Principal Investigator Michale Fee
For the past six years, the Fee laboratory has focused on studying the cellular, circuit, and mechanical underpinnings of songbird vocalizations. In one current project, we are studying nucleus RA, an area that projects directly to motor neurons of the vocal organ. During song, RA neurons each generate a distinctive and reproducible sequence of brief bursts of spikes. Using a new miniature motorized microdrive developed in this lab (see below), we have been able to record from large populations of RA neurons (~50) in the singing bird to understand how premotor activity maps to vocal output.
We are also investigating how the complex sequence of bursts in RA is generated. The primary motor-related input to RA comes from nucleus HVC; previous studies in singing birds found that HVC neurons generate relatively stochastic, dense patterns of spikes. Furthermore, the relationship of these firing patterns to the song suggested that the temporal control of song is organized hierarchically, such that HVC codes for motor patterns on the timescale of song syllables (50-100 ms), and RA codes for motor and acoustic patterns on a much briefer timescale (~10 ms). Thus, the prevailing view in the songbird field has been that burst patterns in RA are largely generated by circuitry intrinsic to RA.
One difficulty with these earlier experiments is that HVC is known to contain at least three classes of neurons: neurons that project to RA, neurons that project to area X (a brain area involved in vocal learning), and local interneurons. We were the first to characterize the firing patterns of antidromically identified HVC neurons in the singing bird, and have shown that RA-projecting HVC neurons are extremely sparsely active - each generating at most a single brief burst of spikes precisely at one time in the song motif. We also found that these neurons burst sequentially, with each neuron bursting at a different time in the song.
What is the causal relationship between the sparse bursts of HVC(RA)neurons and the bursts in downstream nucleus RA? Do bursts in HVC(RA) neurons drive bursts in RA, and if so, is every burst in RA driven from HVC? We are examining these questions in a new head-fixed sleeping bird preparation that permits sophisticated electrophysiological experiments that would be difficult or impossible to do in the freely behaving bird. Recent observations have shown that during sleep, RA neurons generate spontaneous high-frequency bursts, and that the sequence of sleep burst produced by an individual RA neuron can closely resemble the burst sequence of that neuron during singing. We have shown that HVC neurons similarly replay brief snippets of highly sparse song-like sequences during sleep. Furthermore, we have shown that during sleep, nearly every RA burst is driven directly by a small population of HVC(RA). The similarity of song- and sleep- related burst patterns suggest that song-related bursts in RA are likewise directly driven from HVC.
Song Circuits -- Birdsong, a complex yet stereotypic behavior that young birds learn from their fathers, provides an ideal system to study the neural basis of learned behavior. Fee studies this process using sophisticated recording methods to understand the brain areas that are important for song production. He has recently shown that a brain area known as the higher vocal center (HVC) works like the conductor of an orchestra to control the tempo of the song, and he is now working to understand the code by which timing is represented in HVC—whether, for instance, individual neurons activate each other in a ‘chain reaction’ like a cascade of falling dominos to drive the song sequence.