ICO Visual Sciences flashcards

33 cards. Read the term and the definition here. The deck at the top of this page is optional.

Snell's law

At an interface between two media, n₁ sin θ₁ = n₂ sin θ₂, where n is the refractive index of each medium and θ is the angle from the normal.

Gullstrand reduced eye

A simplified schematic eye with a single refracting surface of +60 D, radius 5.55 mm, refractive index 1.333 and axial length 22.22 mm, used for optical calculations.

Spherical equivalent

The single sphere power that places the circle of least confusion on the retina, calculated as sphere plus one-half of the cylindrical power.

Vertex distance correction

When a spectacle lens is moved closer to the eye, its effective power changes; F_contact = F_spectacle divided by (1 − d × F_spectacle), with d in metres.

Jackson cross-cylinder

A lens with equal and opposite cylinder powers 90 degrees apart; flipping it during refraction changes sphere by half the cylinder and rotates the axis 90 degrees.

Purkinje shift

The shift of peak visual sensitivity from about 507 nm in rod-mediated scotopic vision to about 555 nm in cone-mediated photopic vision as illumination increases.

Weber fraction

The smallest detectable change in a stimulus divided by the background level; for photopic luminance discrimination it is roughly 1 to 2 percent.

Critical flicker fusion

The frequency at which a flickering light appears steady; it rises with luminance (Ferry–Porter law) and is about 50 to 60 Hz in the fovea.

Duane's curve

The age-related decline in amplitude of accommodation, falling from about 14 D in childhood to around 2 D by age 50, with the near point receding.

Bowman's layer

An acellular condensation of the anterior corneal stroma lying just beneath the epithelium; it does not regenerate after injury and scars when damaged.

Lamina cribrosa

A sieve-like collagenous structure in the sclera through which retinal ganglion cell axons pass and become myelinated as they form the optic nerve.

Meyer's loop

The temporal loop of the optic radiation that sweeps through the temporal lobe and carries fibres from the contralateral inferior visual field.

Trabecular meshwork

The spongy drainage pathway at the anterior chamber angle through which aqueous humour passes into Schlemm's canal, providing most of the eye's outflow.

Goldmann equation

Intraocular pressure equals the rate of aqueous formation divided by outflow facility, plus episcleral venous pressure; it links pressure to aqueous dynamics.

Corneal deturgescence

The state of relative dehydration maintained by endothelial fluid pumps and barrier function, keeping stromal collagen spacing regular and the cornea transparent.

Phototransduction cascade

Light converts 11-cis retinal to all-trans, activating rhodopsin, transducin and phosphodiesterase; cGMP falls, cyclic nucleotide-gated channels close, and the photoreceptor hyperpolarises.

Tear film osmolarity

A measure of dissolved particle concentration in tears; values above about 308 mOsm/L support a diagnosis of dry eye disease and reflect hyperosmolar stress.

ON bipolar cell

A retinal bipolar cell that depolarises to light because its metabotropic glutamate receptor mGluR6 inverts the sign of the photoreceptor's light response.

Midget ganglion cells

Small retinal ganglion cells projecting to parvocellular layers of the lateral geniculate nucleus; they carry high-acuity and red-green colour information from the fovea.

Simple cell

A primary visual cortex neuron with elongated, oriented receptive fields and distinct ON and OFF subregions, responding best to bars or edges at a specific orientation.

Hering's law

Equal and simultaneous innervation is sent to yoke muscles of the two eyes during conjugate eye movements, so the two eyes move together as a pair.

Alternate cover test

A test in which the cover is moved rapidly between the two eyes, breaking fusion and revealing the total deviation including latent heterophoria.

AC/A ratio

The change in accommodative convergence, in prism dioptres, produced by each dioptre of accommodation change; normal values are about 4 to 6.

Anomalous retinal correspondence

A sensory adaptation in strabismus in which the fovea of one eye pairs with a non-foveal point of the other eye, reducing diplopia and confusion.

Tropicamide

A short-acting antimuscarinic mydriatic and cycloplegic used for fundoscopy and refraction; its effect lasts about four to six hours and it is weaker than cyclopentolate.

Latanoprost

A prostaglandin F2-alpha analogue that lowers intraocular pressure mainly by increasing uveoscleral outflow; it can cause iris darkening and eyelash growth.

ACAID

Anterior chamber-associated immune deviation: antigen introduced into the anterior chamber induces a deviated, suppressive immune response rather than typical inflammation.

Acanthamoeba keratitis

A contact-lens-associated corneal infection, often from tap water, causing pain out of proportion, ring infiltrate and radial keratoneuritis; diagnosed by culture or PCR.

Two-hit hypothesis

Knudson's model in which a tumour suppressor gene such as RB1 requires both alleles to be inactivated before retinoblastoma develops, explaining hereditary bilateral disease.

Leber hereditary optic neuropathy

A maternally inherited optic neuropathy caused by mitochondrial DNA mutations, most often m.11778G>A, producing painless bilateral central vision loss in young adults.

Relative afferent pupillary defect

A reduced pupillary response to light in one eye compared with the other during the swinging flashlight test, indicating asymmetric optic nerve or extensive retinal disease.

Multifocal ERG

A technique that records focal electroretinogram responses from many retinal regions simultaneously, used to map localised macular and retinal dysfunction.

Shaffer grading

A gonioscopic grading system from 0 to 4 that estimates the angle width; grades 0 and 1 indicate a narrow or closed angle with high risk of closure.