Synaptic interactions increase optic flow specificity

Representations of optic flow are encoded in fly tangential neurons by pool ing the signals of many retinotopically organized local motion-sensitive in puts as well as of other tangential cells originating in the ipsi- and cont ralateral half of the brain. In the so called HSE cell, a neuron involved i n optomotor course control, two contralateral input elements, the H1 and H2 cells, mediate distinct EPSPs. These EPSPs frequently elicit spike-like de polarizations in the HSE cell. The synaptic transmission between the H2 and the HSE cell is analysed in detail and shown to be very reliable with resp ect to the amplitude and time-course of the postsynaptic potential. As a co nsequence of its synaptic input, the HSE cell responds best to wide-field m otion, such as that generated on the eyes when the animal turns about its v ertical body axis. It is shown that the specificity of the HSE cell for thi s type of optic flow is much enhanced if rapid membrane depolarizations, su ch as large-amplitude EPSPs or spike-like depolarizations, are taken into a ccount rather than the average membrane potential.