The kinetics of cone visual pigments in man

Genetic manipulation of rod-cone differences in mouse retina

Though rod and cone photoreceptors use similar phototransduction mechanisms, previous model calculations have indicated that the most important differences in their light responses are likely to be differences in amplification of the G-protein cascade, different decay rates of phosphodiesterase (PDE) and pigment phosphorylation, and different rates of turnover of cGMP in darkness. To test this hypothesis, we constructed TrUx;GapOx rods by crossing mice with decreased transduction gain from decreased transducin expression, with mice displaying an increased rate of PDE decay from increased expression of GTPase-activating proteins (GAPs). These two manipulations brought the sensitivity of TrUx;GapOx rods to within a factor of 2 of WT cone sensitivity, after correcting for outer-segment dimensions. These alterations did not, however, change photoreceptor adaptation: rods continued to show increment saturation though at a higher background intensity. These experiments confirm model calculations that rod responses can mimic some (though not all) of the features of cone responses after only a few changes in the properties of transduction proteins.

Light responses of mammalian cones

Cone photoreceptors provide the foundation of most of human visual experience, but because they are smaller and less numerous than rods in most mammalian retinas, much less is known about their physiology. We describe new techniques and approaches which are helping to provide a better understanding of cone function. We focus on several outstanding issues, including the identification of the features of the phototransduction cascade that are responsible for the more rapid kinetics and decreased sensitivity of the cone response, the roles of inner-segment voltage-gated and Ca²⁺-activated channels, the means by which cones remain responsive even in the brightest illumination, mechanisms of cone visual pigment regeneration in constant light, and energy consumption of cones in comparison to that of rods.

The Effect of Blue-blocking Lenses on Photostress Recovery Times

SIGNIFICANCE

The selective reduction in visible wavelengths transmitted through commercially available blue-blocking lenses (BBLs) is known to influence the appearance and contrast detection of objects, particularly at low light levels. This influence may impair the human retinal receptor response time to dynamic light changes during photostress events.

PURPOSE

This study aimed to assess whether BBLs selectively affect photostress recovery times (PSRTs) for chromatic and achromatic stimuli of different Weber contrasts that were viewed on a dark black background.

METHODS

Photostress recovery times were measured in 12 younger participants (18 to 39 years old) with no history of ocular disease or abnormal vision. Photostress recovery times were evaluated for four brands of BBLs, which were compared with a control lens. In these experiments, after exposure to an intense light source for 5 seconds, the time taken to recover vision and correctly identify a computer-generated letter stimulus viewed under low and high luminance levels was determined, which means perception is likely to be governed by mesopic and photopic conditions. Across conditions, the letter stimulus was achromatic and chromatic and could differ in luminance contrast.

RESULTS

Under photopic stimulus conditions, although reducing luminance contrast increased PSRTs, BBLs had no significant effect on PSRTs relative to control lens. However, under mesopic stimulus conditions, BBLs significantly affect PSRTs for both achromatic (F2.006,8.02 = 61.95, P < .0001) and chromatic stimuli (F3,16 =139.01, P < .0001), particularly for blue targets, which had considerably longer PSRTs (38.40 seconds). The brand of BBL was also shown to selectively affect PSRTs, with those with transmittance profiles that block the most blue light having longer PSRTs.

CONCLUSIONS

The present study suggests that, although the color and contrast of the target stimuli affected recovery times, the difference in recovery times between different types of BBLs was noticed only under low-light-level stimulus conditions.

Changes in spatial extent and peak double optical density of human macular pigment with age

The purpose of the present work was to estimate the changes in spatial distribution and optical density of macular pigment (MP) with age. A fundus imaging system with high spatial and spectral resolution was adapted to form an indirect ophthalmoscope. The double optical density at 490 nm of the MP as a function of the location in the retina was obtained for 33 healthy subjects (ages: 21-60 years). There was an increase in spatial extent and decrease in double optical density with age. Furthermore, the spatial distribution of MP showed central areas with irregular shapes and a tendency toward asymmetry.

