Periodic structures and structural gradients in the visual ganglia of the fly

The orderly projection of the visual space onto the visual ganglia and of the planes of the four visual ganglia onto each other is briefly reviewed. The variation of the size of some of the elements in the eye and of the ganglia is then described. This variation follows different gradients for different elements. The gradient of the size of the lenses in the cornea and that of the thickness of the L3 fiber in the lamina have a similar shape, with a maximum near the anterior border of the eye. The thickness of L1 and L2 follow a different rule. A correlation of these gradients with some variations of the efficiency of visual stimuli in different regions of the visual field leads to a tentative proposal for the role of the laminar neurons L1, L2 and L3 in perception.