Spatially-invariant discrimination of visual pattern orientationin the blow fly Phaenicia sericata

Despite a wealth of information regarding visual processing in flies, little is known regarding their pattern discrimination abilities and the neural correlates of pattern recognition. The work presented here provides evidence of learned visual discriminations of complex visual patterns in the blowfly, Phaenicia sericata. A learning and memory assay was developed to determine the pattern-orientation discrimination ability of blowflies. Individual flies were trained to discriminate between pairs of visual patterns that differed in the orientation of their composite lines. During training and subsequent testing trials flies exhibited a preference for the previously-rewarded visual stimulus. Flies learned to discriminate between horizontal and vertical gratings, +45° and -45° gratings, and vertical and +5° gratings. Among four possible underlying mechanisms used to discriminate between these sets of patterns, hypothetical orientation-selective neurons are uniquely capable of discriminating between all sets of discriminated patterns. One previous theory of insect vision suggests that in order for an insect to recognize previously learned visual images, the insect must align the current retinal input with the region of the retina with which the pattern was first viewed and learned (Wehner, 1981; Dill et al., 1993). To address this theory of retinotopic matching, a detailed analysis was made of fly behavior during the discrimination process. Both the spatial structure of the approaches to the visual cues and the range of body orientations used by individual flies demonstrated that Phaenicia approaches the visual cue from an unique vantage point from trial to trial and, therefore, retinotopic matching is not necessary for the recognition of pattern orientation by blowflies. To test for the existence of orientation-selective neurons in Phaenicia, the lobula neuropil was impaled for intracellular recording. Two neurons exhibited directional motion preferences indicative of a possible role in the detection of expanding, oblique edges during approach to visual cues. Two additional neurons with combined responses to both directional motion and the orientation of the stimulus were recorded. These data support the hypothesis that the dipteran lobula processes information regarding the orientation of visual stimuli