Transfer of visual motion information via graded synapses operates linearly in the natural activity range

Synaptic transmission between a graded potential neuron and a spiking neuro n was investigated in vivo using sensory stimulation instead of artificial excitation of the presynaptic neuron. During visual motion stimulation, ind ividual presynaptic and postsynaptic neurons in the brain of the fly were e lectrophysiologically recorded together with concentration changes of presy naptic calcium (Delta [Ca2+](pre)). Preferred-direction motion leads to dep olarization of the presynaptic neuron. It also produces pronounced increase s in [Ca2+](pre) and the postsynaptic spike rate. Motion in the opposite di rection was associated with hyperpolarization of the presynaptic cell but o nly a weak reduction in [Ca2+](pre) and the postsynaptic spike rate. Apart from this rectification, the relationships between presynaptic depolarizati ons, Delta [Ca2+](pre), and postsynaptic spike rates are, on average, linea r over the entire range of activity levels that can be elicited by sensory stimulation. Thus, the inevitably limited range in which the gain of overal l synaptic signal transfer is constant appears to be adjusted to sensory in put strengths.