Learned Discrimination of Pattern Orientation in Walking Flies

To determine the pattern-orientation discrimination ability of blowflies, Phaenicia sericata, a learning/memory assay was developed in which sucrose served as the reward stimulus and was paired with one of two visual gratings of different orientations. Individual, freely walking flies with clipped wings were trained to discriminate between pairs of visual patterns presented in the vertical plane. During training trials, individual flies learned to search preferentially at the rewarded stimulus. In subsequent testing trials, flies continued to exhibit a learned preference for the previously rewarded stimulus, demonstrating an ability to discriminate between the two visual cues. Flies learned to discriminate between horizontal and vertical gratings, +45 degrees (relative to a 0 degrees vertical) and -45 degrees gratings, and vertical and +5 degrees gratings. Individual patterns of learning and locomotive behavior were observed in the pattern of exploration during training trials. The features of the visual cue critical for discrimination of orientation are discussed.