The speed of thought

-October 13, 2016

Your reaction time depends on two things: first, the nature of the event that demands your attention, and second, the speed that signals propagate through your brain.

The human brain operates on three essential timescales. The three are dictated by the response time of the three tightly networked but still somewhat distinct components:

  • The brainstem, which is the oldest part of our brains in evolutionary terms, often called the “reptilian brain” by neuroscientists, is the most automatic; it responds in about 0.03 sec.
  • The limbic system that consists of a bunch of nodule-like doodads in the center of every mammalian and bird brain includes the emotionally-dominated four-F responses—fight, flight, freeze, or mate—and can respond no faster than about 0.2 sec.
  • The outer layer, the part that distinguishes us from most other mammals, called the neocortex requires at least 0.7 sec to register the thoughts that we’re aware of, conscious thoughts.

We could spend hours arguing over what qualifies as a thought. When you see your nemesis down the hall, your face betrays your displeasure in about than 0.03 sec and then it takes about 0.2 sec for you to replace that dragon scowl with feigned indifference. You’re not actually aware of either of these responses for at least 0.7 sec, if ever. Do both reactions qualify as thoughts? Or, to qualify, does a thought have to be conscious, like which beer partners best with a given conversation?

When you hit the brakes, is it a conscious decision? A preponderance of evidence says no, it wasn't conscious in that instant, but it was trained by conscious practice when you learned to drive. Same with deciding whether a pitch is worth swinging at: not conscious but trained from your little league days.

Neuroscience, that fascinating, infant science, has yet to define what a thought is. From the outside looking in, a thought is defined about as well as pornography—you know it when you see someone respond to stimulus. Someday, a thought will be defined as something like the degree of complexity of a network of neurons firing during some timescale. The timescale depends on the speed of signal transmission.

The three response timescales—brainstem, limbic, and neocortex—originate from the speeds of signals in your brain and depend on the number of signals required to generate a particular type of response.

The networks that make up your brain are composed of neurons. Signals are generated in the cell-body, or soma, of the neuron and transmitted on axons. Each neuron has one axon whose length can range from a meter to a fraction of a millimeter depending on its job. The axon connects to other neurons at their dendrites. Neurons typically have thousands of dendrites and axons make an average of 10,000 connections.

The signal itself is an electric pulse called an action potential, an isolated voltage spike of about 100 mV peak-to-peak that lasts about a millisecond (Figure 1).

Figure 1 Action potential: A thought is an electrical pulse. (Graphic by Ransom)

Signals are transmitted from one neuron to another through connectors called synapses. At a synapse, the electrical energy of the action potential triggers release of chemical neurotransmitters at vesicles. The chemicals cross from the axon of the transmitting neuron to the dendrite of the receiver through the synapse. The neurotransmitter chemical energy is converted back into an action potential that propagates into the receiving neuron (Figure 2).

Figure 2 Drawing of a neuron with dendrites on top and axon extending out of the frame (graphic courtesy of the National Institute of Health).

In 1850, Hermann von Helmholtz measured the speed that an action potential travels along an axon: about 30 m/s (which is about 60 mph or 90 ft/s). For comparison, electric fields propagate in wires at about half the speed of light in vacuum, roughly 150 million m/s. Since action potentials propagate along axons by alternate ingress and egress of sodium and potassium ions their speed is limited to chemical diffusion rates. Axons are insulated by a mix of fat and proteins called myelin. The better insulated the axon, the faster the signal. Neural signals range from a few feet per second to about 30% of the speed of sound, with most of those in our brains propagating at about 30 m/s.

To get a handle on thoughts, let's start from the top. Conscious awareness exists at a timescale of heartbeats, about a second. An action potential can travel about 30 m in that time. Our heads are about half a foot from front to back and side to side, so an action potential could make about 200 laps around our brains in the time of one conscious thought. Someday brain imaging like fMRI (functional magnetic resonance imaging) will be capable of mapping the network and estimating the number of neurons involved, but right now the technology’s resolution can't distinguish whether a network involves millions or billions of neurons and can’t resolve timing to less than a second.

At the 0.2 sec. limbic level, a typical signal can travel about 6 m. That's plenty of time for information from your eyes to reach the optical processors in the back of your skull and bounce around back there until a rudimentary image forms that can be sent to your four-f wetware.

Instinctual decisions such as avoiding spiders and snakes or getting out of the way of oncoming trucks, boulders, or saber-toothed tigers, occur on this four-f timescale. But we can train ourselves to make decisions this fast too. By practicing, you can work your response time down to 0.2 sec. For example, when you learned to read, you had to work through each character of every word. The first time you played a guitar chord, you had to set your fingers on each string and then strum. The timescales of your first word and first chord were on the scale of seconds. But after some homework, you learned to read sentences in a few tenths of a second, barely aware of the words that compose them. And if you practiced enough, you can play four chords per bar.

We could reverse engineer the four-f timescale from sports. It takes the ball about 0.2 sec to travel from the pitcher to the catcher, just enough time to distinguish a fastball from a curveball, if you’ve practiced enough.

Finally, that fastest timescale, about 0.03 seconds, is only long enough for a neural signal to travel less than a meter—barely time for them to vault through our sensory processors and deliver rudimentary, cartoon-like, images to our brainstem and cerebellum. Fast enough to maintain our balance or to register the first indicators of disgust or joy. But we can't train ourselves down to this fastest timescale except in a few reptilian-like cases: Before that guy in the cube down the hall became your nemesis, you didn't respond to him with an involuntary scowl, you had to learn to loathe him, well, he had to earn it.

The balance of nature is built on the speed of thought. If saber-toothed tigers' response times were faster than ours, we’d be extinct.

It feels like we make most decisions based on feelings in our four-f guts without deliberation, without any conscious input, as if we lack free will. But we don't, it's just that the deliberation for many of our decisions comes long before the decisions are made, from practice, experience, and the experience of bad decisions that we survived all go into training our guts.

Ransom's next book, The Left Brain Speaks But the Right Brain Laughs: An irreverent (but accurate!) look at the neuroscience of innovation & creativity in art, science & life, is available on November 8, 2016.

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