Vision In Dim Light Research Articles

Nocturnal colour vision in geckos

Nocturnal animals are said to sacrifice colour vision in favour of increased absolute sensitivity. This is true for most vertebrates that possess a dual retina with a single type of rod for colour-blind night vision and multiple types of cone for diurnal colour vision. However, among the nocturnal vertebrates, geckos are unusual because they have no rods but three cone types. Here, we show that geckos use their cones for colour vision in dim light. Two specimens of the nocturnal helmet gecko Tarentola (formerly Geckonia) chazaliae were able to discriminate blue from grey patterns by colour alone. Experiments were performed at 0.002 cd m(-2), a light intensity similar to dim moonlight. We conclude that nocturnal geckos can use cone-based colour vision at very dim light levels when humans rely on colour-blind rod vision.

Crystals of Native and Modified Bovine Rhodopsins and Their Heavy Atom Derivatives

Rhodopsin, the pigment protein responsible for dim-light vision, is a G protein-coupled receptor that converts light absorption into the activation of a G protein, transducin, to initiate the visual response. We have crystallised detergent-solubilised bovine rhodopsin in the native form and after chemical modifications as needles 10-40 microm in cross-section. The crystals belong to the trigonal space group P3(1), with two molecules of rhodopsin per asymmetric unit, related by a non-crystallographic 2-fold axis parallel with the crystallographic screw axis along c (needle axis). The unit cell dimensions are a=103.8 A, c=76.6 A for native rhodopsin, but vary over a wide range after heavy atom derivatisation, with a between 101.5 A and 113.9 A, and c between 76.6 A and 79.2 A. Rhodopsin molecules are packed with the bundle of transmembrane helices tilted from the c-axis by about 100 degrees . The two molecules in the asymmetric unit form contacts along the entire length of their transmembrane helices 5 in an antiparallel orientation, and they are stacked along the needle axis according to the 3-fold screw symmetry. Hence hydrophobic contacts are prominent at protein interfaces both along and normal to the needle axis. The best crystals of native rhodopsin in this crystal form diffracted X-rays from a microfocused synchrotron source to 2.55 A maximum resolution. We describe steps taken to extend the diffraction limit from about 10 A to 2.6 A.

The Visual Cycle of the Cone Photoreceptors of the Retina

The photoreceptors of the eye's retina consist of rods and cones; rods serve vision in dim light, whereas cones serve high-resolution color vision in daylight. The first event in vision is the light-initiated isomerization of 11-cis-retinal, which is attached to rod or cone opsin, to all-trans-retinal. The regeneration of 11-cis-retinal comprises the well-known visual cycle in rods. By using cone-dominant retinas from chickens and ground squirrels, a visual cycle has been discovered in cones that differs radically from that in rods, mainly in the mechanism of isomerization of all-trans-retinol to 11-cis-retinol, and the latter's oxidation to 11-cis-retinal.

The visual cycle of the cone photoreceptors of the retina.

The photoreceptors of the eye's retina consist of rods and cones; rods serve vision in dim light, whereas cones serve high-resolution color vision in daylight. The first event in vision is the light-initiated isomerization of 11-cis-retinal, which is attached to rod or cone opsin, to all-trans-retinal. The regeneration of 11-cis-retinal comprises the well-known visual cycle in rods. By using cone-dominant retinas from chickens and ground squirrels, a visual cycle has been discovered in cones that differs radically from that in rods, mainly in the mechanism of isomerization of all-trans-retinol to 11-cis-retinol, and the latter's oxidation to 11-cis-retinal.

Retinal and optical adaptations for nocturnal vision in the halictid bee Megalopta genalis.