Novel snapshot imaging of photoreceptor bleaching in macaque and human retinas

PURPOSE

Various methods have been used to obtain a topographic map of bleached photopigments in human retinas in the past. The purpose of this study was to determine whether the bleaching topography of the photoreceptors could be obtained by snapshot imaging reflectometry.

METHODS

Four to five fundus photographs of one rhesus monkey and three healthy human subjects were taken by white flashes at intervals of 4 s, with a commercial fundus camera with minimal modifications. The flash-induced reflectance increases (bleaching) were calculated by dividing the reflectance of the first image into the subsequent images, pixel by pixel.

RESULTS

The topography of the bleached macula corresponded well with the anatomical distribution of the cones. The ratio of reflectance changes in the center to that in the surrounding tissue was high for red and low for green and blue images. These results indicate that the reflectivity changes were not artifacts but were derived from changes in the photopigment density in the cones and rods.

CONCLUSIONS

The topography of bleached photoreceptors obtained with a commercial fundus camera from one monkey and three healthy human subjects showed that this technique has potential as a new clinical method for examining photoreceptor function in both normal and diseased retinas.

The macular automated photostress test

PURPOSE

To introduce a standardized macular photostress test using an automated perimeter as a method to quantify macular disease severity and as a tool to distinguish optic neuropathy from macular pathology.

DESIGN

Prospective interventional pilot study.

METHODS

Twenty-five bilaterally pseudophakic subjects aged range, 65 to 84: 15 patients with varying severity of non-neovascular age-related macular degeneration (AMD), five patients with no ocular disease, and five patients with moderate primary open-angle glaucoma (POAG). Previously reported normative values served as controls for this study. Patients underwent foveal threshold testing using the Humphrey Visual Field Perimeter Model 750 (Carl Zeiss Meditec, Inc, Dublin, California, USA). Baseline measurements were compared to threshold sensitivity after photostress at one minute and then two-minute intervals until sensitivity returned to baseline. Main outcome measures were baseline foveal threshold sensitivity, foveal threshold depression, and recovery following photostress.

RESULTS

Automated macular photostress testing in macular disease (AMD) causes a decrease (P < .001) in baseline foveal sensitivity and a delay (P < .001) in recovery time to baseline sensitivity. Optic nerve pathology (POAG) does not affect (P = .343) the foveal response curve.

CONCLUSIONS

The macular automated photostress (MAP) test is an inexpensive, noninvasive, and readily accessible adjunct for evaluating patients with macular disease. This standardized protocol is useful in objectively defining disease severity, may be used to follow response to treatment, and could aid in distinguishing optic neuropathy from macular pathology.

Optical measurement of human retinal macular pigment and its spatial distribution with age

The existence of macular pigment (MP) in human eyes has been found to be of importance in the prevention of age-related macular degeneration (ARMD). A noninvasive technique of two-wavelength imaging fundus reflectometry was developed to measure the density distribution of this yellow pigment in retina in vivo. A total of 55 healthy human subjects were recruited and divided into three groups with average ages and sample sizes of 25.2+/-2.1 (N = 25), 40.2+/-8.3 (N = 13), and 67.5+/-7.1 years (N = 17), respectively. Only the MP distributions in the horizontal meridian of retinas are presented in this report. The results show that the average peak optical density of MP are 0.23+/-0.07, 0.21+/-0.05, and 0.25+/-0.06 from young to old groups, respectively. No significant difference is shown in the statistical t-test between the groups of the peak MP density. The half width of MP spatial distribution (HWMPD) are 4.8 degrees+/-1.5 degrees, 5.1 degrees+/-2.1 degrees and 7.7 degrees+/-2.0 degrees (1 degree = approximately 0.3 mm) from young to old groups, respectively. Mean HWMPD = approximately 5.6 degrees +/- 2.1 degrees. It shows that the area of MP is increased approximately by a rate of 0.06 degrees/yr according to the application of linear regression. However, the cause of the MP extension in the retina with age is unknown.

