Saturation in human cones

Alterations of color vision and pupillary light responses in age-related macular degeneration

Introduction

Age-related macular degeneration (AMD) is the leading cause of irreversible central vision loss in developed countries and one of the leading causes of blindness. In this work, we evaluated color vision and the pupil light reflex (PLR) to assess visual function in patients with early and neovascular AMD (NVAMD) compared with the control group.

Methods

We recruited 34 early patients with dry AMD and classified them into two groups following AREDS: 13 patients with NVAMD and 24 healthy controls. Controls and patients with early dry AMD had visual acuity (VA) best or equal to 20/25 (0.098 logMAR). Color vision was assessed in controls and patients with early dry AMD using the Cambridge Color Test (CCT) 2.0 through the Trivector protocol. The PLR was evaluated using a Ganzfeld, controlled by the RETI-port system. The stimuli consisted of 1s blue (470 nm) and red (631 nm) light flashes presented alternately at 2-min intervals. To assess the cone contribution, we used a red flash at 2.4 log cd.m^-2, with a blue background at 0.78 log cd.m^-2. For rods, we used 470-nm flashes at -3 log cd.m^-2, and for the melanopsin function of ipRGCs, we used 470 nm at 2.4 log cd.m^-2.

Results

Patients with early dry AMD had reduced color discrimination in all three axes: protan (p = 0.0087), deutan (p = 0.0180), and tritan (p = 0.0095) when compared with the control group. The PLR has also been affected in patients with early dry AMD and patients with NVAMD. The amplitude for the melanopsin-driven response was smaller in patients with early dry AMD (p = 0.0485) and NVAMD (p = 0.0035) than in the control group. The melanopsin function was lower in patients with NVAMD (p = 0.0290) than the control group. For the rod-driven response, the latency was lower in the NVAMD group (p = 0.0041) than in the control group. No changes were found in cone-driven responses between the control and AMD groups.

Conclusion

Patients with early dry AMD present diffusely acquired color vision alteration detected by CCT. Rods and melanopsin contributions for PLR are affected in NVAMD. The CCT and the PLR may be considered sensitive tests to evaluate and monitor functional changes in patients with AMD.

Human cone elongation responses can be explained by photoactivated cone opsin and membrane swelling and osmotic response to phosphate produced by RGS9-catalyzed GTPase

Optical coherence tomography has established that human cone photoreceptor outer segments elongate in response to stimuli bleaching large fractions of their visual pigment. Elongation responses are completely described over their 200-fold bleaching range as the sum of two exponentially rising components differing 13-fold in time constants and 4-fold in light sensitivity. Bleaching measurements of individual cones with adaptive optics scanning laser ophthalmoscopy (SLO) suggest that component 2 arises from cone opsin and disk membrane swelling triggered by photoactivation. Application of a model of phototransduction suggests that component 1 corresponds to free phosphate generated by regulator of G-protein signaling 9 (RGS9)-catalyzed hydrolysis of guanosine triphosphate (GTP) in the α-subunit of G protein complexed with phosphodiesterase.

Human cone outer segment (COS) length changes in response to stimuli bleaching up to 99% of L- and M-cone opsins were measured with high resolution, phase-resolved optical coherence tomography (OCT). Responses comprised a fast phase (∼5 ms), during which COSs shrink, and two slower phases (1.5 s), during which COSs elongate. The slower components saturated in amplitude (∼425 nm) and initial rate (∼3 nm ms⁻¹) and are well described over the 200-fold bleaching range as the sum of two exponentially rising functions with time constants of 80 to 90 ms (component 1) and 1,000 to 1,250 ms (component 2). Measurements with adaptive optics reflection densitometry revealed component 2 to be linearly related to cone pigment bleaching, and the hypothesis is proposed that it arises from cone opsin and disk membrane swelling triggered by isomerization and rate-limited by chromophore hydrolysis and its reduction to membrane-localized all-trans retinol. The light sensitivity and kinetics of component 1 suggested that the underlying mechanism is an osmotic response to an amplified soluble by-product of phototransduction. The hypotheses that component 1 corresponds to G-protein subunits dissociating from the membrane, metabolites of cyclic guanosine monophosphate (cGMP) hydrolysis, or by-products of activated guanylate cyclase are rejected, while the hypothesis that it corresponds to phosphate produced by regulator of G-protein signaling 9 (RGS9)-catalyzed hydrolysis of guanosine triphosphate (GTP) in G protein–phosphodiesterase complexes was found to be consistent with the results. These results provide a basis for the assessment with optoretinography of phototransduction in individual cone photoreceptors in health and during disease progression and therapeutic interventions.