The apposition compound eye of a nocturnal bee, the halictid Megalopta genalis, is described for the first time. Compared to the compound eye of the worker honeybee Apis mellifera and the diurnal halictid bee Lasioglossum leucozonium, the eye of M. genalis shows specific retinal and optical adaptations for vision in dim light. The major anatomical adaptations within the eye of the nocturnal bee are (1) nearly twofold larger ommatidial facets and (2) a 4-5 times wider rhabdom diameter than found in the diurnal bees studied. Optically, the apposition eye of M. genalis is 27 times more sensitive to light than the eyes of the diurnal bees. This increased optical sensitivity represents a clear optical adaptation to low light intensities. Although this unique nocturnal apposition eye has a greatly improved ability to catch light, a 27-fold increase in sensitivity alone cannot account for nocturnal vision at light intensities that are 8 log units dimmer than during daytime. New evidence suggests that additional neuronal spatial summation within the first optic ganglion, the lamina, is involved.

Primary events in dim light vision: a chemical and spectroscopic approach toward understanding protein/chromophore interactions in rhodopsin.

The visual pigment rhodopsin (bovine) is a 40 kDa protein consisting of 348 amino acids, and is a prototypical member of the subfamily A of G protein-coupled receptors (GPCRs). This remarkably efficient light-activated protein (quantum yield = 0.67) binds the chromophore 11-cis-retinal covalently by attachment to Lys296 through a protonated Schiff base. The 11-cis geometry of the retinylidene chromophore keeps the partially active opsin protein locked in its inactive state (inverse agonist). Several retinal analogs with defined configurations and stereochemistry have been incorporated into the apoprotein to give rhodopsin analogs. These incorporation results along with the spectroscopic properties of the rhodopsin analogs clarify the mode of entry of the chromophore into the apoprotein and the biologically relevant conformation of the chromophore in the rhodopsin binding site. In addition, difference UV, CD, and photoaffinity labeling studies with a 3-diazo-4-oxo analog of 11-cis-retinal have been used to chart the movement of the retinylidene chromophore through the various intermediate stages of visual transduction.

(1)H and (13)C MAS NMR evidence for pronounced ligand-protein interactions involving the ionone ring of the retinylidene chromophore in rhodopsin.

Rhodopsin is a member of the superfamily of G-protein-coupled receptors. This seven alpha-helix transmembrane protein is the visual pigment of the vertebrate rod photoreceptor cells that mediate dim light vision. In the active binding site of this protein the ligand or chromophore, 11-cis-retinal, is covalently bound via a protonated Schiff base to lysine residue 296. Here we present the complete (1)H and (13)C assignments of the 11-cis-retinylidene chromophore in its ligand-binding site determined with ultra high field magic angle spinning NMR. Native bovine opsin was regenerated with 99% enriched uniformly (13)C-labeled 11-cis-retinal. From the labeled pigment, (13)C carbon chemical shifts could be obtained by using two-dimensional radio frequency-driven dipolar recoupling in a solid-state magic angle spinning homonuclear correlation experiment. The (1)H chemical shifts were assigned by two-dimensional heteronuclear ((1)H-(13)C) dipolar correlation spectroscopy with phase-modulated Lee-Goldburg homonuclear (1)H decoupling applied during the t(1) period. The data indicate nonbonding interactions between the protons of the methyl groups of the retinylidene ionone ring and the protein. These nonbonding interactions are attributed to nearby aromatic acid residues Phe-208, Phe-212, and Trp-265 that are in close contact with, respectively, H-16/H-17 and H-18. Furthermore, binding of the chromophore involves a chiral selection of the ring conformation, resulting in equatorial and axial positions for CH(3)-16 and CH(3)-17.

Photopigment basis for dichromatic color vision in the horse.