Interaction of 11-cis-retinol dehydrogenase with the chromophore of retinal g protein-coupled receptor opsin

Vertebrate opsins in both photoreceptors and the retinal pigment epithelium (RPE) have fundamental roles in the visual process. The visual pigments in photoreceptors are bound to 11-cis-retinal and are responsible for the initiation of visual excitation. Retinochrome-like opsins in the RPE are bound to all-trans-retinal and play an important role in chromophore metabolism. The retinal G protein-coupled receptor (RGR) of the RPE and Müller cells is an abundant opsin that generates 11-cis-retinal by stereospecific photoisomerization of its bound all-trans-retinal chromophore. We have analyzed a 32-kDa protein (p32) that co-purifies with bovine RGR from RPE microsomes. The co-purified p32 was identified by mass spectrometric analysis as 11-cis-retinol dehydrogenase (cRDH), and enzymatic assays have confirmed the isolation of an active cRDH. The co-purified cRDH showed marked substrate preference to 11-cis-retinal and preferred NADH rather than NADPH as the cofactor in reduction reactions. cRDH did not react with endogenous all-trans-retinal bound to RGR but reacted specifically with 11-cis-retinal that was generated by photoisomerization after irradiation of RGR. The reduction of 11-cis-retinal to 11-cis-retinol by cRDH enhanced the net photoisomerization of all-trans-retinal bound to RGR. These results indicate that cRDH is involved in the processing of 11-cis-retinal after irradiation of RGR opsin and suggest that cRDH has a novel role in the visual cycle.

Preferential release of 11-cis-retinol from retinal pigment epithelial cells in the presence of cellular retinaldehyde-binding protein

In photoreceptor cells of the retina, photoisomerization of 11-cis-retinal to all-trans-retinal triggers phototransduction. Regeneration of 11-cis-retinal proceeds via a complex set of reactions in photoreceptors and in adjacent retinal pigment epithelial cells where all-trans-retinol is isomerized to 11-cis-retinol. Our results show that isomerization in vitro only occurs in the presence of apo-cellular retinaldehyde-binding protein. This retinoid-binding protein may drive the reaction by mass action, overcoming the thermodynamically unfavorable isomerization. Furthermore, this 11-cis-retinol/11-cis-retinal-specific binding protein potently stimulates hydrolysis of endogenous 11-cis-retinyl esters but has no effect on hydrolysis of all-trans-retinyl esters. Apo-cellular retinaldehyde-binding protein probably exerts its effect by trapping the 11-cis-retinol product. When retinoid-depleted retinal pigment epithelial microsomes were preincubated with different amounts of all-trans-retinol to form all-trans-retinyl esters and then [3H]all-trans-retinol was added, as predicted, the specific radioactivity of [3H]all-trans-retinyl esters increased during subsequent reaction. However, the specific radioactivity of newly formed 11-cis-retinol stayed constant during the course of the reaction, and it was largely unaffected by expansion of the all-trans-retinyl ester pool during the preincubation. The absence of dilution establishes that most of the ester pool does not participate in isomerization, which in turn suggests that a retinoid intermediate other than all-trans-retinyl ester is on the isomerization reaction pathway.

S-cone contribution to pupillary responses evoked by chromatic flash offset

On a green or red background, the action spectrum of the pupillary responses evoked following the offset of chromatic test flashes shows a prominent short-wavelength lobe and suggests the contribution from photoreceptors other than the previously inferred M- and L-cones (Kimura & Young, Vision Research (1996). 36, 1543-1550), most likely from S-cones. Systematic changes in the shape of the intensity versus amplitude functions with test wavelengths and in the shape of the short-wavelength lobe with response amplitude criteria suggest an antagonistic interaction involving the short- and longer-wavelength photoreceptors.

