic multitasking test: They had to identify which of two possible faces appeared on a computer screen while responding to one of two possible sounds. In just two weeks of training (encompassing eight to twelve practice sessions), the test-takers were able to do both tasks in rapid succession almost as quickly as doing either one on its own. With practice, Marois speculates, the neurons in the brain’s bottleneck regions, primarily in the prefrontal cortex, require fewer signals and less time to produce the right response.

Sometimes our brains actually need to slow down, however. In the retina, the neurons near the center are much shorter than the ones at the edges, and yet somehow all of the signals manage to reach the next layer of neurons in the retina at the same time. One way the body may do this is by holding back certain nerve signals—for instance, by putting less myelin on the relevant axons. Another possible way to make nerve impulses travel more slowly involves growing longer axons, so that signals have a greater distance to travel.

In fact, reducing the speed of thought in just the right places is crucial to the fundamentals of consciousness. Our moment-to-moment awareness of our inner selves and the outer world depends on the thalamus, a region near the core of the brain, which sends out pacemaker-like signals to the brain’s outer layers. Even though some of the axons reaching out from the thalamus are short and some are long, their signals arrive throughout all parts of the brain at the same time—a good thing, since otherwise we would not be able to think straight.

So when Helmholtz recognized that thought moves at a finite rate, faster than a bird but slower than sound, he missed a fundamental difference between the brain and a telegraph. In our heads, speed is not always the most important thing. Sometimes what really matters is timing.