Horses, like other ungulates, are active in the day, at dusk, dawn, and night; and, they have eyes designed to have both high sensitivity for vision in dim light and good visual acuity under higher light levels (Walls, 1942). Typically, daytime activity is associated with the presence of multiple cone classes and color-vision capacity (Jacobs, 1993). Previous studies in other ungulates, such as pigs, goats, cows, sheep and deer, have shown that they have two spectrally different cone types, and hence, at least the photopigment basis for dichromatic color vision (Neitz & Jacobs, 1989; Jacobs, Deegan II, Neitz, Murphy, Miller, & Marchinton, 1994; Jacobs, Deegan II, & Neitz, 1998). Here, electroretinogram flicker photometry was used to measure the spectral sensitivities of the cones in the domestic horse (Equus caballus). Two distinct spectral mechanisms were identified and are consistent with the presence of a short-wavelength-sensitive (S) and a middle-to-long-wavelength-sensitive (M/L) cone. The spectral sensitivity of the S cone was estimated to have a peak of 428 nm, while the M/L cone had a peak of 539 nm. These two cone types would provide the basis for dichromatic color vision consistent with recent results from behavioral testing of horses (Macuda & Timney, 1999; Macuda & Timney, 2000; Timney & Macuda, 2001). The spectral peak of the M/L cone photopigment measured here, in vivo, is similar to that obtained when the gene was sequenced, cloned, and expressed in vitro (Yokoyama & Radlwimmer, 1999). Of the ungulates that have been studied to date, all have the photopigment basis for dichromatic color vision; however, they differ considerably from one another in the spectral tuning of their cone pigments. These differences may represent adaptations to the different visual requirements of different species.

The ChromaGen contact lens system: colour vision test results and subjective responses

The ChromaGen lens system is designed to enhance colour perception in colour vision deficiency (CVD). To investigate its efficacy, 14 CVD subjects were prescribed ChromaGen contact lenses. Colour vision tests (Ishihara, Farnsworth Munsell D-15, Farnsworth Lantern) were administered at baseline, lens dispensing, and after a 2-week lens-wearing trial during which subjective responses were recorded daily using visual analogue scales. ChromaGen lenses significantly reduced Ishihara error rates (p<0.001; ANOVA), particularly for deutan subjects. There was also a significant reduction in errors (p<0.005) on the D-15 test. Conversely, lens wear had no significant effect on Farnsworth Lantern test performance. Subjectively, subjects reported enhanced colour perception, but poor vision in dim light. Judgement of distance and motion were only slightly affected. We conclude that ChromaGen lenses may enhance subjective colour experience and assist in certain colour-related tasks, but are not indicated as an aid for CVD in occupations with colour vision-related restrictions.

Guanine and its retinal distribution in the tapetum of the bigeye tuna, Thunnus obesus

The eye of the bigeye tuna (Thunnus obesus) contains a retinal tapetum composed of guanine. The total amount of the guanine in one eye of the fish (SL=120 cm) was about 88.6 mg. The mean guanine content of the tapetum was approximately 1.25 mg/cm2 of the retinal surface. The highest content of guanine (2.15 mg/cm2) was observed only in the ventro-temporal part of the retina. To distinguish this area from the rest of the eye, we suggested the term ‘locus tapetalis’ for it. The visual accommodation system clearly indicated that the visual axis of the fish is upper-forward and the resulting retinal area for acute vision was suggested to be in the ventro-temporal retina. We discussed that the area centralis of the bigeye tuna may have two functions: to guarantee high visual acuity and to allow for high photo-sensitivity in dim light vision.

Seeing better at night: life style, eye design and the optimum strategy of spatial and temporal summation

Animals which need to see well at night generally have eyes with wide pupils. This optical strategy to improve photon capture may be improved neurally by summing the outputs of neighbouring visual channels (spatial summation) or by increasing the length of time a sample of photons is counted by the eye (temporal summation). These summation strategies only come at the cost of spatial and temporal resolution. A simple analytical model is developed to investigate whether the improved photon catch afforded by summation really improves vision in dim light, or whether the losses in resolution actually make vision worse. The model, developed for both vertebrate camera eyes and arthropod compound eyes, calculates the finest spatial detail perceivable by a given eye design at a specified light intensity and image velocity. Visual performance is calculated for the apposition compound eye of the locust, the superposition compound eye of the dung beetle and the camera eye of the nocturnal toad. The results reveal that spatial and temporal summation is extremely beneficial to vision in dim light, especially in small eyes (e.g. compound eyes), which have a restricted ability to collect photons optically. The model predicts that using optimum spatiotemporal summation the locust can extend its vision to light intensities more than 100 000 times dimmer than if it relied on its optics alone. The relative amounts of spatial and temporal summation predicted to be optimal in dim light depend on the image velocity. Animals which are sedentary and rely on seeing small, slow images (such as the toad) are predicted to rely more on temporal summation and less on spatial summation. The opposite strategy is predicted for animals which need to see large, fast images. The predictions of the model agree very well with the known visual behaviours of nocturnal animals.

