Effects of abnormal light-rearing conditions on retinal physiology in larvae zebrafish.

PURPOSE: Anatomic studies have found that zebrafish retinal neurons develop in a sequential fashion. In addition, exposure to abnormal light-rearing conditions produces deficits in visual behavior of larvae zebrafish, even though there appears to be little effect of the light-rearing conditions on the gross morphology of the retina. The purpose of this study was to assess the effects of abnormal light-rearing conditions on larvae zebrafish retinal physiology. METHODS: Larvae zebrafish (Danio rerio) were exposed to constant light (LL), constant dark (DD), or normal cyclic light (LD) from fertilization to 6 days postfertilization (dpf). After 6 days, the animals were placed into normal cyclic light and tested at 6 to 8, 13 to 15, and 21 to 24 dpf. Electroretinogram (ERG) responses to visual stimuli, consisting of various wavelengths and irradiances, were recorded. Comparisons were made across the three age groups and the three light-rearing conditions. RESULTS: Deficits from the light-rearing conditions were seen immediately after exposure (6 8 dpf). The LL-condition subjects showed the greatest deficit in the UV and short-wavelength areas and the DD-condition subjects showed a slight deficit across the entire spectrum. At 13 to 15 dpf, the LL and DD groups showed an increase in sensitivity and by 21 to 24 dpf, the groups no longer differed from controls. CONCLUSIONS: Abnormal lighting environments can adversely influence the physiological development of the larvae zebrafish retina. The pattern of damage that was seen in zebrafish is similar to that found in other vertebrates, including higher vertebrates. However, unlike higher vertebrates, the zebrafish appears to be capable of regeneration. This suggests that the zebrafish would be a viable model for light environment effects and neural regeneration.     

Three cone systems under white background in the electroretinogram of the cynomolgus monkey

We recorded electroretinograms for 31 monochromatic (400–700 nm) full-field stimuli at five different adaptation levels from anesthetized cynomolgus monkeys. These waveforms were analyzed by means of a principal component analysis to discuss the relationship between three cone systems with a white background. Under no background, spectral response curves of middle-, long- and short-wavelength cone showed peaks at 517, 579 and 435 nm, respectively, which are in agreement with those of the three cone types. Spectral responses of the middle- and long-wavelength cone systems were increased and decreased, respectively, at the region of 530–590 nm as the level of white background light increased. These opposite changes resulted in the shift of the middle- and long-wavelength cone spectral response peaks to 546 and 609 nm, respectively, which suggests interaction between long- and middle-wavelength cone systems. The peak of the short-wavelength cone system shifted to 452 nm because of decreased responses at the region of 400–450 nm. Therefore, the short-wavelength cone system seems to be mediated by a different mechanism from that involving long- and middle-wavelength cone interaction.Abbreviations ITSS increment-threshold spectral sensitivity - L long wavelength - M middle wavelength - PCA principal component analysis - R-G red-green - B-Y blue-yellow - S short wavelength     

APB differentially affects the cone contributions to the zebrafish ERG

APB (DL-2-amino-4-phosphonobutyric acid) has been found to affect the retinal processing of many vertebrate species as evidenced by the suppression of the b-wave component of the electroretinogram (ERG). The present study examined the effects of APB on the cone contributions to the ERG response of adult zebrafish (Danio rerio). ERG responses were obtained from light-adapted adult zebrafish following intravitreal injection of either saline alone or saline with various concentrations of APB ranging from 10 microm to 500 microM. Visual stimuli were 200-ms flashes of various wavelengths and irradiances. Spectral sensitivity functions were calculated from the irradiance versus response amplitude functions of the a-, b-, and d-wave components of the ERG response. Saline had no effects on the ERG response. However, APB had differential effects on the sensitivity of the b- and d-wave components. The effects of APB on the b-wave component were most apparent in the ultraviolet and short-wavelength portions (320-440 nm) of the spectral sensitivity function, although the b-wave was not completely eliminated at these wavelengths. APB-treated subjects were found to possess the same cone mechanisms (L-M and M-S) in the middle- and long-wavelength areas of the spectrum as saline injected subjects, although absolute sensitivity was lower for the APB-injected subjects. Spectral sensitivity based on the d-wave response was affected by APB but only in the short-wavelength region. All results appear to be independent of the APB dose. These results support the notion that glutamate receptors play a specific role in zebrafish visual processing. In addition, the effects of APB support recent anatomical evidence that the zebrafish retina may possess different types of glutamate receptors.     

