Fig. 1.

Hebb’s early concepts of synaptic change during learning. (a) Figure 1 from Hebb's MA thesis [58] shows his early concept of the synaptic changes underlying conditioning. (b) Figure 2 from [59] shows two possible routes of synaptic activity. An axon A with two terminal branches A’ and A” activates B and C. With greater activity of B, the route A’–B will be strengthened, but if there is an interneuron D, which is activated by A’, the route A’–D–C will be strengthened. (c) A possible mechanism of reflex inhibition. This figure was attached to Hebb’s 1934 essay [59]. It shows that his concept of inhibitory synapses was well developed in 1934, but was later deleted from his theory. a, b, c: Reprinted from unpublished papers of D.O Hebb held in McGill University Archives, Montreal, Quebec, file MG1045 [58, 59]. Permission has been obtained from Mary Ellen Hebb to reprint them. a and b are also in: Brown RE, Milner PM. The legacy of Donald O. Hebb: more than the Hebb synapse. Nat Rev Neurosci. 2003;4:1013–9 [23]. Ownership of copyright in original research articles remains with the author, Richard E. Brown