Molecular characterization of a novel short-chain dehydrogenase/reductase that reduces all-trans-retinal

The reduction of all-trans-retinal in photoreceptor outer segments is the first step in the regeneration of bleached visual pigments. We report here the cloning of a dehydrogenase, retSDR1, that belongs to the short-chain dehydrogenase/reductase superfamily and localizes predominantly in cone photoreceptors. retSDR1 expressed in insect cells displayed substrate specificities of the photoreceptor all-trans-retinol dehydrogenase. Homology modeling of retSDR1 using the carbonyl reductase structure as a scaffold predicted a classical Rossmann fold for the nucleotide binding, and an N-terminal extension that could facilitate binding of the enzyme to the cell membranes. The presence of retSDR1 in a subset of inner retinal neurons and in other tissues suggests that the enzyme may also be involved in retinol metabolism outside of photoreceptors.

Reduction of all-trans-retinal limits regeneration of visual pigment in mice

Absorption of photons by pigments in photoreceptor cells results in photoisomerization of the chromophore, 11-cis-retinal, to all-trans-retinal and activation of opsin. Photolysed chromophore is converted back to the 11-cis-configuration via several enzymatic steps in photoreceptor and retinal pigment epithelial cells. We investigated the levels of retinoids in mouse retina during constant illumination and regeneration in the dark as a means of obtaining more information about the rate-limiting step of the visual cycle and about cycle intermediates that could be responsible for desensitization of the visual system. All-trans-retinal accumulated in the retinas during constant illumination and following flash illumination. Decay of all-trans-retinal in the dark following constant illumination occurred without substantial accumulation of all-trans-retinal, generated by constant approximately equal to visual pigment regeneration (t1/2 approximately 5 and t1/2 approximately 7 min, respectively). All-trans-retinal, generated by constant illumination, decayed approximately 3 times more rapidly than that generated by a flash and, as shown previously, the rate of rhodopsin regeneration following a flash was approximately 4 times slower than after constant illumination. The retinyl ester pool (> 95% all-trans-retinyl ester) did not show a statistically significant change in size or composition during illumination. In addition, constant illumination increased the amount of photoreceptor membrane-associated arrestin. The results suggest that the rate-limiting step of the visual cycle is the reduction of all-trans-retinal to all-trans-retinol by all-trans-retinol dehydrogenase. The accumulation of all-trans-retinal during illumination may be responsible, in part, for the reduction in sensitivity of the visual system that accompanies photobleaching and may contribute to the development of retinal pathology associated with light damage and aging.

Fourier models and the loci of adaptation

First measures of sensitivity and the need for a model to interpret them are addressed. Then modeling in the Fourier domain is promoted by a demonstration of how much an approach explains spatial sensitization and its dependence on luminance. Then the retinal illuminance and receptor absorptions produced by various stimuli are derived to foster interpretation of the neural mechanisms underlying various psychophysical phenomena. Finally, the sequence and the anatomical loci of the processes controlling visual sensitivity are addressed. It is concluded that multiplicative adaptation often has effects identical to response compression followed by subtractive adaptation and that, perhaps as a consequence, there is no evidence of retinal gain changes in human cone vision until light levels are well above those available in natural scenes and in most contemporary psychophysical experiments; that contrast gain control fine tunes sensitivity to patterns at all luminances; and that response compression, modulated by subtractive adaptation, predominates in the control of sensitivity in human cone vision.

