An intense blue light stimulus induces a prolonged depolarizing after-potential (PDA) in the peripheral retinula cells, but not in the central retinula cells, of the Drosophila ommatidia, providing the fly has been dark or red adapted and the screening pigments have been genetically removed from the compound eye. Thus, the PDA saturates only the peripheral retinula cells, allowing one to isolate and study extracellularly the summed receptor potentials (SR) of the central retinula cells (Fig. 1). The following lines of evidence support these ideas:
a) The on and off transients, typical of the normal electroretinogram (ERG), are not present in the SR (Fig. 1).
b) The intensity response curve of the intracellularly recorded responses of the peripheral retinula cells fits that of the ERG only after subtraction of the SR contribution from the latter (Fig. 2).
c) The SR action spectrum is different from that of the dark adapted ERG and appears to arise from more than one spectral mechanism (Fig. 3).
