Lateral line nerve fibers do not code bulk water flow direction in turbulent flow.

The discharges of anterior and posterior lateral line nerve afferents were recorded while stimulating goldfish, Carassius auratus, with bulk water flow. With increasing flow velocity lateral line afferents increased their discharge rates. However, an increased response to flow rates occurred even if flow direction was reversed. Thus, individual lateral line afferents did not encode the direction of running water. Frequency spectra of the water motions quantified with particle image velocimetry revealed flow fluctuations that increased with increasing flow velocity. Maximal spectral amplitudes of the flow fluctuations were below 5 Hz (bulk flow velocity 4-15 cms(-1)). The frequency spectra of the firing rates of lateral line afferents also showed an increase in amplitude when fish were exposed to running water. The maximal spectral amplitudes of the recorded data were in the frequency range 3-8 Hz. This suggests that the lateral line afferents mainly responded to the higher frequency fluctuations that developed under flow conditions, but not to the direct current flow or the lower frequency fluctuations. Although individual lateral line afferents encoded neither flow velocity nor flow direction we suggest that higher order lateral line neurons can do so by monitoring flow fluctuations as they move across the surface of the fish.