Quantum Catch Research Articles

Adaptational effects of short wave cone signals on red-green chromatic detection

The red-green chromatic detection mechanism that responds to the difference of L and M cone test signals was isolated in forced-choice experiments. Detection contours in an L, M cone space were measured with 2 Hz Gabor test signals comprising different amplitude ratios of antiphase flickering red and green lights, which formed the 1.2° center of a 7.2° uniform adapting field. To compare the effect of short wave cone adapting levels on the red-green detection sensitivity, thresholds were measured on pairs of adapting fields that were shown to be tritanopic metamers for our individual observers: violet and green adapting lights that produce equal quantal catches in M cones and in L cones but very differently stimulate S cones. The degree to which the adapting field stimulated S cones had little effect on the red-green detection sensitivity, although the red appearance of the adapting field varied considerably owing to the S cone stimulation. Thus, while the S cones may affect the red-green hue dimension, S cone signals appear to have little adaptational effect on red-green detection mediated by the difference of L and M cone test signals.

Photon noise and constant-volume operators.

In an earlier paper [J. Opt. Soc. Am. A 2, 1769 (1985)] a class of nonlinear image processing operators was introduced in which each photoreceptor creates a nonnegative point-spread function whose center height is proportional to its quantum catch and whose volume is constant, so that the local spatial-summation area varies inversely with the local quantum catch. These constant-volume (CV) operators are designed to maximize spatial resolution in the presence of photon noise. In the previous paper it was shown that when CV operators are applied to deterministic images, they produce a surprising range of effects that are reminiscent of human vision, including Mach bands and Weber's-law behavior. In this paper the consequences of applying CV operators to images containing Poisson noise are analyzed. It is shown that a fixed-parameter CV operator can duplicate the global qualitative properties of spatial vision for retinal illuminances ranging from absolute threshold to 1000 Td. Although there are fundamental obstacles to modeling the exact quantitative properties of human spatial vision by CV operators, these operators seem likely to be useful in machine vision.

Foveal sensitivity changes in retinitis pigmentosa

Loss in foveal sensitivity in retinitis pigmentosa (RP) has been attributed to a decrease in quantal catching ability. Using a psychophysical technique (the probe-flash paradigm), we previously found that the results obtained for six RP patients under light adapted conditions were not consistent with a quantal catch hypothesis. To test further this hypothesis twelve RP patients were examined in dark adapted conditions. Probe thresholds were normal for five patients and increased for seven patients. The decreased quantal catching hypothesis was rejected for six of the seven patients.

Chromatic suppression of cone inputs to the luminance flicker mechanism

Eisner and MacLeod [ J. opt. Soc. Am. 71, 705–718 (1981)] showed that intense green and red chromatic adapting fields may suppress respectively the M and L cone input to the luminance mechanism by a factor considerably greater than Weber's law. We obtained evidence for such chromatic suppression by measuring complete detection contours for different ratios of red and green test lights presented in rapid flicker in the center of a uniform field. The detection contours represent thresholds as the quantal modulation of the M and L cones normalized by the quantal catch owing to the field. Luminance flicker mechanisms were identified by sections of the contours where detection was controlled by a linear sum of the M and L cone test signals. The slope of these sections indicated that intense red fields selectively suppressed the L cone input to the luminance mechanism by a factor greater than Weber's law; evidence was much less firm for an analogous suppression of the M cone input by intense green fields. The shape of the detection contours also suggests that intense red fields, which differentially light-adapt the M and L cones, may produce a moderate temporal phase-shift between the M and L cone signals. The shape of the temporal MTF of the M cone and the L cone input to the luminance mechanism may be determined at the cone stage, with the absolute sensitivity (vertical scaling) being partially dependent on selective chromatic suppression of the cone inputs owing to the intense chromatic field. Luminance and red-green chromatic temporal sensitivity functions are presented in terms of the M and L cone quantal modulations. Chromatic sensitivity progressively rises above luminance sensitivity as temporal frequency is gradually lowered below 15 Hz, with the consequence that ‘contrast sensitivity’ may be much higher for color than for luminance.

Short-wavelength-sensitive cones do not contribute to mesopic luminosity.

