Binaural interaction in the medulla and midbrain of toadfish (Opsanus tau)

Abstract

Responses of cells in the toadfish medulla (descending octaval nucleus: DON) and midbrain (torus semicircularis: TS) were studied to investigate binaural interaction and processing. Normally, the two ears of fish cannot be stimulated independently. A method was developed to temporarily inactivate one ear by slightly displacing the saccular otolith on one side (tipping) to change its orientation in space and, therefore, alter the responsiveness of the hair cells. Brain cells were evaluated for directional characteristics and frequency response (1) before otolith tipping, (2) with the otolith tipped, and (3) post-tipping. For DON cells (n=14), contralateral saccular otolith tipping most often resulted in subtle effects consisting of an overall change in responsiveness (±spikes/sec); significant changes in the preferred direction were rare. In the TS (n=20), most cells exhibited changes in responsiveness and in directionality. These experiments demonstrate the existence of excitatory and inhibitory binaural interactions in the medulla and midbrain and show rather complex and unexpected binaural effects on the responsiveness and directional properties of auditory brain cells. [Work supported by the NIH/NIDCD.]