146. Visual Acuity, Visual Field, Light and Dark Adaptations, and Visual Reflexes – Quiz

📌 Professor’s Notes: Quick Revision

High-yield revision notes for rapid review before university examinations.

• Visual acuity is the ability to distinguish two closely placed points as separate. It mainly depends on cone function, especially the high cone density in the fovea. The smallest distance at which two points can be distinguished is called the minimum separable distance.
• Visual acuity is influenced by optical factors such as corneal and lens curvature, lens elasticity, ciliary muscle and suspensory ligaments. Retinal factors include cone density and neuronal convergence. Stimulus factors include size, distance, brightness, colour, contrast, edge sharpness and duration of viewing.
• Snellen chart is used for distance vision. The subject stands 6 metres from the chart and each eye is tested separately. Visual acuity is expressed as d/D; 6/6 is normal, while 6/5 is better than normal and 6/18 indicates reduced acuity. The smallest details of the Snellen letters subtend about 1 minute of arc.
• Near vision is tested using the Jaeger chart, which contains progressively smaller printed text and is read at the usual reading distance.
• The visual field is the entire area seen by one eye while the gaze remains fixed. The overlapping central fields of the two eyes form the binocular field, which is important for depth perception and distance judgment.
• Corresponding retinal points are areas of the two retinas that receive images from the same object. The two foveae are the principal corresponding points. Images falling on non-corresponding points produce diplopia. Persistent ocular misalignment in childhood may cause amblyopia.
• Visual field testing by perimetry helps localize lesions. The normal blind spot corresponds to the optic disc and lies in the temporal visual field.
• Important field defects are: optic nerve lesion → blindness of the affected eye; central optic chiasm lesion → bitemporal hemianopia; optic tract lesion → contralateral homonymous hemianopia; temporal optic radiation lesion → contralateral superior quadrantanopia; parietal optic radiation lesion → contralateral inferior quadrantanopia. Occipital lesions commonly show macular sparing.
• Light adaptation occurs when moving from darkness to bright light and is usually completed within about 5 minutes. Pupillary constriction is the earliest response. Neural adaptation rapidly decreases photoreceptor sensitivity, while rhodopsin bleaching contributes to chemical adaptation.
• Dark adaptation occurs after moving from bright light to darkness. The pupil dilates, and sensitivity progressively increases. Cone adaptation predominates during the first 5–10 minutes, followed by rod adaptation. Rod sensitivity becomes maximal at about 20 minutes and may increase retinal sensitivity by 1,000–10,000 times.
• Rhodopsin regeneration is essential for dark adaptation. Vitamin A is required for retinal formation; deficiency impairs rhodopsin regeneration and causes night blindness. Red light causes relatively little rod bleaching and therefore helps preserve dark adaptation.
• The pupillary light reflex causes constriction of both pupils when light enters one eye. The constriction of the illuminated eye is the direct reflex, while that of the opposite eye is the consensual reflex. The afferent pathway is retina → optic nerve → optic tract → pretectal nucleus; the efferent pathway is Edinger–Westphal nucleus → oculomotor nerve → ciliary ganglion → sphincter pupillae.
• The accommodation reflex for near vision consists of pupillary constriction, ciliary muscle contraction and convergence of both eyes by medial rectus muscles. The pathway involves the visual cortex, Edinger–Westphal nucleus and oculomotor nucleus.
• Argyll Robertson pupil results from a pretectal lesion. Light reflexes are absent, but accommodation is preserved. It was classically associated with neurosyphilis.