Electroretinogram Characteristics and the Spectral Mechanisms of the Median Ocellus and the Lateral Eye in Limulus polyphemus

Abstract
Electrical responses (ERG) to light flashes of various wavelengths and energies were obtained from the dorsal median ocellus and lateral compound eye of Limulus under dark and chromatic light adaptation. Spectral mechanisms were studied by an analyzing response waveforms, e.g. response area, rise, and fall times as functions of amplitude, slopes of amplitude, energy functions, and spectral sensitivity functions obtained by the criterion amplitude method. The data for a single spectral mechanism in the lateral eye are response waveforms independent of wavelength, same slope for response-energy functions at all wavelengths, a spectral sensitivity function with a single maximum near 520 m[mu], and spectral senstivity invariance in chromatic adaptation experiments. The data of 2 spectral mechanisms in the median ocellus are 2 waveform characteristics depending on wavelength, slopes of response-energy functions steeper for short than for long wavelengths, 2 spectral sensitivity peaks (360-and 530-535 m[mu]) when dark-adapted, and selective depression of either spectral sensitivity peak by appropriate chromatic adaptation. The ocellus is 200-320 times more sensitive to UV than to visible light. Both UV and green spectral sensitivity curves agree with Dartnall''s nomogram. The hypothesis is favored that the ocellus contains 2 visual pigments each in a different type of receptor, rather than various absorption bands of a single visual pigment, single visual pigment and a chromatic mask, or fluorescence. With long duration light stimuli a steady-state level followed the transient peak in the ERG from both types of eyes.