Fast-scale adaptive changes of directional tuning in fly tangential cells are explained by a static nonlinearity

The response of vertebrate motion-sensitive neurons to a directional stimulus is affected by the direction of the stimulus that immediately preceded it. These nonlinear effects are also observed for orientation tuning and are typically interpreted as fast-scale adaptive changes. We verified that similar effects are observed for spiking tangential cells in the fly lobula plate. We also investigated the spatial selectivity of these effects by presenting multiple patches at different positions within the receptive field, and found that the effects are strictly local. We modelled the data using elementary operators (linear filters and threshold nonlinearities). A satisfactory account of the results is obtained when an early static nonlinearity acts on the outputs of multiple front-end filters that are subsequently pooled in a spatially restricted manner by the tangential cell. In line with recent studies, these findings emphasize the importance of testing simple nonlinear models before attempting more elaborate interpretations of fast-scale adaptive phenomena in single neurons. We discuss a potential neural implementation of the model based on medullar projections to the lobula plate.