Retinotopic pathways providing motion-selective information to the lobula from peripheral elementary motion-detecting circuits.

Recordings from afferent channels from the medulla supplying deep neuropils of the fly's optic lobes reveal different filter properties among the three classes of afferent neurons: transmedullary cells, T2 neurons, and Y cells. Whereas transmedullary cells respond to local flicker stimuli without discriminating these from directional or oriented motion, the T2 afferent neurons show clear motion orientation selectivity, which corresponds closely with a morphological bias in the orientation of their dendrites and could also be influenced by systems of local recurrent neurons in the medulla. A Y cell having a clearly defined terminal in the lobula, but having dendrite-like processes in the medulla and, possibly, the lobula plate, discriminates the direction of motion and its orientation. These results demonstrate unambiguously that the lobula receives information about motion and that the channels carrying it are distinct from those supplying wide-field motion-selective neurons in the lobula plate. Furthermore, recordings from a newly identified recurrent neuron linking the lobula back to the inner medulla demonstrate that the lobula discriminates nondirectional edge motion from flicker, thereby reflecting a property of this neuropil that is comparable with that of primary visual cortex in cats. The present findings support the proposal that elementary motion detecting circuits supply several parallel channels through the medulla, which segregate to, but are not shared by, the lobula and the lobula plate. The results are discussed in the context of other intracellular recordings from retinotopic neurons and with analogous findings from mammalian visual systems.