Iris Pigmentation Research Articles

Iris pigmentation and AC thresholds.

Data from 160 White children were used to analyze possible associations between iris pigmentation and AC pure-tone thresholds. Iris pigmentation was graded from iris color using glass models of eyes, and AC thresholds were obtained under carefully controlled conditions. Analyses of variance using two groupings of iris color grades showed no evidence of an association between iris color grade and AC thresholds. Furthermore, inspection of arrays of the actual glass eye models, in conjunction with the order of mean thresholds at each test frequency, did not indicate the presence of an association between iris color grades and thresholds. It was concluded that while iris pigmentation may be related to some aspects of hearing ability, it does not appear to be related to AC thresholds in children.

Race, iris pigmentation, and intraocular pressure.

The association of intraocular pressure with age, sex, race, iris pigmentation, systemic blood pressure, and family income was evaluated using data from the Health and Nutrition Examination Survey of 1971-1972. In general, mean intraocular pressure was highest for blacks with brown irides and progressively lower for whites with brown irides, whites with neither brown nor blue irides, and whites with blue irides. Multilinear regression analysis showed positive associations of intraocular pressure with systolic blood pressure (p less than 0.0001), age (p less than 0.0001) and amount of iris pigmentation (p less than 0.0001). The association with iris pigmentation held for both a combined race/iris color variable and for iris color among white persons. When race rather than iris pigmentation was used in the regression equation, it was a weaker (p less than 0.03) but still significant risk factor for higher levels of intraocular pressure. Intraocular pressure was negatively associated with family income (p less than 0.004). Despite the significant associations, the proportion of variance in intraocular pressure that was explained by these variables was small (R2 = 0.06).

Skin reflectance, iris pigmentation and information processing in children

Forty children 8 years of age were selected for either high (54%) or low (<53%) skin reflectance, a variable previously noted to be influenced by α-Melanocyte stimulating hormone (α-MSH). To control for apparent inherited type of melanin density, iris pigmentation was included as a second independent variable, and the design crossed to obtain eight groups of children, representing all combinations of both high and low skin reflectance, light and dark iris pigmentation and sex. Tests of attention reported as sensitive to the infusion of α-MSH in human adults were administered. Auditory Continuous Performance Tests indicated no differential performances between groups. In contrast, Visual Information Processing Tests elicited superior performances from groups with low skin reflectance and more consistently from sub-groups of these with light iris pigmentation, indicating the concurrence of facilitation of the skills with the pigmentary changes previously noted on administration of α-MSH.

The relations of eye colour and smoking to noise-induced permanent threshold shift.

The hearing of 118 otologically normal soldiers 40 years of age or less with at least 8 years service was tested. Their eye colours were also recorded. Each soldier was questioned concerning his medical history, history of noise exposure and smoking habits. Left ear average hearing levels at 3 kHz were poorer in soldiers with eye colours indicating no melanin pigmentation of the iris than those with iris pigmentation (p < 0.05). Effects of smoking were statistically significant at one frequency but difficult to interpret in terms of current theories.

Iris pigmentation and fractionated reaction and reflex time

Recent investigations indicate that an iris pigmentation-motor behavior relationship exists, but may be limited to the reaction time component of a motor task. The absence of differences in peripheral nervous conduction (Wolf and Landers, 1978) suggests that a central nervous system mechanism may be operating, possibly dependent on the characteristics of neuromelanin or catecholamine turnover. Experiment 1 tested this notion by fractionating simple reaction time into premotor and motor components by electromyography. ANOVA revealed that dark-eyed subjects had faster total reaction times and premotor times, but only the premotor time component approached significance ( p < 0.07). Experiment 2 fractionated patellar reflex time of light-eyed and dark-eyed subjects into reflex latency and motor components. There were no eye color differences for any of the reflex time measures. The results of Experiments 1 and 2 support a central nervous system explanation for the iris pigmentation-reaction time phenomenon.

Eye Colour and Susceptibility to Noise-Induced Permanent Threshold Shift

257 3rd-year apprentices were given ear, nose and throat examination, electroacoustic impedance tests and audiometry. Their eye colour was also recorded. Average hearing levels of otologically normal left ears were poorer at 4 kHz (p less than 0.05) for apprentices with eye colours indicating no melanin pigmentation of the iris than for apprentices with melanin iris pigmentation.

The role of iris pigmentation in the perception of an afterimage

Duration, direction-change frequency, and color-change frequency of an induced afterimage were investigated as a function of iris pigmentation (blue or brown) and the wavelength of a light pulse used to bleach retinal photoreceptors. For blue-irised subjects, a yellow (Wratten 9) pulse produced the longest enduring afterimage while a blue-green (Wratten 44A) pulse produced the shortest enduring afterimage. The reverse was true for brown-irised subjects. Neither of the other dependent measures was affected by iris pigmentation or by pulse color. The afterimage duration results were explained in terms of the possible role of the iris in selectively and differentially filtering various wavelengths of light as a function of its pigmentation.