It has been suggested that the short-wavelength-sensitive cones (S cones) play a significant role in the transition from scotopic to photopic vision (the Purkinje shift). We address this issue directly over a 5-log10-unit range of light levels covering scotopic, mesopic, and photopic vision. At each light level we make flicker matches to two reference stimuli by 2-Hz flicker photometry. The two reference lights (441 and 481 nm) differ only in their stimulation of S This novel technique utilizes the different magnitudes of the rod and cone Stiles--Crawford effects. Despite the large difference in S-cone stimulation by the two reference lights (more than 1 log10 unit), the pairs of luminosity functions are indistinguishable at each light level tested. The results indicate that S cones do not contribute to either photopic or mesopic luminosity.

Flicker electroretinogram in retinitis pigmentosa.

Electroretinograms (ERGs) were recorded as a function of flicker frequency from 5 to 50 Hz for 14 retinitis pigmentosa (RP) patients, 12 normal subjects and 1 rod monochromat. Data were analyzed by measuring the angular position of the response maximum, i.e. the phase, as a function of pulse-train frequency. Flicker ERGs obtained from the RP patients showed non-linear, frequency-dependent phase shifts when compared to the normal data. These phase shifts were simulated in a normal observer by attenuating the stimulus luminance by 1 log unit. However, the shape of the waveforms recorded from the normal differed markedly from those recorded from the RP patients. The differences, but not the ratios of the times-to-peak of the positive and negative ERG wavelets were longer in the RP patients than in the normal. These data suggest that the temporal anomalies in the RP flicker ERG are most likely due to changes in the amplitudes and time constants of the ERG components, and not simply to a reduced quantum catch or photoreceptor loss.

Responses of crayfish photoreceptor cells following intense light adaptation.

After intense orange adapting exposures that convert 80% of the rhodopsin in the eye to metarhodopsin, rhabdoms become covered with accessory pigment and appear to lose some microvillar order. Only after a delay of hours or even days is the metarhodopsin replaced by rhodopsin (Cronin and Goldsmith 1984). After 24 h of dark adaptation, when there has been little recovery of visual pigment, the photoreceptor cells have normal resting potentials and input resistances, and the reversal potential of the light response is 10-15 mV (inside positive), unchanged from controls. The log V vs log I curve is shifted about 0.6 log units to the right on the energy axis, quantitatively consistent with the decrease in the probability of quantum catch expected from the lowered concentration of rhodopsin in the rhabdoms. Furthermore, at 24 h the photoreceptors exhibit a broader spectral sensitivity than controls, which is also expected from accumulations of metarhodopsin in the rhabdoms. In three other respects, however, the transduction process appears to be light adapted: The voltage responses are more phasic than those of control photoreceptors. The relatively larger effect (compared to controls) of low extracellular Ca++ (1 mmol/l EGTA) in potentiating the photoresponses suggests that the photoreceptors may have elevated levels of free cytoplasmic Ca++. The saturating depolarization is only about 30% as large as the maximal receptor potentials of contralateral, dark controls, and by that measure the log V-log I curve is shifted downward by 0.54 log units.(ABSTRACT TRUNCATED AT 250 WORDS)

Second-site adaptation in the red-green chromatic pathways

On different chromatic adapting fields, thresholds were measured with a 1.2 deg flash consisting of simultaneous incremental and decremental red and green components that stimulate the M and L cones in any desired ratio. Thresholds were plotted in normalized coordinates in which the quantal change in the M and L cones due to the flash was divided by the quantal catch due to the field. Detection contours for a wide range of test flashes provide evidence for luminance and chromatic mechanisms that respectively respond to the sum and difference of the M and L cone signals. Field color has little influence on the luminance mechanism but strongly affects chromatic detection, with sensitivity being maximal on yellow fields and declining slightly on green fields and declining strongly on red fields. Similar effects were obtained for long (200 msec) and very brief flashes, although the shape of the contours differed considerably. The results provide evidence for a second adaptation site within the red-green chromatic pathways, similar to the second-site in the S cone pathways. Chromatic fields (green and red) polarize the site and reduce sensitivity to chromatic flashes.

