Tetrachromatic input to turtle horizontal cells

Recent physiological experiments support behavioral and morphological evidence for a fourth type of cone in the turtle retina, maximally sensitive in the ultraviolet (UV). This cone type has not yet been included in the models proposed for connectivity between cones and horizontal cells. In this study, we examined the inputs of UV, S, M, and L cones to horizontal cells. We used the high-resolution Dynamic Constant Response Method to measure the spectral sensitivity of horizontal cells without background light and after adaptation to UV, blue (B), green (G), and red (R) light. We concluded the following: (1) Tetrachromatic input to a Y/B horizontal cell was identified. The spectral-sensitivity curves of the cell in three of the adaptation conditions were well represented by L-, M-, and S-cone functions. Adaptation to blue light revealed a peak at 372 nm, the same wavelength location as that determined behaviorally in the turtle. A porphyropsin template could be closely fitted to the sensitivity band in that region, strong evidence for input from a UV cone. (2) The spectral-sensitivity functions of R/G horizontal cells were well represented by the L- and M-cone functions. There was no indication of UV- or S-cone inputs into these cells. (3) The spectral sensitivities of the monophasic horizontal cells were dominated by the L cone. However, the shape of the spectral-sensitivity function depended on the background wavelength, indicating secondary M-cone input. Connectivity models of the outer retina that predict input from all cone types are supported by the finding of tetrachromatic input into Y/B horizontal cells. In contrast, we did not find tetrachromatic input to R/G and monophasic horizontal cells. Chromatic adaptation revealed the spectral-sensitivity function of the turtle UV cone peaking at 372 nm.

Ultraviolet colour opponency in the turtle retina

SUMMARY We have examined the functional architecture of the turtle Pseudemys scripta elegans retina with respect to colour processing, extending spectral stimulation into the ultraviolet, which has not been studied previously in the inner retina. We addressed two questions. (i) Is it possible to deduce the ultraviolet cone spectral sensitivity function through horizontal cell responses? (ii) Is there evidence for tetrachromatic neural mechanisms, i.e. UV/S response opponency? Using a constant response methodology we have isolated the ultraviolet cone input into the S/LM horizontal cell type and described it in fine detail. Monophasic (luminosity), biphasic L/M (red-green) and triphasic S/LM (yellow-blue) horizontal cells responded strongly to ultraviolet light. The blue-adapted spectral sensitivity function of a S/LM cell peaked in the ultraviolet and could be fitted to a porphyropsin cone template with a peak at 372nm. In the inner retina eight different combinations of spectral opponency were found in the centre of the receptive field of ganglion cells. Among amacrine cells the only types found were UVSM−L+ and its reverse. One amacrine and four ganglion cells were also opponent in the receptive field surround. UV/S opponency, seen in three different types of ganglion cell, provides a neural basis for discrimination of ultraviolet colours. In conclusion, the results strongly suggest that there is an ultraviolet channel and a neural basis for tetrachromacy in the turtle retina.

Cone contributions to the photopic spectral sensitivity of the zebrafish ERG

Microspectrophotometry studies show that zebrafish (Danio rerio) possess four cone photopigments. The purpose of this study was to determine the cone contributions to the zebrafish photopic increment threshold spectral-sensitivity function. Electroretinogram (ERG) b-wave responses to monochromatic lights presented on a broadband or chromatic background were obtained. It was found that under the broadband background condition, the zebrafish spectral-sensitivity function showed several peaks that were narrower in sensitivity compared to the cone spectra. The spectral-sensitivity function was modeled with L − M and M − S opponent interactions and nonopponent S- and U-cone mechanisms. Using chromatic adaptation designed to suppress the contribution of the S-cones, a strong U-cone contribution to the spectral-sensitivity function was revealed, and the contributions of the S-cones to the M − S mechanism were reduced. These results show that the b-wave component of the ERG receives input from all four cone types and appears to reflect color opponent mechanisms. Thus, zebrafish may possess the fundamental properties necessary for color vision.

