Vocal learning in humans and songbirds occurs during a sensitive period
in development. Oscine songbirds, such as zebra finches, memorize a tutor
song during the sensory phase and then, using auditory feedback, match their
own vocalizations to the tutor song memory during the sensorimotor phase.
Songbirds possess a set of anatomically distinct brain nuclei that are dedicated
to vocal learning. HVC is a telencephalic song nucleus with auditory and motor
The cellular and neural circuit determinants of sensitive periods, which
are characterized by enhanced plasticity, are not well understood. Activity in
the sensorimotor song control area HVC changes during song learning
(Crandall et al., 2007b). For example, neurons in the HVC of juveniles have
longer, weaker bursts than those in adults. Changes in the circuitry of HVC that
may underlie these developmental changes are not known.
We have examined how population bursts in HVC change during
sensorimotor learning in the zebra finch. First, we found that bursts of activity in
HVC predict stability in song during singing. This led to the hypothesis that
bursts in HVC were stronger in the afternoon when song is known to be more
stable (Deregnaucourt et al., 2005). We found that bursts in HVC increase
each day and during development.
To examine changes in HVC burst activity during song learning, we
recorded ensembles of HVC neurons using multiple tetrode recording in anesthetized juvenile and adult zebra finches. Arrays of tetrodes, such as those
we have used, enable simultaneous recording of many neurons and analysis of
their functional interactions. To identify specific cells in the ensemble, we
antidromically stimulated projections from HVC to other song nuclei and coclustered
antidromic and spontaneous spikes. This method enables the study
of identified HVC projection neurons in the context of the functioning circuit.
With combined tetrode and antidromic methods, we have begun to
investigate interactions among HVC neurons. We have found that both efferent
projections of HVC are active in population bursts, and that the class of neurons
that project to a brain pathway known to generate song variability increases its
bursting activity in adults. We also found that a population of functionally
inhibitory neurons exhibits prolonged bursting in juveniles, which are active in
developing sensory systems during a sensitive period. Collectively, these data
implicate HVC in song learning. We speculate that bursts of activity in HVC, by virtue of a change in inhibition, limit song variability over time during sensorimotor learning.