According to Hebb’s definition (1949), an assembly is characterized by the stronger synaptic connectivity among assembly members than with other neurons. In principle, chains of slow firing neurons, connected with predetermined and fixed synaptic weights can form groups and propagate activity (Abeles, 1991). However, strong, ‘fixed’ connectivity may not be a good model for segregating neuronal groups since synaptic weight distributions are perpetually changing in an activity-dependent fashion in the working brain. In fact, the dynamic range of short-term synaptic plasticity is large and similar to that of long-term plasticity (Marder and Buonomano, 2003), posing problems for the synaptic connection-based definition of cell assemblies. It follows that knowledge of spiking activity is insufficient to properly describe the state of the cortical network unless the distribution of momentary synaptic weights, i.e., the instantaneous functional connection matrix, is also known.