Spectral properties of short-wavelength (blue) cones in the turtle retina.

Long- and medium-wavelength cones in the turtle retina participate in complex neural interactions. They are coupled via excitatory pathways to other cones and receive negative feedback inputs from luminosity-type horizontal cells. Little information has been collected on the S- (short-wavelength or blue) cones because they are scarce in the turtle retina and of smaller dimensions compared to the other cone types. In this paper, flash sensitivity action spectra and photoresponses of seven turtle S-cones were measured in the dark-adapted state and during chromatic background illuminations. The desensitizing action of monochromatic background lights was not uniform across the visible spectrum. A red background was most effective in desensitizing the S-cones to long-wavelength stimuli while a blue background light produced its strongest action on the photoresponses elicited by short-wavelength stimuli. The effects of chromatic adaptation on the S-cone action spectrum and on the kinetics of the small-amplitude photoresponses suggested that the S-cones in the turtle retina were involved in complex neural interactions. These included excitatory inputs probably originating in neighboring L-cones and inhibitory long-wavelength inputs probably mediated by L-type horizontal cells.     

Sensitivity of the human neonate to short- and long-wavelength stimuli

Increment thresholds for long- and short-wavelength stimuli were determined under both white and orange adaptation. The forced-choice preferential looking technique (FPL) was used to test neonates shortly after birth and again at 2 months postnatal. Results were compared to an adult control group. Neonates, 2-month-olds, and adults showed a change in relative sensitivity to the 617 and 453 nm test stimuli with a change in chromatic adaptation, indicating the existence of at least 2 separately adaptable chromatic mechanisms at birth. These results are tentatively discussed in terms of the functioning of rods and short-, middle- and long-wavelength cones. A decrease in all thresholds with age is discussed with regard to attentional differences and postnatal neural development in the visual system.     

Opponent process additivity. II. Yellow/blue equilibria and nonlinear models.

A yellow/blue equilibrium light is one which appears neither yellowish nor bluish (i.e. uniquely red, uniquely green, or achromatic). The spectral locus of the monochromatic greenish equilibrium (around 500 nm) shows little, if any, variation over a luminance range of 2 log₁₀ units. Reddish equilibria are extraspectral, involving mixtures of short- and long-wave light. Their wavelength composition is noninvariant with luminance: a reddish equilibrium light turns bluish-red if luminance is increased with wavelength composition constant.The additive mixture of the reddish and greenish equilibria is again a yellow/blue equilibrium light.We conclude that yellow/blue equilibrium can be described as the zeroing of a nonlinear functional, which is, however, approximately linear in the short-wavelength (“blue”) and middle-wavelength (“green”) cone responses and nonlinear only in the long-wavelength (“red”) cone response. The “red” cones contribute to yellowness, but via a compressive function of luminance. This effect works against the direction of the Bezold-Brücke hue shift.The Jameson-Hurvich yellow/blue chromatic-response function is only approximately correct: the relative values of yellow/blue chromatic response for an equal energy spectrum must vary somewhat with the energy level.     

Investigations of photoreceptor synaptic transmission and light adaptation in the zebrafish visual mutant nrc

PURPOSE. To characterize the retinal physiology of the zebrafish visual mutant no optokinetic response c (nrc) and to identify the genetic map position of the nrc mutation. METHODS. Electroretinograms were recorded from wild-type and nrc zebrafish larvae between 5 to 6 days postfertilization. Responses to flash stimuli, On and Off responses to prolonged light stimuli, and responses to flash stimuli with constant background illumination were characterized. The glutamate agonist, 2-amino-4-phosphonobutyric acid (APB) was used to examine the photoreceptor specific a-wave component of the electroretinogram. Amplified fragment length polymorphism methodology was used to place the nrc mutation on the zebrafish genomic map. RESULTS. nrc and wild-type zebrafish larvae 5 to 6 days postfertilization have similar threshold responses to light, but the b-wave of the nrc electroretinogram is significantly delayed and reduced in amplitude. On and Off responses of nrc larvae to prolonged light have multiple oscillations that do not occur in normal zebrafish larvae after 5 days postfertilization. Analysis of the b-wave demonstrated a light adaptation defect in nrc that causes saturation at background light levels approximately 1 order of magnitude less than those with wild-type larvae. Application of the glutamate analog, APB, uncovered the photoreceptor component of the electroretinogram and revealed a light adaptation defect in nrc photoreceptors. The nrc mutation was placed approximately 0.2 cM from sequence length polymorphism marker Z7504 on linkage group 10. CONCLUSIONS. The zebrafish mutant nrc is a possible model for human retinal disease. nrc has defects in photoreceptor synaptic transmission and light adaptation. The nrc mutant phenotype shows striking similarities with phenotypes of dystrophin glycoprotein complex mutants, including patients with Duchenne/Becker muscular dystrophy. Localization of the nrc mutation now makes it possible to evaluate candidate genes and clone the nrc gene.     

