Photoresponses from Cells in the Fly’s Eye which are not Visual Cells

It is shown that light elicits distinct responses from cells in the fly’s eye which are different from photoreceptors. These cells are designated as “slow cells” because of their sluggish response characteristics. Like the visual cells the slow cells depolarize upon light stimulation, but the time course of their responses is clearly different from that of the photoreceptors. Furthermore, the intensity necessary to evoke a given response amplitude is considerably higher in slow cells than in photoreceptors. Several lines of evidence indicate that the slow cell response is caused by light absorption through the visual pigment rhodopsin in the peripheral photoreceptors R l —6. The slow cells are electrically coupled among each other, as demonstrated by application of local light stimuli and injection of the fluorescent dye Lucifer Yellow . The identity of the slow cells and the mechanism of response generation are discussed.