Effects of rod activity on colour threshold

Vision under mesopic and scotopic illumination

Evidence has accumulated that rod activation under mesopic and scotopic light levels alters visual perception and performance. Here we review the most recent developments in the measurement of rod and cone contributions to mesopic color perception and temporal processing, with a focus on data measured using a four-primary photostimulator method that independently controls rod and cone excitations. We discuss the findings in the context of rod inputs to the three primary retinogeniculate pathways to understand rod contributions to mesopic vision. Additionally, we present evidence that hue perception is possible under scotopic, pure rod-mediated conditions that involves cortical mechanisms.

Assessing rod, cone, and melanopsin contributions to human pupil flicker responses

PURPOSE

We determined the relative contributions of rods, cones, and melanopsin to pupil responses in humans using temporal sinusoidal stimulation for light levels spanning the low mesopic to photopic range.

METHODS

A four-primary Ganzfeld photostimulator controlled flicker stimulations at seven light levels (-2.7 to 2 log cd/m(2)) and five frequencies (0.5-8 Hz). Pupil diameter was measured using a high-resolution eye tracker. Three kinds of sinusoidal photoreceptor modulations were generated using silent substitution: rod modulation, cone modulation, and combined rod and cone modulation in phase (experiment 1) or cone phase shifted (experiment 2) from a fixed rod phase. The melanopsin excitation was computed for each condition. A vector sum model was used to estimate the relative contribution of rods, cones, and melanopsin to the pupil response.

RESULTS

From experiment 1, the pupil frequency response peaked at 1 Hz at two mesopic light levels for the three modulation conditions. Analyzing the rod-cone phase difference for the combined modulations (experiment 2) identified a V-shaped response amplitude with a minimum between 135° and 180°. The pupil response phases increased as cone modulation phase increased. The pupil amplitude increased with increasing light level for cone, and combined (in-phase rod and cone) modulation, but not for the rod modulation.

CONCLUSIONS

These results demonstrate that cone- and rod-pathway contributions are more predominant than melanopsin contribution to the phasic pupil response. The combined rod, cone, and melanopsin inputs to the phasic state of the pupil light reflex follow linear summation.

Scotopic hue percepts in natural scenes

Traditional trichromatic theories of color vision conclude that color perception is not possible under scotopic illumination in which only one type of photoreceptor, rods, is active. The current study demonstrates the existence of scotopic color perception and indicates that perceived hue is influenced by spatial context and top-down processes of color perception. Experiment 1 required observers to report the perceived hue in various natural scene images under purely rod-mediated vision. The results showed that when the test patch had low variation in the luminance distribution and was a decrement in luminance compared to the surrounding area, reddish or orangish percepts were more likely to be reported compared to all other percepts. In contrast, when the test patch had a high variation and was an increment in luminance, the probability of perceiving blue, green, or yellow hues increased. In addition, when observers had a strong, but singular, daylight hue association for the test patch, color percepts were reported more often and hues appeared more saturated compared to patches with no daylight hue association. This suggests that experience in daylight conditions modulates the bottom-up processing for rod-mediated color perception. In Experiment 2, observers reported changes in hue percepts for a test ring surrounded by inducing rings that varied in spatial context. In sum, the results challenge the classic view that rod vision is achromatic and suggest that scotopic hue perception is mediated by cortical mechanisms.

Lateral suppression of mesopic rod and cone flicker detection

This study investigated the mechanisms of flicker detection suppression by measuring mesopic rod and cone critical flicker frequencies (CFFs) at different center and surround illuminance levels. Stimuli were generated with a four-primary photostimulator that provided independent control of rod and cone excitations. The results showed that dim surrounds ≤0.2 Td suppressed cone-mediated CFFs at ≥20 Td but not rod-mediated CFFs. These results can be understood in terms of peak amplitudes of photoreceptor impulse response functions under different stimulation conditions.

Effects of light and dark adaptation of rods on specific-hue threshold

Specific-hue threshold as a function of absolute rod threshold was measured with long-, middle-, and short-wavelength monochromatic test lights presented 17 deg extrafoveally. The measurements were obtained both during the rod phase of long-term dark adaptation and under conditions where the rod receptor system was gradually light adapted from a dark-adapted state by a scotopic background field of increasing retinal illumination. The results show that change in specific-hue threshold with change in absolute rod threshold is not, in general, identical for light and dark adaptation of the rod receptor system. Thus, in the long- and middle-wavelength test regions, the specific-hue threshold could be obtained at higher intensities under the light- as compared to the dark-adaptation condition when absolute rod thresholds were the same. Just the opposite was found for the short-wavelength tests. It is concluded that change in specific-hue threshold with light and dark adaptation of the rod receptor system is not, in general, controlled by the same mechanism.

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.

Summation of rod and S cone signals at threshold in human observers

We examined whether signals from rods and S cones can combine to produce a threshold response. Test flashes of specific wavelengths superposed on a long wavelength adapting field were used to isolate threshold responses from the two receptor systems, simultaneously and at the same retinal location. Dark adaptation experiments and spectral sensitivity determinations indicated that, in the adaptational range from about 1.6 to 2.8 log scot td, 530 nm and 440 nm flashes were detected by rod and S cone photoreceptors, respectively. The intensities of the 530 nm and 440 nm flashes were mixed in various ratios and the increment threshold was then measured with these mixture flashes using the method of constant stimuli. The effects of rod and S cone excitation were found to summate linearly at threshold, under these experimental conditions. Summation occurred presumably at an early stage of the visual process.

