Accommodation (near) reflex

When attention shifts to a near object, three coordinated responses keep its image sharp and single on both foveae: (1) convergence — both medial recti contract so the visual axes meet on the object; (2) accommodation — the ciliary muscle contracts, the zonules slacken and the lens becomes more convex (mainly its anterior surface), increasing refractive power; (3) miosis — pupil constriction increases depth of field and reduces aberrations.

Unlike the light reflex, the near reflex requires the visual cortex. The stimulus is retinal blur and disparity: retina → optic nerve → LGN → V1 → visual association cortex (areas 18/19) and frontal eye fields → descending corticotectal/corticomesencephalic fibres → pretectal and supraoculomotor area of the midbrain → (a) medial rectus subnuclei of the oculomotor nucleus (convergence) and (b) Edinger–Westphal nucleus → CN III → ciliary ganglion → short ciliary nerves → ciliary muscle and sphincter pupillae. The near-response fibres are far more numerous (≈30:1) than pupillary light fibres and run more ventrally in the midbrain, which helps explain light–near dissociation.

Accommodation is measured in dioptres (D = 1/focal length in metres): focusing at 25 cm from infinity requires 4 D. The near point recedes with age as the lens hardens (presbyopia). The amount of convergence is coupled to accommodation (AC/A ratio); excess accommodative effort in uncorrected hyperopic children can trigger accommodative esotropia.