Information storage capacity of synaptic connectivity patterns

How much information can be encoded in synaptic connectivity patterns through formation and elimination of dendritic spines? To answer this question, we perform geometrical analysis and derive an expression for the number of potential synapses, i.e. the number of axons that pass within a spine length of a given dendrite. We evaluate this expression by using our measurements and existing anatomical data, and find that the ratio of actual to potential synapses is 0.26 in mouse neocortex, 0.18 in rat hippocampus, and 0.12–0.23 in macaque visual cortex. Consequently, mammalian synaptic connectivity patterns may encode up to 3–5 bits per synapse. [Copyright &y& Elsevier]