Colour Constancy, Categories, and Spectral Sensitivity

Colour constancy is a multi-stage process whose receptoral and post-receptoral components have been separated by electrophysiological and behavioral studies. A psychophysical study was conducted to determine contributions of the visual processing stage which extracts information about colour categories, and to relate the results to the spectral sensitivity of the chromatic opponent system. Changes in colour appearance with illumination were investigated by using free-viewing and a successive matching method for 40 Munsell hues (saturations: 8 and 4). Subjects were adapted to standard illuminant C and briefly inspected a test chip under a variable illuminant A (tungsten) or g (green). Using criteria similar to those for unique hues, subjects specified ‘typical hues’ under illuminant C: red, yellow, green, blue, and violet (yellow should not contain red or green, green should not contain yellow or blue, etc). For each hue and saturation a dominant wavelength was computed. The degree of colour constancy was then established for all subjects (examined in three series of experiments), all Munsell chips and illuminations, as a one-dimensional Brunswick ratio, BR. The BR was determined by using u‘ v’ coordinates as related to the ratio of perceptual to physical colour changes: BR=1-perceptual/physical. BR was highest for typical (not intermediate) hues and lowest for hues resembling variable illuminants. The BR as a function of dominant wavelength resembles the spectral sensitivity function of chromatic opponency. These findings are interpreted as contributions of high-order opponent interactions to colour constancy.

Spectral sensitivity of monocularly deprived cats

The reports of rod-dominated psychophysical spectral sensitivity from the deprived eye of monocularly lid-sutured (MD) monkeys are intriguing but difficult to reconcile with the absence of any reported deprivation effects in retina. As most studies of MD retina have been from cat, we have examined psychophysically the increment threshold spectral sensitivity of MD cats using both reaction time and simultaneous two-choice behavioral procedures. Although the deprived eyes exhibited an absolute increment threshold sensitivity deficit, both rod and cone spectral sensitivity functions were obtained on large white backgrounds. This normal transition from rod to cone vision, as background luminance increased, was also found in threshold vs. intensity functions. Using their deprived eye, some cats exhibited a rod spectral sensitivity function when a smaller, normally photopic, background was used providing some support for a hypothesis that the rod-dominated spectral sensitivity observed in monkey may represent detection of scattered stimulus light. Alternatively monocular deprivation may reveal a rod-dominated mechanism which exists in monkey but not in cat.

Light-evoked contraction of red absorbing cones in the Xenopus retina is maximally sensitive to green light

To test the hypothesis that light-evoked cone contraction in eye cups from Xenopus laevis is controlled through a direct mechanism initiated by the cone's own photopigment, we conducted spectral-sensitivity experiments. We estimate that initiation of contraction of red absorbing cones (611 nm) is 1.5 log units more sensitive to green (533 nm) than red (650 nm) light stimuli. The difference is comparable to that predicted from the spectral-sensitivity function of the green absorbing, principal rod (523 nm). Furthermore, 480-nm and 580-nm stimuli which are absorbed nearly equally by the principal rod have indistinguishable effects on cone contraction. We also found that light blockade of nighttime cone elongation is much more sensitive to green than to red light stimuli. Our observations are inconsistent with the hypothesis tested, and suggest that light-regulated cone motility is controlled through an indirect mechanism initiated primarily by the green absorbing, principal rod.

Behavioral and electrophysiological sensitivity to temporally modulated visual stimuli in the ground squirrel

Behavioral and electrophysiological methods were used to measure sensitivity to flickering lights in a dichromatic species, the California ground squirrel (Spermophilus beecheyi). Discrimination tests were used to determine spectral sensitivity at stimulus frequencies from 5–50 Hz and increment threshold spectral sensitivity. The contributions of retinal mechanisms to these capacities were assessed by recording the responses of optic nerve fibers to temporally modulated monochromatic lights. In the ground squirrel, as in the human, the shape of the spectral-sensitivity function depends on the temporal frequency of the stimulus. Results from single-unit recording show that all of the classes of optic nerve fibers in the ground squirrel are highly phase-locked to the stimulus for modulation rates as high as 50 Hz. Neither the responses of photoreceptors nor any class of optic nerve fiber can singly account for the behavioral results. The electrophysiological results are also counter to models which propose that temporally dependent changes in the spectral sensitivity of spectrally opponent fibers account for the behavior. The temporal resolution of the optic nerve fibers exceeds that of the behaving animal suggesting that retinal mechanisms do not limit behavioral temporal resolution.