Diverse speed response properties of motion sensitive neurons in the fly's optic lobe.

Speed and acceleration are fundamental components of visual motion that animals can use to interpret the world. Behavioral studies have established that insects discriminate speed largely independently of contrast and spatial frequency, and physiological recordings suggest that a subset of premotor descending neurons is in this sense speed-selective. Neural substrates and mechanisms of speed selectivity in insects, however, are unknown. Using blow flies Phaenicia sericata, intracellular recordings and dye-fills were obtained from medulla and lobula complex neurons which, though not necessarily speed-selective themselves, are positioned to participate in circuits that produce speed-selectivity in descending neurons. Stimulation with sinusoidally varied grating motion (0-200 degrees /s) provided a range of instantaneous velocities and accelerations. The resulting speed response profiles are indicative of four distinct speed ranges, supporting the hypothesis that the spatiotemporal tuning of mid-level neurons contains sufficient diversity to account for the emergence of speed selectivity at the descending neuron level. This type of mechanism has been proposed to explain speed discrimination in both insects and mammals, but has seemed less likely for insects due to possible constraints on small brains. Two additional recordings are suggestive of acceleration-selectivity, a potentially useful visual capability that is of uncertain functional significance for arthropods.