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@PHDTHESIS{Tellers:538269,
author = {Tellers, Philipp},
othercontributors = {Wagner, Hermann and Spehr, Marc},
title = {{R}epresentation of spatial and spectro-temporal cues in
the midbrain and forebrain of {N}orth {A}merican barn owls
({T}yto furcata pratincola)},
school = {Aachen, Techn. Hochsch.},
type = {Dissertation},
address = {Aachen},
publisher = {Publikationsserver der RWTH Aachen University},
reportid = {RWTH-2015-05159},
pages = {XI, 122, CXXXVI [Bl.] : Ill., graph. Darst.},
year = {2015},
note = {Aachen, Techn. Hochsch., Diss., 2015},
abstract = {The barn owl is a crepuscular and nocturnal bird of prey
that relies mainly on its acoustic system for the
identification and localization of potential prey. The barn
owl is able to localize even faint sounds in a natural
environment precisely. Like mammals, barn owls use the
interaural time difference (ITD) for the localization of the
azimuthal sound source position. In the barn owl’s
auditory system, ITD is processed in two separate pathways,
the midbrain and forebrain pathways, which are both able to
mediate an accurate representation of stimulus direction.
While the azimuthal position of the sound source is
represented by the ITD, stimulus identity is assumed to be
represented by the spectro-temporal structure of the
stimulus. In general, ITD is encoded by the response rate of
auditory neurons in the barn owl, while the temporal
response pattern of auditory neurons is often correlated to
the spectro-temporal structure of the stimulus. During my
PhD thesis, I analyzed the representation of both the ITD
and the spectro-temporal structure of acoustic stimuli in
the ICX and the auditory arcopallium (AAr) of the barn owl.
The ICX belongs to the midbrain pathway, while the AAr is a
forebrain nucleus. The results obtained from neurons located
in the two nuclei were compared to check whether the
encoding of stimulus parameters differed beyond a previously
reported reorganization of ITD encoding in the forebrain
pathway that is due to the combination of low- and
high-frequency components with different best ITDs. Both ITD
processing pathways integrate the output of narrowly tuned
neurons of the core of the IC (ICC). The output of ICC
neurons with different best frequencies is integrated to
solve ambiguities in the representation of ITD. The data
presented in the current thesis show major differences in
the encoding of ITD in the ICX and the AAr. While response
variability generally increased in single neurons during
across-frequency integration, AAr neurons exhibited a
significantly higher variability in their response rates
compared to ICX neurons. In the midbrain branch of the
auditory pathway, ITD is known to be represented by the
maximum response rate of single neurons. Typically, each
neuron responds only to one ITD maximally, i.e., the best
ITD. The reported increase in response variability of AAr
neurons was accompanied by a lower suppression of
side-peaks, causing a more ambiguous representation of the
best ITD in single neurons. Furthermore, the broad
main-peaks and the reported response variability of most
neurons resulted in a less accurate encoding of the best ITD
in the AAr compared to the two IC nuclei. The reported
increase in the response variability of AAr neurons is
presumably a consequence of the more complex encoding of
ITD. While across-frequency integration in the midbrain is
restricted to frequencies above 2 kHz and supposedly
phase-dependent – i.e., carrier sensitive – this thesis
showed that ITD encoding in the AAr exhibited both high- and
low-frequency carrier-sensitive elements as well as a
substantial low-pass carrier-tolerant component. The
low-pass carrier-tolerant response component originated from
the envelope of high-pass $(\>3$ kHz) stimulus components,
thus indicating that barn owls, like mammals, are able to
extract carrier-sensitive and carrier-tolerant ITDs from the
same frequency range. In accordance with previous studies,
this thesis also showed that the low-frequency
carrier-tolerant component exhibits larger best ITDs
compared to the carrier-sensitive components, indicating an
integration of inputs with different best ITDs in AAr
neurons.Whether the reported differences in ITD encoding of
the ICX and the AAr were related to a change in function was
analyzed in the final parts of this thesis. Thereby, the
focus was on two aspects, the time course of ITD encoding
and the representation of the spectro-temporal stimulus
structure. ITD representation in the AAr was delayed
compared to the ICX. ICX neurons exhibited lower latencies
compared to AAr neurons and strong onset – i.e., phasic
– responses to acoustic stimuli. By contrast, besides the
higher latencies, AAr neurons typically exhibited a more
sustained representation of ITD that in many neurons lasted
beyond the stimulus offset. These results are consistent
with behavioral studies that reported an involvement of the
AAr in the memory-based localization of auditory signals.
Furthermore, the response rate of a subpopulation of AAr
neurons steadily increased with stimulus duration,
indicating the existence of a temporal integration mechanism
in the forebrain pathway, which might be the reason for the
sustained representation of ITD.The changes in the encoding
of ITD from the midbrain to the forebrain were accompanied
by a differing representation of the spectro-temporal
stimulus structure. Spike generation was locked to the
occurrence of certain spectro-temporal stimulus structures
in neuronal subpopulations of both the ICX and the AAr.
Thus, neurons of both nuclei were able to encode the
spectro-temporal structure of the stimuli in their response
pattern. However, the ratio of neurons that exhibited a
correlation between the response pattern and the acoustic
stimulus was higher in the ICX compared to the AAr. The
generation of spikes in response to certain stimulus
structures was also more reliable in single ICX neurons,
while AAr neurons exhibited a higher temporal precision in
the spiking. Furthermore, single ICX neurons were, in
contrast to AAr neurons, able to represent both the best ITD
and the spectro-temporal stimulus structure accurately.
These results conform to previous published data.
Electrophysiological studies already reported that neurons
in the mammalian forebrain neglect the encoding of
spectro-temporal stimulus structure supposedly in favor of
the representation of abstract entities in the acoustic
stimulus, like echoes or ambient noise.In summary, this
thesis clearly showed that the encoding of ITD in the ICX
and the AAr exhibits major differences. The accurate and
unambiguous representation of ITD, the rather low response
variability and the integration across inputs with similar
best ITD in the ICX of the barn owl are in accordance with
previous studies that assumed a place code-like
representation of ITD in the ICX of the barn owl. By
contrast, the high response variability, the worse
representation of best ITD and the frequency dependence of
the best ITD reported for single AAr neurons makes it
unlikely that ITD is encoded in a place code, which depends
on an accurate and, more importantly unambiguous
representation of best ITD. The observed differences in both
the time course of ITD representation and the encoding of
spectro-temporal stimulus structure presumably reflect the
different function of the midbrain and forebrain ITD
processing pathway, with the former responsible for the
mediating of fast and accurate head saccades toward salient
stimuli and the latter more involved in the sustained,
memory-related representation of stimulus direction.},
cin = {162110 / 163310 / 160000},
ddc = {570},
cid = {$I:(DE-82)162110_20140620$ / $I:(DE-82)163310_20140620$ /
$I:(DE-82)160000_20140620$},
typ = {PUB:(DE-HGF)11},
urn = {urn:nbn:de:hbz:82-rwth-2015-051599},
doi = {10.18154/RWTH-2015-05159},
url = {https://publications.rwth-aachen.de/record/538269},
}
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