New research has shed light on the size of neural connections, estimating the memory capacity of the brain far higher than common estimates. Investigators from the Salk Institute for Biological Studies used advanced microscopy and computational algorithms to image rat brains and reconstruct the connectivity, shapes, volumes and surface area of the brain tissue down to a nanomolecular level. They found that the difference in the sizes of the pairs of synapses were very small, on average, only about eight percent different in size. In computer terms, 26 sizes of synapses correspond to about 4.7 “bits” of information. Previously, it was thought that the brain was capable of just one to two bits for short and long memory storage in the hippocampus.
In a computational reconstruction of brain tissue in the hippocampus (see picture), the researchers found the unusual occurrence of two synapses from the axon of one neuron (translucent black strip) forming onto two spines on the same dendrite of a second neuron (yellow). Separate terminals from one neuron’s axon are in synaptic contact with two spines (arrows) on the same dendrite of a second neuron in the hippocampus. The spine head volumes, synaptic contact areas (red), neck diameters (gray), and number of presynaptic vesicles (white spheres) of these two synapses are nearly identical.
The findings offer insight into the brain’s surprising efficiency, according to the research team. They also suggested that the data could also help computer scientists build ultraprecise and energy-efficient computers that employ “deep learning” and artificial neural nets capable of sophisticated learning and analysis.