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encoding
of complex motor behaviors; auditory feedback and vocal learning organization of the song motor network The
motor pathway for song production consists of a bilaterally symmetrical
descending set of pre-motor forebrain nuclei (HVc and RA), which
ultimately innervate the motor- or premotor neurons for muscles
that control respiration and the vocal organ (Syrinx). By recording
neural activity simultaneously in left and right HVc of awake,
vocalizing male zebra finches, we have shown that vocal premotor
neurons are highly synchronized during specific portions of the
song motor response. Because synchronization occurs independently
of the recording sites within HVc in either hemisphere, we believe
that most HVc motor units are globally synchronized. The lack
of direct interhemispheric connections between forebrain song
control nuclei (birds do not have a corpus callosum) and the pattern
of synchronous activity suggest that each nucleus receives song-specific
patterned inputs from thalamus or midbrain: These inputs are likely to be critical
in determining song parameters related to syllable sequencing
and timing and suggest an important role for these input structures
in the determination of song structure. We have been investigating,
in collaboration with Eric Vu (Barrow Neurological Institute),
the nature of interhemispheric synchronization and coordination
during singing by combining short stimulation pulses, which disrupt
song behavior, with extracellular recordings in HVc (Vu, Schmidt
and Mazurek, 1998). auditory responses in the song system In
addition to its motor properties, HVc also receives direct auditory
inputs from the auditory forebrain and acts as a gateway for auditory
information flow into the song system. Recordings from anesthetized
songbirds have shown that HVc contains some of the most complex
auditory neurons known. By directly comparing auditory responses
from the same electrode in awake and anesthetized zebra finches,
we have shown that HVc auditory neurons are completely suppressed
in the awake, behaving animal (Schmidt and Konishi, 1998). (a) Representation of a typical zebra finch song highlighting
the highly stereotyped nature of this behavior. A given song
bout is typically preceded by one or more introductory notes
(i) which are followed by a stereotyped sequence of syllables
(S1 --> S4), known as motifs, which are repeated several
times (two in this example). (b) Quantitative representation of synchronization of song
premotor activity. Top, Spectrogram of the first motif of a
canonical song used as reference to compute the sliding cross
correlation. Middle and bottom, Color representation of the
sliding cross correlation matrix for song premotor activity
for the 1st and 2nd motif (bottom panel). Each cross correlation
is normalized to the autocorrelation to show the correlation
value r for each cross correlation time lag and cross correlation
matrices are color coded to illustrate the different r values
with red representing high levels of correlation and blue representing
negative r values or values near 0. Comparison of the cross
correlation matrix between the first and second motif show that
the overall correlation pattern between hemispheres is highly
conserved across motifs. Significant cross correlation values
(shown in red) occur at or near 0 ms time lag and occur in a
discreet syllable specific pattern. Cross correlation matrix
for the 1st and 2nd motif both represent the mean from 22 different
songs sung over two days. This unexpected result is of general
interest because it suggests that the behavioral state of an animal
may play an important role in modulating , in some cases even
suppressing, sensory flow into higher cortical areas. In songbirds,
the suppression of auditory responses seems to occur within HVc,
as it is not observed in auditory input stages to this nucleus.
Recent findings in our lab as well as that of Dan Margoliash (Dave
et al. (1998) Science 282:2250-2254 ) suggest that
sleep can mimic the effect of anesthesia on auditory gating. We
are presently investigating the mechanism(s) by which different
behavioral states may modulate the opening and closing of this
gate. Behavioral states that are of particularly interest to us
include sleep, attention as well as the motor act of singing.
We are also interested in whether gating is developmentally regulated
during song learning. Gating of auditory responses in HVc would
prevent young juveniles from storing the song template into song
system-specific areas and we hypothesize such gating should be
absent in juvenile birds during the early auditory phase of song
learning. Understanding the cellular bases of this phenomenon
may provide us with insights into the regulatory mechanisms of
critical periods during vocal learning. synaptic organization of HVc To
begin to analyze possible cellular mechanisms of auditory gating,
as well as understand the organization of the vocal motor network,
we are characterizing the synaptic organization of HVc. In particular,
we are studying how synaptic inputs from known input structures
may modulate, or suppress, neural responses in different subtypes
of HVc neurons. This work is performed using both sharp and whole
cell electrode recording techniques in slice as well as restrained
whole animal preparations.
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