In age-related macular degeneration (AMD), the processing of fine details in a visual scene, based on a high spatial frequency processing, is impaired, while the processing of global shapes, based on a low spatial frequency processing, is relatively well preserved. The present fMRI study aimed to investigate the residual abilities and functional brain changes of spatial frequency processing in visual scenes in AMD patients. AMD patients and normally sighted elderly participants performed a categorization task using large black and white photographs of scenes (indoors vs. outdoors) filtered in low and high spatial frequencies, and nonfiltered. The study also explored the effect of luminance contrast on the processing of high spatial frequencies. The contrast across scenes was either unmodified or equalized using a root-mean-square contrast normalization in order to increase contrast in high-pass filtered scenes. Performance was lower for high-pass filtered scenes than for low-pass and nonfiltered scenes, for both AMD patients and controls. The deficit for processing high spatial frequencies was more pronounced in AMD patients than in controls and was associated with lower activity for patients than controls not only in the occipital areas dedicated to central and peripheral visual fields but also in a distant cerebral region specialized for scene perception, the parahippocampal place area. Increasing the contrast improved the processing of high spatial frequency content and spurred activation of the occipital cortex for AMD patients. These findings may lead to new perspectives for rehabilitation procedures for AMD patients.

An earlier analysis, which yielded an optimal pair of blue and green primaries (436 & 490 nm) for tritanomaloscopy, is reevaluated. That analysis minimized population variance in the mid-match points of color normals by taking into account, for a set of blue and green tritanopic metamers, the contributions of the lens and macular pigment variances and of matching range. The revision to the matching-range contribution takes into account the effect, neglected in the original analysis, of the varying angle between the blue–green primary mixture lines and the corresponding cyan test and yellow desaturant mixture lines. Use is made of new measurements of the macular pigment absorbance spectrum, a new estimate of the lens absorbance spectrum, the population variances of the lens and macular pigment, and of matching-range data for a current Moreland equation. Tritanopic metamers are derived from a revised set of cone fundamentals. The net effect of all of these revisions on the specification of optimal primaries is small (440 and 488 nm). However, larger changes are involved in the choice of test and desaturant wavelengths.

Although inositol trisphosphate (InsP3) is a key substance in phototransduction of invertebrate photoreceptors, its intracellular concentration remains unknown. The purpose of this study was to assay its concentration and the concentration of its precursor, phosphatidylinositol bisphosphate (PtdInsP2), within squid photoreceptors. Rhabdomeric membranes were purified and their PtdInsP2 content measured with a phosphate assay after the extracted phospholipids were deacylated and separated by ion-exchange chromatography. At least 75% of the total PtdInsP2 found in the retinal homogenate was associated with the plasma membranes of the rhabdomeric microvilli, where PtdInsP2 was 3.1 ± 0.7% of the total phospholipids, a level comparable to values published for rat brain. In terms of rhodopsin, microvillar membranes contained 3.7 ± 0.9 mol PtdInsP2/mol rho. The InsP3 content of living retinas was measured with a radioreceptor assay. The basal content of dark-adapted retinas was 0.15 ± 0.05 InsP3/rho, equivalent to 30 ± 9 nmol/g tissue that is about twice that of rat brains. Flash illumination (≈lms in duration) that photoactivated 1% of rhodopsin increased the level about fivefold to 0.68 ± 0.22 lnsP3/rho. Corresponding decrease in PtdInsP2 was undetectable as it was within measurement errors. For PtdInsP2, the measured content corresponds to 5.6 ± 1.4 mM within the volume of rhabdomere. Maximal light-induced concentration of InsP3 is calculated to be 1.2 ± 0.4 mM within the cytoplasm of the distal segment. Each photoactivated rhodopsin leads to the formation of 500 InsP3 molecules when measured 15 s after the flash. Thus, high concentration of InsP3 in these cells is primarily due to restricted intracellular volumes rather than to high amplification by the enzyme cascade. The InsP3 concentration within squid photoreceptors is the highest yet reported for any transducing cell and may indicate the involvement of relatively low affinity receptors or channels during invertebrate phototransduction.