Rod-cone interaction in form detection

Using a Wright colorimeter, absolute threshold, absolute form threshold and specific form threshold were measured during long-term dark adaptation in the extrafoveal retina. The specific form threshold was found to fall markedly at about the cone-rod break but thereafter rose steeply. Furthermore, during the rod phase of the dark adaptation the form percept of the small, slender rectangular test field changed qualitatively from a line or rectangle to a circular field at all mesopic intensities. The results indicate that light signals from rods may both facilitate and suppress cone-mediated information about form, and that the rod system may completely dominate the perception of form several log units above the absolute dark-adapted cone threshold when the eye is dark adapted.

Dark adaptation of foveal cones during the cone-plateau period

Following substantial bleaching by "white" light, absolute threshold, relative spectral sensitivity and sensation of hue of monochromatic lights were measured at the central fovea during the cone-plateau period. The absolute-threshold level was found to increase and then decrease markedly, the relative spectral sensitivity remained invariant, while the sensation of hues of monochromatic lights from the long- and middle-wave regions of the spectrum changed toward hues of shorter wavelengths.

Rod suppression of cone-mediated information about colour and form during dark adaptation

Following substantial bleaches, the specific form and hue thresholds were measured during dark adaptation with a test stimulus of 1 x 2 degrees at 40 degrees extrafoveally. The wavelength of the test field was varied between runs. The results show that both thresholds started to rise at about the cone-rod break of the dark-adaptation curve, irrespective of wavelength used in the test. Furthermore, the specific threshold for form was found to rise when a scotopic stimulus was superimposed on a photopic test flash. On the other hand, both thresholds remained at the cone-plateau level when the test flash was confined within the rod-free fovea. In order to explain the rise in the specific thresholds, it is suggested that signals from rods generated directly in response to the test stimulus may suppress both cone-mediated form and colour. It is also suggested that this type of rod-cone interaction represents a general characteristic involved in several kinds of visual information processing.

Spatial properties of rod-cone interactions in flicker and hue detection

Rod-cone interactions in flicker and hue detection were compared to examine the hypothesis that they are mediated by mechanisms with different spatial properties. Flicker and hue thresholds for a 1 deg test stimulus (TS) were measured as a function of background luminance and diameter. Flicker thresholds were reduced from their dark-adapted value by an 11 deg diameter background, but not by a 1 deg background. These results, in agreement with previous work, demonstrate that light adaptation of rods surrounding the TS is necessary to eliminate their effect on cone flicker thresholds. In contrast, hue thresholds were reduced from their dark-adapted value to a comparable degree by 1 and 11 deg backgrounds, indicating that light adaptation of the rods stimulated by the TS is sufficient to abolish the rod-cone hue interaction. Our results support the contention that the rod-cone flicker and hue interactions are mediated by different mechanisms. We also demonstrated that changing the detection task while keeping stimulus parameters constant is sufficient to shift between these two types of rod-cone interactions.

Mechanisms of rod-cone interaction: evidence from congenital stationary nightblindness

The dark-adapted rod system can elevate cone-mediated thresholds for flicker detection as well as thresholds for the detection of hue. We examined these two types of rod-cone interactions in two individuals with congenital stationary nightblindness (CSNB), a retinal disorder in which rod outer segment function is intact, but in which a defect occurs in the transmission of rod signals within the retina. The two types of rod-cone interaction were differentially affected by the retinal pathology; the rod-cone flicker interaction was normal, but the rod-cone hue interaction was absent. These results provide evidence that, despite similarities in the adaptational properties of these two types of rod-cone interaction, they are mediated by different visual mechanisms.

Spectral sensitivity and wavelength discrimination of the human peripheral visual field

Spectral sensitivity and wavelength discrimination are determined along the nasal horizontal meridian of the human peripheral retina. The target size as a function of eccentricity is varied according to a particular cortical magnification factor. Spectral sensitivity is measured by flicker photometry parameterized for the flicker frequency (10-20 Hz) and is found to be independent of the eccentricity (0-80 degrees) for 20-Hz flicker photometry after correction of the foveal spectral sensitivity for macular pigment absorption. This 20-Hz function is chosen as being representative for the peripheral luminous-efficiency function and is used in the wavelength-discrimination experiments. The peripheral retina can perform wavelength discrimination up to an eccentricity of 80 degrees. If field-size scaling according to the eccentricity-dependent cone density, the cortical magnification factor, or the reciprocal of the interganglion cell distance is applied, then wavelength-discrimination performance from 8 degrees to 80 degrees eccentricity is roughly the same. Foveal wavelength discrimination is considerably better than peripheral wavelength discrimination.

Inhibitory influence of unstimulated rods in the human retina: evidence provided by examining cone flicker

In the parafoveal retina of human observers, cone-mediated sensitivity to flicker decreases as rods become progressively more dark-adapted. This effect is greatest when a rod response to flicker is precluded. These results indicate that rods tonically inhibit cone pathways in the dark.

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.

Bezold-Brücke phenomenon of the far peripheral retina

The Bezold-Brücke phenomenon was measured during the cone-plateau period of the long-term dark-adaptation curve at 25 degrees, 40 degrees and 60 degrees in the nasal field of view, and at 40 degrees and 70 degrees in the temporal field. In striking contrast to previous measurements of the B-B phenomenon the present results generally show that an increase of the luminance level in the middle- and long-wave regions of the spectrum produces, respectively, a trend toward green and red instead of toward yellow. The present results are explained on neural rather than on photochemical mechanisms.

Effect of retinal location on cone critical flicker frequency

This experiment investigated the differential sensitivity of various areas of the retina using flicker. For 12 subjects testing was carried out in the fovea, and 5 degrees and 6 degrees temporal to the fovea using a wavelength of 555 nm. Testing was done both in the presence of a surround beam and in its absence. In all cases, there was a 2.5- to 4-fold increase in the amount of energy needed to perceive flicker as testing was shifted from the fovea to the periphery. A number of possible explanations are suggested to account for these findings.