Photoreceptor visual fields, ommatidial array, and receptor axon projections in the polarisation-sensitive dorsal rim area of the cricket compound eye

We made intracellular recordings from the photoreceptors of the polarisatio n-sensitive dorsal rim area of the cricket compound eye combined with dye m arking. By measuring visual field sizes and optical axes in different parts of the dorsal rim area, we assessed the optical properties of the ommatidi a. Due to the large angular sensitivities (median about 20 degrees) and the high sampling frequency (about 1 per degree), the visual fields overlap ex tensively, such that a given portion of the sky is viewed simultaneously by a large number of ommatidia. By comparing the dye markings in the retina a nd in the optic lobe, the axon projections of the retinula cells were exami ned. Receptors R1, R2, R5 and R6 project to the lamina, whereas R7 projects to the medulla. The microvilli orientation of the two projection types dif fer by 90 degrees indicating the two analyser channels that give antagonist ic input to polarisation-sensitive interneurons. Using the retinal marking pattern as an indicator for the quality of the intracellular recordings, th e polarisation sensitivity of the photoreceptors was re-examined. The polar isation sensitivity of recordings from dye-coupled cells was much lower (me dian: 4.5) than that of recordings in which only one cell was marked (media n: 9.8), indicating that artefactual electrical coupling between photorecep tors can significantly deteriorate polarisation sensitivity. The physiologi cal value of polarisation sensitivity in the cricket dorsal rim area is thu s typically about 10.