INTERMITTENT STIMULATION BY LIGHT : III. THE RELATION BETWEEN INTENSITY AND CRITICAL FUSION FREQUENCY FOR DIFFERENT RETINAL LOCATIONS

When measurements of the critical fusion frequency for white light over a large range of intensities are made with the rod-free area of the fovea, the relation between critical frequency and log I is given by a single sigmoid curve, the middle portion of which approximates a straight line whose slope is 11.0. This single relation must be a function of the foveal cones. When the measurements are made with a retinal area placed 5° from the fovea, and therefore containing both rods and cones, the relation between critical frequency and log I shows two clearly separated sections. At the lower intensities the relation is sigmoid and reaches an upper level at about 10 cycles per second, which is maintained for 1.25 log units, and is followed by another sigmoid relationship at the higher intensities similar to the one given by the rod-free area alone. These two parts of the data are obviously separate functions of the rods at low intensities and of the cones at high intensities. This is further borne out by similar measurements made with retinal areas 15° and 20° from the fovea where the ratio of rods to cones is anatomically greater than at 5°. The two sections of the data come out farther apart on the intensity scale, the rod portion being at lower intensities and the cone portion at higher intensities than at 5°. The general form of the relation between critical frequency and intensity is therefore determined by the relative predominance of the cones and the rods in the retinal area used for the measurements.     

INTERMITTENT STIMULATION BY LIGHT : V. THE RELATION BETWEEN INTENSITY AND CRITICAL FREQUENCY FOR DIFFERENT PARTS OF THE SPECTRUM

1. An optical system is described which furnishes large flickering fields whose brightness, even when reduced with monochromatic filters, is capable of covering the complete range of the relation between critical frequency and intensity. 2. For a centrally located test field of 19° diameter, with light from different parts of the spectrum, the data divide into a low intensity section identified with rod function, and a high intensity section identified with cone function. The transition between the two sections is marked by an inflection point which is sharp, except for 450 and 490 mµ where, though clearly present, it is somewhat rounded. 3. The intensity range covered by the flicker function is smallest in the red, and increases steadily as the wave-length decreases. The increase is due entirely to the extent of the low intensity, rod section which is smallest (non-existent for S. S.) in the red and largest in the violet. The high intensity cone portion for all colors is in the same position on the intensity axis, and the only effect of decreasing wave-length is to shift the rod section to lower intensities without changing its shape. 4. The measurements are faithfully described by two similar equations, one for the rods and one for the cones, both equations being derived from the general stationary state equation already used for various visual functions.     

THE RELATION BETWEEN VISUAL ACUITY AND ILLUMINATION

1. Visual acuity varies in a definite manner with the illumination. At low intensities visual acuity increases slowly in proportion to log I; at higher intensities it increases nearly ten times more rapidly in relation to log I; at the highest illuminations it remains constant regardless of the changes in log I. 2. These variations in visual acuity measure the variations in the resolving power of the retina. The retina is a surface composed of discrete rods and cones. Therefore its resolving power depends on the number of elements present in a unit area. The changes in visual acuity then presuppose that the number of elements in the retina is variable. This cannot be true anatomically; therefore it must be assumed functionally. 3. To explain on such a basis the variations of visual acuity, it is postulated that the thresholds of the cones and of the rods are distributed in relation to the illumination in a statistical manner similar to that of other populations. In addition the rods as a whole have thresholds lower than the cones. Then at low intensities the increase in visual acuity depends on the augmentation of the functional rod population which accompanies intensity increase; and at higher intensities the increase in visual acuity depends on the augmentation of the functional cone population. The number of cones per unit foveal area is much greater than the number of rods per unit peripheral area, which accounts for the relative rates of increase of rod and cone visual acuity with intensity. At the highest illuminations all the cones are functional and no increase in visual acuity is possible. 4. If this division into rod visual acuity and cone visual acuity is correct, a completely color-blind person should have only rod visual acuity. It is shown by a study of the data of two such individuals that this is true. 5. The rod and cone threshold distribution has been presented as a purely statistical assumption. It can be shown, however, that it is really a necessary consequence of a photochemical system which has already been used to describe other properties of vision. This system consists of a photosensitive material in reversible relation with its precursors which are its products of decomposition as well. 6. On the basis of these and other data it is shown that a minimal retinal area in the fovea, which can mediate all the steps in such functions as visual acuity, intensity discrimination, and color vision, contains about 540 cones. Certain suggestions with regard to a quantitative mechanism for color vision are then correlated with these findings, and are shown to be in harmony with accurately known phenomena in related fields of physiology.     

