Consensual pupillary light response in the red-eared slider turtle (Trachemys scripta elegans)

Purpose of this study was to determine if the turtle has a consensual pupillary light response (cPLR), and if so, to compare it to its direct pupillary light response (dPLR). One eye was illuminated with different intensities of light over a four log range while keeping the other eye in darkness. In the eye directly illuminated, pupil diameter was reduced by as much as ∼31%. In the eye not stimulated by light, pupil diameter was also reduced but less to ∼11%. When compared to the directly illuminated eye, this generated a ratio, cPLR-dPLR, equal to 0.35. Ratio of slopes for log/linear fits to plots of pupil changes versus retinal irradiance for non-illuminated (−1.27) to illuminated (−3.94) eyes closely matched at 0.32. cPLR had time constants ranging from 0.60 to 1.20 min; however, they were comparable and not statistically different from those of the dPLR, which ranged from 1.41 to 2.00 min. Application of mydriatic drugs to the directly illuminated eye also supported presence of a cPLR. Drugs reduced pupil constriction by ∼9% for the dPLR and slowed its time constant to 9.58 min while simultaneous enhancing constriction by ∼6% for the cPLR. Time constant for the cPLR at 1.75 min, however, was not changed. Results support that turtle possesses a cPLR although less strong than its dPLR.     

Uveoscleral outflow following cyclodialysis in the monkey eye using a fluorescent tracer

Cyclodialysis was performed in one eye of each of eight cynomolgus monkeys. Two days later, the intraocular pressure was 1.6 +/- 0.7 mmHg in eyes with cyclodialysis and 12.0 +/- 0.7 mmHg in fellow control eyes. 10(-4) M fluorescein-isothiocyanate dextran (70,000 molecular weight) was perfused into the anterior chamber of each eye for 30 min. The eyes were enucleated and dissected into sclera, choroid, retina, iris, and ocular fluid. Samples were homogenized and centrifuged, and the fluorescence of the supernatant was measured. Expressed as equivalent volumes of aqueous, the rate of anterior chamber movement of tracer via uveoscleral pathways was 1.40 +/- 0.17 microliter/min in cyclodialysis eyes and 0.34 +/- 0.10 microliter/min in control eyes. Cyclodialysis results in a fourfold increase in uveoscleral outflow, contributing to the observed hypotony.     

Increased vascular permeability in the rabbit iris induced by prostaglandin E1

Prostaglandin E₁ (PGE₁) (25 g) was applied topically to the eyes of albino rabbits, and lanthanum was used as an electron microscopic tracer to study possible vascular permeability changes in the iris of these eyes. Lanthanum was injected directly into the blood circulation 15min after application of the PG, and the eyes were enucleated 30 or 60 min later. PGE₁Was found to induce increased vascular permeability. Heavy deposits of the tracer were found in the walls of iridial venules, especially between the endothelium and its basement lamina. The highest concentration of lanthanum was found in close relation to the interendothelial clefts. Occasionally, gaps between neighbouring endothelial cells were observed. In control eyes, not treated with PG, no extravasation of the tracer was found. The study indicates that the iridial blood vessels contribute to the production of the plasmoid aqueous humour induced by PGE₁ in the rabbit eye.This paper was presented in part at the 7th Annual Meeting of the European Club for Ophthalmic Fine Structure in Ystad, Sweden on April 20 and 21, 1979     

The color appearance of stimuli detected via short-wavelength-sensitive cones: comparisons with visual adaptation and visual field data for peri- or post-menopausal women under 70 years of age

Dynamics of foveal light adaptation for vision mediated via short-wavelength-sensitive (SWS) cones were compared for two groups of healthy amenorrheic (peri- or post-menopausal) women not using hormonal medication. Each subject was assigned to a group based on the color name—“lavender” (∼2/3 of all subjects) or “white” (∼1/3 of all subjects)—chosen in a forced-response paradigm to best describe a threshold-level 440-nm test presented on a larger 3.6 log td 580-nm background that had been viewed for ∼5 min. During the first 20-30 s after this 3.6 log td background abruptly replaced a much dimmer background, the threshold elevations (relative to the steady-state levels measured at ∼5 min) were significantly greater for the lavender-naming subjects than for the white-naming subjects. However, exponential rates of recovery were indistinguishable for the two groups. A viable interpretation is that the gain of the visual response at background onset is greater for lavender-naming subjects than for white-naming subjects at or distal to a site where responses from middle-wavelength-sensitive and long-wavelength-sensitive (MWS and LWS) cones oppose responses from SWS cones. In addition, the color names derived from foveal testing were related systematically to extrafoveal sensitivities measured with Short Wavelength Automated Perimetry (SWAP), in a manner suggesting that response gain and/or response speed may be greater for lavender-naming subjects in the direction of increased SWS response also. Evidence from other subject populations suggests that the choice of color name and the dynamics of visual response each can be affected by alterations (particularly reductions) of estrogen synthesis and response.     