Spatial sensitization of the B cone pathways

Spatial sensitization functions were determined under chromatic adaptation conditions designed to isolate the short-wavelength-sensitive pathways (B cone pathways). Spatial sensitization occurs over large retinal dimensions for B cone detected targets and can be produced both by signals from B cones and by signals from R and/or G cones. Field sensitivity measures demonstrate that the mechanism producing sensitization on short-wavelength backgrounds has the spectral sensitivity of B cones. The field sensitivity on long-wavelength backgrounds does not correspond to any receptor spectral sensitivity or any pi mechanism, but suggests that opponent interaction between medium and long-wavelength receptors produces sensitization of B cone detected targets.     

Colored light stimuli in ERG for differential diagnosis of cone dystrophies]

We recorded electroretinograms (ERG) with white and color stimuli in normal persons and four patients with cone dystrophies. Kodak Wratten-filters in blue, blue-green, green, yellow and red were used for the color flashes. ERGs to all color stimuli were recorded at dark and light adapted conditions with different stimulus intensities, to 30 Hz flicker stimulation and with special filtering for oscillatory potentials. Selective blue cone-responses were obtained using strong blue flashes at a yellow background. Patients with cone dystrophies showed slightly to moderately reduced responses at dark adaptation to blue, blue-green, green and yellow stimuli. Red stimuli elicited only small responses with a markedly delayed b-wave implicit time. The light adapted recordings, flicker responses and oscillatory potentials were reduced to all color stimuli. However, differences between patients with cone dystrophies could be detected concerning the responses to red and green. In two patients the responses were reduced to the same degree to all color stimuli. Another patient had very small responses and no oscillatory potentials to red, but his responses were only moderately reduced to green. A patient with combined red-green cone and rod dystrophy had a blue cone hypersensitivity. Responses to blue and blue-green were large at all stimulus conditions, but responses to all other stimuli were much smaller. The blue cone ERG showed a prominent blue cone response. ERG recording to colored stimuli allows a separation of retinal dysfunction in patients with cone dystrophies.     

Color electroretinography

Electroretinograms to white and color stimuli were recorded in four normal subjects and nine subjects with different cone dysfunctions, including protanopia, cone dystrophy, cone dystrophy with supernormal b-waves at dark adaptation, cone dystrophy with missing b-waves during light adaptation and rod-cone dystrophy with blue cone hypersensitivity. Color stimuli were obtained with Kodak Wratten filters in blue, blue-green, green, yellow and red. Electroretinograms to all stimuli were recorded during dark and light adaptation with different stimulus intensities and to 30-Hz flicker stimulation. In protanopia, responses to red during light adaptation and flicker stimulation were reduced. All cone dystrophies showed reduced amplitudes and prolonged implicit times to red when dark adapted. The light-adapted responses were equally reduced to all color stimuli in cone dystrophy and cone dystrophy with supernormal b-waves. Contrary to other cone dystrophies, in cone dystrophy with missing b-waves, responses to red were severely reduced and responses to green were preserved, indicating a predominantly red cone dysfunction. Blue cone hypersensitivity was clearly distinct from other dystrophies in having large responses to blue and blue-green and much smaller responses to all other colors in all stimulus conditions. The electroretinogram with color stimuli allowed separation of different cone dysfunctions and identification of new retinal dysfunction syndromes.     