Movement of retinal along cone and rod photoreceptors

Single isolated photoreceptors can be taken through a visual cycle of light adaptation by bleaching visual pigment, followed by dark adaptation when supplied with 11-cis retinal. Light adaptation after bleaching is manifested by faster response kinetics and a permanent reduction in sensitivity to light flashes, presumed to be due to the presence of bleached visual pigment. The recovery of flash sensitivity during dark adaptation is assumed to be due to regeneration of visual pigment to pre-bleach levels. In previous work, the outer segments of bleached, light-adapted cells were exposed to 11-cis retinal. In the present work, the cell bodies of bleached photoreceptors were exposed. We report a marked difference between rods and cones. Bleached cones recover sensitivity when their cell bodies are exposed to 11-cis retinal. Bleached rods do not. These results imply that retinal can move freely along the cone photoreceptor, but retinal either is not taken up by the rod cell body or retinal cannot move from the rod cell body to the rod outer segment. The free transfer of retinal along cone but not along rod photoreceptors could explain why, during dark adaptation in the retina, cones have access to a store of 11-cis retinal which is not available to rods. Additional experiments investigated the movement of retinal along bleached rod outer segments. The results indicate that retinal can move along the rod outer segment, but that this movement is slow, occurring at about the same rate as the regeneration of visual pigment.

Mapping cone photopigment optical density

The distribution of cone photopigment across the retina affects the amount of light captured by cones at each retinal location. Cone photopigment optical density is measured in two ways, with reflectometry and/or with color matching. Color matching measures a higher optical density than does reflectometry. Control experiments confirm that large-field color matches measure photopigment optical density toward their outer edge. There is qualitative agreement as to photopigment distribution from both techniques near the fovea. Beyond 1 deg, color matching indicates little decrease in photopigment with increasing eccentricity, whereas retinal densitometry shows a steep decline in photopigment. The decrease in perifoveal optical density measured with reflectometry is attributed to the decrease in cone coverage from fovea to perifovea as rods and interphotoreceptor spaces increase. Differences among subjects in photopigment distribution near the fovea, measured with both techniques, reflect differences in the specialization of the foveal center for cone length and/or photopigment concentration per cone, which are factors influencing results from both techniques.

Reflectometry with a scanning laser ophthalmoscope

We describe noninvasive techniques to optimize reflectometry measurements, particularly retinal densitometry, which measures the photopigment density difference. With these techniques unwanted scattered light is greatly reduced, and the retina is visualized during measurements. Thus results may be compared for each retinal location, and visible artifacts are minimized. The density difference measurements of the cone photopigment depend on the optical configuration of the apparatus. The cone photopigment density difference is greatest near the fovea and for most observers decreases rapidly with eccentricity. A research version for reflectometry and psychophysics of the scanning laser ophthalmoscope is described.

Dark-adaptation mechanisms of the long-wave foveal cones

The ordinary long-term rod and cone dark-adaptation curves have generally been assumed to follow a single exponential rate of recovery. However, in two previous papers on rod dark-adaptation (Stabell et al., 1986a, b), the recovery curve was found to consist of three different sections. The results of the present paper show the same type of recovery function with three different sections for the long-term dark-adaptation curve of the long-wave cone system. During the major, middle section log cone threshold, like log rod threshold, is linearly related to the logarithm of the concentration of bleached photopigment. Presupposing that the bleached cone photopigment acts as a ligand, the change in threshold level obtained during the middle section of the dark-adaptation curve is well described by the change in activity rate of an allosteric, postively cooperative enzyme built as a dimer.

Human macular pigment assessed by imaging fundus reflectometry

A computerized, television-based, imaging fundus reflectometer was used to obtain estimates of the spatial distribution of macular pigment (xanthophylls) from seven normal subjects. Digitized images of the bleached macula of each subject were acquired at illuminating wavelengths from 462 to 697 nm. An analysis of spectral reflectances indicated that differences in short-wavelength reflectance between the foveal center and parafovea were influenced by spatial variations in melanin and oxyhemoglobin absorption as well as by the distribution of macular pigment. To provide an estimate of the spatial distribution of macular pigment alone, we have corrected fundus images obtained at 462 nm for the effect of melanin and oxyhemoglobin absorption. The spatial variation in macular pigment double density across the horizontal and vertical meridians of the retina was well described by Gaussian functions. The peak double densities for the individual subjects ranged from 0.22 to 0.45 and the standard deviations of the Gaussian functions averaged approx. 1 degree.

