Human Vision and Light

UV Radiation and Vision

• UV radiation damages the cornea and anterior structures
• Polycarbonate absorbs most UV radiation
• Visible light has a wavelength of 400-700 nm

Photoreceptors in the Retina

• There are two types of photoreceptors: cones and rods
• Cones are responsible for color vision and are concentrated in the fovea centralis
• Rods are responsible for peripheral and night vision

Rods

• Absence of color discrimination
• Poor acuity (~20/200)
• Best discrimination about 30 minutes after darkness
• Predominate in peripheral vision
• Peripheral vision is more sensitive in dim conditions
• Motion is better detected in peripheral vision
• Rods shed and are recycled by the RPE (retinal pigment epithelium)

Retinitis Pigmentosa (RP)

• A genetic disorder characterized by the death of photoreceptors
• Bony spicules may be present in the retina
• All disc waste remains in the eye and damages the retina
• Some RP patients may have healthy-looking retinas but have trouble seeing; ERG (electroretinogram) is needed to diagnose
• Not all RP cases are genetic

Rhodopsin

• A photopigment found in rod discs
• Each eye has 120 million rods, each with 1,000 discs, and each disc has 10,000 molecules of rhodopsin
• The large number of rhodopsin molecules allows the eye to capture light and increases sensitivity at nighttime
• Weber's fraction (ΔI/I) = 0.14, representing the minimum change in stimuli needed to alter vision

Cones

• About 6-7 million cones per eye
• Remain constant with age
• Color sensitive
• Good acuity (20/20)
• Adapt rapidly
• Weber's fraction (ΔI/I) = 0.015, representing better contrast sensitivity than rods

Cone Photopigments

• 3 photopigments in the eye: erythrolabe (red), chlorolabe (green), and cyanolabe (blue)
• Erythrolabe: peaks around 557 nm, most present (64%), and concentrated in the fovea centralis
• Chlorolabe: peaks around 530 nm, about 32% of all cones, and concentrated in the fovea centralis
• Cyanolabe: peaks around 426 nm, about 5-10% of all cones, and concentrated outside the fovea centralis

Dark Adaptation

• Sensitivity increases by 5 log units in about 35 minutes (100,000 more sensitive)
• First arm of the curve (cone section) plateaus after 10 minutes
• First arm of the curve breaks after 12 minutes (rod-cone break)
• The break represents the time when rods become more sensitive than cones
• Second arm of the curve plateaus after 35 minutes
• Represents the scotopic threshold for the specific wavelength

Other Retinal Disorders

• Congenital Stationary Night Blindness (CSNB): a genetic disorder affecting rod function, primarily X-linked recessive
• Traumatic Retinopathy: mimics RP, caused by traumatic injuries
• Syphilis: can cause retinal damage
• Rubella: can cause abortions, eruptions in skin, and can be mistaken with RP
• Thioridazine Retinopathy: an anti-psychotic medication causing crystalline deposits in the retina, confused with RP

Argus II: Bionic Eye

• A device that gives black and white detection, outlines, borderlines, and patterns to RP patients
• Artificial retina; helps RP patients

Medications Affecting the Eye

• Acne medication (Isotretinoin): can cause dry eyes and sudden decrease in night vision
• Antihistamines: can cause angle-closure glaucoma in some people
• Antimalarial drugs (Chloroquine, Hydroxychloroquine): can cause retinal toxicity, leading to vision loss
• Corticosteroids (Prednisone): increase the risk of cataract and glaucoma (steroid responders)
• Erectile dysfunction drugs (Sildenafil, Tadalafil, Vardenafil): can cause blurred vision, sensitivity to light, and seeing a blue tinge
• Ethambutol (Myambutol): an antibiotic used to treat TB, associated with optic neuropathy problems
• Tamoxifen (Nolvadex): an antiestrogen drug used to treat breast cancer, can lead to crystalline retinopathy
• Topiramate (Topamax): an anticonvulsant used to treat seizure disorders, associated with angle-closure glaucoma
• Vitamin A deficiency: required for synthesis of rhodopsin, can occur due to dietary restrictions or diseases