Directional encoding by fish auditory systems

This paper reviews and discusses several investigations of the peripheral n eural code for the directional axis of acoustical particle motion in the sa ccule of two fishes: goldfish (Carassius auratus) and toadfish (Opsanus tau ). Most saccular afferents are directional in the manner of hair cells, hav ing a cosine-shaped directional response pattern. The saccular sensory epit helia are orientated almost vertically in a parasagittal plane. In the hori zontal plane, these epithelia are orientated obliquely with respect to the midline. Hair-cell stercocilia project perpendicularly Thus, directional re sponse patterns of saccular afferents tend to be orientated in azimuth para llel to the orientation of the epithelia in the head. The oblique angle of the toadfish saccule is greater than that of the goldfish, and the range of best directions in the horizontal plane for each species reflects those di ffering orientations. The azimuth of acoustical particle motion could be co mputed by comparing the relative activation of the two saccules, as is the case for the ears of most terrestrial vertebrates. The spatial patterns of saccular hair-cell orientation of most fishes thus appear to have little fu nction in azimuthal source location, lout for toadfish are probably most im portant for determining the elevation of monopole sources.