Harnessing the Sun to See Anew

Light drives vision by directly activating opsin-based visual pigments in rod and cone photoreceptors. In this issue of Neuron, Morshedian et al. (2019) show that light also drives regeneration of the cone visual pigments via an elegant biochemical mechanism in Müller glial cells of the neural retina that can contribute to sustained cone function under daytime conditions.

Restoration of high-sensitivity and adapting vision with a cone opsin

Inherited and age-related retinal degenerative diseases cause progressive loss of rod and cone photoreceptors, leading to blindness, but spare downstream retinal neurons, which can be targeted for optogenetic therapy. However, optogenetic approaches have been limited by either low light sensitivity or slow kinetics, and lack adaptation to changes in ambient light, and not been shown to restore object vision. We find that the vertebrate medium wavelength cone opsin (MW-opsin) overcomes these limitations and supports vision in dim light. MW-opsin enables an otherwise blind retinitis pigmenotosa mouse to discriminate temporal and spatial light patterns displayed on a standard LCD computer tablet, displays adaption to changes in ambient light, and restores open-field novel object exploration under incidental room light. By contrast, rhodopsin, which is similar in sensitivity but slower in light response and has greater rundown, fails these tests. Thus, MW-opsin provides the speed, sensitivity and adaptation needed to restore patterned vision.

In vivo optophysiology reveals that G-protein activation triggers osmotic swelling and increased light scattering of rod photoreceptors

The light responses of rod and cone photoreceptors have been studied electrophysiologically for decades, largely with ex vivo approaches that disrupt the photoreceptors' subretinal microenvironment. Here we report the use of optical coherence tomography (OCT) to measure light-driven signals of rod photoreceptors in vivo. Visible light stimulation over a 200-fold intensity range caused correlated rod outer segment (OS) elongation and increased light scattering in wild-type mice, but not in mice lacking the rod G-protein alpha subunit, transducin (Gα_t), revealing these responses to be triggered by phototransduction. For stimuli that photoactivated one rhodopsin per Gα_t the rod OS swelling response reached a saturated elongation of 10.0 ± 2.1%, at a maximum rate of 0.11% s^-1 Analyzing swelling as osmotically driven water influx, we find the H₂O membrane permeability of the rod OS to be (2.6 ± 0.4) × 10^-5 cm⋅s^-1, comparable to that of other cells lacking aquaporin expression. Application of Van't Hoff's law reveals that complete activation of phototransduction generates a potentially harmful 20% increase in OS osmotic pressure. The increased backscattering from the base of the OS is explained by a model combining cytoplasmic swelling, translocation of dissociated G-protein subunits from the disc membranes into the cytoplasm, and a relatively higher H₂O permeability of nascent discs in the basal rod OS. Translocation of phototransduction components out of the OS may protect rods from osmotic stress, which could be especially harmful in disease conditions that affect rod OS structural integrity.

An Analysis of Visual Adaptation and Contrast Perception for Tone Mapping

Tone Mapping is the problem of compressing the range of a High-Dynamic Range image so that it can be displayed in a Low-Dynamic Range screen, without losing or introducing novel details: The final image should produce in the observer a sensation as close as possible to the perception produced by the real-world scene. We propose a tone mapping operator with two stages. The first stage is a global method that implements visual adaptation, based on experiments on human perception, in particular we point out the importance of cone saturation. The second stage performs local contrast enhancement, based on a variational model inspired by color vision phenomenology. We evaluate this method with a metric validated by psychophysical experiments and, in terms of this metric, our method compares very well with the state of the art.

