Song circuit in bird brain contains map of space and time

Because the timescale of the calcium indicator they used could not resolve the fast firing patterns of the interneurons, they combined multi-channel electrophysiology with local field potentials (LFP) recordings, which can reflect synchronous ensemble neural activity over roughly 100 μm. They found that HVC neurons fired in synchrony about 30 times per second, or 30 Hz. [...]LFPs recorded from nearby electrodes were more similar than those recorded from electrodes separated by about 200 μm. Averaging over all recording sites in all birds, they found that synchronous neural activity at 30 Hz occurred across a distance of 108-125 μm. Taken together with the calcium imaging results, these findings indicate that HVC activity during singing is correlated over a distance of roughly 100 μm. When comparing the timing of activity in the two cell types, the researchers found that projection neurons fire during peaks in the 30 Hz LFP, whereas interneurons fire in the troughs. According to the authors, the recurrence of these length scales and timescales raises the possibility that spatial clustering and 30 Hz rhythmic alternation of excitatory and inhibitory neural activity may be a fundamental feature of motor circuits and could be critical for the sequential organization of behavior.