THE VISIBILITY OF MONOCHROMATIC RADIATION AND THE ABSORPTION SPECTRUM OF VISUAL PURPLE

1. After a consideration of the existing data and of the sources of error involved, an arrangement of apparatus, free from these errors, is described for measuring the relative energy necessary in different portions of the spectrum in order to produce a colorless sensation in the eye. 2. Following certain reasoning, it is shown that the reciprocal of this relative energy at any wave-length is proportional to the absorption coefficient of a sensitive substance in the eye. The absorption spectrum of this substance is then mapped out. 3. The curve representing the visibility of the spectrum at very low intensities has exactly the same shape as that for the visibility at high intensities involving color vision. The only difference between them is their position in the spectrum, that at high intensities being 48 µµ farther toward the red. 4. The possibility is considered that the sensitive substances responsible for the two visibility curves are identical, and reasons are developed for the failure to demonstrate optically the presence of a colored substance in the cones. The shift of the high intensity visibility curve toward the red is explained in terms of Kundt''s rule for the progressive shift of the absorption maximum of a substance in solvents of increasing refractive index and density. 5. Assuming Kundt''s rule, it is deduced that the absorption spectrum of visual purple as measured directly in water solution should not coincide with its position in the rods, because of the greater density and refractive index of the rods. It is then shown that, measured by the position of the visibility curve at low intensities, this shift toward the red actually occurs, and is about 7 or 8 µµ in extent. Examination of the older data consistently confirms this difference of position between the curves representing visibility at low intensities and those representing the absorption spectrum of visual purple in water solution. 6. It is therefore held as a possible hypothesis, capable of direct, experimental verification, that the same substance—visual purple—whose absorption maximum in water solution is at 503 µµ, is dissolved in the rods where its absorption maximum is at 511 µµ, and in the cones where its maximum is at 554 µµ (or at 540 µµ, if macular absorption is taken into account, as indeed it must be).     

INTENSITY DISCRIMINATION IN THE HUMAN EYE : I. THE RELATION OFΔI/ITO INTENSITY

New measurements of the brightness difference sensibility of the eye corroborate the data of previous workers which show that ΔI/I decreases as I increases. Contrary to previous report, ΔI/I does not normally increase again at high intensities, but instead decreases steadily, approaching a finite limiting value, which depends on the area of the test-field and on the brightness of the surrounding field. On a logarithmic plot, the data of ΔI/I against I for test-fields below 2° are continuous, whereas those for test-fields above 2° show a sharp discontinuity in the region of intensity in which ΔI/I decreases rapidly. This discontinuity is shown to divide the data into predominantly rod function at low intensities, and predominantly cone function at high intensities. Fields below 2° give higher values of ΔI/I at all intensities, when compared with larger fields. Fields greater than one or two degrees differ from one another principally on the low intensity side of the break. Changes in area above this limit are therefore mainly effective by changing the number of rods concerned. This is confirmed by experiments controlling the relative numbers of rods and cones with lights of different wavelength and with different retinal locations. At high intensities ΔI/I is extremely sensitive to changes in brightness of surrounding visual fields, except for large test-fields which effectually furnish their own surrounds. This sensitivity is especially marked for fields of less than half a degree in diameter. Although the effect is most conspicuous for high intensities, the surround brightness seems to affect the relation between variables as a whole, except in very small fields where absence of a surround of adequate brightness results in the distortion of the theoretical relation otherwise found. The theoretical relationship for intensity discrimination derived by Hecht is shown to fit practically all of the data. Changes in experimental variables such as retinal image area, wavelength, fixation, and criterion may be described as affecting the numerical quantities of this relationship.     

