Angular sensitivity of blowfly photoreceptors: broadening by artificial electrical coupling

Summary1.Electrical coupling between R1–6 photoreceptors was investigated by measuring angular sensitivities and quantum bumps.2.Recordings were made from two extreme types of cells: Type a: cells with a diffraction-like angular sensitivity profile (see Smakman et al. 1984). Only large bumps could be obtained from these cells. Type b: cells with large, asymmetrical sidebands in the angular sensitivity profile. Large and small bumps could be recorded from these cells, specifically with off-axis illumination.3.The position of the small sidebands of the type a cells depends strongly on wavelength, as expected for a diffraction curve. The position of the large asymmetrical sidebands in the angular sensitivity profile of the type b cells was found to be independent of wavelength, indicating that these sidebands are not caused by the diffraction pattern of the facet lens.4.The angular position of the large asymmetrical sidebands corresponds with the position of neighbouring photoreceptors, suggesting electrical interactions between R1–6 photoreceptors.5.The dependence of this electrical coupling on microelectrode properties was investigated. It was possible to change the degree of electrical coupling by selection of electrodes.6.The difference in properties of the two cell types encountered are interpreted as an indication that some R1–6 photoreceptors are artificially electrically coupled while others are not. The correlation of the electrical coupling with electrode types and the possible artificial origins of coupling are discussed.