Retinal arteriolar and capillary vascular reactivity in response to isoxic hypercapnia

The aim of the study was to compare the magnitude of vascular reactivity of the retinal arterioles in terms of percentage change to that of the retinal capillaries using a novel, standardized methodology to provoke isoxic hypercapnia. Ten healthy subjects (mean age 25 years, range 21-31) were recruited. Subjects attended a single visit comprising two study sessions separated by 30 min. Subjects were fitted with a sequential re-breathing circuit connected to a computer-controlled gas blender. Each session consisted of breathing at rest for 10 min (baseline), increase of P_ETCO₂ (maximum partial pressure of CO₂ during expiration) by 15% above baseline whilst maintaining isoxia for 20 min, and returning to baseline conditions for 10 min. Retinal hemodynamic measurements were performed using the Canon Laser Blood Flowmeter and the Heidelberg Retina Flowmeter in random order across sessions. Retinal arteriolar diameter, blood velocity and flow increased by 3.3%, 16.9% and 24.9% (p < 0.001), respectively, during isoxic hypercapnia. There was also an increase of capillary blood flow of 34.8%, 21.6%, 24.9% (p ≤ 0.006) at the optic nerve head neuroretinal rim, nasal macula and fovea, respectively. The coefficient of repeatability (COR) was 5% of the average P_ETCO₂ both at baseline and during isoxic hypercapnia and was 10% and 7% of the average P_ETO₂ (minimum partial pressure of oxygen at end exhalation), respectively. The overall magnitude of retinal capillary vascular reactivity was equivalent to the arteriolar vascular reactivity with respect to percentage change of flow. The magnitude of isoxic hypercapnia was repeatable.     

Retinal thinning in tree shrews with induced high myopia: optical coherence tomography and histological assessment

This study determined retinal thinning in a mammalian model of high myopia using optical coherence tomography (OCT) and histological sections from the same retinal tissue. High myopia was induced in three tree shrews (Tupaia belangeri) by deprivation of form vision via lid suture of one eye, with the other eye a control. Ocular biometry data was obtained by Ascan ultrasonography, keratometry and retinoscopy. The Zeiss StratusOCT was used to obtain Bscans in vivo across the retina. Subsequently, eyes were enucleated and retinas fixed, dehydrated, embedded and sectioned. Treated eyes developed a high degree of axial myopia (−15.9 ± 2.3 D; n = 3). The OCT analysis showed that in myopic eyes the nasal retina thinned more than the temporal retina relative to the disc (p = 0.005). Histology showed that the retinas in the myopic eyes comprise all layers but were thinner than the retinas in normal and control eyes. Detailed thickness measurements in corresponding locations of myopic and control eyes in superior nasal retina using longitudinal reflectivity profiles from OCT and semithin vertical histological sections showed the percentage of retinal thinning in the myopic eyes was similar between methods (OCT 15.34 ± 5.69%; histology 17.61 ± 3.02%; p = 0.10). Analysis of retinal layers revealed that the inner plexiform, inner nuclear and outer plexiform layers thin the most. Cell density measurements showed all neuronal cell types are involved in retinal thinning. The results indicate that in vivo OCT measurements can accurately detect retinal thinning in high myopia.     

Influence of age, gender and iris color on mechanical and chemical sensitivity of the cornea and conjunctiva