Axon diameters of retinal ganglion cells were measured from electron micrographs of the nerve fiber layer of the cat. Three adult retinae were examined which had mean axonal diameters of 1.18 ± 0.86 (n = 5553), 1.12 ± 0.79 (n = 7265), and 1.47 ±1.11 μm (n = 10,867). Cumulative histograms from several locations adjacent to the optic disc were unimodal (modal peaks: 0.6–0.8 μm). This unimodal distribution, however, did not reflect the regional differences in axonal diameters found throughout the retina. In many locations, especially those related to axons of the temporal retina, axon diameter distributions were clearly bimodal or even trimodal (modal peaks: 0.6–0.8, 1.4–2.1, and 3.3 μm). Measurements from one retina indicated that the mean diameters of axons arising from the area centralis and visual streak (0.94 ± 0.63 and 0.98 ± 0.68, respectively) were not significantly different from each other; however, when compared to other areas around the optic disc, the percentage of fibers with diameters between 1.5–2.0 μm was highest in the sample adjacent to the area centralis. Axons temporal to the optic disc were found to be on average larger than those nasal to the optic disc; similarly superior axons were larger than inferior axons. Axonal distributions at the retinal periphery were found to be significantly different from those at the optic disc (p ≤ 0.05) and contained a higher percentage of medium-sized axons and fewer small axons. In each of the three retinae the proportions small, medium, and large axons were respectively γ: 46; 47; 48, β: 50; 49; 48, and α: 4; 4; 4; regional differences in the proportions of each axonal class are compared to previously published ganglion cell density maps. Differences between axonal bundles within each sample location were not significantly different; however, in one retina axons in the scleral half of the fiber layer were significantly larger (P ≤ 0.01) than axons in the vitreal half of the nerve fiber layer adjacent to the optic disc. When compared to the axonal diameter distributions found within the optic nerve (Cottee et al., 1991) and optic tract (Reese et al., 1991), our data indicates that the diameter of retinal axons may increase by up to 30% along the length of the visual pathway.

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.

We have examined the retinotopy in the superior colliculus of pigmented and albino ferrets using both anatomical and electrophysiological methods. While the distribution of contralaterally projecting retinotectal ganglion cells is characterized by the presence of an area centralis superimposed on a visual streak in both strains, the ipsilateral projection from temporal hemiretina is strongly reduced in albinos. In spite of the significantly altered retinotectal projection pattern, the collicular visual field map in the albino ferret reveals the same characteristics as in the pigmented animal with a strongly enlarged representation of the center of visual space. An areal comparison between retinotectal ganglion cell distribution and collicular areal magnification shows that the increase in areal magnification factor between the periphery and the representation of the central visual hemifield exceeds the corresponding increase in retinal ganglion cell density between peripheral retina and area centralis by a factor of three in pigmented and a factor of four in albino ferrets. The areal magnification factor of the representation of the retinal visual streak does not exceed the increase in retinotectal ganglion cell density. Thus, our results suggest that the representation of visual space in the superior colliculus of albino and pigmented ferrets does not simply follow the retinotectal ganglion cell density, but that there is an enhanced representation of the frontal central visual field. The possibility is discussed that the collicular visual field map may be determined either by both retinotectal and corticotectal projections or by the colliculus' intrinsic structure.

Visual evoked responses to dichoptically presented multifocal stimuli were recorded for 92 eyes. Two stimulus variants were explored: temporally sparse and rapidly contrast reversing. We used hierarchal decomposition (HD) to represent the multifocal responses in terms of a small number of potentially unique component waveforms that are interrelated in a multivariate linear autoregressive (MLAR) relationship. The HD method exploits temporal correlations over a range of delays in the responses to estimate parallel, feedforward and feedback relationships between the HD components. Three HD components having temporal interrelationships constrained (at P < 0.05) to a moving ∼20 ms window could describe the multifocal responses well (median r2-values up to 90%). HD components were similar for both stimulus types and the component waveforms were temporally correlated, especially the first and third components. The data set was large enough to estimate separate HD components for each multifocal stimulus region. The component waveforms differed somewhat by region but the MLAR relationships were similar. At short delays parallel processing dominated. At longer delays the proportion of response drives that were attributed to feedback and feedforward relationships grew. Overall HD analysis seems to provide an informed summary of multifocal responses and insights into their sources.

