The Duplex Nature of the Retina of the Nocturnal Gecko as Reflected in the Electroretinogram

Abstract

A b s t r a c t The effect of light and dark adaptation on the electrical activity in two species of nocturnal gecko, Hemidactylus turcicus and Tarentola mauritanica was studied. The electroretinogram of both species changes from the scotopic type in the dark-adapted state to the photopic type after strong light adaptation. For the scotopic response fusion frequencies up to 18 flashes per see. are obtained in both species. For the photopic response fusion frequencies up to 5 ° flashes per see. are seen in Tarentola, and up to 25 flashes per see. in Hemidactylus.Proceeding from dark to light adaptation the increment threshold (dI) is measured at different levels of adaptive illumination (I). At low levels of illumination the dI/I ratio is found to be small and at high levels of illumination to be large. No difference in the dI/I ratio is obtained for test lights of 462 and 605 m#. During dark adaptation the change of threshold after exposure to moderate and weak lights (up to 108 times dark threshold) is rather fast. After light adaptation to strong light (106 times dark threshold) duplex dark adaptation curves are seen with a break separating a fast and a slow phase of dark adaptation. The significance of these results from a retina which possesses sense cells of only one type is discussed.

The Visual Cells and Visual Pigments of the Vertebrate Eye

“…It is of more than passing interest to note that the spectral mechanisms are apparently different for the parietal and lateral eyes of lizards. HAMASAKI (1969) noted such a difference for the green iguana and a difference was previously found for Lacerta sicula campestris (DODT andSCHERER, 1967, 1968).…”

“…It is unfortunate that CROZIER and WOLF did not justify, with histological evidence, the assertion of a pure-rod retina in their gecko, or else use a morc genuinely nocturnal genera of geckos. This criticism does not apply to the work of DODT and JESSEN (1961) who employed Hemidactylus turcicus and Tarentola mauritanica. The visual cells of these geckos were studied by TANSLEY (1959) who found typical gecko rods with large outer segments, paraboloids, no oildroplets and the type A singles, type B doubles and type C doubles.…”

Intracellular recordings from gecko photoreceptors during light and dark adaptation.

“…All the other species thus far studied, with the exception of the gecko, have cones as well as rods, and above rod saturation cones can mediate visual responsiveness. In the gecko, the smaller rod (see Methods) may have conelike adaptation properties and may function at light levels above saturation of the larger rod (Dodt and Jessen, 1961 pass ranges of higher light intensity (Boynton and Whitten, 1970;Dowling and Ripps, 1972;Normann and Werblin, 1974). Although it has recently been proposed that much of the change in sensitivity of such photoreceptors during light adaptation derives directly from bleaching of visual pigment (Boynton and Whitten, 1970), substantial shifts of tlae voltage-intensity curve occur already at background intensities which do not bleach a significant fraction of the photopigment (Dowling andRipps, 1972, Normann andWerblin, 1974).…”

Photochemistry of Vision

“…This similarity led CROZIER and WOLF to question some of the implications of the duplicity theory of vision. This criticism does not apply to the work of DODT and JESSEN (1961) who employed Hemidactylus turcicus and Tarentola mauritanica. The Sphaerodactylidae, in general, are not strictly nocturnal and GOIN and GOIN, (1962) stated that this family of geckos tends to be more active during the day than are most geckos.…”