Human rhodopsin measurement using a T.V.-based imaging fundus reflectometer

An imaging fundus reflectometer for in vivo mapping of rhodopsin levels is described. The instrument is based on a high-sensitivity television camera attached to a Zeiss fundus camera, which enables areas of retina of angular subtense 25 degrees to be examined at a resolution of about 1 degree. Digital processing techniques are used to average the video signal spatially and temporally and to analyse the spatial information. Measurements with an artificial eye indicate that performance is comparable to that of photomultiplier-based systems. Rhodopsin levels and regeneration data for a normal human subject are presented; these are consistent with published values. The map of visual pigment levels derived from these normal data is contrasted with that for a subject with a patchy retinal dysfunction (autosomal dominant retinitis pigmentosa).

Densitometric measurement of human cone photopigment kinetics

We measured the radiance response function for steady state bleaching lights and the regeneration of the cone visual photopigments using the continuous recording densitometer described by v. Norren and v.d. Kraats. Measurements made on 5 deuteranopes, 1 protanope and 2 color-normal observers were similar. The radiance response function was steeper than the function predicted by a simple first-order kinetic equation. For a measured density (ca 0.3) we evaluated whether high stray light (ca 47.5%) and high two-way optical density (ca 1.3) could account for the deviation from the prediction of a first-order equation. Such a model was rejected because these parameters predicted different estimates of the time course of regeneration for different test wavelengths (554 and 605 nm). Statistical analysis of the regeneration data revealed a highly significant non-linearity. A model in which the rate of regeneration increases as the proportion of bleached photopigment decreases is required to explain both the radiance function and the regeneration data.

Determination of human cone pigment density difference spectra in spatially resolved regions of the fovea

A new method was developed to measure spectrally and spatially resolved cone pigment optical density difference in normal human subjects. Using digitized television images of human retinas scanned before and after bleaching the cone pigments with a bright light, unique high-resolution images of cone pigment density difference were produced. The spectral peak density difference within the fovea was found to be at 560 nm. These measurements demonstrate a decrease in cone pigment optical density difference with increased distance from the subject's central fixation point in the fovea. The cone pigment density difference was asymmetrically distributed in the fovea with higher amounts on the nasal side.

Dark adaptation with interposed white adapting fields

It is proposed that dark adaptation following a moderate pigment bleach may nearly as well be carried out (and more conveniently) under low room lighting conditions as in complete darkness. To test this idea, dark adaptation curves were determined either immediately after the termination of a 3 min, 4.1 log td white pre-exposure field, or following 10 or 15 min of additional exposure to one of three low-level photopic (2.9, 2.4, 1.8 log td) backgrounds of white light. Dark thresholds measured after the additional exposure fell rapidly and reached the rod plateau of the normal dark adaptation curve with a maximal delay of 1.5 min (for the 10 min backgrounds) or 6.5 min (for the 15 min backgrounds). For the time to be spent in the dark, this meant a savings of 8.5 min. At smaller delays savings were even greater. The difference between savings and delay indicates whether or not an interposed background is feasible.

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.

Cone pigments in acute posterior multifocal placoid pigment epitheliopathy

We measured cone pigment density and kinetics with a reflection retinal densitometer during the evolution and resolution of posterior pole lesions in a 17-year-old patient with acute posterior multifocal placoid pigment epitheliopathy. Three weeks after onset, density was markedly reduced and regeneration complex and nonexponential. Pigment density increased, and kinetics returned to normal as the disease resolved.

Paradoxical pupil in congenital achromatopsia

Children with congenital achromatopsia possess an interesting paradoxical pupillary constriction to darkness that has not been previously described. This paper describes three children in whom this paradoxical pupillary reaction was documented with infrared pupillometry. The literature on the pupil in achromatopsia is reviewed and a tentative testable model of the reaction is hypothesized.