The application of chromatic dark-adapted kinetic perimetry to retinal diseases

PURPOSE: To demonstrate the value of a 2-color perimetric procedure for determining cone and rod system contributions to the dark-adapted kinetic visual field (VF). DESIGN: Prospective evaluation of perimetric testing procedure. PARTICIPANTS: Five patients with retinal diseases and 6 visually normal individuals. METHODS: Long- and short-wavelength stimuli were presented under dark-adapted conditions in a Goldmann perimeter. Visual fields were measured for the II and V test target sizes with a long-wavelength filter (cut-on at 600 nm) and a short-wavelength filter (cutoff at 510 nm). Light intensities through these filters were matched scotopically for the rod system by producing equal peripheral boundaries on 6 visually normal individuals. To validate the application of this procedure, we tested a patient with congenital achromatopsia and another patient with congenital stationary night blindness (CSNB). We then tested 2 patients with retinitis pigmentosa (RP) and 1 patient with Usher's syndrome to determine the cone and rod contributions to their VF isopters. MAIN OUTCOME MEASURES: Isopters for long- and short-wavelength test stimuli, and the appearance of the test stimuli, whether reported as chromatic or achromatic. RESULTS: The patient with congenital achromatopsia showed superimposed isopters for the 2 stimuli, which were reported as achromatic, demonstrating that the peripheral field boundaries were rod mediated. The patient with CSNB showed an isopter in response to the long-wavelength stimulus that was considerably larger than that in response to the short-wavelength stimulus, both stimuli reported as chromatic, showing that the cone system determined peripheral thresholds for both stimuli. In 2 patients with RP, we observed a mixed pattern of cone or rod system detection of the chromatic stimuli. The peripheral isopters were rod mediated, whereas the cone system determined the central field isopters. In an Usher's syndrome patient, cones mediated both the peripheral and the central field isopters. CONCLUSIONS: A 2-color dark-adapted Goldmann perimetric procedure was able to determine whether the VF isopters were rod or cone mediated in 5 patients with various forms of retinal disease.     

Lighting conditions and retinal development in goldfish: photoreceptor number and structure

The retinas of 63 goldfish were examined after varying durations of exposure to one of three environmental lighting conditions beginning before hatching: constant light (340 lux), cyclic light (12 hr 320 lux, 12 hr dark) and constant dark. Up to 8 months, no effects of constant light or dark on photoreceptor numbers or structure were apparent. Densities of rod and cone nuclei were normal and all retinal layers appeared normal by light microscopy. Exposure to constant light for 12 months or longer resulted in a reduction in rod density by 37%. Cone numbers were unaffected by constant light, even with exposures of 3 yr, and rod and cone outer segments were normal in length at 11-20 months under all environmental conditions. Due to poor survival, only one animal was available for quantitative examination from the group reared in constant dark 12 months or longer. Photoreceptor size and number in this retina were similar to those in the constant light condition. The results suggest that the formation and maturation of rods and cones in goldfish retina is unaffected by rearing in constant light. However, long-term exposure (greater than or equal to 12 months) may disrupt maintenance of differentiated rods.     

Transient tritanopia in migraine: evidence for a large-field retinal abnormality in blue-yellow opponent pathways

PURPOSE: To determine whether the magnitude of transient tritanopia (TT) differs between migraine and control groups. TT is a retinal phenomenon characterized by a paradoxical reduction in sensitivity to short-wavelength (purple) stimuli after extinction of long-wavelength (yellow) adapting displays. A group difference in the magnitude of TT would provide evidence for a retinal contribution to the S-cone-specific color-processing abnormalities that have been reported in migraine. METHODS: Thirty-two migraineurs and 32 age- and sex-matched control participants were tested with a four-alternative, forced-choice procedure to determine S-cone increment and decrement detection thresholds before and after adaptation to a long-wavelength (yellow) display and a neutral (white) display. Migraine history, migraine triggers, and pattern sensitivity were also assessed. RESULTS: Both groups' detection thresholds for increment (purple) S-cone stimuli were increased after extinction of the long-wavelength adapting display compared with the neutral display, demonstrating TT. This loss of sensitivity was significantly greater in the migraine group. In contrast, loss of sensitivity to decrement (yellow) S-cone stimuli was less marked and did not differ between the groups. The magnitude of TT correlated positively with indices of pattern sensitivity and susceptibility to visually triggered migraines but not with migraine history. CONCLUSIONS: These results demonstrate that abnormalities in a specific retinal circuit contribute to decreased short-wavelength sensitivity after adaptation in migraine. As thresholds did not correlate with indices of migraine history, it is unlikely that this finding reflects cumulative damage induced by repeated migraine episodes.     

