Since brain data transmitted as spikes scale at an
exponential, or at least supralinear, function of synapse density,
as shown here in two-dimensional networks, the mammalian
brain in three dimensions and higher density is likely to scale
information transfer at an even higher function of synapses.
In the mammalian brain, synapses per neuron are estimated
at 8000 in the mouse cortex, 18 000 in the rat hippocampus
and 50 000 in the human cortex (14). The 6000 synapses
per neuron that we achieved in our low density planar rat
hippocampal cultures suggest a good approximation to the
in vivo density. The supralinear increase in the spike rate
with connectivity is highly suggestive of the notion that
the brain’s processing advantage comes not only from its
high degree of connectivity but also from the even greater
increase in information flow (spike rate) that is enabled
by that connectivity. The Internet provides a metaphor
for the scaling of information processing, as it is claimed
that Internet information flow scales supralinearly with the
number of connections, perhaps as the square (Metcalfe’s law),
exponentially or nlog(n) (Briscoe et al 2006). Even though
our brain is slow by computer standards, a clear processing
advantage is evident by raising the 50 000 connections/neuron
to a higher power.
Acknowledgment
This work was supported in part by NIH NS52233.