Introduction

One of the great concepts in contemporary neuroscience, the Hebbian synapse has been described as ‘The hypothesis that memory is associated with use-dependent synaptic modification’1 and ‘the seat of plasticity and change’, where ‘relating specific neural events to the specific processes of learning and memory…’ takes place.2 While theories of learning have their origins in 17th century philosophy,3 during the 19th century a more neuroscientific approach developed, and by 1890 the famous American psychologist William James had clearly envisaged the idea of neural learning when he wrote: “When two elementary brain-processes have been active together or in immediate succession, one of them, on re-occurring, tends to propagate its excitement into the other”4 (James's emphasis). However, the more specific concept that use results in changes in nerve connections, and that these changes in turn form the basis for learning and memory, was first proposed by Tanzi in Italy in 1893, then by Cajal in Spain 2 years later and by Kappers et al after a further 30 years (for reviews see Jacobson3 and Grossman5).

But, aside from these writers, it was another illustrious American psychologist, Donald Hebb, who is considered to have made ‘the first explicit statement of the physiological learning rule for synaptic modification that has since become known as the Hebb synapse’.6 In 1949, in his influential book The Organization of Behavior, Hebb summarised his ‘neurophysiological postulate’: “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”7 (Hebb's emphasis). Later he went on to develop this theory …