Measuring communication in neural networks

Population neuronal activity measured by the local field potential (LFP) may measure how well long-range synaptic input can drive local spiking due to changes in excitability. However, the need to causally control circuit activity while measuring its large-scale effects means this hypothesis has not been directly tested. We used large-scale recordings in frontal cortex (PFC and Brodmann areas 4 and 6) of awake monkeys to measure responses to optogenetic stimulation in posterior parietal cortex (Brodmann areas 5 and 7).

We investigated parietal-frontal neuronal coupling—the efficacy of synaptic input from parietal cortex on frontal cortical neurons. To measure
neuronal coupling, we optogenetically-stimulated neurons in the posterior parietal cortex and measured synaptically-mediated spiking responses in anatomically specific sites in the frontal cortex. The likelihood that parietal stimulation would elicit a spiking response in frontal cortex measured neuronal coupling. Interestingly, the instantaneous strength of neuronal coupling varied in time and was predicted by phase of local field potential fluctuations at particular frequencies. These results measure dynamic neuronal communication across a large-scale circuit and demonstrate that LFP fluctuations reflect the strength of neuronal coupling measured by excitability to synaptic input.