Directional variant and invariant hearing thresholds in the rainbow trout (Salmo Gairdneri)

The gas-filled bladder, the fish middle-ear analogue, transforms sound pressure (p) into pulsations, which generate displacement waves with body-fixed orientation that stimulate the nearby labyrinths. Moreover, in addition to this indirect stimulation, the otolith organs are stimulated directly by the particle motion vector (v) in line with the source. For pure tones the vector sum of both stimulations results in elliptical displacement orbits, which are nearly identical for left and right ears. We hypothesize that these orbits are centrally analysed, either by breakdown into the original, direct and indirect components, the segregation hypothesis, or by analysis of their characteristic form and orientation, the orbit hypothesis. This provides fish with spatial hearing. Analysing the orbit form and orientation would constitute an essentially monaural mechanism of directional hearing that should show strong and characteristic azimuthal anisotropies. To investigate these anisotropies we have studied the rainbow trout Salmo gairdneri psychophysically with respect to directional-detection and directional-masking paradigms involving heart rate conditioning. We employed superpositioning of standing waves in a large tank for total control of a probe signal and a narrow-band noise masker With a sinusoidal p and v probe and a p and v (omnidirectional) noise masker, or with a pure v probe and a (‘uni’directional) p noise masker, trout proved to be equally sensitive to all directions. Only when the indirect stimulation was mimicked by a v stimulus, were we able to demonstrate anisotropy. This indicates that trout indeed use the orbit lengths in the detection and masking tasks. The results can be explained if binaural convergence plays a role, under the assumption that left and right orbits differ appreciably. This might be the case if the divergence in orientation between the indirect stimulations reaching the right and left labyrinths were much larger than would be expected from the geometrical relationships between the two labyrinths and the swimbladder