Adaptive plasticity during the development of colour vision

Colour vision greatly enhances the discriminatory and cognitive capabilities of visual systems and is found in a great majority of vertebrates and many invertebrates. However, colour coding visual systems are confronted with the fact that the external stimuli are ambiguous because they are subject to constant variations of luminance and spectral composition. Furthermore, the transmittance of the ocular media, the spectral sensitivity of visual pigments and the ratio of spectral cone types are also variable. This results in a situation where there is no fixed relationship between a stimulus and a colour percept. Colour constancy has been identified as a powerful mechanism to deal with this set of problems; however, it is active only in a short-term time range. Changes covering longer periods of time require additional tuning mechanisms at the photoreceptor level or at postreceptoral stages of chromatic processing. We have used the trichromatic blue acara (Aequidens pulcher, Cichlidae) as a model system and studied retinal morphology and physiology, and visually evoked behaviour after rearing fish for 1-2 years under various conditions including near monochromatic lights (spectral deprivation) and two intensities of white light (controls). In general, long-term exposure to long wavelengths light had lesser effects than light of middle and short wavelengths. Within the cone photoreceptors, spectral deprivation did not change the absorption characteristics of the visual pigments. By contrast, the outer segment length of middle and long-wave-sensitive cones was markedly increased in the blue rearing group. Furthermore, in the same group, we observed a loss of 65% short-wave-sensitive cones after 2 years. These changes may be interpreted as manifestations of compensatory mechanisms aimed at restoring the balance between the chromatic channels. At the horizontal cell level, the connectivity between short-wave-sensitive cones and the H2 cone horizontal cells, and the spinule dynamics were both affected in the blue light group. This observation rules out the role of spinules as sites of chromatic feedback synapses. The light-evoked responses of H2 horizontal cells were also sensitive to spectral deprivation showing a shift of the neutral point towards short wavelengths in the blue rearing group. Interestingly, we also found an intensity effect because in the group reared in bright white light the neutral point was more towards longer wavelength than in the dim light group. Like the changes in the cones, the reactions of horizontal cells to spectral deprivation in the long wave domain can be characterised as compensatory. We also tested the spectral sensitivity of the various experimental groups of blue acara in visually evoked behaviour using the optomotor response paradigm. In this case, the changes in the relative spectral sensitivity were more complex and could not be explained by a simple extrapolation of the adaptive and compensatory processes in the outer retina. We conclude that the inner retina, and/or the optic tectum are also involved and react to the changes of the spectral environment. In summary, we have shown a considerable developmental plasticity in the colour vision system of the blue acara, where epigenetic adaptive processes at various levels of the visual system respond to the specific spectral composition of the surroundings and provide a powerful mechanism to ensure functional colour vision in different visual environments. We suggest that processes involving an active fine-tuning of the photoreceptors and the postreceptoral processing of chromatic information during ontogenetic development are a general feature of all colour vision systems. Such mechanisms would establish a functional balance between the various chromatic channels. This appears to be an essential condition for the cognitive systems to extract the relevant and stable information from the unstable and changing stimulus situation.     

Interaction between the short-wavelength cone and rod systems in the electroretinogram of the cynomolgus monkey

We recorded electroretinograms from anesthetized cynomolgus monkeys by means of three monochromatic (435, 524 or 579 nm) full-field stimuli presented with a monochromatic background light. The color of the background light was changed with 29 interference filters (400–700 nm). Waveforms were analyzed by means of principal component analysis to investigate interaction between the short-wavelength cone and other systems. With the same test lights, responses were also recorded during dark adaptation after 20,000-lux full bleach. The shape of the short-wavelength cone electroretinogram changed as a function of the wavelength of monochromatic background light. The change was clearer around 500-nm background light, to which the rod system is highly sensitive. Further, short-wavelength cone responses developed during the first 15 min in the dark; however, they became smaller with the recovery of the rod system. These findings suggest that there is interaction between the shortwavelength cone and rod systems. In addition, the results of waveform analysis confirmed the interaction between the long-wavelength and middle-wavelength cone systems.Abbreviations PCA principal component analysis     

Visual detection of UV cues by adult zebrafish (Danio rerio)

Visual sensitivity to ultraviolet (UV) light is widespread in the animal kingdom. Many studies on UV vision have utilized physiological and/or anatomical methods to determine whether animals are visually sensitive to UV wavelengths. However, ultimately behavioral methods can reveal whether retinal UV sensitivity results in perceptual detection of UV stimuli. For the widely studied zebrafish (Danio rerio), the adult retina possesses cone photoreceptors that are sensitive to UV light. Here, we used a behavioral technique, the escape response assay, to test whether adult zebrafish can visually detect and behaviorally respond to visual cues that reflect UV. We found that adult zebrafish robustly respond to UV reflecting cues under UV light while showing no responses to the same cues under no UV light. From our results, we confirm that adult zebrafish can visually detect UV reflecting cues and show that UV perceptual sensitivity is functional in adult zebrafish. Our study highlights the utility of the fish escape response assay for UV visual behavior research.     