Time course of adaptation to stimuli presented along cardinal lines in color space

Visual sensitivity is a process that allows the visual system to maintain optimal response over a wide range of ambient light levels and chromaticities. Several studies have used variants of the probe-flash paradigm to show that the time course of adaptation to abrupt changes in ambient luminance depends on both receptoral and postreceptoral mechanisms. Though a few studies have explored how these processes govern adaptation to color changes, most of this effort has targeted the L-M-cone pathway. The purpose of our work was to use the probe-flash paradigm to more fully explore light adaptation in both the L-M- and the S-cone pathways. We measured sensitivity to chromatic probes presented after the onset of a 2-s chromatic flash. Test and flash stimuli were spatially coextensive 2 degrees fields presented in Maxwellian view. Flash stimuli were presented as excursions from white and could extended in one of two directions along an equiluminant L-M-cone or S-cone line. Probes were presented as excursions from the adapting flash chromaticity and could extend either toward the spectrum locus or toward white. For both color lines, the data show a fast and slow adaptation component, although this was less evident in the S-cone data. The fast and slow components were modeled as first- and second-site adaptive processes, respectively. We find that the time course of adaptation is different for the two cardinal pathways. In addition, the time course for S-cone stimulation is polarity dependent. Our results characterize the rapid time course of adaptation in the chromatic pathways and reveal that the mechanics of adaptation within the S-cone pathway are distinct from those in the L-M-cone pathways.

Probed-sinewave paradigm: a test of models of light-adaptation dynamics

Studies of light adaptation have, in general, employed either aperiodic or periodic stimuli. In earlier work, models originally developed to predict the results from one tradition failed to predict results from the other but the models from the two traditions could be merged to predict phenomena from both. To further test these merged models, a paradigm combining both types of stimuli was used. The threshold for a brief flash (the probe) was measured at various phases on a background that was varied sinusoidally in time. The probe threshold depends upon the phase at which it is presented for all background frequencies used, 0-16 Hz. These threshold variations are not well described by a sinewave; the peak threshold is > 180 deg out of phase with the trough threshold. Further, the positions of the peaks and troughs shift fairly abruptly at background modulations of 4-8 Hz. The difference between the peak and trough thresholds varies as a function of temporal frequency in a manner approximating the temporal contrast sensitivity function. The dc level (mean threshold) does not. The peak-trough difference dominates at low frequencies of background modulation, while the dc level dominates for higher frequencies. Existing models of light adaptation do not predict the key features of the data.

Testing a computational model of light-adaptation dynamics

Experiments from the periodic and aperiodic traditions were used to guide the development of a quantitatively valid model of light adaptation dynamics. Temporal contrast sensitivity data were collected over a range of 3 log units of mean luminance for sinusoids of 2 to 50 Hz. Probe thresholds on flashed backgrounds were collected over a range of stimulus-onset asynchronies and background intensities from 0.1 to 1000 td. All experiments were performed foveally in the photopic range and used a consistent stimulus paradigm and psychophysical method. The resulting model represents a merging of elements from both traditions, and consists of a frequency-dependent front-end followed by a subtractive process and static nonlinearity.

Adaptation mechanisms in spatial vision--II. Flash thresholds and background adaptation

To examine how the mechanisms of light adaptation affect spatial pattern vision, contrast detection thresholds were measured for sinusoidal (increment-Gabor) probes on flashed backgrounds in the presence of steady adapting backgrounds. The thresholds for all spatial frequencies (1-12 c/deg), flashed-background intensities (dark to 4 log td) and adapting-background intensities (dark to 4 log td) were adequately described by a simple model consisting of a compressive nonlinearity (a modified Naka-Rushton function), a subtractive adaptation factor, and a multiplicative adaptation factor. For all five subjects the compressive nonlinearity was found to vary systematically with spatial frequency; for all but one subject, the subtractive and multiplicative factors were found to be relatively constant.

