ELECTRORETINOGRAM OF FRESH-WATER TURTLE: QUANTITATIVE RESPONSES TO COLOR SHIFT

Abstract

In the excised, dark-adapted turtle eye, a sudden shift in illumination from one color to another elicits an electric response, representing the off-effect of one receptor and the on-effect of another. This shift response was studied quantitatively in relation to the difference in wave length between two stimulating beams. The fixed color was provided by one of six interference filters (wave length maximum, 512, 538, 570, 637, 647, 672 m[upsilon]), and the variable color by a monochromator. Intensities of the two beams were adjusted to make the on-effects equal and of constant size through each series of tests across the spectral range 510-675 m[upsilon]. The difference in wave length between the two beams was progressively increased in small steps. The average relative size of the shift response was plotted against wave length. The response increased in a step-wise fashion as the wave lengths diverged. The plateaus were approximately from 675 to 640 m[mu] from 625 to 600 m[mu], from 580 to 550 m[mu] and from 520 to 510 m[mu]. These results agree with those previously reported, based on spectral sensitivity curves (Jour. Neurophysiol., 1958, 21: 45-61) in suggesting that the turtle retina has 3 types of receptors in the cone system with peak sensitivities in the red, orange and yellow-green, and a few rods with peak sensitivity in the blue-green.