Modeling and measuring lateral line excitation patterns to changing dipole source locations.

In order to determine excitation patterns to the lateral line system from a nearby 50 Hz oscillating sphere, dipole flow field equations were used to model the spatial distribution of pressures along a linear array of lateral line canal pores. Modeled predictions were then compared to pressure distributions measured for the same dipole source with a miniature hydrophone placed in a small test tank used for neurophysiological experiments. Finally, neural responses from posterior lateral line nerve fibers in the goldfish were measured in the test tank to demonstrate that modeled and measured pressure gradient patterns were encoded by the lateral line periphery. Response patterns to a 50 Hz dipole source that slowly changed location along the length of the fish included (1) peaks and valleys in spike-rate responses corresponding to changes in pressure gradient amplitudes, (2) 180 degrees phase-shifts corresponding to reversals in the direction of the pressure gradient and (3) distance-dependent changes in the locations of peaks, valleys and 180 degrees phase-shifts. Modeled pressure gradient patterns also predict that the number of neural amplitude peaks and phase transitions will vary as a function of neuromast orientation and axis of source oscillation. The faithful way in which the lateral line periphery encodes pressure gradient patterns has implications for how source location and distance might be encoded by excitation patterns in the CNS. Phase-shift information may be important for (1) inhibitory/excitatory sculpting of receptive fields and (2) unambiguously encoding source distance so that increases in source distance are not confused with decreases in source amplitude.