Modeling the dynamics of light adaptation: the merging of two traditions

Light adaptation has been studied using both aperiodic and periodic stimuli. Two well-documented phenomena are described: the background-onset effect (from an aperiodic-stimulus tradition) and high-temporal-frequency linearity (from the periodic-stimulus tradition). These phenomena have been explained within two different theoretical frameworks. Here we briefly review those frameworks. We then show that the models developed to predict the phenomenon from one tradition cannot predict the phenomenon from the other tradition, but that the models from the two traditions can be merged into a class of models that predicts both phenomena.

The effect of adaptation on the differential sensitivity of the S-cone color system

This paper presents a psychophysical dissection of the S-cone color system. Experiments were guided by a skeletal model that assumed a first stage consisting of S-, M- and L-cones, and a second stage of the opponent combination of the S and L+M signals. The response of the S-cone system was isolated by measuring difference thresholds between lights that were equiluminant tritanopic confusion pairs and thus differed only in S-cone excitation. Two types of mechanisms that control sensitivity in the S-cone system were identified: (i) static mechanisms that have a restricted range and thus limit discrimination to a small range of inputs; and (ii) adaptive mechanisms that change the state of the system in response to changes in steady illumination, so that the system is sensitive to small changes from the adapting light. These mechanisms were localized by lights that stimulated the S-cone system while keeping the signal constant at either the S, the L+M, or the post-opponent stage. The response function of the static mechanism was estimated by measuring difference thresholds at judgment points other than the steady adapting light. This procedure was repeated at a number of adaptation lights to examine the properties of adaptive mechanisms. The data were consistent with an elaborated model that included identical multiplicative gain control mechanisms in the S and L+M pre-opponent branches, and a post-opponent static sigmoidal nonlinearity with different amounts of compression for positive and negative opponent inputs.

Subtractive processes in light adaptation

We measured the time course of light adaptation in foveal vision following the onset of an adapting background. Several adaptational steps in the low to mid photopic range were examined. The time course of multiplicative and subtractive components of the adaptation were extracted from the data. Unlike previous findings there were no subtractive changes for several hundred milliseconds following light onset, and the process took 10-15 sec to reach steady state. It seems likely that the fast component previously observed results from effectively instantaneous center-surround antagonism, and that our measurements reflect a second subtractive process involving the slow loss of the d.c. signal over time.

The role of spatial filtering in sensitivity regulation

The role of spatial filtering in controlling sensitivity to increments is hard to evaluate under normal viewing conditions because eye movements lead to a confounding of spatial and temporal transients. We measured sensitivity to increments on different sized backgrounds in photopic and scotopic vision when the backgrounds were stabilized on the retina, thus eliminating temporal transients. The saturating effect of small fields on photopic thresholds was preserved under these conditions indicating that spatial filtering by retinal cells is critical in maintaining photopic sensitivity. Some effect of spatial pattern on sensitivity in stabilized vision was also observed in scotopic vision, although it was much smaller than was observed in photopic vision. The interaction effects between rod and cone systems that are observed with small backgrounds were also preserved in stabilized vision, implicating a very peripheral site for the generation of these interactions.

The time-course of multiplicative and subtractive adaptation process

This paper examines, for foveal cone vision, the processes which mediate the transition to a steady state of adaptation following a change of illumination. In the steady state, the signal from an adapting field is attenuated not only by a multiplicative factor (reduction in gain) but also by a subtractive signal. We show that the multiplicative change is accomplished very rapidly following the onset of an adapting field (within about 50 msec). Much of the subtractive change is also accomplished rapidly, but it takes several sec to complete. At the offset of the field, the multiplicative process takes over 200 msec to recover. This slower time-course at offset may be a consequence of receptoral persistence.

