MOTION MOUNTAIN

the story of the brain 249
The capacity of the brain
Ref. 225, Ref. 226
“Computers areboring. Theycangiveonly
answers.
(Wrongly) attributed toPabloPicasso”
The human brain is built in such a way that its fluctuations cannot destroy its contents.
The brain is well protected by the skull for exactly this reason. In addition, the brain
literally grows connections, called synapses, between its various neurons, which are the
cells doing the signal processing.The neuron is the basic processing element of the brain,
performing the basic classification. It can only do two things: to fire and not to fire. (It
is possible that the time at which a neuron fires also carries information; this question is
not yet settled. )The neuron fires depending on its input, which comes via the synapses
from hundreds of other neurons. A neuron is thus an element that can distinguish the
inputs it receives into two cases: those leading to firing and those that do not. Neurons
are thus classifiers of the simplest type, able only to distinguish between two situations.
Every time we store something in our long term memory, such as a phone number,
the connection strength of existing synapses is changed or new synapses are grown.The
connections between the neurons are much stronger than the fluctuations in the brain.
Only strong disturbances, such as a blocked blood vessel or a brain lesion, can destroy
neurons and lead to loss of memory.
As a whole, the brain provides an extremely efficient memory. Despite intense efforts,
engineers have not yet been able to build a memory with the capacity of the brain in
the same volume. Let us estimated this memory capacity. By multiplying the number of
neurons, about10 11 ,* by the average number of synapses per neuron, about 100, and also
by the estimated average number of bits stored in every synapse, about 10**, we arrive at
a conservative estimate for the storage capacity of the brain of about
M rewritable ≈10 14 bit≈10 4 GB . (95)
(Onebyte, abbreviated B, is the usual name for eight bits of information.) Note that evolutionhasmanagedtoputasmanyneuronsinthebrainastherearestarsinthegalaxy,and
thatif we add all thedendrite lengths,we get a total lengthof about10 11 m, whichcorrespondsto
thedistanceto fromtheEarthto theSun. Our brain truly isastronomically
complex.
However, this standard estimate of10 14 bits is not really correct! It assumes that the
only component storing information in the brain is the synapse strength. Therefore it
only measures theerasable storage capacity of the brain. In fact, information is also stored
in the structure of the brain, i.e., in the exact configuration in which every cell is connected
to other cells. Most of this structure is fixed at the age of about two years, but it
continues to develop at a lower level for the rest of human life. Assuming that for each
of theNcells withnconnections there arefn connection possibilities, this write once
Challenge 253 e capacity of the brain can be estimated as roughlyN√fnfn logfn bits. ForN=10 11 ,
*Thenumberofneuronsseemstobeconstant,andfixedatbirth.Thegrowthofinterconnectionsishighest
betweenage oneandthree,whenit issaid toreachupto10 7 newconnections persecond.
**Thisisanaverage.Sometypesofsynapsesinthebrain,inthehippocampus,areknowntostoreonlyone
bit.
Motion Mountain – The Adventure of Physics copyright © Christoph Schiller June 1990–November 2015 free pdf file available at www.motionmountain.net