Neuropeptide modulation of photosensitivity. II. Physiological and anatomical effects of substance P on the lateral eye of Limulus

A system of efferent substance P-like immunoreactive fibers innervates the ommatidia of the Limulus lateral eye. Thus, we tested the physiological effects of substance P on the lateral eye by measuring the electroretinogram, a population potential reflecting the photoreceptors' response to light, under different experimental conditions. Substance P had no direct effect on the photoreceptors, but it induced an increase in their responsiveness to test flashes of light. The latency, magnitude, and duration of this reversible modulatory effect was dose-dependent. The lateral eye displays an endogenous circadian rhythm in its responsiveness to light. Application of exogenous substance P in the daytime causes an immediate rise as well as an increase in the nocturnal peak, while injection of one of its antagonists (D-Pro2, D-Phe7, D-Trp9 substance P) in the afternoon retards the normal rise in sensitivity and reduces the nighttime levels. Passive incubation with substance P antibodies at midnight caused a drop to diurnal levels of photosensitivity. Short-term changes in photosensitivity, similar in their nature to the substance P-induced ones, were caused by arousing the subjects. Arousal had an effect on the ongoing circadian rhythm similar to that of substance P application. Thus, the substance P efferent system may regulate neural responsiveness in both a short-term, environmentally induced manner, as well as for level setting in a circadian fashion. The mechanism for substance P-induced increases in photosensitivity involves changes in ommatidial structure: contraction of distal pigment cells, resulting in an increased aperture, and contraction of the retinular cells and rhabdom, resulting in a wider diameter of the latter. These structural modifications result in a greater angle of acceptance and increased light quantum catch.

Intrinsic noise in locust photoreceptors.

1. In locust photoreceptors, the amplitude of the response to light pulses lasting less than 20 ms depends solely upon the number of absorbed photons, which can be estimated at low intensities by counting quantum bumps. Consequently, each receptor can be operated as a calibrated photon counter. 2. Three types of noise in receptor responses have been identified--extrinsic or photon noise and two types of intrinsic noise, dark noise (spontaneous activity) and transducer noise (noise in the transduction mechanism). The methods by which the noise sources are measured and identified involves measuring the responses to a train of flashes of constant intensity and converting these voltage values into a series of equivalent quantum catches. Because photon absorptions follow the Poisson distribution, the variance among equivalent catches due to photon noise equals the mean catch, and any excess variance represents intrinsic noise. 3. Dark noise is negligible: spontaneous signals (quantum bumps produced in darkness) occur less than ten times per hour at 25 degrees C, and the combined effects of membrane and electrode noise are unimportant at all but the highest intensities. 4. At low intensities transducer noise is responsible for more than 50% of all receptor noise (variance), and this rises to 90% when bright stimuli are presented to the dark-adapted eye. 5. Two simple models of transduction indicate that variations in the amplitudes and latencies of responses to single photons are a major source of transducer noise. 6. Transducer noise would be difficult to detect from an analysis of response noise alone, without knowledge of absolute photon catch, because in some important respects it mimics photon noise, e.g. it lowers the quantum efficiency without violating the square root relationship relating increment thresholds to mean intensity.

Sensitivity losses in a long wavelength sensitive mechanism of patients with retinitis pigmentosa

The foveal vision of certain patients afflicted with retinitis pigmentosa may be mediated solely by a long wavelength sensitivity (1ws) mechanism, because (1) increment threshold data could always be fitted to a single template curve with unity slope, (2) test and field sensitivities differ only by a proportionality constant over the visible spectrum, (3) intense background fields have additive (quantum catch) effects on thresholds, and (4) wavelength differences of bipartite foveal stimuli could not be distinguished. Both test and field sensitivities were appreciably lower than the lws sensitivity values for unaffected deuteranopes. The action spectrum was similar (but not identical) to estimates of the erythrolabe absorption spectrum. The results provide a better understanding of the photopic vision of patients with retinitis pigmentosa and further evidence that their effective optical density of residual cones is reduced.

Sexual dimorphism in a photoreceptor

The recent observation of fluorescence emission from photoreceptor cells in flies1 offers a new opportunity to map the distribution of the various spectral types in a highly organized retinal mosaic. Previous studies had led to the conclusion that in each ommatidium the two central receptor cells are quite distinct from their six neighbours in terms of anatomical projection2,3, visual pigment and physiology4. We show here that in a restricted, well defined region of the male eye only, one of these central cells resembles its six neighbours not only in having the same visual pigment and physiological properties, but also in sending its axon to the same neuropile. A clue to the function of this sexually dimorphic retinal organization is given by the fact that these male-specific cells dominate the frontal-dorsal part of the retina (fovea) which is used by the male to track females in aerobatic chases5,6. We suggest that the male fovea, already known to drive sex-specific higher-order neurones7, has sacrificed colour vision for other more vital kinds of information processing such as improving quantum catch and possibly also contrast transfer or movement detection.