Effects of iris pigmentation on response of ocular pressure to timolol

Timolol ophthalmic solution effectively reduced mean intraocular pressure (IOP) in Negro and Caucasian patients with chronic open angle glaucoma and untreated IOP of 22 mm Hg or higher. No differences in pressure lowering response were observed among the races although the Negroes required somewhat higher timolol concentrations to produce similar reductions from baseline IOP. Negro patients also appeared to have a more severe disease as indicated by the number of prior antiglaucoma drugs required and the untreated (baseline) pressures. An unexpected finding was the significantly greater number of Caucasians with dark irides who were discontinued from therapy because of inadequate pressure reduction.

Human eye color and reaction time.

Response latencies to a visual stimulus were determined for 44 male and 82 female Caucasians. Those with dark eyes had significantly shorter simple (one stimulus) reaction times. Complex (one of four stimuli) reaction times showed a similar, but nonsignificant, trend. Data from an additional 206 male Caucasians on a one-trial non-practiced reaction task followed the same nonsignificant trend. Darker iris pigmentation apparently correlates with shorter reaction times but more strongly on practiced tasks and on those which do not involve accuracy.

Iris pigmentation and visual-geometric illusions.

Blur or degrading of the retinal image has been shown to be a factor in the formation of visual-geometric illusions where intersecting line elements are present. Light irises allow more scattered light within the eye, which results in more image degradation than found in dark-eyed subjects. Measurements on 755 observers show that illusion magnitude varies as a function of iris pigmentation for a configuration with intersecting line elements (Müller-Lyer illusion), but not for a configuration devoid of such features (Ebbinghaus illusion).

Normal values in clinical electrooculography. IV. Analysis of two dimensionless EOG parameters and their relation to other variables.

Two EOG parameters - The Arden ratio (A) and an expression devised by Gliem (G) - from a sample of normal human subjects are studied. Their relations to the EOG potential and time parameters and to sex, age, pupillary diameter, degree of iris pigmentation, refractive error and axial length, ocular protrusion and interpupillary distance were assessed. Right eye and left eye distributions were congruent, although individual differences were sometimes appreciable, especially in the case of G. The two ratios were positively correlated and both showed a negative correlation to the interval between the dark trough and the light peak. Correlations to the EOG potential parameters, especially the light induced potential rise of the dark adapted eye, were also established. In the case of A, a higher level was disclosed in the male half of the sample. A negative age correlation, predominant in the female part of the sample, characterized both ratios. The two ratios were positively correlated to the pupil diameter, negatively correlated to the degree of refractive errror, and positively correlated to the degree of ocular protrusion. The consequences for the clinical EOG test are discussed, and it is concluded that the present EOG procedure is a qualitative rather than a quantitative test.

IRIS Pigmentation and Reactive Motor Performance

Worthy's (1974) hypothesis that brown-eyed subjects respond faster than blue-eyed subjects on reactive motor tasks was tested. Experiment 1 compared dark-brown and blue-eyed subjects on rotary pursuit and choice response time tasks. There was a tendency for the predicted effect, but it was only evident for choice responding where speed was the principal performance criterion. In Experiment 2 iris pigmentation effects were found to be limited to the reaction-time component, but not the movement time component, of a simple motor response. Differences in the filtering of lightwaves and in CNS neuro-pigmentation are mechanisms commonly used to explain these findings. Iris pigmentation effects were found for both auditory and visual stimuli, thus supporting the neuro-pigmentation explanation.

NORMAL VALUES IN CLINICAL ELECTROOCULOGRAPHY

The following EOG potential and time parameters from 72 normal subjects were analysed: base value B, dark trough D, light peak L, light induced potential rise L-D, interval between beginning of dark adaptation and occurrence of dark through d and interval between dark trough and light peak l. Their relations to sex, age pupillary diameter, degree of iris pigmentation, refractive error, axial length, corneal curvature and diameter, ocular protrusion and interpupillary distance were assessed. Right eye and left eye samples of the EOG parameters were congruent, although individual differences were sometimes appreciable. The levels of B and D were higher in the female half of the sample. A positive correlation existed between age and D level. L-D was negatively correlated to the degree of refractive error and positively correlated to the ocular protrusion. A positive correlation was found between d and the four potential parameters, and there was a positive correlation between age and l. Practical consequences of the statistical analysis relating to the interpretation of such EOG data are discussed.

Iris colour and relationship of tyrosinase activity to adrenergic innervation.

IRIS colour to some degree depends on the sympathetic innervation of the eye. Chance clinical observations of humans and experimental studies in animals have demonstrated consistently a gradual lessening of colour intensity in the iris after interruption of the sympathetic pathways to the eye1,2. Similarly, full iris pigmentation fails to develop when the sympathetic innervation to the eye is absent from birth3. Interest in the relationship of sympathetic innervation to iris colour has increased with reports of direct adrenergic innervation to iris stromal melanocytes4–6. For mammals this is extraordinary in that most melanocytes, for example those of the skin, have no innervation. We have investigated this relationship using tyrosinase activity as an index of melanin synthesis since melanin is the chief pigment of the iris and tyrosinase is the rate-limiting enzyme in the production of melanin7. In turn, iris tyrosinase activity was measured in normal rabbits and in rabbits in which the sympathetic pathways to the eye have been interrupted.

