Responses to mechanically and visually cued water waves in the nervous system of the medicinal leech.

Sensitivity to water waves is a key modality by which aquatic predators can detect and localize their prey. For one such predator - the medicinal leech, Hirudo verbana - behavioral responses to visual and mechanical cues from water waves are well documented. Here, we quantitatively characterized the response patterns of a multisensory interneuron, the S cell, to mechanically and visually cued water waves. As a function of frequency, the response profile of the S cell replicated key features of the behavioral prey localization profile in both visual and mechanical modalities. In terms of overall firing rate, the S cell response was not direction selective, and although the direction of spike propagation within the S cell system did follow the direction of wave propagation under certain circumstances, it is unlikely that downstream neuronal targets can use this information. Accordingly, we propose a role for the S cell in the detection of waves but not in the localization of their source. We demonstrated that neither the head brain nor the tail brain are required for the S cell to respond to visually cued water waves.