The outer retina and tapetum lucidum of the snook Centropomus undecimalis (Teleostei).

The snook Centropomus undecimalis is a nocturnal fish inhabiting turbid estuaries, bays, and inshore waters. Its retina is duplex, containing rods and short single, long single, and double cones. Cones are arranged in a square mosaic; the rods are extremely numerous and outer segments are short and staggered in depth. Ellipsoid mitochondria undergo morphological alterations along the vitreo–scleral axis including the appearance of elongate cristae and the development of electron-dense matrices which nearly obscure the cristae. Peripheral mitochondria give off tubular elements in the calycal processes. Pigment epithelial cells have long processes which extend vitread to the cone ellipsoids and contain tapetal spheres (0.3 to 0.5 μm). The processes form a diffusely reflecting tapetum about 80 μm thick. In the scotopic eye the tapetum is uncovered when the retinal pigment retreats into the cell bases. The reflecting material is a triglyceride, largely glyceryl tridocosahexaenoate. Its high refractive index (n = 1.50) and the close packing of the spheres cause light to be backscattered. Reflection from the white lipid tapeta is about 50%; quantal catch in the retina because of the tapetum is enhanced by a factor of about 1.5.

A field-additive pathway detects brief-duration, long-wavelength incremental flashes

This paper reports tests of the hypothesis that a 10 msec, 667 nm, foveal test flash, presented upon a steady, 8° background, is detected by a pathway whose sensitivity is monotonically related to a linear functional of photoreceptor quantum catch. The hypothesis is tested by (1) measuring incrementthreshold curves for various background wavelenghts, and (2) measuring threshold upon mixtures of two backgrounds with different wavelengths. The data are consistent, within measurement error, with the hypothesis. We argue, on grounds of plausibility, that the pathway's sensitivity to the 10 msec test flash is controlled not by an arbitrary linear functional, but by the quantum catch of a single class of photoreceptors.

Stiles's II5 color mechanism: tests of field displacement and field additivity properties.

The II5 color mechanism was isolated in two observers with a 667 nm, 200 ms, 1 deg foveal test flash. Stiles's field displacement law and the law of field additivity were tested with increment thresholds upon monochromatic and bichromatic adapting fields. The data of both observers obey the displacement law and exhibit no systematic deviations from additivity. Our results cannot reject the hypothesis that the first log unit of adaptation observed under II5-isolation conditions is determined exclusively by the quantum catch rate of the long-wavelength-sensitive cones.

Detection of long-duration, long-wavelength incremental flashes by a chromatically coded pathway

This paper reports tests of the hypothesis that a 200 msec 667 nm, foveal test flash, presented upon a steady, 8° background, is detected by a pathway whose sensitivity is monotonically related to a linear functional of photoreceptor quantum catch. The measurement conditions used in this paper are the same as those used by Stiles in his π-mechanism analysis. The hypothesis is tested by measuring increment-threshold curves for various background wavelengths and for various mixtures of two backgrounds. The results clearly reject the hypothesis. We propose two models that are consistent with the measurements reported here as well as measurements—using a 10 msec test flash—reported previously. We then discuss related empirical and theoretical work.

Dark‐adaptation in abnormal (RCS) rats studied electroretinographically.

1. Electroretinogram (e.r.g.) responses recorded from dark-reared rats with inherited retinal dystrophy (RCS) showed progressive decline in b-wave ampliture and prolongation of the time to the peak of the b-wave with age when compared with records obtained from dark-reared normal albino rats. 2. Dark-adaptation was followed in RCS and normal rats by recording the light intensity needed to evoke a criterion e.r.g. response at different time intervals after bleaching and 90% of the rhodopsin. 3. In normal rats, dark-adaptation was governed by two mechanisms. The first 25--35 min of recovery was determined by cones. The second branch, determined by the recovery of rods, lasted for about 3 hr and proceeded along an exponential time course with time constant of 41.4 +/- 2.4 min (S.E. of mean). 4. In RCS rats, the time course of the dark-adaptation after a 90% bleach depended on age. In 25--30 day old rats the recovery curve had at least three breaks separating three different mechanisms. Rats, 35--40 days old, exhibited double exponential recovery curves, while 45--70 day old rats recovered along a single exponential curve similar in time course to the cone branch of dark-adaptation found in normal rats. 5. Action spectra obtained from RCS rats at different time intervals of the recovery curve showed that in young rats, 25--30 days old, small e.r.g. responses recorded before bleaching and at the end of the recovery period were determined by rhodopsin while those recorded during the first part of the recovery from 90% bleach were determined by a combination of rods and cones. In RCS rats of advanced age (45--70 days old), rhodopsin was the major contributor to the e.r.g. responses recorded either before bleaching or at the end of the recovery period. 6. The gradual deterioration with age of the e.r.g. in RCS rats cannot be explained by either the decrease in quantum catch due to the decrease in rhodopsin content or by the linear relationship between log e.r.g. threshold and pigment concentration. 7. Using estimates of rhodopsin density within surviving rods obtained from retinal densitometry, it was shown that in RCS rats where more than 30% of normal levels of rhodopsin was located within the functioning rods, the log intensity needed for a criterion e.r.g. response measured at the end of the recovery period from a 90% bleach was linearly related to the fraction of 'functional' rhodopsin. 8. No simple relationship between log e.r.g. threshold and rhodopsin concentration could be found during the course of recovery in the dark from a strong bleaching exposure in RCS rats of all ages.

