That balance is critical to survival. Neurons are far more ef;cient than computers,
but despite that, the brain still consumes
a tremendous amount of energy. While
accounting for just 2 percent of our body
weight, the human brain devours 20 percent of the calories that we eat.
Functionally, most neurons have a lot
of properties similar to those of transistors. Both act as switches that can either
transmit or not transmit electrical pulses,
depending on signals they receive. The
trade-offs that have evolved in humans
could not be more different from those
that engineers made in designing conventional computers, however. Engineers
chose accuracy. Brains, shaped by natural
selection, minimize energy consumption
at all costs. Skinny neurons require less
energy, so evolution shrank them, and
brains adapted to operate barely above
the noise threshold.
With great ef;ciency, though, came a lot
of mistakes. Ideally, for example, neurons
should fire off electric spikes only when
they receive signals from other cells telling them to do so. But the brain’s skinniest neurons sometimes send out random
spikes triggered by ion channel proteins’
popping open accidentally. The smaller the
neuron, the more sensitive it is to these random channel openings, and the more often
these hiccups occur. The brain’s smallest
neurons operate “at the limit of biophysics,” Laughlin says. In 2005 he found that
shrinking those neurons a tiny bit more
meant they would burp out more than 100
random spikes per second.
This ;aky behavior places a fundamental
limit on how we function. Compensating
for random neural noise has shaped the
human brain—and human intelligence—
from the bottom up: the size and shape
of neurons, the wiring pattern of neural
circuits, and even the language of spikes
that encodes information. In the most basic
sense, the brain manages noise by using
large numbers of neurons whenever it can.
It makes important decisions (such as “Is
that a lion or a tabby cat?”) by having sizable groups of neurons compete with each
other—a shouting match between the
lion neurons and the tabby cat neurons
in which the accidental silence (or spontaneous outburst) of a few nerve cells is
overwhelmed by thousands of others. The
winners silence the losers so that ambiguous, and possibly misleading, information
is not sent to other brain areas.
The brain also ;lters out errors using a
neural code based on coincidences in timing. Consider the “Bill Clinton cells” that
neuroscientists have found in the brain’s
medial temporal lobe. These neurons ;re
whenever you see a picture of Bill Clinton,
hear his voice, or read his name. (You have
similar neurons for each of the hundreds of
other people you are familiar with.) A Clinton neuron might give off a spike whenever
