Frequency coding of particle motion by saccular afferents of a teleost fish.

The saccule is known to play an important role in hearing in fishes. In this study we investigated spatial frequency selectivity of single saccular afferents in a teleost fish (the sleeper goby, Dormitator latifrons) to acoustic particle motion at 50-400 Hz. Saccular afferents have similar distributions of best sensitivity (-90.0 to -54.8 dB re. 1 g, mean +/- s.d.=-81.1+/-8.0 dB) and characteristic frequencies (<or=50-400 Hz, median=80 Hz) along the longitudinal, side-to-side and dorsoventral axes of fish. They were lowpass, bandpass or broadly tuned to low frequencies with Q(50%) at 15 dB above threshold in ranges from 0.28 to 3.30 (1.46+/-0.71), 0.18 to 2.54 (1.36+/-0.78), and 0.41 to 4.26 (2.25+/-0.1.12) along the three axes, showing slightly greater frequency tuning in the vertical axis than horizontal axes. At supra threshold we found tuning plasticity, i.e. best frequencies of saccular afferents shifted to high frequencies as stimulus level increased. Isolevel rate-frequency curves were asymmetrical with shallow slopes at the low-frequency edge and steep slopes at the high-frequency edge. Saccular afferents of the sleeper goby have similar capabilities of coding particle motion frequencies in the three orthogonal axes. Results from this and other studies suggest that (1) the saccule is the major hearing organ in this species, (2) the saccule of this species is capable of encoding sound frequencies in three dimensional space, and (3) saccular afferents in fishes without accessory auditory structures exhibit similar frequency selectivity in response to particle motion.