Representation of binaural masking level difference in the inferior colliculus of the barn owl (tyto alba)
Auditory detection and localization is important for animals and humans and crucial for barn owls. In natural environments, the spatial separation of signal and interfering noise improves detectability of the signal. In psychoacoustics experiments this is achieved by changing the spatial attributes of the sound, for example by shifting the phase of the tone or the noise in one ear. The improvement resulting from this shift was called “Binaural Masking Level Difference“ (BMLD). I recorded the activity of neurons in the central and external nuclei of the inferior colliculus (ICC and ICX, respectively) of the barn owl. These neurons are sensitive to a variation of the interaural delay. I tested three masking configurations: 1) the noise N and the tone S were set at the best interaural delays (BD), resulting in the NBDSBD configuration; 2) the noise was set at the best interaural delay but the tone was set at the best interaural delay with 180° phase shift, or the worst interaural delay (WD), resulting in the NBDSWD configuration, 3) the noise was set at the best interaural delay with 180° phase shift and the tone was set at the best delay (NWDSBD configuration). The noise level was held constant while the tone level was varied. The detection threshold was determined using the index of discrimination according to signal detection theory. BMLD is restricted to frequencies that allows conservation of time by phase locking (<3 kHz in mammals). In the owl phase locking is observed up to 9 kHz. Therefore, I expected BMLDs at higher frequencies than 3 kHz. In the first part of the study, I show exactly this. Due to distinct neural stations in the owl inferior colliculus, it was possible to address the hierarchy of detection and localization at the neuronal level. The ICC neurons are narrowly tuned to frequency and had lower detection thresholds than the ICX neurons. Since ICC neurons responded maximally to more than one ITD in their ITD function, they could not signal a specific position in the space. Integration of many frequency channels from ICC neurons enables the ICX neurons to represent a specific position in space. I propose that there is a division of labor between ICC and ICX: detection happens at the level of the ICC, while correlates of localization are available at the level of ICX. This is consistent with psychophysical studies. In psychophysics, the inversion of tone or noise results in unmasking. I studied both unmasking scenarios. I measured responses of IC neurons to masking scenarios with a phase shift in tone and analyzed the firing rate of neurons to the masking stimuli, while in another experiment I shifted the phase of the noise. Inversion of tone resulted in a mean improvement of about 12 dB in detection threshold in ICC neurons . Inversion of noise led to 10 dB BMLD in these neurons. The BMLDs in ICX neurons were larger than those in ICC neurons. The results from the ICC neurons are comparable to psychophysical observations in humans. I also tested the effects of the noise level on BMLDs produced by tone or noise inversion. An increase in the noise level resulted in an increase in the magnitude of the BMLD. This result is again comparable to psychophysical observations in humans. The contribution of tone and noise in driving the neurons differs at different signal to noise ratios and is different at different masking configurations. At low tone levels the neurons were driven only by the noise while at high tone levels neurons were driven mainly by the tone, indicating occlusion. The contribution of the noise in driving the neurons was larger at higher noise levels. In the NBDSBD configuration the masking is the result of an excitatory interaction between noise and tone, while in NBDSWD and NWDSBD the interaction is inhibitory.During the analysis I observed changes in spike pattern (especially spike latency) by changing the tone level in masking stimuli. According to the information theory, I found that response onset transmits more information than the spike count.
info:eu-repo/classification/ddc/590, Tiere (Zoologie), Hearing, owl, BMLD, Binaural hearing, localization, detection
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