Electrical and adaptive properties of rod photoreceptors in Bufo marinus. I. Effects of altered extracellular Ca2+ levels.

The effects of altering extracellular Ca(2+) levels on the electrical and adaptive properties of toad rods have been examined. The retina was continually superfused in control (1.6 mM Ca(2+)) or test ringer's solutions, and rod electrical activity was recorded intracellularly. Low-calcium ringer's (10(-9)M Ca(2+)) superfused for up to 6 min caused a substantial depolarization of the resting membrane potential, an increase in light-evoked response amplitudes, and a change in the waveform of the light-evoked responses. High Ca(2+) ringer's (3.2 mM) hyperpolarized the cell membrane and decreased response amplitudes. However, under conditions of either low or high Ca(2+) superfusion for up to 6 min, in both dark-adapted and partially light-adapted states, receptor sensitivity was virtually unaffected; i.e., the V-log I curve for the receptor potential was always located on the intensity scale at a position predicted by the prevailing light level, not by Ca(2+) concentration. Thus, we speculate that cytosol Ca(2+) concentration is capable of regulating membrane potential levels and light-evoked response amplitudes, but not the major component of rod sensitivity. Low Ca(2+) ringer's also shortened the period of receptor response saturation after a bright but nonbleaching light flash, hence accelerating the onset of both membrane potential and sensitivity recovery during dark adaptation. Exposure of the retina to low Ca(2+) (10(-9)M) ringer's for long periods (7-15 min) caused dark-adapted rods to lose responsiveness. Response amplitudes gradually decreased, and the rods became desensitized. These severe conditions of low Ca(2+) caused changes in the dark-adapted rod that mimic those observed in rods during light adaptation. We suggest that loss of receptor sensitivity during prolonged exposure to low Ca(2+) ringer's results from a decrease of intracellular (intradisk) stores of Ca(2+); i.e., less Ca(2+) is thereby released per quantum catch.

Efferent optic nerve fibers mediate circadian rhythms in the Limulus eye.

When the horseshoe crab is kept in constant darkness, the lateral eye produces larger electroretinographic and optic nerve responses at night than during the day. These circadian rhythms are mediated by synchronous bursts of efferent impulses in the optic nerve trunk. The endogenous efferent activity appears to increase both the gain and the quantum catch of the photoreceptors.

Saturation in human cones

Increment threshold intensities were measured for a foveally presented 20msec test flash which fell on a flashed 4.1° white background field. The test field was varied in color (red or green) and size (1° or 0.57°). In one condition (green 0.57° test) the onset asynchrony of the background and test fields was also varied. The increment threshold measurements directly demonstrate saturation of theπ 4 and π 5 color mechanisms and, based on the cone signal equation given by Alpern, Rushton and Torii (1970c), provide estimates of the semi-saturation constant σ. For a given color mechanism, a single value of σ was found to be adequate for all test flash sizes and asynchrony intervals tested; however, these values of σ are about 100 times smaller than those found by Alpern et al. using the contrast-flash technique. The Alpern et al. cone signal equation is essentially a simple gain control model. It is shown that when light intensities are correctly expressed in terms of quantum catch, the gain control model can account explicitly for both the observed cone mechanism saturation in the absence of pigment bleaching and the absence of saturation in the presence of bleaching by steady backgrounds.