Light-microscopic immunocytochemistry was utilized to localize the different populations of substance P-immunoreactive (SP-IR) neurons in the hamster retina. Based on observation of 2505 SP-IR neurons in transverse sections, 34% were amacrine cells whose pear-shaped or round cell bodies (7–8 μm) were situated in the inner half of the inner nuclear layer (INL) or in the inner plexiform layer (IPL), while 66% of SP-IR somata (6–20 μm) were located in the ganglion cell layer (GCL) which were interpreted to be displaced amacrine cells and retinal ganglion cells (RGCs). At least three types of SP-IR amacrine cells were identified. The SP-IR processes were distributed in strata 1, 3, and 5 with the densest plexus in stratum 5 of the inner plexiform layer. In the wholemounted retina, the SP-IR cells were found to be distributed throughout the entire retina and their mean number was estimated to be 4224 ± 76. Two experiments were performed to clarify whether any of the SP-IR neurons in the GCL were RGCs. The first experiment demonstrated the presence of SP-IR RGCs by retrogradely labeling the RGCs and subsequently staining the SP-IR cells in the retina using immunocytochemistry. The second experiment identified SP-IR central projections of RGCs to the contralateral dorsal lateral geniculate nucleus. This projection disappeared following removal of the contralateral eye. The number of SP-IR RGCs was estimated following optic nerve section. At 2 months after sectioning the optic nerve, the total number of SP-IR neurons in the GCL reduced from 4224 ± 76 to a mean of 1192 ± 139. Assuming that all SP-IR neurons in the GCL which disappeared after nerve section were RGCs, the number of SP-IR RGCs was estimated to be 3032, representing 3–4% of the total RGCs. In summary, findings of the present study provide evidence for the existence of SP-IR RGCs in the hamster retina.

Congenital achromatopsia is a stationary retinal disorder with autosomal recessive inheritance. It is characterized by significant attenuation of cone-photoreceptor function. Symptoms include photophobia, nystagmus, and poor visual acuity from birth. Unlike in cone or cone–rod dystrophies, the retinal fundus usually appears normal. Here we describe two siblings with congenital achromatopsia, who exhibit different ophthalmic phenotypes. History was taken, and ophthalmic examinations were performed in a 7-year-old girl and her 5-year-old brother, who were referred to our department because of poor visual acuity. Two of their grandparents were brother and sister, suggesting an autosomal recessive transmission in inheritance. They have been followed for more than 13 years since the initial evaluation. Symptoms, visual acuity, and kinetic visual field were very similar to each other, consistent with findings of typical congenital achromatopsia. However, color-vision tests suggested that the brother had residual color discrimination, but the sister did not. The siblings had different full-field electroretinographic and spectral-sensitivity findings: residual cone functions were detected in only the brother, in agreement with his residual color vision. They also had different findings of retinal fundi and ocular refractions: the sister had bilaterally atrophic-appearing macular lesions and myopic errors. In contrast, the brother remains hyperopia and has exhibited no specific retinal findings until age 18 years. The causes why both complete and incomplete achromats occur in the siblings are uncertain but might be caused by modifying effects of sex-related genes or by environmental factors influencing certain gene regulations in cone photoreceptors.

In the inner plexiform layer, amacrine cells receive glutamatergic input from bipolar cells. Glutamate can depolarize amacrine cells by activation of ionotropic glutamate receptors or mediate potentially more diverse changes via activation of G protein-coupled metabotropic glutamate receptors (mGluR5). Here, we asked whether selective activation of metabotropic glutamate receptor 5 is linked to modulation of the voltage-gated Ca2+ channels expressed by cultured GABAergic amacrine cells. To address this, we performed whole-cell voltage clamp experiments, primarily in the perforated-patch configuration. We found that agonists selective for mGluR5, including (RS)-2-chloro-5-hydroxyphenylglycine (CHPG), enhanced the amplitude of the voltage-dependent Ca2+ current. The voltage-dependent Ca2+ current and CHPG-dependent current enhancement were blocked by nifedipine, indicating that L-type Ca2+ channels, specifically, were being modulated. We have previously shown that activation of mGluR5 produces Ca2+ elevations in cultured amacrine cells (Sosa et al., 2002). Loading the cells with 5 mM BAPTA inhibited the mGluR5-dependent enhancement, suggesting that the cytosolic Ca2+ elevations are required for modulation of the current. Although activation of mGluR5 is typically linked to activation of protein kinase C, we found that direct activation of this kinase leads to inhibition of the Ca2+ current, indicating that stimulation of this enzyme is not responsible for the mGluR5-dependent enhancement. Interestingly, direct stimulation of protein kinase A produced an enhancement of the Ca2+ current similar to that observed with activation of mGluR5. Thus, activation of mGluR5 may modulate the L-type voltage-gated Ca2+ current in these GABAergic amacrine cells via activation of protein kinase A, possibly via direct activation of a Ca2+-dependent adenylate cyclase.

