From my notes on December 22, 2015.

Hebbian theory via Wikipedia

Hebbian theory is a theory in neuroscience that proposes an explanation for the adaptation of neurons in the brain during the learning process. It describes a basic mechanism for synaptic plasticity, where an increase in synaptic efficacy arises from the presynaptic cell’s repeated and persistent stimulation of the postsynaptic cell. Introduced by Donald Hebb in his 1949 book The Organization of Behavior,1 the theory is also called Hebb’s rule, Hebb’s postulate, and cell assembly theory. Hebb states it as follows:

Let us assume that the persistence or repetition of a reverberatory activity (or “trace”) tends to induce lasting cellular changes that add to its stability.… When an axon of cell A is near enough to excite a cell B and repeatedly or persistently takes part in firing it, some growth process or metabolic change takes place in one or both cells such that A ’​s efficiency, as one of the cells firing B, is increased.1

The theory is often summarized by Carla Shatz’s phrase: “Cells that fire together, wire together”.2 However, this summary should not be taken literally. Hebb emphasized that cell A needs to “take part in firing” cell B, and such causality can only occur if cell A fires just before, not at the same time as, cell B. This important aspect of causation in Hebb’s work foreshadowed what is now known about spike-timing-dependent plasticity, which requires temporal precedence.3 The theory attempts to explain associative or Hebbian learning, in which simultaneous activation of cells leads to pronounced increases in synaptic strength between those cells, and provides a biological basis for errorless learning methods for education and memory rehabilitation.

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