Insulin Receptor Signaling in Cones

In humans, age-related macular degeneration and diabetic retinopathy are the most common disorders affecting cones. In retinitis pigmentosa (RP), cone cell death precedes rod cell death. Systemic administration of insulin delays the death of cones in RP mouse models lacking rods. To date there are no studies on the insulin receptor signaling in cones; however, mRNA levels of IR signaling proteins are significantly higher in cone-dominant neural retina leucine zipper (Nrl) knock-out mouse retinas compared with wild type rod-dominant retinas. We previously reported that conditional deletion of the p85α subunit of phosphoinositide 3-kinase (PI3K) in cones resulted in age-related cone degeneration, and the phenotype was not rescued by healthy rods, raising the question of why cones are not protected by the rod-derived cone survival factors. Interestingly, systemic administration of insulin has been shown to delay the death of cones in mouse models of RP lacking rods. These observations led to the hypothesis that cones may have their own endogenous neuroprotective pathway, or rod-derived cone survival factors may be signaled through cone PI3K. To test this hypothesis we generated p85α^−/−/Nrl^−/− double knock-out mice and also rhodopsin mutant mice lacking p85α and examined the effect of the p85α subunit of PI3K on cone survival. We found that the rate of cone degeneration is significantly faster in both of these models compared with respective mice with competent p85α. These studies suggest that cones may have their own endogenous PI3K-mediated neuroprotective pathway in addition to the cone viability survival signals derived from rods.     

TEMPERATURE AND CRITICAL ILLUMINATION FOR REACTION TO FLICKERING LIGHT : III. SUNFISH

For the sunfish Enneacanthus the mean value of the critical illumination for response to visual flicker at constant flash frequency (with light time = dark time) is related to temperature by the Arrhenius equation. The temperature characteristic for 1/I_m is different above and below 20°C. In each range (12° to 20°; 20° to 30°) the temperature characteristic is the same for rod and cone segments of the duplex flicker response contour: 8,200 and 14,400. This makes it difficult, if not impossible, to consider that the two groups of elements are organized in a significantly different way chemically. For the presumptively rod-connected elements implicated in response to flicker, the curve is markedly discontinuous, so that the high and low temperature parts are dislocated; whereas for the cones they are not. This is entirely consistent with other (e.g., genetic) evidence pointing to their separate physical substrata. The uncommon exhibition of a higher µ over a higher range of temperature, previously found, however, in a few cases, together with the different relations of rod and cone effects to the critical temperature, explain aspects of these data which in earlier incomplete measurements were found to be puzzling.     

Functional Comparison of Rod and Cone Gαt on the Regulation of Light Sensitivity

The signaling cascades mediated by G protein-coupled receptors (GPCRs) exhibit a wide spectrum of spatial and temporal response properties to fulfill diverse physiological demands. However, the mechanisms that shape the signaling response of the GPCR are not well understood. In this study, we replaced cone transducin α (cTα) for rod transducin α (rTα) in rod photoreceptors of transgenic mice, which also express S opsin, to evaluate the role of Gα subtype on signal amplification from different GPCRs in the same cell; such analysis may explain functional differences between retinal rod and cone photoreceptors. We showed that ectopically expressed cTα 1) forms a heterotrimeric complex with rod Gβ₁γ₁, 2) substitutes equally for rTα in generating photoresponses initiated by either rhodopsin or S-cone opsin, and 3) exhibited similar light-activated translocation as endogenous rTα in rods and endogenous cTα in cones. Thus, rTα and cTα appear functionally interchangeable. Interestingly, light sensitivity appeared to correlate with the concentration of cTα when expression is reduced below 35% of normal. However, quantification of endogenous cTα concentration in cones showed a higher level to rTα in rods. Thus, reduced sensitivity in cones cannot be explained by reduced coupling efficiency between the GPCR and G protein or a lower concentration of G protein in cones versus rods.     