Comments on the relations between auditory fatigue and iris pigmentation.

It has been reported that subjects with highly pigmented irises (brown) experience significantly less temporary threshold shift (TTS) than subjects with less pigmented irises (blue), and that those with green-gray pigmentation display intermediate amounts of TTS. TOTA and BOCCI noted the high correlation between the melanin content in the stria vascularis and that found in the pigmentation of the iris; they attributed their TTS differences across eye colour to the protective effects of melanin. TTS data are reported in this paper as a function of eye colour for exposure stimulus parameters almost identical to those used by TOTA and BOCCI (1 000 HZ at 110 dB SPL for 3 min). The present results do not support the hypothesis that individuals with highly pigmented irises (brown-eyed) are more resistant to auditory fatigue than those with less pigmentation of the iris (blue-eyed). Median TTS at 20 sec post-exposure among brown, green-gray, and blue iris categories did not differ by more than 1.8 dB, and the median TTS at 2 min post-exposure among those iris categories did not differ by more than 0.2 dB.

Malformation Syndromes

Seasonal affective disorder (SAD) is a polyfactorial and polygenetic disorder that involves biological and psychological sub-mechanisms that differentially involve depression, seasonality, circadian rhythms, retinal sensitivity, iris pigmentation, sleep factors, and the neurotransmitters involved with these systems. Within the framework of the polyfactorial conceptualization of SAD, we review the possible contributions of vitamin D3 with respect to the aforementioned sub-mechanisms. We hypothesize that rather than functioning primarily as a proximal or direct sub-mechanism in the etiology of SAD, vitamin D likely functions in a more foundational and regulative role in potentiating the sub-mechanisms associated with the depressive and seasonality factors. There are several reasons for this position: 1. vitamin D levels fluctuate in the body seasonally, with a lag, in direct relation to seasonally-available sunlight; 2. lower vitamin D levels have been observed in depressed patients (as well as in patients with other psychiatric disorders) compared to controls; 3. vitamin D levels in the central nervous system affect the production of both serotonin and dopamine; and 4. vitamin D and vitamin D responsive elements are found throughout the midbrain regions and are especially concentrated in the hypothalamus, a region that encompasses the circadian timing systems and much of its neural circuitry. We also consider the variable of skin pigmentation as this may affect levels of vitamin D in the body. We hypothesize that people with darker skin pigmentation may experience greater risks for lower vitamin D levels that, especially following their migration to regions of higher latitude, could contribute to the emergence of SAD and other psychiatric and physical health problems.

Laser photocoagulation of the iris.

Irides of rabbit, cat, monkey, and human eyes were exposed to ruby laser coagulation in an attempt to produce iridotomies. One monkey iris was exposed to the argon laser. The ability to produce an iridotomy was directly related to the degree of iris pigmentation. There was no observed damage to the corneas, lenses, or retinas of animals exposed. A method for producing iridotomy in the tan-brown iris by ruby photocoagulation had a high success rate.

Letters to the Editor

Dr. Dancis's letter underlines the complexities encountered in studying the various metabolic alterations resulting from a derangement of the internal environment as seen in phenylketonuria. It is a clinical fact that phenylketonuric individuals have diluted hair and iris pigmentation and that this defect is reversible by restriction in phenylalanine intake or supplementation of a normal diet by tyrosine. The inhibition by phenylalanine of tyrosinase derived from embryonic skin (Saunders, et al.), mushrooms (Dancis and Balis), or mammalian melanoma (Miyamoto and Fitzpatrick) suggests but one of several possible sites at which melanin formation may be inhibited in the phenylketonuric patient.

Waardenburg's syndrome with fundus and other anomalies.

Waardenburg's syndrome is a rare disease of autosomal dominant inheritance. It is characterized by lateral displacement of the inner canthi and the inferior lacrimal puncta, prominence of the root of the nose and of the medial portions of the brows, congenital deafness, white forelock, and heterochromia iridum (one blue eye and one brown eye). It has not been emphasized previously that the characteristic variations in iris pigmentation are paralleled by the patterns of pigmentation of the fundus. Study of 14 new cases of Waardenburg's syndrome in patients of different racial origin reveals that the hypochromic iris (usually light blue) is characteristically associated with a homolateral blond or albinoid fundus. When the iris hypochromia happens to be bilateral, fundus hypochromia is characteristically also bilateral. The hyperchromic iris (usually shades of brown) is characteristically associated with a homolateral brunet fundus. Unilateral iris bicolor (patches of brown and light blue) is sometimes associated

Iris pigmentation and mydriatics.

Iris pigmentation and mydriatics.