The Optic Lobes

Each ommatidia provides six receptor axons (from the R1-R6 photoreceptors) that terminate in six separate columns (optic cartridges) of the lamina. The R1-R6 photoreceptors in an ommatidium have different optical alignments. However, six R1-R6 photoreceptors distributed amongst six different ommatidia share the same optical alignment. The axons of six optically coherent photorecpetors converge to a single optic cartridge. Each optic cartridge represents a single visual sampling point and gives rise to a set of 5 monopolar cells and the T1 basket cell. The axons of these six neurons from a cartridge project coherently across the first optic chiasma accompanied by the axons of R7 and R8 photoreceptors that share the same optical alignment.

Each group of 8 axons from the lamina and retina comprises the inputs to a medulla column. There are as many columns as there are optic cartridges or ommatidia. Successive chiasmata link the lamina to the outer medulla, and the inner medulla to the lobula, lobula plate. Each neuropil of the lobula complex receives an isomorphic arrangement of retinotopic input neurons. Thus, throughout the optic lobes the arrangement of columns is representitive of the arrangement of ommatidia and, hence, visual sampling points. Dendritic trees of individual neurons can be precisely mapped within these columnar neuropils and related out to specific areas of the eye's visual field.

The lobula plate has provided a rich source of knowledge about motion detecting mechanisms in large flies (e.g. Calliphora and Phaenicia) because of its large tangential neurons that receive inputs from chromatic motion-sensitive interneurons and which are easily accessed by microelectrodes.

The lobula consists mainly of palisades of columnar neurons which, in larger flies, have small receptive fields and are orientation-selective, rather like pyramidal neurons in the cat's visual cortex. These smaller neurons are arranged as retinotopic arrays, each made up of an isomorphic ensemble of one morphological species of columnar cell. In larger flies, about 30 different types of columnar cells have been recognized, and thus 30 ensembles. Each cell type targets an optic glomerulus in the ventro-lateral deutocerebrum. Thus, each glomerulus receives all the axons of a specific ensemble. The dendrites of descending neurons embrace and invade optic glomeruli in a manner reminiscent of output neurons from olfactory glomeruli.

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