Fig. 7

Buzsaki’s [26] extension of Hebb’s Cell Assembly and Phase Sequence. (a) Hebb’s [77] diagram of a cell assembly. Arrows represent transitions between individual assemblies. The direction of activity flow across assemblies (edges) is determined by the stronger synaptic strengths among assembly members relative to other connections (not shown). The same assembly can participate in a sequence more than once (e.g., pathway 1, 4 indicates recurring transitions). (b) Top: long sequence of two characters (e.g., dot and dash). Its embedded information is virtually impossible to recover. Bottom: same exact sequence as above after adding syntactic segmentation (space = stop-start punctuation) between the short strings of characters. The Morse code message reads: “segmentation of information is essence of coding.” By analogy, segmentation or “chunking” of neuronal assemblies can be brought about by salient external stimulus sequences, brain-initiated, modality-specific synchronizing- blanking mechanisms, internally generated oscillations, or other syntactical mechanisms. (c) Reader-defined cell assemblies. Neurons that fire within the time integrating window of a reader mechanism define an assembly (irrespective of whether assembly members are connected synaptically or not). Readers a, b, c, and w may receive inputs from many neurons (1 to n) by way of synapses differing in strength but respond only to a combination of spiking neurons to which they are most strongly connected (e.g., reader a responds preferentially to co-firing of neurons 1, 5, and 9 at t1, even though it may be synaptically innervated by neurons 2, 6, and 10 as well; at t2, neuron b fires in response to the discharge of neurons 2, 6, and 10). Synaptic strengths between neurons vary as a function of the spiking history of both postsynaptic and presynaptic neuron (short-term plasticity). The response of the reader neuron, therefore, depends on both the identity of the spiking upstream neurons and the constellation of current synaptic weights (“synapsembles”). Reader mechanism q has a longer time integrator and, therefore, can link together assemblies to neural “words,” reading out a new quality not present in the individual representations of a, b, and c. Reprinted from Buzsaki G. Neural syntax: cell assemblies, synapsembles, and readers. Neuron. 2010;68:362–85 [26] Copyright (2010), with permission from Elsevier