The purpose of this study was to establish the influence of age, gender and iris color on the mechanical and chemical sensitivity of the cornea and the conjunctiva. In 57 healthy subjects (27 males, 30 females; ages between 23 and 71 years), sensory thresholds to mechanical and chemical stimulation were measured in the central cornea and the temporal conjunctiva using a Belmonte's gas esthesiometer. Mechanical stimulation consisted of warmed air pulses of 3s duration at different flow rates (40-200 ml/min). For chemical (acidic) stimulation, 3-s warmed gas pulses containing 10% to 80% CO(2) in air were applied, at a flow below mechanical threshold flow. Corneal and conjunctival thresholds to mechanical and chemical stimuli increased with age. Premenopausal women were more sensitive to corneal stimulation than men of similar ages but overall differences in mechanical and chemical threshold between men and women were not significant. Individuals with blue eyes had significantly lower corneal chemical thresholds than those with brown or green eyes. Multiple linear regression analysis evidenced that corneal mechanical threshold depends on age and iris color according to the equation 58.2 ml/min+1.3.(Age-23 years)-9.7 if blue eyes. For chemical threshold, the equation was 28.6%CO(2)+05.(Age-23 years)-12.5 if blue eyes. In the case of the conjunctiva, only age determined mechanical and chemical thresholds according to the equations: 78.2 ml/min+1.4.(Age-23 years) and 41.6%CO(2)+0.8.(Age-23 years), respectively. Therefore, normal values of mechanical and chemical thresholds of the cornea and conjunctiva measured with the Belmonte gas esthesiometer can be predicted according to age and iris color. Variations of sensitivity with age, iris color and gender may reflect differences in innervation density and neural responsiveness associated with the hormonal status.     

Light intensity modulates corneal power and refraction in the chick eye exposed to continuous light

Continuous exposure of chicks to light was shown to result in severe hyperopia, accompanied by anterior segment changes, such as severe corneal flattening. Since rearing chicks in complete darkness results only in mild hyperopia and minor changes in corneal curvature, we hypothesized that light intensity may play a role in the development of refractive changes under continuous light illumination. To test this hypothesis, we examined the effects of rearing chicks under various continuous light intensities. More specifically, we investigated the refractive parameters of the chicks’ eyes, and avoided light cycling effects on ocular development. To this end, thirty-eight chicks were reared under 24-h incandescent illumination, at three different light intensities: 10,000 lux (n = 13), 500 lux (n = 12), and 50 lux (n = 13). Their eyes underwent repeated retinoscopy, keratometry, and ultrasound biometry, as well as caliper measurements of enucleated eyes. Both refraction and corneal refractive power were found to be correlated with light intensity. On day 90 after hatching, exposure to light intensities of 10 000, 500, and 50 lux resulted in hyperopia of +11.97 ± 3.7 (mean ± SD) +7.9 ± 4.08 and +0.63 ± 3.61 diopters (D), respectively. Under those intensities, corneal refractive power was 46.10 ± 3.62, 49.72 ± 4.16, and 56.88 ± 4.92 D, respectively. Axial length did not differ significantly among the groups. The vitreous chamber was significantly deeper in the high than in the low-intensity groups. Thus, during the early life of chicks exposed to continuous lighting, light intensity affects the vitreous chamber depth as well as the anterior segment parameters, most notably the cornea. The higher the intensity, the more severe was the corneal flattening observed and the hyperopia that developed, whereas continuous illumination at low intensities resulted in emmetropia. Thus, light intensity is an important factor that should be taken into account when studying refractive development.     

Age-dependence of the optomechanical responses of ex vivo human lenses from India and the USA, and the force required to produce these in a lens stretcher: the similarity to in vivo disaccommodation

The purpose of this study was to study the age-dependence of the optomechanical properties of human lenses during simulated disaccommodation in a mechanical lens stretcher, designed to determine accommodative forces as a function of stretch distance, to compare the results with in vivo disaccommodation and to examine whether differences exist between eyes harvested in the USA and India.Postmortem human eyes obtained in the USA (n = 46, age = 6-83 years) and India (n = 91, age = 1 day-85 years) were mounted in an optomechanical lens stretching system and dissected to expose the lens complete with its accommodating framework, including zonules, ciliary body, anterior vitreous and a segmented rim of sclera. Disaccommodation was simulated through radial stretching of the sectioned globe by 2 mm in increments of 0.25 mm. The load, inner ciliary ring diameter, lens equatorial diameter, central thickness and power were measured at each step. Changes in these parameters were examined as a function of age, as were the dimension/load and power/load responses.Unstretched lens diameter and thickness increased over the whole age range examined and were indistinguishable from those of in vivo lenses as well as those of in vitro lenses freed from zonular attachments. Stretching increased the diameter and decreased the thickness in all lenses examined but the amount of change decreased with age. Unstretched lens power decreased with age and the accommodative amplitude decreased to zero by age 45-50. The load required to produce maximum stretch was independent of age (median 80 mN) whereas the change in lens diameter and power per unit load decreased significantly with age.The age related changes in the properties of human lenses, as observed in the lens stretching device, are similar to those observed in vivo and are consistent with the classical Helmholtz theory of accommodation. The response of lens diameter and power to disaccommodative (stretching) forces decreases with age, consistent with lens nuclear stiffening.     