VISUAL ACUITY AND ILLUMINATION IN DIFFERENT SPECTRAL REGIONS

The relation between visual acuity and illumination was measured in red and blue light, using a broken circle or C and a grating as test objects. The red light data fall on single continuous curves representing pure cone vision. The blue light data fall on two distinct curves with a transition at about 0.03 photons. Values below this intensity represent pure rod vision. Those immediately above represent the cooperative activity of rods and cones, and yield higher visual acuities than either. Pure cone vision in this intensity region is given by central fixation (C test object). All the rest of the values above this transition region represent pure cone vision. In blue light the rod data with the C lie about 1.5 log units lower on the intensity axis (cone scale) than they do in white light, while with the grating they lie about 1.0 log unit lower than in white light. Both the pure rod and cone data with the C test object are precisely described by one form of the stationary state equation. With the grating test object and a non-limiting pupil, the pure rod and cone data are described by another form of the same equation in which the curve is half as steep. The introduction of a small pupil, which limits maximum visual acuity, makes the relation between visual acuity and illumination appear steeper. Determinations of maximum visual acuities under a variety of conditions show that for the grating the pupil has to be larger, the longer the wavelength of the light, in order for the pupil not to be the limiting factor. Similar measurements with the C show that when intensity discrimination at the retina is experimentally made the limiting factor in resolution, visual acuity is improved by conditions designed to increase image contrast. However, intensity discrimination cannot be the limiting factor for the ordinary test object resolution because the conditions designed to improve image contrast do not improve maximum visual acuity, while those which reduce image contrast do not produce proportional reductions of visual acuity.     

THE FLICKER RESPONSE CONTOUR FOR THE GECKO (ROD RETINA)

The flicker response contour for the gecko Sphaerodactylus (retina with only rods) agrees in all essential respects (intensity range, shape) with that for the turtle Pseudemys (cone retina), as determined under equivalent conditions with the same apparatus. With experimentally determined correction for the expansion of the iris at the very lowest intensities, the F - log I contour for the gecko is a simple probability integral. Its maximum F is lower than that for other animals; this means simply a smaller number of available sensory elements. The quantitative parallelism in the magnitudes of the intensities at the inflection of F - log I and the shape constants for rod and cone animals show that assumptions from comparative histological evidence concerning the properties of rods and cones in relation to visual performance may be quite misleading.     

Low Activation and Fast Inactivation of Transducin in Carp Cones

Cone photoreceptors show lower light sensitivity and briefer light responses than rod photoreceptors. The light detection signal in these cells is amplified through a phototransduction cascade. The first step of amplification in the cascade is the activation of a GTP-binding protein, transducin (Tr), by light-activated visual pigment (R*). We quantified transducin activation by measuring the binding of GTPγS in purified carp rod and cone membrane preparations with the use of a rapid quench apparatus and found that transducin activation by an R* molecule is ∼5 times less efficient in cones than in rods. Transducin activation terminated in less than 1 s in cones, more quickly than in rods. The rate of GTP hydrolysis in Tr*, and thus the rate of Tr* inactivation, was ∼25 times higher in cones than in rods. This faster inactivation of Tr* ensures briefer light responses in cones. The expression level of RGS9 was found to be ∼20 times higher in cones than in rods, which explains higher GTP hydrolytic activity and, thus, faster Tr* inactivation in cones than in rods. Although carp rods and cones express rod- or cone-versions of visual pigment and transducin, these molecules themselves do not seem to induce the differences significantly in the transducin activation and Tr* inactivation in rods and cones. Instead, the differences seem to be brought about in a rod or cone cell-type specific manner.     

Light-dependent Changes in Outer Segment Free-Ca2+ Concentration in Salamander Cone Photoreceptors