Global motion integration mechanisms can utilize signals defined by purely chromatic information. Is global motion integration sensitive to the polarity of such color signals? To answer this question, we employed isoluminant random dot kinematograms (RDKs) that contain a single chromatic contrast polarity or two different polarities. Single-polarity RDKs consisted of local motion signals with either a positive or a negative S or L–M component, while in the different-polarity RDKs, half the dots had a positive S or L–M component, and the other half had a negative S or L–M component. In all RDKs, the polarity and the motion direction of the local signals were uncorrelated. Observers discriminated between 50% coherent motion and random motion, and contrast thresholds were obtained for 81% correct responses. Contrast thresholds were obtained for three different dot densities (50, 100, and 200 dots). We report two main findings: (1) dependence on dot density is similar for both contrast polarities (+S vs. −S, +LM vs. −LM) but slightly steeper for S in comparison to LM and (2) thresholds for different-polarity RDKs are significantly higher than for single-polarity RDKs, which is inconsistent with a polarity-blind integration mechanism. We conclude that early motion integration mechanisms are sensitive to the polarity of the local motion signals and do not automatically integrate information across different polarities.

The cellular composition of the inner nuclear layer (INL) is largely conserved among mammals. Studies of rabbit, monkey, and mouse retinas have shown that bipolar, amacrine, Müller, and horizontal cells make up constant fractions of the INL (42, 35, 20, and 3%, respectively); these proportions remain relatively constant at all retinal eccentricities. The purpose of our study was to test whether the organization of cat retina is similar to that of other mammalian retinas. Fixed retinas were embedded in plastic, serially sectioned at a thickness of 1 μm, stained, and imaged at high power in the light microscope. Bipolar, amacrine, Müller, and horizontal cells were classified and counted according to established morphological criteria. Additional sets of sections were processed for protein kinase C and calretinin immunoreactivity to determine the relative fraction of rod bipolar and AII amacrine cells. Our results show that the organization of INL in the cat retina contains species-specific alterations in the composition of the INL tied to the large fraction of rod photoreceptors. Compared with other mammalian retinas, cat retinas show an expansion of the rod pathway with rod bipolar cells accounting for about 70% of all bipolar cells and AII cells accounting for nearly a quarter of all amacrine cells. Our results suggest that evolutionary pressures in cats over time have refined their retinal organization to suit its ecological niche.

To investigate the influence of voltage-sensitive conductances in shaping light-evoked responses of retinal bipolar cells, whole-cell recordings were made in the slice preparation of the tiger salamander, Ambystoma tigrinum. To study contrast encoding, the retina was stimulated with 0.5-s steps of negative and positive contrasts of variable magnitude. In the main, responses recorded under voltage- and current-clamp modes were remarkably similar. In general agreement with past results in the intact retina, the contrast/response curves were relatively steep for small contrasts, thus showing high contrast gain; the dynamic range was narrow, and responses tended to saturate at relatively small contrasts. For ON and OFF cells, linear regression analysis showed that the current response accounted for 83–93% of the variance of the voltage response. Analysis of specific parameters of the contrast/response curve showed that contrast gain was marginally higher for voltage than current in three of four cases, while no significant differences were found for half-maximal contrast (C50), dynamic range, or contrast dominance. In sum, the overall similarity between current and voltage responses indicates that voltage-sensitive conductances do not play a major role in determining the shape of the bipolar cell's contrast response in the light-adapted retina. The salient characteristics of the contrast response of bipolars apparently arise between the level of the cone voltage response and the postsynaptic current of bipolar cells, via the transformation between cone voltage and transmitter release and/or via the interaction between the neurotransmitter glutamate and its postsynaptic receptors on bipolar cells.