Photopigments and pseudo-pigments

The spectral properties of photopigments are often inferred from physiological measures of spectral sensitivity. From these measures pigments have recently been inferred with properties not corresponding to those of any pigment measured by spectrophotometry. It is proposed that in all these cases the properties of single pigments are not being measured; rather, they are pseudo pigments resulting from a specific form of neural interaction involving several real, known, photopigments. These pseudo-pigments were accepted as being related to single, but unusual, pigments largely because they obeyed the commonly accepted but incomplete criteria for identifying single pigments. Starting from the essential features of a pseudo-pigment, an analysis is developed; the conclusion is that pseudo-pigments can be resolved into standard photopigments, as identified by spectrophotometry, and that these interact in a specified form. A method is described for identifying the number andλ_max's of the photopigments involved in any physiological spectral sensitivity function, as well as certain details of the physiological interactions involved.     

The relationship between peripapillary crescent and axial length: Implications for differential eye growth

We evaluated the relationship between the size of the peripapillary crescent and the axial length (AL) of the eye as well as the fine structure of the peripapillary crescent in selected eyes. Infrared fundus imaging and spectral domain optical coherence tomography (SDOCT) (Spectralis HRA + OCT, Heidelberg Engineering, Germany) centered at the fovea were performed on 72 healthy adults. On the infrared fundus images, we measured (a) the distance between the foveola and the temporal edge of the optic disc (FOD) and (b) the distance between the foveola and the temporal edge of the peripapillary crescent (FOC) (if present). A peripapillary crescent presented at the nasal margin of the disc in 64% of the subjects. The FOD and FOC were 4.22 mm ± 0.46 and 3.97 mm ± 0.25, respectively. Only the FOD was significantly correlated with axial length. As AL increased by 10%, the FOD increased by 13%, the outer neural retina only expanded by 4% (as indicated by the FOC). This result emphasizes that retinal stretching may not mirror scleral growth, and the existence in some eyes of a difference between the photoreceptor margin and retinal pigment epithelium (RPE) margin suggests that within the retina there could be slippage during eye growth.     

Raman spectroscopy for differential diagnosis of endophthalmitis and uveitis in rabbit iris in vitro

We developed a diagnostic tool to differentiate between endophthalmitis and uveitis using Raman spectroscopy. Twenty-two New Zealand rabbits with endophthalmitis induced by Staphylococcus aureus (10 animals), noninfectious uveitis induced by lipopolysaccharide from Escherichia coli (10 animals) and controls (two animals) were analyzed. Twenty-four hours after the eyes were inoculated, iris tissue was dissected and subjected to dispersive Raman spectroscopy using an excitation source at 830 nm and a spectrograph/CCD camera to detect a Raman signal with an integration time of 50 s. With the collected spectra of endophthalmitis and uveitis, we developed a routine to classify spectra in each specimen using principal components analysis, using a leave-one-out cross-validation procedure. The mean Raman spectra of tissues with uveitis and endophthalmitis showed several bands in the region of 800-1800 cm⁻¹, which have been attributed to nucleic acids, amino acids, proteins, and lipids. The bands at 1004, 1339, and 1555 cm⁻¹ differed significantly (t-test, p < 0.05) between diseases. The principal components PC3 and PC4 differed significantly (ANOVA, p < 0.05) for the two tissue types, indicating that these PCs can be used to discriminate between the two diseases using Mahalanobis distance as a discriminator. This technique is useful for differentiating the spectral bands of uveitis and endophthalmitis, and the diagnostic model showed sensitivity of 89%, specificity of 100%, and accuracy of 92% using the leave-one-out cross-validation procedure. These results may be clinically relevant for differentiating endophthalmitis from uveitis, and this approach may become a noninvasive method to optimize the diagnosis of inflammatory and infectious vitreoretinal diseases.     

Effects of speed, age, and amblyopia on the perception of motion-defined form

We determined the effect of dot speed on the typical and atypical development of motion-defined form perception. Monocular motion coherence thresholds for orientation discrimination of motion-defined rectangles were determined at slow (0.1 deg/s), medium (0.9 deg/s) and fast (5.0 deg/s) dot speeds. First we examined typical development from age 4 to 31 years. We found that performance was most immature at the slow speed and in the youngest group of children (4-6 years). Next we measured motion-defined form perception in the amblyopic and fellow eyes of patients with amblyopia. Deficits were found in both eyes and were most pronounced at the slow speed. These results demonstrate the importance of dot speed to the development of motion-defined form perception. Implications regarding sensitive periods and the neural correlates of motion-defined form perception are discussed.     