Simultaneous measurements of photocurrent and outer segment Ca²⁺ were made from isolated salamander cone photoreceptors. While recording the photocurrent from the inner segment, which was drawn into a suction pipette, a laser spot confocal technique was employed to evoke fluorescence from the outer segment of a cone loaded with the Ca²⁺ indicator fluo-3. When a dark-adapted cone was exposed to the intense illumination of the laser, the circulating current was completely suppressed and fluo-3 fluorescence rapidly declined. In the more numerous red-sensitive cones this light-induced decay in fluo-3 fluorescence was best fitted as the sum of two decaying exponentials with time constants of 43 ± 2.4 and 640 ± 55 ms (mean ± SEM, n = 25) and unequal amplitudes: the faster component was 1.7-fold larger than the slower. In blue-sensitive cones, the decay in fluorescence was slower, with time constants of 140 ± 30 and 1,400 ± 300 ms, and nearly equal amplitudes. Calibration of fluo-3 fluorescence in situ from red-sensitive cones allowed the calculation of the free-Ca²⁺ concentration, yielding values of 410 ± 37 nM in the dark-adapted outer segment and 5.5 ± 2.4 nM after saturating illumination (mean ± SEM, n = 8). Photopigment bleaching by the laser resulted in a considerable reduction in light sensitivity and a maintained decrease in outer segment Ca²⁺ concentration. When the photopigment was regenerated by applying exogenous 11-cis-retinal, both the light sensitivity and fluo-3 fluorescence recovered rapidly to near dark-adapted levels. Regeneration of the photopigment allowed repeated measurements of fluo-3 fluorescence to be made from a single red-sensitive cone during adaptation to steady light over a range of intensities. These measurements demonstrated that the outer segment Ca²⁺ concentration declines in a graded manner during adaptation to background light, varying linearly with the magnitude of the circulating current.     

THE RELATION BETWEEN VISUAL ACUITY AND ILLUMINATION

1. An apparatus for measuring the visual acuity of the eye at different illuminations is described. The test object is continuously variable in size and is presented at a fixed distance from the eye in the center of a 30° field. Observation of the field is through an artificial pupil. The maximum intensity obtainable is more than enough to cover the complete physiological range for the eye with white light though only 110 watts are consumed by the source. Means for varying the intensity over a range of 1:10¹⁰ in small steps are provided. 2. The relation of visual acuity and illumination for two trained observers was measured, using two different types of test object, a broken circle and a grating. The measurements with both test objects show a break at a visual acuity of 0.16, all values below that being mediated by the rods and those above by the cones. The grating gives higher visual acuities at intensities less than about 30 photons and lower visual acuities above that. The maximum visual acuity attainable with the grating under the same conditions is about 30 per cent lower than that with the C. It is shown that the limiting factor in the resolution of the eye for the grating is the diameter of the pupil when it is less than 2.3 mm. and the size of the central cones when the pupil is larger than that. The value of the diameter of the cone derived on that basis from the visual acuity data agrees with that derived from direct cone count in a unit of area. 3. The data for the cones made with both test objects are adequately described by one and the same form of the stationary state equation derived by Hecht for the photoreceptor system. This fact, together with certain considerations about the difference in the nature of the two test objects with regard to the resolvable area, leads to the conclusion that detail perception is a function of a distance rather than an area. All the data for the rods can likewise be described by another variety of the same equation, although the data are too fragmentary to make the choice of the form as certain as might be desired.     

Quantitative Aspects of cGMP Phosphodiesterase Activation in Carp Rods and Cones

Cones are less light-sensitive than rods. We showed previously in carp that more light (>100-fold) is required in cones than in rods to activate 50% of cGMP phosphodiesterase (PDE). The lower effectiveness of PDE activation in carp cones is due partly to the fact that the activation rate of transducin (Tr) by light-activated visual pigment (R*) is 5-fold lower in carp cones than in rods. In this study, we tried to explain the remaining difference. First, we examined the efficiency of activation of PDE by activated Tr (Tr*). By activating PDE with known concentrations of the active (guanosine 5′-Ο-(γ-thio)triphosphate (GTPγS)-bound) form of Tr*, we found that Tr* activated PDE at a similar efficiency in rods and cones. Next, we examined the contribution of R* and Tr* lifetimes. In a comparison of PDE activation in the presence (with GTP) and absence (with GTPγS) of Tr* inactivation, PDE activation required more light (and was therefore less effective) when Tr* was inactivated in both rod and cone membranes. This is probably because inactivation of Tr* shortened its lifetime, thereby reducing the number of activated PDE molecules. The effect of Tr* inactivation was larger in cones, probably because the lifetime of Tr* is shorter in cones than in rods. The shorter lifetimes of Tr* and R* in cones seem to explain the remaining difference in the effectiveness of PDE activation between rods and cones.     