Thalamic control of cat lateral suprasylvian visual area: relation to patchy association projections from area 18.

In this study, we examined functional contributions of major subdivisions of the lateral geniculate nucleus to the cat's lateral suprasylvian visual area (LS) in relation to the patchy horizontal distributions of association inputs. Multiple-unit activity driven via the contralateral eye was assessed during reversible blockade of the retinotopically corresponding part of layer A, the C layers as a group, or the medial interlaminar nucleus (MIN). Inactivating each of these targets reduced activity at some cortical sites, with inactivation of layer A having, on average, the largest effect. Activity was rarely abolished by inactivation of a single target, indicating that most LS sites receive multiple inputs. Dependence on layer A was strongly correlated with the horizontal distribution of association inputs from area 18. Closely spaced injections of anatomical tracers into extensive regions of area 18 resulted in patches of terminal label in lateral suprasylvian cortex. Activity inside the patches was relatively dependent on layer A, whereas that outside the patches was not. Dependence on the MIN and layer A were negatively correlated, suggesting that inputs dominated by the MIN and layer A were concentrated in independent sets of patches. These results indicate that the anatomically observed patchy projections reflect the functional consequences of geniculate lamination. The A layers are high-acuity relays, whereas the MIN is probably a specialization for dim-light vision (Lee et al., 1984; Lee et al., 1992). We propose that the partial overlap of inputs dominated by the A layers and the MIN allows dynamic shifts in their relative contributions to LS responses, optimizing the balance of high-acuity and high-sensitivity channels over a wide range of illumination conditions.

The steric trigger in rhodopsin activation

Rhodopsin is the seven transmembrane helix receptor responsible for dim light vision in vertebrate rod cells. The protein has structural homology with the other G protein-coupled receptors, which suggests that the tertiary structures and activation mechanisms are likely to be similar. However, rhodopsin is unique in several respects. The most striking is the fact that the receptor “ligand”, 11-cis retinal, is covalently bound to the protein and is converted from an “antagonist” to an “agonist” upon absorption of light. NMR studies of rhodopsin and its primary photoproduct, bathorhodopsin, have generated structural constraints that enabled docking of the 11-cis and all-trans retinal chromophores into a low-resolution model of the protein proposed by Baldwin. These studies also suggest a mechanism for how retinal isomerization leads to rhodopsin activation. More recently, mutagenesis studies have extended these results by showing how the selectivity of the retinal-binding site can be modified to favor the all-trans over the 11-cis isomer. The structural constraints produced from these studies, when placed in the context of a high-resolution model of the protein, provide a coherent picture of the activation mechanism, which we show involves a direct steric interaction between the retinal chromophore and transmembrane helix 3 in the region of Gly121.

In the eye of the beholder: Visual pigments and inherited variation in human vision

