MECHANISMS OF DENDRITIC INTEGRATION UNDERLYING GAIN-CONTROL IN FLY MOTION-SENSITIVE INTERNEURONS

In the compensatory optomotor response of the fly the interesting phen omenon of gain control has been observed by Reichardt and colleagues ( Reichardt et al., 1983): The amplitude of the response tends to satura te with increasing stimulus size, but different saturation plateaus ar e assumed with different velocities at which the stimulus is moving. T his characteristic can already be found in the motion-sensitive large field neurons of the fly optic lobes that play a role in mediating thi s behavioral response (Hausen, 1982; Reichardt et al., 1983; Egelhaaf, 1985; Haag et al., 1992). To account for gain control a model was pro posed involving shunting inhibition of these cells by another cell, th e so-called pool cell (Reichardt et al., 1983), both cells sharing com mon input from an array of local motion detectors. This article descri bes an alternative model which only requires dendritic integration of the output signals of two types of local motion detectors with opposit e polarity. The explanation of gain control relies on recent findings that these input elements are not perfectly directionally selective an d that their direction selectivity is a function of pattern velocity. As a consequence, the resulting postsynaptic potential in the dendrite of the integrating cell saturates with increasing pattern size at a l evel between the excitatory and inhibitory reversal potentials. The ex act value of saturation is then set by the activation ratio of excitat ory and inhibitory input elements which in turn is a function of other stimulus parameters such as pattern velocity. Thus, the apparently co mplex phenomenon of gain control can be simply explained by the biophy sics of dendritic integration in conjunction with the properties of th e motion-sensitive input elements.