How are long-term memories acquired and stored in the brain? How can a diverse range of stimuli trigger a perception and memory recall? These are just a few of the fundamental questions asked by D.O. Hebb who, in 1948, proposed the model above to address them. Arrows represent groups of neurons organized in a closed "cell assembly" which was initially created and then maintained by repeated correlations in firing patterns. The reverberating nature of activity across this assembly, indicated in red, is the neural representation of a memory. The meandering nature of activity propagation helps preserve the memory even when any single cell group (arrow) is refractory to stimulation, and the entire assembly can be activated (recalled) by stimuli impinging on any one of the cell groups. Based on activity patterns, cell assemblies can be added to or pruned back. Watch the video to see a simulation of this process.