At reduced luminance levels, the visual system integrates light over extended periods of time. Although the general effects of this process are known, specific changes in the visual cortex have not been identified. We have studied the physiological changes that occur during a transition from high to low luminance by measurements of single neurons in the cat's primary visual cortex. Under low-luminance conditions, we find increased latencies, expanded temporal responses, and a loss of temporal structure. This results in temporal-frequency tuning curves that are peaked at relatively low frequencies. To examine parallel perceptual changes, we compared perceived temporal frequency in human subjects under high- and low-luminance conditions. Low-luminance flickering patterns are perceived to modulate at relatively high rates. This occurs even though peak sensitivity is shifted to relatively low temporal frequencies. To explore further the perceptual component, we measured perceived temporal frequency in human subjects with unilateral optic neuritis for whom optic nerve transmission is known to be relatively slow and generally similar to the normal physiological state under low luminance. These subjects also perceive relatively high modulation rates through their affected eye. Considered together, these results demonstrate an inverse relationship between the physiological and the perceptual consequences of reduced stimulus luminance. This relationship may be accounted for by shifts of neuronal population responses between high- and low-luminance levels.

The retinal topography of the cat's optic tract was determined by means of injections of the enzyme horseradish peroxidase (HRP) into the tract. This analysis was accomplished by the subtraction of all HRP injection sites not labeling a defined retinal area from those injection sites which resulted in ganglion cell labeling (Venn diagram analysis). Using this method, the following correspondences were demonstrated for the ipsilateral and contralateral projections: superior retina represented in medial optic tract; inferior retina in lateral tract; and area centralis in a dorsocentral location (which was part of a larger area representing the visual streak). The temporal raphe was represented in the ipsilateral tract as a band curving from the area centralis region toward the dorsomedial border of the tract. Contralateral fibers from a region superior to the optic disc were found to be displaced with respect to the general retinal representation in the optic tract and this appeared to be related to retinal development. The ratio of contralateral to ipsilateral fibers was determined and found to be nonuniform within the tract.

Injection of HRP into the optic tract of the cat also allowed the axons from labeled retinal ganglion cells to be traced within the retina and optic disc. Axons from ganglion cells lying temporal to the raphe curve around the area centralis enter the optic disc on the lateral and inferior aspects. Ganglion cells lying nasal to the raphe send their axons more directly to enter the optic disc on its superior aspect. A schema is proposed whereby the retina is mapped onto the optic tract.

We have used wild-type mice and mice possessing defects in specific retinal circuits in order to more clearly define functional circuits of the inner retina. The retina of the nob mouse lacks communication between photoreceptors and depolarizing bipolar cells (DBCs). Thus, all light driven activity in the nob mouse is mediated via remaining hyperpolarizing bipolar cell (HBC) circuits. Transducin null (Trα−/−) mice lack rod photoreceptor activity and thus remaining retinal circuits are solely generated via cone photoreceptor activity. Activation in inner retinal circuits in each of these mice was identified by monitoring light-induced expression of an immediate early gene, c-fos. The number of cells expressing c-fos in the inner retina was dependent upon stimulus intensity and was altered in a systematic fashion in mice with known retinal mutations. To determine whether c-fos is activated via circuits other than photoreceptors in the outer retina, we examined c-fos expression in tulp1−/− mice that lack photoreceptors in the outer retina; these mice showed virtually no c-fos activity following light exposure. Double-labeling immunohistochemical studies were carried out to more clearly define the population of c-fos expressing amacrine cells. Our results indicate that c-fos may be used to map functional circuits in the retina.

Previous studies of rod photoreceptors in vivo have employed a paired-flash electroretinographic (ERG) technique to determine rod response properties. To test whether absence versus presence of the ERG b-wave affects the photoreceptor response derived by the paired-flash method, we examined paired-flash-derived responses obtained from nob mice, a mutant strain with a defect in signal transduction between photoreceptors and ON bipolar cells that causes a lack of the b-wave. Normal littermates of the nob mice served as controls. The normalized amplitude-intensity relation of the derived response determined in nob mice at the near-peak time of 86 ms was similar to that determined for the controls. The full time course of the derived rod response was obtained for test flash strengths ranging from 0.11 to 17.38 scotopic cd s m−2 (sc cd s m−2). Time-course data obtained from nob and control mice exhibited significant but generally modest differences. With saturating test flash strengths, half-recovery times for the derived response of nobversus control mice differed by ∼60 ms or less about the combined (nob and control) average respective values. Time course data also were obtained before versus after intravitreal injection of l-2-amino-4-phosphonobutyrate (APB) (which blocks transmission from photoreceptors to depolarizing bipolar cells) and of cis 2,3-piperidine dicarboxylic acid (PDA) (which blocks transmission to OFF bipolar cells, and to horizontal, amacrine and ganglion cells). Neither APB nor PDA substantially affected derived responses obtained from nob or control mice. The results provide quantitative information on the effect of b-wave removal on the paired-flash-derived response in mouse. They argue against a substantial skewing effect of the b-wave on the paired-flash-derived response obtained in normal mice and are consistent with the notion that, to good approximation, this derived response represents the isolated flash response of the photoreceptors in both nob and normal mice.

