How fast is the speed of thought? - Seena Mathew

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Your mortal enemy has captured you and hooked you up to a bizarre experiment. He's extended your nervous system with one very long neuron to a target about 70 meters away. At some point, he's going to fire an arrow. If you can then think a thought to the target before the arrow hits it, he'll let you go. So who wins that race? Seena Mathew examines the speed of thought.

Lesson by Seena Mathew, directed by Andrew Foerster.

Your mortal enemy has captured you
and hooked you up to a bizarre experiment.

He’s extended your nervous system
with one very long neuron

to a target about 70 meters away.

At some point,
he’s going to fire an arrow.

If you can then think a thought
to the target before the arrow hits it,

he’ll let you go.

So who wins that race?

In order to answer, we have to examine
the hardware of thought: neurons.

The human brain has about 86 billion
of these cells.

They transmit signals down their axons
by way of electrical impulses,

or action potentials.

One neuron can then pass that signal
to the next at a synapse

by way of chemical neurotransmitters.

The signal is received
by the next neuron’s dendrites,

propagated down its axon,
and passed further along.

So, the key factors that determine
how quickly you think

include how long it takes to generate
an initial action potential;

propagate it down the length of the axon;
and transport it through the synapse.

We must also factor in the number
of neurons involved

and the distance the signal has to travel.

Let’s see what this looks like in a simple
pathway— your knee-jerk reflex.

A strike to your patellar tendon
triggers an electrical impulse

that travels up a sensory neuron
to your spine.

There the signal branches,
and for the sake of simplicity,

we’ll consider the segment that jumps
into a motor neuron

to journey back down your leg.

The total length of the neurons
in that pathway

is about 1 meter in someone
who is 5 foot 5 inches,

and on average it takes
15 to 30 milliseconds from strike to kick.

Speed is distance divided by time,

so this signal travels somewhere
between 120 to 240 kilometers per hour.

The initial action potential accounts
for 1 to 5 milliseconds

and synaptic transmissions only take
.1 to .5 milliseconds,

so the bulk of that time
is spent within the axons.

This is consistent with research findings

that the average individual neuron sends
signals at around 180 kilometers per hour.

But speeds can be boosted with myelination
and increased axon diameter.

Myelin is a fatty sheath
that insulates an axon,

preventing electrical currents
from leaking out.

Meanwhile, axons with larger diameters
offer less internal resistance.

These compounded factors can raise
the speed of an action potential

as high as 432 kilometers per hour.

There’s plenty of variation:
some people think faster than others,

and your own speed of thought changes
throughout your lifetime.

In particular, as you reach old age,

the myelin sheath covering your axons
wears down,

and other neuronal structures degrade.

Back to the dastardly experiment.

Arrows shot from recurve bows fly,
on average,

around 240 kilometers per hour.

Which means that given a sufficiently
long, myelinated or large-diameter neuron,

your thoughts actually could win the race.

But… there’s a wrinkle.

The arrow and thought don’t leave
the gate at the same time;

first the arrow fires,
then once you perceive it,

your signal can start down its path.

Processing images or music,
participating in inner speech,

and recalling memories all require
complicated neural pathways

that are nowhere close to the linearity
of the knee-jerk reflex.

The speed at which these thoughts
occur is mostly consistent,

with variations based on myelination
and axon diameter.

But the duration of a thought will vary
significantly depending on its routes,

pitstops, and destination.

In this case, when you perceive
a threatening stimulus,

you’ll invoke a fear startle response.

Similar to the knee-jerk response,

a startle can be involuntary
and quite fast.

If the string twangs loud enough,

you might react
in less than 65 milliseconds.

More likely though, your startle reaction
will be based on sight.

Our eyes can process an image
as quickly as 13 milliseconds,

but computation of what you’re seeing
and determining the danger it poses

can take as long
as 180 to 200 milliseconds.

In that time the arrow will have gained
a head start of about 13 meters.

The target is far enough away

that you’ve got just enough
of a chance to catch up,

if you can quickly, and quite literally,
think your way out.