The photostress recovery test in the clinical assessment of visual function

To distinguish optic nerve conduction defects from macular disease in patients with otherwise unexplained loss of central vision we first determined the best visual acuity with correction at distance in unilateral defects. The normal eye was tested first and photostressed for ten seconds by looking at an ordinary penlight held 2 to 3 cm from the eye. The time required to read three letters on three Snellen test lines just larger than the best acuity was used as the end point. In 63 eyes with maculopathy the recovery time was prolonged. Prolonged recovery time was not observed in 20 patients who had optic nerve disease.

The density and photosensitivity of human rhodopsin in the living retina

1. The visual pigment in a 5 degrees circular patch of the living human retina 18 degrees temporal from the fovea was studied with the Rushton retinal densitometer. The measuring light (570 nm) was selected to obviate artifacts from colour photoproducts.2. The action spectrum of a 10% bleach agrees well with the action spectrum at absolute threshold for the same patch of retina. The quantized C.I.E. scotopic spectral sensitivity curve is a good description of both spectra. Therefore, the visual pigment studied must be human rhodopsin.3. Its density has been estimated in five different ways. The results are in reasonable agreement. The optical density of human rhodopsin in vivo is about 0.35 (common logarithmic units) at its gamma(max.)4. The photosensitivity of human rhodopsin in vivo was determined by studying its rate of bleaching in response to steps of monochromatic light exposed to the dark adapted eye, by measuring the amount bleached in the steady state by monochromatic lights as well as the amount bleached by 10 sec flashes of white light.5. The results obtained by the different methods are in good agreement with each other and with previous estimates made by others using white light.6. The photosensitivity of human rhodopsin in vivo [epsilongamma(max) = 62,000 to 120,000 l./cm mole] is much higher than expected from in vitro measurements.

Dark adaptation and visual pigment regeneration in human cones

Foveal threshold elevation and red-green cone pigment regeneration have been studied in the dark after a wide range of bleaches in normal man with a view to probing the limits of the application of the Dowling-Rushton relation: i.e., the direct proportionality between log threshold elevation and fraction of unregenerated pigment. Cone pigment regeneration (and threshold recovery) is much faster after short bleaches than expected from the kinetics of a simple monomolecular reaction. Recovery is faster after a fixed (short) duration bleach the weaker it is. Except for the first 30 s after relatively weak bleaches and the entire recovery after a very brief (<0.001 s) saturating bright flash which bleaches a little more than 50%, the results are accurately fit by the Dowling-Rushton relation over the entire range tested with only one arbitrary constant (the proportionality factor). Theory predicts too low threshold in comparison with what is obtained, for both of these exceptions

Rhodopsin bleaching signals in essential night blindness

1. The dark-adaptation curves of two subjects with essential night blindness revealed no evidence for functioning rod vision. Cone vision was normal.2. The photopupillomotor dark adaptation, and flash intensity response amplitude curves on one of these subjects confirmed the absence of rod function.3. However, there is the normal amount of rhodopsin in their rods with normal kinetics.4. Cone pigment kinetics are also nearly normal. After a full bleach, log threshold elevation of the foveal cones is linearly related to pigment regeneration. The constant of proportionality is about 3.0 as it is in the normal retina.5. After a full rhodopsin bleach, the contralateral pupil size recovered its full dark value along a curve which followed the regeneration of rhodopsin.6. The results in (5) are identical to those previously found on normal subjects.7. With the exception of a very small response attributed to the contribution of cones, no significant changes in pupil size were evoked by uniform ganzfeld steady backgrounds until the intensity of retinal illuminance was so high that appreciable rhodopsin was bleached. This contrast to the changes evoked by weak steady backgrounds in the normal eye.8. Therefore, rod bleaching signals are normal in such retinas but rod signals evoked by real lights are not functional. This supports Rushton's concept as to how bleaching signals influence retinal sensitivity as opposed to the view of Barlow.9. The defect in essential night blindness very probably involves the rod automatic gain control, but because of (4) the cone gain control must be normal.10. Therefore, rod and cone gain control mechanisms must be independent in these night blind retinas and, by analogy, in the normal retina as well.