Large field motion detection in goldfish, measured in the optomotor response, is based on the L-cone type, and is therefore color-blind (Schaerer & Neumeyer, 1996). In experiments using a two-choice training procedure, we investigated now whether the same holds for the detection of a small moving object (size: 8 mm diameter; velocity: 7 cm/s). In initial experiments, we found that goldfish did not discriminate between a moving and a stationary stimulus, obviously not taking attention to the cue “moving.” Therefore, random dot patterns were used in which the stimulus was visible only when moving. Using black and white random dot patterns with variable contrast between 0.2 and 1, we found that the fish could see motion only with high (0.8) contrast. In the decisive experiment, a red-green random dot pattern was used. By keeping the intensity of the red dots constant and reducing the intensity of the green dots, a narrow intensity range was found in which goldfish could no longer discriminate between the moving random dot stimulus in random dot surround and the stationary random dot pattern. The same was the case when a red moving disk was presented in green surround. This is the evidence that object motion is red-green color blind, i.e., color information cannot be used to detect the moving object. Calculations of the cone excitation values revealed that the M-cone type is decisive, as this cone type (and not the L-cone type) is not modulated by that particular red-green pattern in which the moving stimulus was invisible.

In the absence of a direct and specific marker for basket cells, the aim of this paper was to use available data to estimate the density of basket cell synaptic input to smooth and spiny neurons within layer IV of cat striate visual cortex (area 17). A linear quantitative analysis of layer IV basket cell connectivity data suggests that on average basket cells (1) comprise 25–35% of all GABAergic neurons in layer IV (3552–4736 cells mm−3), (2) account for 30–41% of all putative inhibitory dendritic synapses of layer IV spiny stellate cells (145–195 synapses cell−1) and a similar proportion of layer IV basket cells (25–37%, 71–107 synapses cell−1), and (3) provide each layer IV spiny cell with 13–45 axons and each layer IV basket cell with 6–29 axons. These estimates suggest that basket cells may be less common and provide a smaller proportion of the dendritic synaptic input to layer IV spiny and smooth neurons than previously thought. In addition, the analysis indicates that a layer IV spiny stellate cell may receive on average as many synapses and axons from layer IV basket cells as from lateral geniculate relay cells. Based on this potential numerical similarity, a geniculate-basket synaptic pairing in a spine-shaft microcircuit is hypothesized. This microcircuit could implement a type of local (dendritic) push–pull interaction underlying subfield antagonism.

People with normal trichromatic color vision experience variegated hue percepts under dim illuminations where only rod photoreceptors mediate vision. Here, hue perceptions were determined for persons with congenital color vision deficiencies over a wide range of light levels, including very low light levels where rods alone mediate vision. Deuteranomalous trichromats, deuteranopes and protanopes served as observers. The appearances of 24 paper color samples from the OSA Uniform Color Scales were gauged under successively dimmer illuminations from 10 to 0.0003 Lux (1.0 to −3.5 log Lux). Triads of samples were chosen representing each of eight basic color categories; “red,” “pink,” “orange,” “yellow,” “green,” “blue,” “purple,” and “gray.” Samples within each triad varied in lightness. Observers sorted samples into groups that they could categorize with specific color names. Above −0.5 log Lux, the dichromatic and anomalous trichromatic observers sorted the samples into the original representative color groups, with some exceptions. At light levels where rods alone mediate vision, the color names assigned by the deuteranomalous trichromats were similar to the color names used by color normals; higher scotopic reflectance samples were classified as blue-green-grey and lower reflectance samples as red-orange. Color names reported by the dichromats at the dimmest light levels had extensive overlap in their sample scotopic lightness distributions. Dichromats did not assign scotopic color names based on the sample scotopic lightness, as did deuteranomalous trichromats and colour-normals. We reasoned that the reduction in color gamut that a dichromat experiences at photopic light levels leads to a limited association of rod color perception with objects differing in scotopic reflectance.