How fast is a thought?

A "thinking" neuron labelled with a fluorescent protein.

I am constantly inspired by my students. Today, when talking about light, one of my best students asked me, “What is faster, the speed of light or the speed of thought?” Of course, the most concise answer would be “the speed of light – by a long shot;” however, the complete answer is much more interesting.

Thinking occurs by the chemical signaling of nerves – one to another – through a network of molecular and chemical cascades, making the means of neural signaling intrinsically different than the propagation of light. While light is described by the particle wave duality (for explanation, click here) and can travel through many materials (most notably, a vacuum) in the form of a traveling wave, a neuron must use direct chemical signaling in order to propagate a signal, such as a thought. This implies the introduction of many variables which influence the speed of the propogation, and which we can examine one at a time.

First, the speed of a neuron, or nerve cell, is dependent on the surrounding chemical environment. To understand this, we must first understand how the signal is transmitted to begin with. The membranous surface of a neuron is covered with tiny channels which selectively allow positively and negatively charged ions to get through. The rapid influx of ions causes an event known as depolarization, which in turn causes more channels to open, followed by a feedback system designed to restore the balance of the neuron to its initial voltage. The feedback system is directed along the length of the neuron, like a wave, so that the voltage changes quickly move from one end to the other. Remember, this only resembles a wave, in reality it is a rapid and local movement of positive and negative charges (simulation of nerve propagation). This process is called an action potential. If ions (positively and negatively charged small molecules) are either too high or too low in concentration around the neuron, it will affect the rate of the action potential, thereby changing the speed of transmission.

Second, the speed of propagation is dependent on the neuron’s particular surface characteristics. Some neurons are myelinated, meaning that they have a fatty layer wrapping around certain areas, causing the neuron to resemble a candy cane or a striped sweater. These neurons tend to conduct more rapidly, as the signal can be forced to “jump” the stripes, thereby cutting down on the distance traveled. These neurons, which include the neurons which transmit information about reflexes and body movements to and from the brain, optimally transmit a signal between 5 and 120 m/s. The difference in these numbers reflects the relationship between the diameter of the nerve and its ability to rapidly transmit the action potential. But myelinated nerves are not “thinking” signals. They serve to conduct information from the muscles and receptors of the body to the central nervous system. Indeed, even this type of signal is very rapid compared to the speed of thought.

The neurons responsible for thinking, which reside within the gray matter of the brain, are not myelinated. They are very thin, and transmit at speeds around 0.5 m/s. Compare this number to the speed of light, which ideally is 300,000,000 m/s. That means that on a human’s best day, the speed of thought is 600,000,000 times slower than the speed of light. This holds even if your name happens to be Albert Einstein!

For More Information:
Neuroscience Core Concepts
Teaching Resources from the Society for Neuroscience
Book Chapter: Biological Basis of Behavior