SYNAPTIC
DARWINISM
This is the
"Synaptic Darwinism" website of Paul Adams and Kingsley Cox, who
work at the Kalypso Mind/Brain Center, and in the Department of
Neurobiology at Stony Brook University. | Outline | "Synaptic Darwinism" is a general approach to
certain problems of brain function. The starting point is the observation that
both in neuroscience and in general biology one sees a similar process: the
spontaneous development of complex structures which are well adapted to equally
complex environments. Perhaps rather similar processes are at work in both
cases. This does NOT mean that the brain processes information using DNA, though
both DNA and brain hardware may employ similar logic. Several authors, notably
Gerald Edelman, have proposed that the brain uses Darwinian strategies. But
"Synaptic Darwinism" invokes a quite different, more specific, and
more powerful, analogy. It proposes that the replicating entities in the brain
are arrays of synapses. Instead of a particular polynucleotide sequence, one has
a row of synapses. A connection is made up of many such functionally identical
synapses, and as a result of the electrical activity of these 2 neurons,
synapses can be added to the connection, or removed. These new synapses are
replicas of the existing synapses, since they connect the same neurons. However,
no replication process is perfect. An imperfect replica of a specific synapse is
still a synapse, but it no longer exactly connects the same particular pair of
neurons. The imperfect replica now connects one of the original pair to a
neighbor of the the other member of the pair. These well-defined replication and
mutation operations might ultimately be largely responsible for wiring up the
entire brain, as a result of electrical activities caused by an animal's
interaction with its environment.
There is recent electrophysiological evidence
that events resembling synaptic "replication" and "mutation"
do occur in the brain during learning - or, more precisely, in brain slices
during procedures that mimick real learning. There is also recent evidence that
mature neurons can form sprouts, which make novel synapses, during recovery from
injury or even normal learning. If the same electrical events control both
connection strengthening and novel synapse formation, the novel synapses are
equivalent to "mutations". This view of synapse formation overcomes a
serious limitation of conventional models of brain wiring. The bulk of the brain
is composed not of computing elements (synapses and impulse-initiation sites)
but of wires. If each neuron were connected to every other neuron, the wires
would have to be of subatomic dimensions, and electrical spikes would have to
travel faster than light. So the actual connections are just a tiny subset of
the possible connections. Since the computing power of the brain lies largely in
the detailed connections, either connections have to be pre-specified by the
accumulated creeping wisdom of gene-based evolution, or they have to be
constantly shuffled around and evaluated on-line. "Synaptic Darwinism"
claims that most of the shuffling and evaluation is done rapidly at the level of
synapses rather than slowly at the level of genes. This flexibility comes at a
price. Experimental new connections degrade current performance. They may even
proliferate until wiring becomes almost random. Thus synaptic mutation must be
kept in check. One way to do this is to make replication more perfect, and this
appears to have been a major theme in vertebrate evolution. Another is to only
allow modification of connections across which electrical activity is
appropriate. This would require rather elaborate circuitry, but this
hypothetical circuitry corresponds remarkably closely to the most puzzling and
characteristic features of neocortex. Quite astonishingly, a large neocortex
undergoing Synaptic Darwinism must dream, sleep and, during the day, be
conscious. Although the ideas and arguments underlying "Synaptic
Darwinism" are plausible, in agreement with much recent data, and
compatible with more conventional "neural network" approaches to brain
function, they are still speculative. continue... |
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