Role of acoustic striae in hearing: reflexive responses to elevated sound-sources

This report is the fourth in a series describing the results of ablation-behavior experiments directed to the ascending output of the cochlear nuclei as it is conducted centrally within the acoustic striae. This fourth report focuses on the unique physiology of the fusiform or 'output' cells of the dorsal cochlear nucleus whose axons course through the dorsal acoustic stria (DAS). Because electrophysiological studies have shown that the cues for sensing the elevation of a sound source would seem to be best analyzed by the dorsal cochlear nucleus and projected centrally via its DAS, we tested normal cats and cats deprived of DAS for their ability to orient to elevated sources of broad-band noise. For behavioral testing, we made use of reflexive or unconditioned orienting responses to elevated sound sources using a similar method to one we have used previously for azimuth testing (Thompson GC, Masterton RB. Brainstem auditory pathways involved in reflexive head orientation to sound. J Neurophysiol 1978;41:1183-1202). The results show that cats deprived of their DAS do indeed have a marked deficit in their ability to orient to an elevated sound source. Further behavioral testing indicated that this deficit is not the secondary result of an attentional or peripheral motor deficit. Although the present results do not prove that the reflexive deficit is strictly auditory in nature, the deficit is notable in that it is the only one yet known to result from a lesion of the dorsal cochlear nucleus or its central projections.