Frequenzintegration im akustischen System der Schleiereule: Experimentelle und theoretische Charakterisierung der Mechanismen im Arcopallium
Final Report Abstract
This project tackled the question how sound localization is represented in the barn owl’s forebrain. The data were compared to the representation of such information in the barn owl's midbrain. 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 precisely localize faint sounds in a natural environment. 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. Both, the representation of the ITD and the spectro-temporal structure of acoustic stimuli in the midbrain nucleus ICX and the telencephalic auditory arcopallium (AAr) were examined. 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), located in the midbrain. The data showed 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, it was shown in this project 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, it was also shown 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. 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.
Publications
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(2012) Transformation from a pure time delay to a mixed time and phase delay representation in the auditory forebrain pathway. J Neurosci 32: 5911-5923
Vonderschen K and Wagner H
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(2013) Neuroethology of prey capture in the barn owl (Tyto alba L.) J Physiol (Paris) 107: 51-61
Wagner, H, Kettler L, Orlowski J, Tellers P
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(2013) Side peak suppression in responses of an across-frequency integration model to stimuli of varying bandwidth as demonstrated analytically and by implementation. J Comput Neurosci.
Goeckel T, Führ H, Lakemeyer G, Wagner H
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(2014) Detecting interaural time differences and remodeling their representation. Trends Neurosci. 37: 289-300
Vonderschen K, Wagner H
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(2014) Estimating characteristic phase and delay from broadband interaural time difference tuning curves. J Comput Neurosci. 2014 Oct 4. [Epub ahead of print]
Lehmann J, Tellers P, Wagner H, Führ H