Recovery of the blood-aqueous barrier after topical chemical irritation in the rabbit eye

An acute irritative response in the rabbit eye, taking the form of a rise in the intraocular pressure (IOP), miosis, and breakdown of the blood-aqueous barrier, was elicited by topical application of 1% neutral formaldehyde. The peak rise in IOP was 22.1 ± 2.5 mmHg and occurred within 14.2 ± 1.9 min, after which IOP returned to normal values in 45.4 ± 5.2 min. An increased amount of i.v.-injected Evans Blue was found in the aqueous humour when injected 15 min after the irritation (119.0 ± 21.2 g/ml, compared with 2.1 ± 1.4 g/ml in intact eyes;P < 0.001), and the protein concentration in the aqueous humour was also increased, to 11.3 ±1.4 mg/ml (P < 0.001). When Evans Blue was injected 60 min after the irritation, no statistically significant difference was found from normal in the amounts (13.7 ± 7.8 g/ml) in the aqueous humour, although the protein concentration in the aqueous humour was again found to be elevated (7.9 ± 1.6 mg/ml,P < 0.01). Histologically, the Evans Blue was shown to leak through both the ciliary and the iris barriers at 15 min. At 60 min only occasional Evans Blue leakage was demonstrated between the nonpigmented epithelial cells and only minor fluorescence was seen in iris stroma. The histological findings were in good agreement with Evans Blue analyses in the aqueous humour. The miosis lasted over 60 min in the formaldehyde-treated eyes. After sympathectomy the rise in IOP was more rapid at the beginning of the response, suggesting greater sensitivity of the sympathectomized eyes to the irritative stimuli. However, the miosis had recovered by 60 min after irritation in the sympathectomized eyes. In the contralateral eyes changes in IOP and in the integrity of the blood-aqueous barrier were observed. The present results suggest that the breakdown of the blood-aqueous barrier after sensory nerve stimulation is transient and recovery rapid.     

Immunohistochemical evaluation of the integrity of the blood-aqueous barrier in normal and rubeotic human eyes

Background: The integrity of the blood-aqueous barrier (BAB) was analyzed using an immunohistochemical technique for the demonstration of albumin. Methods: Paraffin sections of 36 normal eyes obtained from eye banks or at autopsy (mean age 62.8 ± 15.2 years) and 46 eyes with marked iris neovascularization (mean age 54.6 ± 25.3 years) were formalin-fixed and examined using rabbit anti-human albumin. Results: In normal eyes, albumin staining was found in the iris stroma inside and outside the iris vessels but was not detected across the anterior iris border; albumin was present in the anterior chamber in one eye, but not internal to the nonpigmented ciliary epithelium. In rubeotic eyes, albumin staining extended along the anteri or iris in all 46 cases; albumin was demonstrated in the anterior chamber in 31 eyes and along to the nonpigmented ciliary epithelium in 13 eyes. Differences between normal and rubeotic eyes were significant for intensity of albumin staining in the iris stroma and for presence of albumin along the anterior iris, within the anterior chamber, and along the ciliary epithelium (P < 0.001, ² test). Conclusion: Our findings indicate that the BAB may be less resistant to leakage in the iris stroma than at the ciliary epithelium. BAB breakdown in rubeotic eyes occurred mainly in the iris; the ciliary epithelium was much less involved. Immunohistochemical staining for albumin appears to be useful for evaluating the integrity of the BAB in human pathologic specimens.     

Total ocular, anterior corneal and lenticular higher order aberrations in hyperopic, myopic and emmetropic eyes

