Motion sensitive interneurons in the optomotor system of the fly

The three horizontal cells of the lobula plate of the blowflyCalliphora erythrocephala were studied anatomically and physiologically by means of cobalt impregnations and intracellular recordings combined with Procion and Lucifer Yellow injections. The cells are termed north, equatorial and south horizontal cell (HSN, HSE, HSS) and are major output neurons of the optic lobe. 1. The dendritic arborizations of the HSN, HSE, HSS reside in a thin anterior layer of the lobula plate and extend over the dorsal, equatorial and ventral parts of this neuropil, respectively. Due to the retinotopic organization of the optic lobe, these parts correspond anatomically to respective regions of the ipsilateral visual field. Homologue horizontal cells in both lobula plates of the same animal and in different animals are highly variable with respect to their individual dendritic branching patterns. They are extraordinarily constant, on the other hand, with regard to the position and size of their dendritic fields as well as their dendritic branching density distributions. Each cell covers about 40% of the total area of the lobula plate and shows the highest dendritic density near the lateral margin of the neuropil which subserves the frontal eye region. The axons of the horizontal cells are relatively short and large in diameter; they terminate in the posterior ventrolateral protocerebrum. 2. The horizontal cells are directionally selective motion sensitive visual interneurons responding preferentially to progressive (front to back) motion in the ipsilateral visual field with graded depolarization of their axons and superimposed action potentials. Stimulation with motion in the reverse direction leads to hyperpolarizing graded responses. The HSE and HSN are additionally activated by regressive motion in the contralateral visual field.