INTENSITY DISCRIMINATION IN THE HUMAN EYE : II. THE RELATION BETWEENΔI/IAND INTENSITY FOR DIFFERENT PARTS OF THE SPECTRUM

1. A new apparatus is described for measuring visual intensity discrimination over a large range of intensities, with white light and with selected portions of the spectrum. With it measurements were made of the intensity ΔI which is just perceptible when it is added for a short time to a portion of a field of intensity I to which the eye has been adapted. 2. For white and for all colors the fraction ΔI/I decreases as I increases and reaches an asymptotic minimum value at high values of I. In addition, with white light the relation between ΔI/I and I shows two sections, one at low intensities and the other at high intensities, the two being separated by an abrupt transition. These findings are contrary to the generally accepted measurements of Koenig and Brodhun; however, they confirm the recent work of Steinhardt, as well as the older work of Blanchard and of Aubert. The abrupt transition is in keeping with the Duplicity theory which attributes the two sections to the functions of the rods and cones respectively. 3. Measurements with five parts of the spectrum amplify these relationships in terms of the different spectral sensibilities of the rods and cones. With extreme red light the relation of ΔI/I to I shows only a high intensity section corresponding to cone function, while with other colors the low intensity rod section appears and increases in extent as the light used moves toward the violet end of the spectrum. 4. Like most of the previously published data from various sources, the present numerical data are all described with precision by the theory which supposes that intensity discrimination is determined by the initial photochemical and chemical events in the rods and cones.     

CRITICAL ILLUMINATION AND FLICKER FREQUENCY IN RELATED FISHES

Flicker response curves have been obtained at 21.5°C. for three genera of fresh water teleosts: Enneacanthus (sunfish), Xiphophorus (swordtail), Platypoecilius (Platy), by the determination of mean critical intensities for response at fixed flicker frequencies, and for a certain homogeneous group of backcross hybrids of swordtail x Platy (Black Helleri). The curves exhibit marked differences in form and proportions. The same type of analysis is applicable to each, however. A low intensity rod-governed section has added to it a more extensive cone portion. Each part is accurately described by the equation F = F_max./(1 + e ^{-p log-p logI/I_i}), where F = flicker frequency, I = associated mean critical intensity, and I_i is the intensity at the inflection point of the sigmoid curve relating F to log I. There is no correlation between quantitative features of the rod and cone portions. Threshold intensities, p, I_i, and F_max. are separately and independently determined. The hybrid Black Helleri show quantitative agreement with the Xiphophorus parental stock in the values of p for rods and cones, and in the cone F_max.; the rod F_max. is very similar to that for the Platy stock; the general level of effective intensities is rather like that of the Platy form. This provides, among other things, a new kind of support for the duplicity doctrine. Various races of Platypoecilius maculatus, and P. variatus, give closely agreeing values of I_m at different flicker frequencies; and two species of sunfish also agree. The effect of cross-breeding is thus not a superficial thing. It indicates the possibility of further genetic investigation. The variability of the critical intensity for response to flicker follows the rules previously found to hold for other forms. The variation is the expression of a property of the tested organism. It is shown that, on the assumption of a frequency distribution of receptor element thresholds as a function of log I, with fluctuation in the excitabilities of the marginally excited elements, it is to be expected that the dispersion of critical flicker frequencies in repeated measurements will pass through a maximum as log I is increased, whereas the dispersion of critical intensities will be proportional to I_m; and that the proportionality factor in the case of different organisms bears no relation to the form or position of the respective curves relating mean critical intensity to flicker frequency. These deductions agree with the experimental findings.     

