ICO Visual Sciences key terms

A monochromatic optical defect in which rays passing through the periphery of a spherical surface focus at a different point from paraxial rays, blurring the image and reducing retinal image quality, especially when the pupil is large.

The dispersion-related focusing error that occurs because short wavelengths are refracted more strongly than long wavelengths, producing longitudinal and transverse color fringes and reducing polychromatic image quality in the eye.

The measure of how strongly light rays converge or diverge at a given plane, expressed as the reciprocal of the distance in meters; used to trace image formation through refracting surfaces and lenses.

A simplified schematic model representing the eye as a single refracting surface separating air from a medium of uniform refractive index, allowing quick calculation of retinal image size and refractive error.

One of the paired cardinal points of an optical system through which an incident ray passes without changing its direction; in the eye it defines the angle subtended by an object at the retina.

The conjugate point of the retina for an unaccommodated eye: the farthest distance at which a target is seen clearly without accommodation, whose location defines the magnitude and sign of refractive error.

The active increase in dioptric power of the eye produced by contraction of the ciliary muscle, which relaxes zonular tension and allows the crystalline lens to become more spherical and convex.

The maximum increase in dioptric power the eye can achieve through accommodation, measured as the difference between the far point and near point vergences and declining progressively with age.

The three-dimensional intensity distribution formed at the retina by a point source of light after passing through the eye's optics; its width quantifies optical blur and its shape reveals aberrations.

A frequency-domain description of how strongly each spatial frequency's contrast is preserved by an optical system, derived from the point spread function and used to quantify retinal image quality.

A curve plotting the reciprocal of the minimum detectable contrast against spatial frequency, peaking near 3 to 6 cycles per degree and reflecting both optical and neural limits on vision.

The rate at which luminance varies across space, usually expressed in cycles per degree of visual angle; high spatial frequencies carry fine detail and low frequencies carry coarse structure.

A perceived bright or dark band at the border between regions of differing luminance, produced by lateral inhibition in the retina and demonstrating that brightness perception is not a direct copy of the stimulus.

A neural mechanism in which activity in one neuron suppresses the activity of neighboring neurons, sharpening spatial contrast and contributing to phenomena such as Mach bands and simultaneous contrast.

The region of the visual field whose stimulation alters the firing of a given visual neuron; retinal ganglion cells typically show center-surround organization that enhances edge detection and contrast.

The biochemical cascade in photoreceptors in which absorbed photons activate rhodopsin or cone opsin, triggering transducin and phosphodiesterase to lower cyclic GMP and close cation channels, producing hyperpolarization.

The G-protein-coupled visual pigment of rod photoreceptors, composed of opsin bound to the chromophore 11-cis-retinal; absorption of light isomerizes the chromophore and initiates the phototransduction cascade.

The steady inward sodium and calcium current through cyclic GMP-gated channels in darkness that keeps photoreceptors depolarized and tonically releasing glutamate; light suppresses this current and hyperpolarizes the cell.

The principle that vision is served by two receptor systems: scotopic rods, which are highly sensitive and color-blind, and photopic cones, which are less sensitive but support high acuity and color vision.

The change in relative luminous efficiency toward shorter wavelengths as illumination falls from photopic to scotopic levels, reflecting the shift from cone to rod mediation of vision.

The lowest rate at which an intermittently presented light appears steady rather than flickering; it rises with luminance, stimulus size, and retinal eccentricity and reflects temporal resolution.

A single layer of nonregenerating hexagonal cells on the posterior corneal surface that actively transports fluid and ions out of the stroma, maintaining corneal clarity and normal thickness.

The thickest corneal layer, consisting of regularly arranged collagen lamellae, keratocytes, and proteoglycans; its uniform spacing and relative dehydration are essential for corneal transparency.

The active process by which the corneal endothelium removes fluid from the stroma, counteracting the stroma's tendency to swell and preserving the ordered collagen spacing required for transparency.

A spongy tissue at the anterior chamber angle through which aqueous humor passes on its way to Schlemm's canal; increased resistance here raises intraocular pressure in open-angle glaucoma.

A clear fluid secreted by the ciliary epithelium that supplies nutrients to avascular ocular tissues and maintains intraocular pressure through a balance of production and outflow.

The fluid pressure within the eye, determined mainly by the rate of aqueous production relative to outflow resistance and episcleral venous pressure; elevated levels are a major risk factor for glaucoma.

A small central retinal depression containing the highest density of cones and lacking overlying inner retinal layers, specialized for high-acuity and color vision.

A retinal neuron whose axon leaves the eye in the optic nerve; it integrates signals from bipolar and amacrine cells and encodes features such as contrast, color, and motion.

The X-shaped crossing of the optic nerves where fibers from the nasal retina decussate to the opposite side while temporal fibers remain ipsilateral, creating the basis for binocular visual field representation.

The thalamic relay nucleus that receives retinal input via the optic tract and projects to the primary visual cortex; its six layers segregate input by eye and by cell type.

The fiber bundle carrying axons from the lateral geniculate nucleus through the temporal and parietal white matter to the primary visual cortex, with lesions producing characteristic visual field defects.

A rapid, ballistic eye movement that shifts the line of sight between fixation targets; saccades are measured for latency, accuracy, and peak velocity in clinical and research testing.

A slow conjugate eye movement that keeps a moving target on or near the fovea, requiring intact cortical and cerebellar control; its gain and symmetry are clinically informative.

A short-latency reflex in which vestibular input drives compensatory eye movements opposite to head motion, stabilizing the retinal image during head rotation.

An involuntary rhythmic oscillation of the eyes, classified by waveform, direction, and conjugate nature; it may be congenital or acquired and often indicates vestibular, cerebellar, or visual pathway dysfunction.

A condition in which the refractive error differs between the two eyes by a clinically meaningful amount, which can impair binocular fusion and predispose to amblyopia in children.

A reduction in best-corrected visual acuity in one or both eyes caused by abnormal visual experience during development, without detectable structural abnormality of the eye.

The perception of depth derived from horizontal disparity between the images received by the two eyes, representing the highest grade of binocular cooperation.

The concentration of an inhaled anesthetic at which half of subjects do not respond to a standard surgical stimulus; used to compare potency and to titrate anesthetic depth.