Directional asymmetry in responses of local interneurons in the crayfish deutocerebrum to hydrodynamic stimulation of the lateral antennular flagellum.

We have recorded spiking responses from single, bimodally sensitive local interneurons (Type I) in the crayfish deutocerebrum to hydrodynamic and odorant stimuli flowing in two directions past the lateral antennular flagellum. Changing the direction of seamless introductions (meaning, with minimal variations of fluid velocity magnitude) of odorant flow past the flagellum, from proximal-->distal to distal-->proximal, did not consistently affect the dose-dependent responses of Type I neurons. By contrast, changing the direction of an abruptly initiated flow of water (or odorant) past the flagellum resulted in consistently larger numbers of spikes in response to this hydrodynamic stimulation when the flow direction was proximal-->distal. This response asymmetry is discussed in relation to its possible relevance regarding antennular flicking behavior. The putative involvement of flagellar hydrodynamic receptors, the beaked hairs, and the hydrodynamic flow asymmetries they are exposed to, are examined theoretically in the accompanying paper.