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Main Research Site: http://schmidt.bio.upenn.edu/schm
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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