Donald Hebb:

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Hebb's hypothesis is that the basis for neural development lay in a selective strengthening or inibition of synapses between neurons. Synapses that get used are reinforced, while synapses that are not used are inhibited. This dual process molds the structure of the brain in a darwinian fashion. Metabolic change therefore occurs in the brain all the time. These synaptic changes are the basis for all learning and memory.
Besides advancing the learning rule for synaptic modification, the books defines the notion of the brain as a connectionist device and the notion that within the brain regions of interconnected self-reinforcing subnets of neurons (or "cell assemblies") form for long periods of time.
The brain is an evolutionary system: genes determine only its initial configuration, experience molds the brain according to darwinian principles of selection.
The selective strenghtening of the synapses causes the brain to organize itself into cell assemblies, each assembly representing a fragment of a concept, each assembly overlapping others so that concepts are naturally linked into larger concepts. Each resonating cell assembly behaves like a rule: triggered by an event, will fire for a while at a higher rate.
Psychological conditioning is ubiquitous in animals because it is a property of individual neurons.

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