During the 1950s electron microscopy enabled us to peer into micro-circuitry where the majority of signal processing probably takes place in the brain. This was a major step forward in understanding brain structure and function that resulted in the empirical confirmation of the chemical synapse. However, revealing the rules of functional connectivity at the micron level was painful at best and was mostly beyond our capabilities. New techniques are now enabling us to begin unlocking the mysteries of connectivity and signal processing at the micron level in the brain (see “Other related blog posts” below). New research that contributes to revealing the structure and function of brain micro-circuitry was recently reported in the paper “Functional specificity of local synaptic connections in neocortical networks” (published May 5, 2011 in Nature).
The research team recorded responses to visual stimuli in every neuron found in a 285 micrometers by 285 micrometers by 90 micrometers volume of mouse brain that spanned layers 2/3 in the primary visual cortex. They presented oriented grating patterns that drifted in different directions and, in addition, they presented natural visual scenes from film clips. Finally, the team individually recorded the simultaneous electrical activity from up to four identified neurons. This enabled them to determine if the neurons were synaptically connected.
The results show that connections between neighboring neurons (less than 50 micrometers apart) are highly specific (not random). Visually driven neurons were more likely to connect with each other and the probability of connection increased with increases in the similarity of their responses to visual stimuli. Furthermore, neurons with similar responses to natural scenes showed more robust probabilities of being synaptically connected than neurons with similar responses to drifting gratings. These data are crucial for constraining our models of brain function. It will be important to determine to what extent the findings are specific to the species (mouse) and/or brain region (layers 2/3 visual cortex) explored and to what extent they my be stated as general principles of brain structure and functional organization.
Other related blog posts:
The Connectome: Automated Submicron Reconstruction of Brain Circuitry