TRANSITION FROM SINGLE TO MULTIPLE FREQUENCY CHANNELS IN THE PROCESSING OF BINAURAL DISPARITY CUES IN THE OWL'S MIDBRAIN.
Interaural level(ILD) and time (ITD)
differences are the primary cues for the barn owl to localize the vertical and
horizontal positions of sound, respectively.
Distinct anatomical pathways in the brainstem process ILD and ITD
separately and reach the external nucleus of the inferior colliculus(ICX). We analysed neuronal selectivity to ILD and
ITD in the ICX and its input nucleus, the lateral shell of the central nucleus
of the inferior colliculus(ICL).
ICL neurons responded to monoaural stimulation of one or both ears. ILD functions were either bell-shaped or monotonic. ICL neurons could be divided into two groups according to their noise ITD curves: one with multiple peaks of identical height(type 1, "phase ambiguous") and the other with a main peak and smaller secondary peaks (typell). All neurons with monotonic ILD function belonged to type 1. The peak width of bell-shaped ILD functions was narrower in type than in type1. In type ll cells, the peak was sharper with noise stimuli than with tone bursts. As reported earlier, ICX neurons were broadly tuned to frequency and responded maximally to noise at a unique ITD. They responded exclusively to binaural stimuli with a range of ILD. Monoaural stimulation evoked no response or inhibited the cells. ILD curves of ICX neurons were thus sharply peaked.
These results suggest that ILD information is first combined with ITD information in type 1 ICL cells and then multiple frequency inputs begin to converge in type ll ICL cells and eliminate phase ambiguity. The results also suggest that convergence of different frequency channels contributes to neuronal selectivity to ILD and explain the behavioral observation that in the vertical direction, barn owls locate noise sounds much better than tones.
(Supported by the Uehara Memorial Foundation and NIH).