Finding the Engram: A Pathway for Song Memory in Zebra Finches

Finding memory traces, also called engrams, has been a major goal in the neuroscience field for decades. Although episodic memory -- the memory of autobiographical events -- is known to rely on the hippocampus for its formation, procedural memory -- the memory of motor skills -- does not require hippocampus and the exact nature and mechanism of it has remained largely unknown. Vocal learning is a form of procedural learning of a sequence of vocal movements from a social model, a rare trait detected in only few animal species including songbirds and humans. The learning of vocal production is guided by the retention of the memory of the social model's vocal behavior. In this dissertation, I used song learning in zebra finches as the animal model to study the neural basis of song memory. I used a newly developed spatiotemporally specific optogenetic method combined with neuron population-specific genetic lesion to target a neural pathway of zebra finches and examined its role in song memory. Through this series of experiments, I showed that 1) imposing artificial activity in this pathway results in birds singing songs with temporal structure conforming to the imposed activity, suggesting a mechanism for encoding the temporal structure of song; 2) imposing activity paired with live bird tutoring cause the birds to learn only from the imposed activity, but not from the live bird tutor, suggesting this pathway is either able to override other pathways for acquiring song memory, or a non-redundant pathway for encoding the temporal structure of song; 3) genetic lesioning of cells in this pathway precludes birds from learning from a tutor, but does not affect song learning if birds received tutoring before lesioning, suggesting this pathway is necessary for acquiring song memory and that memory transmitted via this pathway is not stored within but downstream of it. This study is the first case showing artificial activity imposed in a neural pathway implants memories that subsequently guide the learning of a motor skill. In Part I of this dissertation, I introduce memory and strategies that can be used to find engram, song learning of zebra finches and previous work in search of the engram of song memory and discuss the rationale of my design of experiments. In Part II, I present in three separate chapters experiments I conducted to examine of the role of a neural pathway of zebra finches in song memory.