Directional response properties of saccular afferents of the toadfish, Opsanus tau

The displacement sensitivity, frequency response, and directional response properties of primary saccular afferents of toadfish (Opsanus tau) were studied in response to a simulation of acoustic particle motion for which displacement magnitudes and directions were manipulated in azimuth and elevation. Stimuli were 50, 100, and 200 Hz sinusoidal, translatory oscillations of the animal at various axes in the horizontal and midsagittal planes. Thresholds in these planes defined a cell's characteristic axis (the axis having the lowest threshold) in spherical coordinates. Recordings were made from afferents in rostral, middle, and caudal bundles of the saccular nerve. The most sensitive saccular afferents responded with a phase-locked response to displacements as small as 0.1 nm. This sensitivity rivals that of the mammalian cochlea and is probably common to the sacculi and other otolith organs of most fishes. Most afferents showed lower thresholds at 100 Hz than at 50 or 200 Hz. Eighty percent of afferents have three-dimensional directional properties that would be expected if they innervated a group of hair cells having the same directional orientation on the saccular epithelium. Of the afferents that are not perfectly directional, most appear to innervate just two groups of hair cells having different orientations. The directional characteristics of afferents are qualitatively correlated with anatomically defined patterns of hair cell orientation on the saccule. In general, azimuths of best sensitivity tend to lie parallel to the plane of the otolith and sensory epithelium. Elevations of best sensitivity correspond well with hair cell orientation patterns in different regions of the saccular epithelium. Directional hearing in the horizontal plane probably depends upon the processing of interaural differences in overall response magnitude. These response differences arise from the gross orientations of the sacculi and are represented, in part, as time differences among nonspontaneous afferents that show level-dependent phase angles of synchronization. Directional hearing in the vertical plane may be derived from the processing of across-afferent profiles of activity within each saccule. Fishes were probably the first vertebrates to solve problems in sound source localization, and we suggest that their solutions formed a model for those of their terrestrial inheritors.