Total ocular higher order aberrations and corneal topography of myopic, emmetropic and hyperopic eyes of 675 adolescents (16.9 ± 0.7 years) were measured after cycloplegia using COAS aberrometer and Medmont videokeratoscope. Corneal higher order aberrations were computed from the corneal topography maps and lenticular (internal) higher order aberrations derived by subtraction of corneal aberrations from total ocular aberrations. Aberrations were measured for a pupil diameter of 5 mm. Multivariate analysis of variance followed by multiple regression analysis found significant difference in the fourth order aberrations (SA RMS, primary spherical aberration coefficient) between the refractive error groups. Hyperopic eyes (+0.083 ± 0.05 μm) had more positive total ocular primary spherical aberration compared to emmetropic (+0.036 ± 0.04 μm) and myopic eyes (low myopia = +0.038 ± 0.05 μm, moderate myopia = +0.026 ± 0.06 μm) (p < 0.05). No difference was observed for the anterior corneal spherical aberration. Significantly less negative lenticular spherical aberration was observed for the hyperopic eyes (−0.038 ± 0.05 μm) than myopic (low myopia = −0.088 ± 0.04 μm, moderate myopia = −0.095 ± 0.05 μm) and emmetropic eyes (−0.081 ± 0.04 μm) (p < 0.05). These findings suggest the existence of differences in the characteristics of the crystalline lens (asphericity, curvature and gradient refractive index) of hyperopic eyes versus other eyes.     

The ultrastructure of parapapillary chorioretinal atrophy in eyes with secondary angle closure glaucoma

Background: The present study was performed to investigate the ultrastructure of deep retinal layers and choroid corresponding to the parapapillary chorioretinal atrophy in eyes with secondary angle-closure glaucoma. Methods: The glaucomatous eyes included two eyes enucleated due to iris ring melanoma with high intraocular pressure and one eye with neovascular glaucoma enucleated due to ocular pain. The control eyes included one eye enucleated due to choroidal malignant melanoma with normal intraocular pressure and one eye enucleated during surgery for supramandibular carcinoma. These eyes were studied with light and electron microscopy. Results: In the region of parapapillary chorioretinal atrophy of glaucomatous eyes, the retinal pigment epithelial cells showed degenerative changes, such as loss of basal in foldings and microvilli, degenerated mitochondria, vacuolar degeneration and irregular distribution of melanin granules. The photoreceptors were decreased in number in this area of glaucomatous eyes. The lumen of the choriocapillary vessels adjacent to the optic nerve was collapsed. Conclusion: These results elucidate the fine structures of deep retina and choroid in the region of parapapillary chorioretinal atrophy of glaucomatous eyes, and suggest that the reduced choroidal perfusion might be the pathogenetic mechanism of glaucomatous parapapillary chorioretinal atrophy.     

Histological study on transient ocular hypertension after laser iridotomy in rabbits

We performed argon-laser iridotomy on pigmented rabbits and examined histologically the tissue of the anterior chamber angle, the iris, and the ciliary body. After laser iridotomy, 9/10 eyes showed an elevation in transient intraocular pressure. Deposits of blood plasma with fibrin were observed in the tissue of the circumferential anterior chamber angle from eyes enucleated at a period of high intraocular pressure. In the eyes in which ocular pressures returned to the baseline, the quantity of blood plasma was small. In the laser-irradiated iris and iris processes, cells constructing the vessel wall were degenerated, and blood plasma and fibrin exuded. In and around the ciliary body, fibrin was not demonstrated. These observations suggest that argon-laser irradiation to the iris leads to a breakdown of the blood-aqueous barrier mainly in the laser-treated iris, and that deposition of blood plasma with fibrin in the anterior chamber angle tissue may cause transient intraocular pressure elevations after laser iridotomy in rabbits.     

Physiology of Normal and Abnormal Colour Vision

The physiology of colour vision is reviewed in the light of recent neurophysiological and anatomical studies in vertebrate retinas. The physiological correlate of trichromasy is the existence of three classes of cone cells, each possessing exclusively one of three photopigments with maximum spectral sensitivity at either 440 nm, 540 nm or 570 nm. The electrical responses of single cells of the various types within the retina and lateral geniculate nucleus exhibit both spatial and chromatic coding which is opponent in character. The horizontal and amacrine cells show sophisticated responses which demonstrate their important roles in the processing of information about colour vision. Human abnormal colour vision is reviewed in the light of recent psychophysical studies. Red-green dichromasy is due to the absence of one of the two photopigments normally active in the red-green range of the spectrum. In the red-green anomalous trichromasies one of the photopigments active in that range is replaced by an abnormal photopigment although the spectral loci of the peak sensitivity of these photopigments and the shape of their absorption curves have not yet been accurately identified. The traditional association of mono-chromasy with a rod-only retina has been challenged by the finding of cone cells in histological preparation of eyes of achromats, and the psychophysical evidence that more than one receptor type participates in visual function. One of these receptor types is the normal rod but the other may be a normal blue-sensitive cone or a cone filled with rhodopsin, the rod photopigment.