Figure 1.

Schematic representations of two Hebbian postulates underlying memory formation in the brain. (A) Schematics of Hebbian associative plasticity. Cells that fire together wire together. From left to right: Presynaptic cell (black) does not activate post synaptic cell when firing on own. When two presynaptic cells fire simultaneously post synaptic cell fires action potential. This coincident firing of two presynaptic cells leads to synaptic strengthening. Now when the same presynaptic cell fire on own (red) it now activates the post synaptic cell. (B) Schematic of the second Hebbian postulate: reverberating ensemble of neurons. From left to right: The network may reverberate when a sensory signal (black arrow) enters at one node in an interconnected neurons ensemble (grey small arrows). This signal will remain active in the ensemble by moving from neuron to neuron within the network. The path of transmission will vary depending upon neurons’ history of activity, current synaptic strengths and other sensory signals entering the network. Thus, reverberation may follow different paths at each iteration. This reverberant activity can become coincident with new neural signals (middle, black arrows), thereby facilitating ‘Hebbian postulate one’ plasticity at some synapses in the network (in red).