Jeremy Nathans Howard Hughes Medical Institute Departments of Molecular Biology and Genetics, Neuroscience, and Ophthalmology Johns Hopkins University School of Medicine Baltimore, Maryland 21205 Each of us inhabits an internally consistent visual world. In everyday life we take the constancy of this world for granted and assume that others share it. Although this assumption is roughly valid when averaged over the popu- lation, psychophysical experiments over the past two cen- turies have revealed significant differences among individ- ual humans in many aspects of visual function. One source of these differences has recently been identified at a mo- lecular level: sequence variation in the genes encoding the visual pigments, the light-absorbing proteins in the rods and cones that initiate phototransduction. In humans there are four visual pigments. Each visual pigment consists of an integral membrane apoprotein bound to a chromophore, 1 l-cis retinal. Rhodopsin, the rod pigment, mediates vision in dim light. The three cone pigments, referred to as blue, green, and red, mediate color vision and function only in bright light. The absorption spectra of the four human visual pigments are shown in Figure 1. Inherited Variation in Co/or Vision The most common variation in human color vision arises from a serinelalanine polymorphism at position 180 in the red pigment gene. The absorption spectrum of the red(Ser- 180) allele is red-shifted 4-5 nm relative to the red(Ala-180) allele (Merbs and Nathans, 1992a; Asenjo et al., 1994). Males who carry the red(Ser-180) allele have a corre- spondingly greater sensitivity to long wavelength lights than do those who carry the red(Ala-180) allele (Wind- erickx et al., 1992b). In the Caucasian gene pool, the red(Ser-180) and red(Ala-180) alleles are found at frequen- cies of 82% and 38%, respectively. The common variations in red-green color vision that are colloquially referred to as color blindness arise from loss of either the red or the green cone pigment (dichro- macy) or from the production of a visual pigment with a shifted absorption spectrum (anomalous trichromacy). The high frequencies of these variations in males-approxi- mately 2% for dichromacy and, depending on the popula- tion, between 2% and 8% for anomalous trichromacy- reflect the X chromosome location of the red and green pigment genes, a relaxation of selective pressure for nor- mal trichromacy, and an unusually facile mutational mech- anism. The red and green pigment genes are organized in a head-to-tail tandem array in which the transcription units and intergenic spacer DNA are 98% identical at the nucleotide level, an arrangement that predisposes them to homologous but unequal recombination (reviewed by Nathans et al., 1992). Unequal intergenic recombination generates variant arrays in which entire repeat units are

Symptomatic abnormalities of dark adaptation in patients with age-related Bruch's membrane change.

Some patients with age-related changes at the level of Bruch's membrane and good visual acuity report poor vision in dim light, fading vision in bright light, and a central scotoma noticeable in the dark. Ophthalmic examination, scotopic thresholds, and dark adaptation kinetics were recorded in 12 eyes of 12 patients with such symptoms. All had macular drusen which were hypofluorescent on fluorescein angiography in nine subjects, and six had evidence of prolonged choroidal filling on fluorescein angiography. Scotopic thresholds were depressed in six patients who all experienced a central scotoma in the dark or poor night vision. The kinetics of dark adaptation were abnormal in all 10 patients in whom reliable measurements were possible. The findings suggest that visual symptoms reflect abnormality of both scotopic sensitivity and the time course of dark adaptation in patients with age-related Bruch's membrane change.

Acuity-sensitivity trade-offs of X and Y cells in the cat lateral geniculate complex: role of the medial interlaminar nucleus in scotopic vision.

1. The cat medial interlaminar nucleus (MIN) receives inputs almost exclusively from tapetal retina, suggesting that the MIN has a special role in dim-light vision. In this study we compared the sensitivities of cells in the MIN with those in layers A and magnocellular C of the lateral geniculate nucleus (LGNd), using drifting sinusoidal gratings to determine contrast thresholds as a function of spatial frequency and retinal adaptation level over the entire scotopic range. 2. About one-half of the cells recorded in the MIN and layer A had brisk responses that could be nulled by properly positioned, counterphased sinusoidal gratings, and were classified as X cells. The rest of the cells in the MIN and layer A, as well as all cells recorded in layer C, were Y cells. 3. MIN cells had higher contrast sensitivity than layer A cells for low spatial frequencies (0.15 cycles/deg and below) over a wide range of adaptation levels, both overall and for separate comparisons within X or Y cells. Layer C Y cells were intermediate in sensitivity between MIN and layer A Y cells. For low spatial frequencies, Y cells as a group were more sensitive than X cells, whereas the reverse was true for high spatial frequencies. 4. These data enable one to determine the lowest adaptation level at which stimuli of a given contrast can be detected for a given structure. At the lowest spatial frequencies, the MIN can function at adaptation levels approximately 1 log unit below layer A, averaged over all stimulus contrasts. In contrast, the tapetum lowers luminance threshold by at most 0.16 log unit. 5. For scotopic conditions and eccentricities within 15 degrees of the area centralis, contrast sensitivity decreases with eccentricity for low spatial frequencies and remains flat or slightly increases for high spatial frequencies. This relationship, which is opposite to that found for photopic vision, is strongest for MIN Y cells. 6. These data support the hypothesis that the retinal conflict between sensitivity and acuity is ameliorated in the CNS through separate thalamic relays with different degrees of afferent convergence. MIN cells have higher luminance sensitivity than layer A cells, but at the expense of acuity. Layer C appears to occupy an intermediate position in this trade-off.