Response saturation of short-wavelength cone pathways controlled by color-opponent mechanisms

Response saturation of the short-wavelength cone pathways was measured with foveal, violet test flashes on steady or flashed violet fields. These violet stimuli were presented on an intense steady yellow “auxiliary” field that suppressed the Π₄ and Π₅ mechanisms. At given violet field intensities the increment threshold curve became very steep, demonstrating response saturation of the short-wavelength pathways. This response saturation could be greatly reduced or eliminated by flashing the yellow auxiliary field, rather than having it steady, or by adding a flashed yellow field to the steady yellow auxiliary field. The response saturation was strongly promoted by presenting a negative yellow field flash (i.e. a decrement of the steady yellow field) with the positive violet field flash. The results show that the response saturation is controlled, not by independently acting short-wavelength cones, but by cancellative mechanisms that receive signals of opposite sign from the short-wavelength cones and the middle- and long-wavelength cones.     

Turtle C-type horizontal cells act as push-pull devices

Chromaticity (C-type) horizontal cells in the retina of cold-blooded vertebrates receive antagonistic inputs from cone photoreceptors of different spectral types leading to color opponency. The relative contribution of each spectral type of cones can be selectively altered by chromatic background illumination. Therefore, the spectral properties of C-type horizontal cells are expected to change when the intensity and color of ambient illumination are altered. In this study, we investigated the effects of chromatic background lights upon color opponency in Red/Green (RGH) and Yellow/Blue (YBH) C-type horizontal cells in the everted eyecup preparation of the turtle Mauremys caspica. Photoresponses were elicited by long-wavelength and short-wavelength light stimuli in the dark-adapted state and under conditions of chromatic background illumination. We found that the total voltage range. within which graded depolarizing and the hyperpolarizing photoresponses could be elicited, either increased or decreased depending upon the color of the background light. However, the maximal and minimal potential levels determined respectively by long-wavelength and short-wavelength light stimuli of supersaturating intensity remained unchanged, regardless of the wavelength and intensity of the background. These findings indicate that turtle C-type horizontal cells operate as push-pull devices. A sufficiently bright short-wavelength stimulus can push them all the way to the maximal hyperpolarizing level while a very bright long-wavelength stimulus can pull them towards the most depolarizing potential.     

Visual pigment coexpression in all cones of two rodents,the Siberian hamster,and the pouched mouse

PURPOSE: To decide whether the identical topography of short- and middle-wavelength cone photoreceptors in two species of rodents reflects the presence of both opsins in all cone cells. METHODS: Double-label immunocytochemistry using antibodies directed against short-wavelength (S)-and middle- to long-wavelength (M/L)-sensitive opsin were used to determine the presence of visual pigments in cones of two species of rodents, the Siberian hamster (Phodopus sungorus) and the pouched mouse (Saccostomus campestris) from South Africa. Topographical distribution was determined from retinal whole-mounts, and the colocalization of visual pigments was examined using confocal laser scanning microscopy. Opsin colocalization was also confirmed in consecutive semithin tangential sections. RESULTS: The immunocytochemical results demonstrate that in both the Siberian hamster and the pouched mouse all retinal cones contain two visual pigments. No dorsoventral gradient in the differential expression of the two opsins is observed. CONCLUSIONS: The retina of the Siberian hamster and the pouched mouse is the first example to show a uniform coexpression of M and S cone opsins in all cones, without any topographical gradient in opsin expression. This finding makes these two species good models for the study of molecular control mechanisms in opsin coexpression in rodents, and renders them suitable as sources of dual cones for future investigations on the role and neural connections of this cone type.     

Rod sensitivity relative to cone sensitivity in retinitis pigmentosa.

The present study evaluates rod sensitivity relative to cone sensitivity in retinitis pigmentosa patients. From perimetric measures of absolute thresholds to long- and short-wavelength stimuli, we find the results can be characterized in three different ways. For one group of patients it appears that only cone function is present in all parts of the visual field. For another group of patients, despite large losses in absolute sensitivity, rod sensitivity still exceeds cone sensitivity by at least the normal factor, suggesting concomitant rod and cone sensitivity losses. For a third group of patients it appears that central retinal thresholds are determined primarily by cones, midperipheral retinal thresholds determined by a composite of rod and cone function, and far- peripheral thresholds determined predominately by rods. The argument is put forward that the different groups of threshold results represent different disease mechanisms rather than different stages in the progression of a single disease process.