Quality over Quantity: synaptic strength an important factor in neuronal relationships

A new study has shown that few, strong synaptic connections between pairs of primary visual cortex (V1) neurons create correlated responses to visual stimuli, while many weaker connections link neurons with uncorrelated responses.

The research was led by Professor Thomas Mrsic-Flogel, whose research group at Biozentrum, Universität Basel, aims to understand the mechanisms that neural networks use to encode sensory information.

This recent work, published in Nature, shows that weak synaptic connections far outnumber strong ones between pyramidal cells of Layer 2/3 of the mouse V1. However, the majority of local excitation comes from the smaller number of strong inputs in the microcircuits related to visual processing.

Further, there is a greater correlation in responses from neurons with a similar spatial receptive field (RF) structure.

Professor Thomas Mrsic-Flogel said:

"We wanted to see if there are rules that explain how neurons connect in complex networks comprising millions of neurons. It turns out that one of the rules is quite simple. Like-minded neurons are strongly coupled, while neurons that behave very differently from each other connect weakly or not at all."

These results suggest that uncommon but strong synaptic connections have an important role to play in cortical processing. This leads to strongly tuned excitation and neuronal selectivity (through amplification and prolonged responses to specific visual features).

More research will be necessary to clarify the function of the large number of weak connections. The authors of this study believe they may have a role to play in learning. It is possible that these already existing connections could be strengthened rapidly should the neurons need to change their behaviour.

The research used two-photon in vivo calcium imaging and whole-cell in vitro recordings to elucidate the relationships between synaptic strength and neuronal response.

Cossell L., Florencia Iacaruso M., Muir D. R., Houlton R., Sader E. N., Ko H., Hofer S. B., Mrsic-Flogel T. D. (2015) Functional organisation of excitatory synaptic strength in primary visual cortex Nature, 518:399-403 doi: 10.1038/nature14182