Receptive field patterns in photoreceptors of the fly

Abstract

1. The shape of the receptive field in the photoreceptors of the flesh fly,Boettcherisca peregrina, compound eye was contoured by using the intracellular responses to illumination from different positions around the compound eye. 2. The shapes of these contours were classified into six patterns. Type A was roughly round and small, corresponding to the normal receptive field. Type B was similar to type A in shape but was larger. Type C was large and stellate in shape with spokes radiating along thex, y andz axes of the alignment of the onimatidia. This type was classified into three sub-groups: type C1 with spokes along all of thex, y andz axes; type C2 with spokes along thex andy axes; and type C3 with spokes along thex andy axes except in an upward direction. Types D, E and F were very long and extended in one direction. The direction of extension was horizontal, oblique and vertical in types D, E and F, respectively. Type E was classified into two sub-groups, E1 and E2, based on the direction of the oblique extension. One or more areas with high sensitivity (subpeaks) at the tip of the extending field in these large fields were found. 3. Type A was most frequent, followed by type B. Types D, E and F were more common than type C. 4. In contrast to the spectral sensitivity determined by axial illumination, the second peaks of the blue range were found to be diminished when measured by non-axial illumination, even if their positions were in the peripheral area or the sub-peaks. 5. The size of the acceptance angle was smaller in types A and C than in the other types. The angle in type A was smallest at the antero-medial region of the compound eye and became larger with increasing distance laterally from the antero-medial region. 6. The different receptive field types were not evenly distributed throughout the eye. The cells of types A and B were found mainly in the antero-medial region but were scattered over the compound eye. The cells of type C were limited to a small area in the antero-medial region.The cells of types D, E and F were distributed throughout the lateral region of the compound eye. 7. The receptive fields in light- and dark-adapted eyes showed no difference in their patterns, although the acceptance angle of each peak within the receptive field became slightly narrow under light-adaptation. This result may suggest that migration of the screening pigment is negligible. 8. No facilitatory or inhibitory interactions appeared between the responses elicited by illuminations from two different positions within the wide receptive field. The responses were summative. 9. Latencies of the responses were the same under axial or non-axial illumination in type A cells with the usual, small receptive field. In contrast, the responses of cells with wide receptive fields showed step-like prolongation of the latency with increasing distance of the illuminating position from on-axis. The phase of the polarization sensitivitiy curves shifted when the receptive field measured under illumination from each position corresponding to the occurrence of the step-like prolongation of the latency indicating that the wide receptive field is composed of inputs from different photoreceptors. 10. The present study suggests that the origin of the wide receptive field in the photoreceptor is neuronal.