Nanoconnectomic upper bound on the variability of synaptic plasticity

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Abstract

Information in a computer is quantified by the number of bits that can be stored and recovered. An important question about the brain is how much information can be stored at a synapse through synaptic plasticity, which depends on the history of probabilistic synaptic activity. The strong correlation between size and efficacy of a synapse allowed us to estimate the variability of synaptic plasticity. In an EM reconstruction of hippocampal neuropil we found single axons making two or more synaptic contacts onto the same dendrites, having shared histories of presynaptic and postsynaptic activity. The spine heads and neck diameters, but not neck lengths, of these pairs were nearly identical in size. We found that there is a minimum of 26 distinguishable synaptic strengths, corresponding to storing 4.7 bits of information at each synapse. Because of stochastic variability of synaptic activation the observed precision requires averaging activity over several minutes.

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Thomas M Bartol

Howard Hughes Medical Institute, Salk Institute for Biological Studies, La Jolla, United States

For correspondence
bartol@salk.edu

Competing interests
The authors declare that no competing interests exist.
Cailey Bromer

Howard Hughes Medical Institute, Salk Institute for Biological Studies, La Jolla, United States

Competing interests
The authors declare that no competing interests exist.
Justin P Kinney

Massachusetts Institute of Technology, Cambridge, United States

Competing interests
The authors declare that no competing interests exist.
Micheal A Chirillo

Center for Learning and Memory, Department of Neuroscience, The University of Texas at Austin, Austin, United States

Competing interests
The authors declare that no competing interests exist.
Jennifer N Bourne

University of Colorado Denver, Denver, United States

Competing interests
The authors declare that no competing interests exist.
Kristen M Harris

Center for Learning and Memory, Department of Neuroscience, The University of Texas at Austin, Austin, United States

Competing interests
The authors declare that no competing interests exist.
Terrence J Sejnowski

Howard Hughes Medical Institute, Salk Institute for Biological Studies, La Jolla, United States

Competing interests
The authors declare that no competing interests exist.

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