Visual pigment from a pure-cone retina

The electroretinogram of a diurnal gecko

Using the electroretinogram as the criterion of retinal activity the flicker fusion frequency, course of dark adaptation, and spectral sensitivity of the pure cone retina of the diurnal gecko, Phelsuma inunguis, were investigated. Both the curve relating flicker fusion frequency to stimulus intensity and that relating the amplitude of the flicker response to stimulus intensity showed a break as the intensity was increased. The dark adaptation curve was that typical of cone retinae; there was no break, adaptation was relatively rapid, and there was a total increase of sensitivity of only about 3 log units. The spectral sensitivity curve showed two maxima, a major one at about 560 mµ and another at about 460 mµ. Chromatic adaptation with red and blue lights demonstrated the presence of two independent mechanisms. Although red adaptation could not have had a direct effect on the pigment responsible for the "blue" mechanism the sensitivity of this mechanism was depressed by red adaptation. The possible relationships of the two mechanisms are discussed.

Spectral mechanisms in the tree squirrel retina

The retina of the gray squirrel (Sciurus carolinensis) contains rods and cones in a ratio of about 2:3. The spectral mechanisms in this retina were examined in behavioral and electrophysiological experiments. Tests of color vision revealed that this animal has a spectral neutral point at about 500 nm and, thus, dichromatic color vision. Recordings made from single optic nerve fibers and results obtained from an analysis of the flicker photometric electroretinogram (ERG) indicated that vision in the gray squirrel is based on three spectral mechanisms. One of these, presumably rod-based, has peak sensitivity at about 502 nm. The other two mechanisms reflect the presence of two classes of cone having average peak sensitivity of about 444 nm and 543 nm.

Anatomical evidence for cone and rod-like receptors in the gray squirrel, ground squirrel, and prairie dog retinas

In the gray squirrel (Sciurus carolinensis), the prairie dog (Cynomys ludovicianus), and the Mexican and 13-line ground squirrels (Citellus mexicanus and C. tridecemlineatus) there exist two distinct classes of photo-receptors that have cone-like and rod-like anatomical features respectively. These two receptor classes were previously known to exist in the gray squirrel, but only the cone-like (C) receptor had been observed in the other species. We have now found small numbers of rod-like (R) receptors in the other species as well. R-receptors comprise about 40% of the receptors in the gray squirrel, 10% of the receptors in the prairie dog, and 4-5% of the receptors in the two species of ground squirrel. This paper describes certain light and electron microscopic features of these two receptor classes including their synaptic connections with second-order cells and with each other. We find that the C-receptor has a morphology and synaptic organization characteristic of other mammalian cones. However, the R-receptor differs from other mammalian rods in certain morphological respects, and its synaptic organization has both cone and rod characteristics as well as some unusual features.

Electrophysiological evidence for rod-like receptors in the gray squirrel, ground squirrel and prairie dog retinas

Spectral sensitivities of the gray squirrel, Mexican and 13-line ground squirrel and prairie dog were determined by electroretinography under both dark- and light-adapted conditions. The dark-adapted spectral sensitivity function obtained from intact eyes of these species peaks between 515-525 nm; however, when corrected for lens absorption or recorded from the lensless eye, it peaks near 500 nm and closely matches in shape a rhodopsin nomogram curve (lambda max equals 502 nm). Upon light adaptation all these retinas become relatively more sensitive to long-wave stimuli (i.e., they show a small Purkinje shift). The light-adapted spectral sensitivity function is broader than that obtained from the dark-adapted eye, especially toward the longer wavelengths. Weconclude that in all these species the dark-adapted spectral sensitivity is mediated by a single, rhodopsin-like photopigment and that light-adapted sensitivity is mediated by two (or more) photopigments.

A purkinje shift in the spectral sensitivity of grey squirrels

1. The light-adapted spectral sensitivity of the grey squirrel has been determined by an automated training method at a level about 6 log units above the squirrel's absolute threshold.2. The maximum sensitivity is near 555 nm, under light-adapted conditions, compared with the dark-adapted maximum near 500 nm found by a similar method.3. Neither the light-adapted nor the dark-adapted behavioural threshold agrees with electrophysiological findings using single flash techniques, but there is agreement with e.r.g. results obtained with sinusoidal stimuli.