Abstract
Humans have the ability to learn and execute a wide range of movement sequences that are integral to everyday life, from playing the piano to speaking, but little is known about how these behaviors are represented within the brain. Songbirds offer an excellent experimental model to study the organization of identified cortical circuits underlying a complex learned motor behavior. Towards that end, we focused on the cortical premotor nucleus HVC (proper name), which contains the neural circuitry enabling song production (Vu et al., 1994; Hahnloser et al., 2002; Long and Fee, 2008). HVC premotor neurons send primary axons to the forebrain motor nucleus RA (robust nucleus of the arcopallium) which ultimately drive song. In addition to these efferent outputs, RA-projecting HVC neurons also send axon collaterals within HVC (Gurney and Katz, 1981; Mooney, 2000), which are thought to form a synaptic chain capable of enabling precise sequences (Long et al, 2010). Despite their implications for a range of HVC circuit models (Fee et al., 2004; Amador et al., 2013), the fine structure of these processes is poorly understood. To address this issue, we examined the morphology of premotor neuron axons by delivering an intracellular tracer (Neurobiotin) in vivo using juxtacellular (loose patch) stimulation (Pinault, 1996; Narayanan et al., 2014), coupled with 2-photon imaging (Margrie et al., 2003) in order to specifically target premotor neurons in that nucleus. The axonal arbors were then reconstructed using a semi-automated process (Oberlaender et al., 2007). In addition to a quantification of the arborization patterns and putative target neurons, we are also interested in whether the projection patterns of single premotor neurons can suggest topographic structure, especially in light of recent evidence suggesting that connectivity within HVC may exhibit a directional bias (Stauffer et al., 2012; Poole et al., 2012; Day and Nick, 2013).
