DENDRITIC COMPUTATION OF DIRECTION SELECTIVITY AND GAIN-CONTROL IN VISUAL INTERNEURONS

The extraction of motion information from time varying retinal images is a fundamental task of visual systems. Accordingly, neurons that sel ectively respond to visual motion are found in almost all species inve stigated so far. Despite its general importance, the cellular mechanis ms underlying direction selectivity are not yet understood in most sys tems. Blocking inhibitory input to fly visual interneurons by picrotox inin (PTX), we demonstrate that their direction selectivity arises lar gely from interactions between postsynaptic signals elicited by excita tory and inhibitory input elements, which are themselves only weakly t uned to opposite directions of motion. Their joint activation by prefe rred as well as null direction motion leads to a mixed reversal potent ial at which the postsynaptic response settles for large field stimuli . Assuming the activation ratio of these opponent inputs to be a funct ion of pattern velocity can explain how the postsynaptic membrane pote ntial saturates with increasing pattern size at different levels for d ifferent pattern velocities (''gain control''). Accordingly, we find t hat after blocking the inhibitory input by PTX, gain control is abolis hed.