Retinula cell responses in a moth superposition eye

Abstract

The lack of photoreceptor cell movement upon stimulation in a group of moths made possible the direct electrophysiological measurement of the photoreceptor properties in a moth superposition eye. The 13–16 retinula cells of the ommatidium are usually coupled in twos or threes. There are strong antagonistic electrical interactions at the receptor level. The measured acceptance angles (Δ ρ ) of the best units are in the range 1.6-1.8°. Spectral sensitivity peaks lie near 380 and 520 nm. Some units have positive-going responses to some colours and negative-going responses to others. Polarization sensitivity is usually low, but is high if tested at a suitable wavelength in a unit showing colour opponency. The dynamic range (10-90%, length of slope) of the V /lg I curve is over 100-fold intensity range for purely ultraviolet or green-sensitive units; over 10 4 -fold for some units with responses in opposite directions to different wavelengths, and over a 10 6 -fold intensity range for units with negative-going responses at all wavelengths. The wide ranges are attributed to strong antagonistic interactons between receptors. Despite the superposition optics, the retinula cells are approximately the same sensitivity (in terms of millivolts per peak axial photon) as those of a diurnal apposition eye under the same conditions. The electrical interaction between different receptors has the effect of reducing the responses to light that is unpolarized, white, diffuse, or simultaneously applied, and of emphasizing responses to light that is polarized, coloured, localized or successively applied. Tests with a movable point source and a surround reveal several effects of the opponency between receptors by a single mechanism. Edges are emphasized by spatial interaction and differences between colours are emphasized by interaction within ommatidia. On a green background, small objects of a different colour will stand out. With certain wavelength combinations, the temporal properties are enhanced. The adaptive advantage of the diurnal superposition eye, from these measurements, is that it provides a higher intensity at the photoreceptors that in turn makes possible a greater degree of inhibitory spatial and temporal interaction at the primary photoreceptor level.