Interaction of Sound Pressure and Particle Acceleration in the Excitation of the Lateral-Line Neuromasts of Sprats

In clupeids a pressure-sensitive inner ear is hydrodynamically connected to the lateral-line and both sound pressures and particle accelerations stimulate the receptors of the lateral-line neuromasts. Experiments on sprats were done using transients to confirm and extend quantitative information about these stimuli and their interaction with each other. Recordings from nerves from neuromasts were made and stimuli needed to evoke constant responses compared. It was shown that there were responses to both directions of water velocity inside the canal; that the most sensitive range of frequency of the receptors with respect to the velocity of water in the canal was between 20 and 100 Hz; that the ratio between the sensitivity to outside pressure changes and the sensitivity to acceleration in the outside medium was approximately as predicted by earlier mechanical observations; that the change of these sensitivity ratios at different positions in the infraorbital canal was as predicted and that the responses to simultaneous pressure and acceleration transients were synchronous. The velocities expected at each neuromast in the infraorbital canal of a sprat have been calculated for various positions of a fish around a vibrating source with characteristics approximating to the tail of another sprat. Significant changes in the relative excitation of different neuromasts, as a result of the mixing of pressure-induced and acceleration-induced velocities, occur in two situations: these are changes of distance and changes of the rate of change of the acceleration of the source surface. Possible biological advantages, including the ranging of sources, of mixing inputs from the pressure and acceleration components of a sound field are discussed.