Rod and cone pigments of the Atlantic guitarfish,Rhinobatos lentiginosus Garman

Using both extraction- and micro-spectrophotometric (MSP) methods the visual pigment(s) from the rods and cones of the Atlantic guitarfish, Rhinobatos lentiginosus, were shown to be spectrally similar, if not identical (lambda max = 498-499 nm). Color vision, therefore, is unlikely unless mediated via colored oil droplets in the inner segments. The identical lambda max for the rod and cone pigments suggest that vision in both dim and bright light may correlate with the underwater spectrum over the depths and the times of day that guitarfish are active. The primary advantage of the blue-green sensitive visual pigments, we suggest, is to enhance the contrast of targets silhouetted against the background spacelight.

Intra- and interspecific changes in retinal morphology among mesopelagic and demersal teleosts from the slope waters of New Zealand

Retinae from mesopelagic teleosts with adult ranges in the shallow, mid and deep mesopelagic zones, respectively, were examined by light microscopy. Retinal characteristics were described, and photoreceptor densities, outer segment dimensions, and convergence ratios measured from transverse sections. Juveniles of all species had lower photoreceptor densities, outer segment lengths and convergence ratios than adults. In species with multiple banks of photoreceptors, additional banks were added as the retina increased in size. A positive correlation was found between the degree of retinal specialisation for vision in dim light, and the depth of occurrence. The retina of each specimen was given a rank based on log unit changes in photoreceptor density and convergence ratio, the length of photoreceptor outer segments and the presence or absence of multiple banks of photoreceptors. Higher ranks (indicating greater retinal specialisation) were found among species occurring at greater depths. Among species showing a change in depth preference with growth, there was a corresponding increase in retinal rank. It is suggested that the proposed system of ranks has application in predicting the depth of occurrence of a species with a given pattern of retinal morphology.

