ELECTRORETINOGRAM OF FRESH-WATER TURTLE: FORM AND SPECTRAL SENSITIVITY

Abstract

The form and spectral sensitivity of the gross electroretinogram (erg) have been determined in excised, dark-adapted eyes of fresh-water turtles (Emvdidae), animals with predominantly-cone retinas. Electric responses were recorded for 0.2 second flashes at 24 wave lengths between 430 and 750 m[mu], at 7 different levels of intensity covering about 3 log units. At moderate-to-bright intensities, the erg displayed typical a- and b-waves. This confirms the observations of other workers that cones as well as rods produce b-waves. With red stimuli the b-wave possessed two components; with short wave-length stimuli, only one. The a-wave appeared largest with green stimuli. At extremely low intensities, the b-wave was often followed by another positive deflection, apparently an off-effect. Spectral sensitivity was determined at several levels of illumination. At high levels, peak sensitivity lay in the red, at approximately 645 m[mu]. At low levels, it lay somewhere in the blue or green. These two peaks appear to represent the activities of cones and rods. At moderate intensities, the cone sensitivity curve displayed shoulders suggesting the activities of two additional types of cones with peak sensitivities at about 620 mu and 575 m[mu].