Visual Pigments of Goldfish Cones : Spectral Properties and Dichroism

Freshly isolated retinal photoreceptors of goldfish were studied microspectrophotometrically. Absolute absorptance spectra obtained from dark-adapted cone outer segments reaffirm the existence of three spectrally distinct cone types with absorption maxima at 455 ± 3,530 ± 3, and 625 ± 5 nm. These types were found often recognizable by gross cellular morphology. Side-illuminated cone outer segments were dichroic. The measured dichroic ratio for the main absorption band of each type was 2–3:1. Rapidly bleached cells revealed spectral and dichroic transitions in regions near 400–410, 435–455, and 350–360 nm. These photoproducts decay about fivefold as fast as the intermediates in frog rods. The spectral maxima of photoproducts, combined with other evidence, indicate that retinene₂ is the chromophore of all three cone pigments. The average specific optical density for goldfish cone outer segments was found to be 0.0124 ± 0.0015/µm. The spectra of the blue-, and green-absorbing cones appeared to match porphyropsin standards with half-band width Δν = 4,832 ± 100 cm^–1. The red-absorbing spectrum was found narrower, having Δν = 3,625 ± 100 cm^–1. The results are consistent with the notion that visual pigment concentration within the outer segments is about the same for frog rods and goldfish cones, but that the blue-, and green-absorbing pigments possess molar extinctions of 30,000 liter/mol cm. The red-absorbing pigment was found to have extinction of 40,000 liter/mol cm, assuming invariance of oscillator strength among the three cone spectra.     

The Frequency-Response Electroretinogram Distinguishes Cone and Abnormal Rod Function in rd12 Mice

Early studies on Rpe65 knockout mice reported that remaining visual function was attributable to cone function. However, this finding has been challenged more and more as time has passed. Electroretinograms (ERGs) showed that rd12 mice, a spontaneous animal model of RPE65 Leber’s congenital amaurosis, had sizeable photopic responses. Unfortunately, the recorded ERG waveform was difficult to interpret because of a remarkably delayed peak-time, which resembles a rod response more than a cone response. Here, we compare flicker ERGs in animals with normal rod and cone function (C57BL/6J mice), pure rod function (cpfl5 mice), and pure cone function (Rho^-/- mice) under different adaptation levels and stimulus intensities. These responses were then compared with those obtained from rd12 mice. Our results showed that normal rods respond to low frequency flicker (5 and 15 Hz) and that normal cones respond to both low and high frequency flicker (5–35 Hz). As was seen in cpfl5 mice, rd12 mice had recordable responses to low frequency flicker (5 and 15Hz), but not to high frequency flicker (25 and 35 Hz). We hypothesize that abnormal rods may be the source of residual vision in rd12 mice, which is proved correct here with double mutant rd12mice. In this study, we show, for the first time, that frequency-response ERGs can effectively distinguish cone- and rod-driven responses in the rd12 mouse. It is another simple and valid method for evaluating the respective contributions of retinal rods and cones.     

THE VISUAL DISCRIMINATION OF INTENSITY AND THE WEBER-FECHNER LAW

1. A study of the historical development of the Weber-Fechner law shows that it fails to describe intensity perception; first, because it is based on observations which do not record intensity discrimination accurately, and second, because it omits the essentially discontinuous nature of the recognition of intensity differences. 2. There is presented a series of data, assembled from various sources, which proves that in the visual discrimination of intensity the threshold difference ΔI bears no constant relation to the intensity I. The evidence shows unequivocally that as the intensity rises, the ratio See PDF for Equation first decreases and then increases. 3. The data are then subjected to analysis in terms of a photochemical system already proposed for the visual activity of the rods and cones. It is found that for the retinal elements to discriminate between one intensity and the next perceptible one, the transition from one to the other must involve the decomposition of a constant amount of photosensitive material. 4. The magnitude of this unitary increment in the quantity of photochemical action is greater for the rods than for the cones. Therefore, below a certain critical illumination—the cone threshold—intensity discrimination is controlled by the rods alone, but above this point it is determined by the cones alone. 5. The unitary increments in retinal photochemical action may be interpreted as being recorded by each rod and cone; or as conditioning the variability of the retinal cells so that each increment involves a constant increase in the number of active elements; or as a combination of the two interpretations. 6. Comparison with critical data of such diverse nature as dark adaptation, absolute thresholds, and visual acuity shows that the analysis is consistent with well established facts of vision.