Prentice Award Lecture

00937002/85/6207—0427$02.00/0 AMERICAN JOURNAL or OPTOMETRY & PHYSIOLOGICAL OPTICS copyright © 1985 AMERICAN ACADEMY or OPTOMETRY | awards/honors A Vol. 62, No. 7, pp. 427-438 Printed in U. S.A. Prentice Award Lecture: A Simple Retinal Mechanism That Has Complex and Profound Effects on Perception TOM N. CORNSWEETq&#39; School of Social Sciences, University of California, Irvine, California In the November 1984 issue of Scientific American, Lederman published an article called “The Value of Fundamental Science”; his first sentence says a lot of what I want to say. “One takes up fundamental science out of a sense of pure excitement, out of joy at enhancing human culture, out of awe at the heritage handed down by generations of masters and out of a need to publish first and become famous.“ The work I want to tell you about is exciting to me, and now, with this honor, I feel famous. It is unusual for a professional academy like this one, whose primary goal is and should be the advancement of a profession, to give its highest honor to people doing basic research. That says something special about this academy and its members. I feel highly honored by this award, and I am also very grateful. I suppose all of us like to think that the work we do is impor- tant, but not many of us are lucky enough to have somebody else say that they think what we do Is worth doing. That is very flattering and gratifying. Thank you. I would like to tell you about a theory of a Very Simple retinal mechanism that seems to explain many apparently complicated and un- related visual phenomena. I think it is interest- q18 because, even if the theory turns out to be ‘q0118 In the sense that perhaps there is no such mechanism actually in the retina, it does what 3“ 800d_theories do in that it reveals that what We previously thought were unrelated phenom- 9118 can all be manifestations of the same mech- T The Prentice Award Lecture, presented at the An- flufll Meeting of the American Academy of Optometry ‘HES; Louis, Missouri, December, 1984. ‘ ceived November 29, 1984. Pl‘-D-. Member of Faculty. anism; and, especially if some of the phenomena seemed complicated and the mechanism is sim- ple, then the world is easier to understand and our thinking is simplified. To me, that step in science, where a new theory seems to take a lot of messy pieces and melt them down to a single clear drop, is the essence of what is esthetically pleasing about science. (The next step, of course, is that your successors, or you if you’re lucky, look deeply into the drop and see a whole new set of messy pieces.) Most of the visual phenomena that I will talk about are probably familiar to you, but I will refresh your memory about them as we go along. They are light and dark adaptation, Weber’s law, Ricco’s law, brightness constancy, receptive field center-surround antagonism, Mach bands, and changes in acuity with luminance. Some of these phenomena are necessarily related to each other, in that if you have one, you almost have to have the other, and some are not necessarily related, but all of them can be explained by one simple retinal mechanism. And I would like to introduce the mechanism by talking about still another phenomenon that it explains, one that may actually be the fundamental reason why the mechanism evolved, and so may be the reason why all of the other phenomena occur. This fundamental phenomenon doesn’t have a name. It is the fact that people are a whole lot better at detecting objects in bright light than you would expect on the basis of their vision in dim light, and vice versa, that is, people do a whole lot better in dim light than you would expect them to do on the basis of how well they do in bright light. Let me explain that. A basic property of objects in the world is that they reflect fixed proportions of the light falling on them. Suppose your eyes were pointed at an 427

Cat medial interlaminar nucleus: retinotopy, relation to tapetum and implications for scotopic vision.

The medial interlaminar nucleus (MIN) of the cat was electrophysiologically mapped in sufficient detail to resolve individual laminae and to allow reconstruction of isoazimuth and isoelevation lines in coronal, sagittal, and horizontal planes. The electrophysiologically defined laminar pattern was in agreement with that revealed anatomically in the same animal, as well as with the general pattern revealed by anatomical methods in several unmapped nuclei. The MIN is made up of three distinct layers, each receiving inputs from one hemi-retina but none representing an entire hemifield. We confirm the findings of Guillery et al. (19) that the contralateral hemifield is represented in layers 1 and 2 through the contralateral and ipsilateral eyes, respectively, and that layer 3 represents the ipsilateral hemifield through the contralateral eye. Elevation and absolute value of azimuth are represented continuously through the MIN. When an isoazimuth line crosses the border between layer 3 and either layer 1 or 2, the absolute value of azimuth is maintained but the sign of the azimuth changes. Adjacent points on either side of this border represent mirror symmetrical visual directions on opposite sides of the vertical meridian. This indicates that the distance from the vertical meridian is an independently coded parameter within the geniculate complex. There is virtually no nasotemporal overlap in any layer of the MIN. The function relating magnification (mm3 per steradian) to eccentricity is strikingly similar to the function relating retinal ganglion cell density to eccentricity, suggesting that a constant fraction of retinal ganglion cells project to the MIN at all eccentricities. Most of the volume of each MIN layer is devoted to lower visual fields. Analysis of the geniculate retinotopic maps of Sanderson (46) reveals no equivalent bias toward lower visual fields in the dorsal lateral geniculate nucleus. The MIN represents a region of retina roughly coincident with the tapetum, suggesting a role of the MIN in dim-light vision.