ABSTRACT
In this work we discuss Hebb’s old ideas about cell assemblies in the light of recent results concerning temporal structure and correlations in neural signals. We want to give a conceptual (and necessarily only rough) picture of how ideas about “binding by synchronization”, “synfire chains”, “local and global assemblies”, “short and long term memory” and “behaviour” might be integrated into a coherent model of brain functioning based on neuronal assemblies. KEYWORDS: Cell assemblies; Synchronization; Gamma-oscillations; Synfire chains; Memory; Behaviour
1. CELL ASSEMBLIES AND ASSOCIATIVE MEMORIES
1.1. Cell Assemblies
Cell assemblies were introduced by Donald Hebb with the intention of providing a functional, and at the same time a structural model for cortical processes and neuronal representations of external events (Hebb, 1949). According to Hebb’s ideas, stimuli, objects, things, but also more abstract entities, like concepts, contextual relations, ideas (and so on) are considered to be represented in the brain by simultaneous activation of large groups of neurones, which are connected by relatively numerous and/or strong mutual excitatory synapses. Each single neurone may belong to many different cell assemblies. The determinant of an assembly is the connectivity structure between cells. This defines which cells lend support to each others’ firing, and hence have a higher probability of becoming coactivated in a reliable manner in response to different versions of the same stimulus. If an external stimulus excites a sufficiently large subset of cells of an assembly, then the whole assembly can “ignite” or “fire”, because recurrent activity,
distributed via the specific mutual connections, can also raise above threshold those cells which are not (or are only weakly) stimulated externally. This can be viewed as an elementary associative process, where the firing of externally driven cells represents the key information, and triggers the firing of cells representing information addressed by, but not yet contained in the key. In a similar way, some kind of short term memory is also supported. If continuously changing sub-groups of cells produce maintained activation of other groups, the activation within the assembly may survive for some time after its ignition, even if the external stimulation has already vanished.