Lateral interactions in the control of visual sensitivity

Threshold vs intensity curves for cone vision, measured in the parafoveal retina, quickly saturate if the adapting background is made small (e.g. 19' at 5 degrees eccentricity). Log increment threshold increases at a rate of about 3:1 with log background illuminance at levels as low as 10 td. This shows that lateral interactions are an important process in preserving differential sensitivity in cone vision across the wide range of illuminances over which it normally operates. Parallels between light and dark adaptation in the effect of field size were explored, since effects of comparable magnitude are observed in both. Backgrounds and bleaches equated for their effects at one field size do not have equal effects on threshold at other field sizes, however, with small-area bleaches raising threshold more than predicted. This failure of equivalence was also revealed in a second experiment, in which recovery of sensitivity following small area bleaches was measured in the presence of large steady background fields, which have the effect of lowering threshold. Thresholds following the small bleach were lowered less than expected on the basis of the "equivalent background" hypothesis, a result which we take to mean that signals from bleached cones exceed those produced by a background which has an equivalent effect on threshold (the "equivalent background"). Control experiments examined whether rods contribute to the overloading of cone response by small fields and the possible contribution of such central adaptation processes as spatial frequency adaptation.

Effects of bleaching and backgrounds on the flash response of the cone system

1. Increment-threshold functions for flashed backgrounds were measured in the human fovea under several conditions: (1) during dark adaptation following full bleaches, (2) in the presence of steady adapting backgrounds and (3) 500 msec following extinction of adapting backgrounds.2. To prevent the intense flashed backgrounds from interfering with the course of dark adaptation the inter-trial interval was continuously increased during dark adaptation. This technique may prove generally useful for presenting suprathreshold stimuli during dark adaptation.3. All the increment-threshold functions measured during dark adaptation were found to be roughly shape invariant and continuously accelerating when plotted in log-log co-ordinates. Furthermore, in order to translate a function obtained at any given time into coincidence with a function obtained at any other time, it had to be translated vertically and horizontally the same number of log units. This is equivalent to adding or removing neutral density filters from in front of the eye.4. The increment-threshold functions obtained with steady adapting backgrounds were also continuously accelerating, but could not be brought into coincidence by equal vertical and horizontal translation. However, this became possible again if the adapting background was extinguished during presentation of the flashed background.5. These results contradict the equivalent-background hypothesis. None the less, they suggest that under present conditions the effects of bleaches and backgrounds may be similar except that steady backgrounds provide additional quanta which drive the visual system part of the way up its intensity-response function.6. The conclusions above were supported by applying a simple model based on the equation R = R(max). I(n) / (I(n) + I(1) (n)), which has frequently been used to describe the peak responses of retinal neurones to flashed stimuli. Virtually all of the data reported here were fitted by this simple model with R(max) held constant.7. The parameters estimated from the model imply that the flash responses measured in the present experiments differ in at least one fundamental way from receptor responses. Even after taking into account changes in the half saturation constant I(1), steady backgrounds were found to be much less effective than flashed backgrounds in driving the visual system up its intensity-response function. A subtractive inhibitory network prior to the non-linear stages responsible for threshold saturation could explain this result.

The derivation of nerve signals from contrast flash data. A re-analysis

This paper presents a new analysis of the contrast flash data of Alpern et al. (1970a--d). It was prompted by the criticism of Wandell (1976) who pointed out that Alpern et al., main conclusion, i.e, that the inhibitory signal N (phi) elicited by the contrast flash (phi) takes the form (formula: see text), would imply an unrealistic excitatory photo response. The present analysis shows the data to be consistent with an inhibitory signal of the form (formula: see text).

Apparent saturation of blue-sensitive cones occurs at a color-opponent stage

Response saturation of blue-sensitive cone pathways was studied by measuring increment thresholds for violet test flashes on flashed violet fields in the presence of a steady yellow "auxiliary" field of constant radiance. Adding intense yellow field flashes to the violet field flash could eliminate or reduce response saturation (greatly reduce threshold), whereas "negative" yellow field flashes drove the mechanism to further saturation. The response saturation is thus not, in general, controlled exclusively by independent blue-sensitive cones but by spectrally opponent mechanisms that receive opposite-signed signals from blue-sensitive cones and from green-or red-sensitive cones. These results add to a growing number of studies that demonstrate that detection of signals from blue-sensitive cones is largely through a color-opponent pathway.