Ulf Stabell

Variation in density of macular pigmentation and in short-wave cone sensitivity with eccentricity

Using both absolute threshold and flicker techniques, we measured the relative spectral sensitivity at different eccentricities during the cone-plateau period of the long-term dark-adaptation curve. With both techniques the relative spectral sensitivity tended to increase with eccentricity in the short-wave region of the spectrum. The results suggest that the relatively high short-wave sensitivity of the extrafovea obtained with the threshold technique, is due both to variation in density of macular pigmentation nd in short-wave cone sensitivity, while the relatively high short-wave sensitivity obtained with the flicker technique is due to variation in density of macular pigmentation alone. The results were used as a basis for analyzing the variation in density of macular pigmentation and in short-wave cone sensitivity with eccentricity.

Rod and cone contributions to peripheral colour vision

Abstract The changes in chromaticities were measured, in a dark-adapted state, when monochromatic lights were moved from the fovea to 2.5° and 7.5° temporally. In order to analyze the contributions of the rod and cone mechanisms to these changes, the chromaticities were measured during dark adaptation following high light adaptation. For several min during the cone-plateau period, the colourmixture functions remained invariant and the outlines of the test field appeared distinct and clearcut. Just prior to the cone-rod break of the dark-adaptation curve, however, the chromaticities generally started to change and the shape of the test field became somewhat blurred. The evidence is judged to indicate that the chromaticities measured during the period of invariance are due to pure cone activity, while the shift in chromaticities measured during the further stay in the dark is due to the effect of rod intrusion.

Wavelength discrimination of peripheral cones and its change with rod intrusion.

Abstract Following high light adaptation, wavelength-discrimination functions of the peripheral retina were obtained during the cone-plateau period of the long-term dark-adaptation curve and after 30 min dark adaptation at 3, 10 and 100 photopic td. The wavelength-discrimination curves obtained during the cone-plateau period were found to be strikingly similar to the wavelength-discrimination functions recorded at the rod-free fovea and thus characteristic of normal trichromatic vision. During the rod phase of the dark-adaptation curve, wavelength discrimination tended to deteriorate in the short and middle regions of the spectrum and to improve in the red region.

The effect of rod acitvity on colour matching functions

Abstract When an achromatic scotopic stimulus was superimposed upon a red test light in a dark-adapted eye, the hue of the test light changed toward yellow. Moreover, when the chromaticities of a test beam were measured twice during dark adaptation: (1) at the moment when the cone threshold and (2) when the sensitivity of the rods were assumed to have reached their absolute dark-adapted values, colours of orange and green-yellow were found to change toward yellow, while colours of blue-green and violet were found to change toward blue. The results appear to oppose the assumption of the Duplicity Theory of vision that the rod activity contributes a constant achromatic colour quality to the visual sensation, but are consistent with the suggestion that the chromatic response of rods may change as a function of selective chromatic stimulation of cones.

Absolute spectral sensitivity at different eccentricities

Absolute spectral-threshold functions were measured during the cone-plateau period and in a dark-adapted state at 0, 6, 17, 28, 45, and 65 degrees temporally to the fovea. It was found that, when the photopic functions were brought together at 660 nm, they closely coincided in the 520-700-nm region of the spectrum, irrespective of location, suggesting that the relative spectral sensitivity and the weighted contributions of the middle-and long-wave cone photopigments remain invariant across the retina. On the other hand, the results suggest that the relative contribution of the short-wave cone mechanism increases between fovea and 17 degrees, stays essentially constant between 17 and 28 degrees, and decreases between 28 and 65 degrees. Furthermore, the results suggest that the absolute sensitivity of the middle- and long-wave cones decreases between fovea and 65 degrees, whereas the absolute sensitivity of the rods increased form fovea to 17 degrees and decreases between 17 and 65 degrees degrees. Finally, the log difference between the absolute dark-adapted cone and the rod threshold was found to increase between fovea and 45 degrees and to decrease between 45 and 65 degrees.

Color vision in the peripheral retina under photopic conditions

Chromaticities of spectral colors were measured during the cone-plateau period at 17 degrees, 25 degrees, 40 degrees and 60 degrees in the nasal field of view and at 40 degrees and 70 degrees in the temporal field. The results obtained in the nasal field show a progressive contraction of the color gamut with distance from the fovea with maximum shrinkage in the middle-wave region. Color discrimination in the temporal field of view was found to be much better developed than in the nasal field. Thus, all the primary hues were clearly observable at 70 degrees temporally. The exceptionally good color discrimination obtained in the present study is explained on the assumption that only cones are effectively excited upon stimulation during the cone-plateau period.

Mechanisms of chromatic rod vision in scotopic illumination

After viewing a coloured patch for 30 sec, successive contrast colours were triggered by stimulating either rods or cones. The conditions were arranged so that the rod and cone stimuli matched both with respect to chromaticness and brightness in a chromatically neutral state of adaptation. The results showed that the contrast colours triggered by rods were strikingly similar to those triggered by cones. Yet, the scotopic contrast colours, as compared with the photopic ones, were generally found to be somewhat displaced toward blue. This displacement was attributed to the difference in test conditions. Thus, it was suggested that, although rods may excite all the different types of spectrally opponent cells, they generally tend to prefer the short-wave cells. Moreover, it was concluded that the scotopic successive contrast colours are triggered by rod signals feeding into the primary rod pathway and therefore must originate centrally to the receptor level.

Peripheral colour vision : Effects of rod intrusion at different eccentricities

Chromaticities of monochromatic lights from different parts of the spectrum were measured both during the cone-plateau period of the long-term dark-adaptation curve and in a completely dark-adapted state. The measurements were obtained at 3, 8, 30 and 65 deg in the temporal field of view and at 1, 2, 3 and 4 log units above the cone-plateau level. The results show that cone-mediated colours obtained during the cone-plateau period in general are desaturated when rod signals intrude during long-term dark adaptation. The desaturation effect of rods obtained at high mesopic illumination levels was found to increase when the test field was moved from 3 to 30 deg but to reduce markedly between 30 and 65 deg. Surprisingly, the desaturation was clearly observable even at a retinal illumination of 20,000 ph td. The desaturation effect of rods is explained by the suggestion that differences in ongoing activity rates of the different types of spectrally opponent cells become levelled out to some extent when light signals from rods intrude during dark adaptation.

Color-vision mechanisms of the extrafoveal retina

Wavelength discrimination, spectral sensitivity as well as color-matching performance were measured at the fovea and at different eccentricities in the peripheral retina. The results show that the underlying mechanisms of color vision in the normal peripheral retina are different from those of the classic forms of congenital color blindness. On the other hand, a close correspondence was found between color-vision characteristics obtained in the extrafoveal retina and in patients with acquired color-vision defects due to diseases of the optic nerve, suggesting that the loss of color discrimination with eccentricity and during progression of these diseases has a common underlying basis.

Effects of rod activity on color threshold

Abstract The specific thresholds of blue and red lights, measured extrafoveally in a dark-adapted state, were obtained at intensities below the cone-plateau level of the long-term dark-adaptation curve, while the specific thresholds of green lights were obtained at intensities above the cone-plateau level. Furthermore, the specific threshold of a deep red light measured in a completely dark-adapted state was found to drop when an achromatic scotopic stimulus was superimposed. It is suggested that rod activity may both suppress and facilitate chromatic-related cone activity.