Novel approaches for the investigation of sound localization in mammals

The ability to localize sounds in space is important to mammals in terms of awareness of the environment and social contact with each other. In many mammals, and particularly in humans, localization of sound sources in the horizontal plane is achieved by an extraordinary sensitivity to interaural time differences (ITDs). Auditory signals from sound sources, which are not centrally located in front of the listener travel different distances to the ears and thereby generate ITDs. These ITDs are first processed by binaural sensitive neurons of the superior olivary complex (SOC) in the brainstem. Despite decades of research on this topic, the underlying mechanisms of ITD processing are still an issue of strong controversy and the processing of concurrent sounds for example is not well understood. Here I used in vivo extra-cellular single cell recordings in the dorsal nucleus of the lateral lemniscus (DNLL) to pursue three novel approaches for the investigation of ITD processing in gerbils, a well-established animal model for sound localization. The first study focuses on the ITD processing of static pure tones in the DNLL. I found that the low frequency neurons of the DNLL express an ITD sensitivity that closely resembles the one seen in the SOC. Tracer injections into the DNLL confirmed the strong direct inputs of the SOC to the DNLL. These findings support the population of DNLL neurons as a suitable novel approach to study the general mechanism of ITD processing, especially given the technical difficulties in recording from neurons in the SOC. The discharge rate of the ITD-sensitive DNLL neurons was strongly modulated over the physiological relevant range of ITDs. However, for the majority of these neurons the maximal discharge rates were clearly outside this range. These findings contradict the possible encoding of physiological relevant ITDs by the maximal discharge of single neurons. In contrast, these data support the more recent hypothesis that the discharge rate averaged over a population of ITD-sensitive neurons encodes the location of low frequency sounds. In the second study, I investigated the ITD processing of two concurrent sound sources, extending the classical approach of using only a single sound source. As concurrent sound sources a pure tone and background noise were chosen. The data show that concurrent white noise has a high impact on the response to tones and vice versa. The discharge rate to tones was mostly suppressed by the noise. The discharge rate to the noise was suppressed or enhanced by the tone depending on the ITD of the tone. Investigating the responses to monaural stimulation and to tone stimulation with concurrent spectrally filtered noise, I found that the ITD sensitivity of DNLL neurons strongly depends on the spectral compositions, the ITDs, and the levels of the concurrent sound sources. Two different mechanisms that mediate these findings were identified: monaural across-frequency interactions and temporal interactions at the level of the coincidence detector. Simulations of simple coincidence detector models (in cooperation with Christian Leibold) suggested this interpretation. In the third study of my thesis, the temporal resolution of binaural motion was analyzed. Particularly, it was investigated how fast the neuronal system can follow changes of the ITD. Here, psychophysical experiments in humans and electrophysiological recordings in the gerbil DNLL were performed using identical acoustic stimulation. Although the binaural system has previously been described as sluggish, the binaural response of ITD-sensitive DNLL neurons was found to follow fast changes of ITDs. Furthermore, in psychophysical experiments in humans, the binaural performance was better than expected when using a novel plausible motion stimulus. These data suggest that the binaural system can follow changes of the binaural cues much faster than previously reported and almost as fast as the monaural system, given a physiological useful stimulus. In summary, the results presented here establish the ITD-sensitive DNLL neurons as a novel approach for the investigation of ITD processing. In addition, the usage of more complex and naturalistic stimuli is a promising and necessary approach for opening the field for further studies regarding a better understanding of the hearing process

The representation of respiratory movements in the inferior olive

It has been previously proposed that the inferior olive (IO) acts as a 'comparator' signalling to the cerebellum differences (errors) between signals conveying the command for movement from 'higher motor centres' and the activity these signals evoke at spinal 'centres' which also are in reciept of peripheral inputs (Oscarsson, 1973). Validation of this hypothesis has been hindered by the lack of direct access to the command signals. A review of the literature revealed that the respiratory system, as a source of centrally initiated automatic movements, offers a unique opportunity for studying both the command and outcome for a naturally occurring movement. The experiments in the first section of the thesis reinvestigate the interactions between the central respiratory drive and segmental (chest wall) reflexes. Muscle afferent activity was also monitored in intercostal nerve filaments to define proprioceptive inputs to the spinal cord and brainstem during artificial ventilation. Against this background the IO was explored by extracellular recording to seek evidence of central and peripheral-related respiratory activities. Three principal types of respiratory related activities were encountered in the dorsal accessory olive (DAO); 1), respiratory phased, mass activity; 2), respiratory phased, low frequency activity; 3), respiratory pump locked activity. The spatial location of these activities was found to correspond to a narrow, longitudinal strip in the DAO representing the segmental projection of the spino-olivary neurones located in the thoracic spinal cord, as identified anatomically by Matsushita et al. (1992). The source of these activities was investigated by cold block of the cervical spinal cord at C3. Whereas the central respiratory activity was abolished in spinal motoneurones, it persisted in the DAO indicating that the DAO receives a phasic respiratory input of supraspinal origin. Lowering the CO2 while holding the mechanical conditions in the thorax constant, resulted in changes in the olivary discharge, further indicating that the central command is fed to the olive, thus supporting the view that the olivary neurones act as a comparator signalling differences between the command for movement and the outcome

Physiology, Psychoacoustics and Cognition in Normal and Impaired Hearing

​The International Symposium on Hearing is a prestigious, triennial gathering where world-class scientists present and discuss the most recent advances in the field of human and animal hearing research. The 2015 edition will particularly focus on integrative approaches linking physiological, psychophysical and cognitive aspects of normal and impaired hearing. Like previous editions, the proceedings will contain about 50 chapters ranging from basic to applied research, and of interest to neuroscientists, psychologists, audiologists, engineers, otolaryngologists, and artificial intelligence researchers.

Physiology, Psychoacoustics and Cognition in Normal and Impaired Hearing

otorhinolaryngology; neurosciences; hearin