T3

Color, high-resolution stationary shapes, movement, and stereoscopic depth are processed in some subcortical visual pathways, and from Vi onward in separate information channels. These individual aspects must be integrated to achieve visual perception. In diurnally active primates like humans, over half of the cortex is involved in processing visual information. On a simplified scale, the parietal cortex analyzes the "where" and involves motor systems, and the temporal cortex takes care of the "what" of visual input comparing it with memory.

Axons of the optic tract (especially those of M and y cells) also project to subcortical regions of the brain such as the pretectal region, which regulates the diameter of the pupils (see below); the superior colliculi (^ B), which are involved in oculomotor function (^ p.360);

the hypothalamus, which is responsible for cir-cadian rhythms (^ p. 334).

The pupillary reflex is induced by sudden exposure of the retina to light (^ p. 350). The signal is relayed to the pretectal region; from here, a parasympathetic signal flows via the Edinger-Westphal nucleus, the ciliary ganglion and the oculomotor nerve, and induces narrowing of the pupils (miosis) within less than 1 s. Since both pupils respond simultaneously even if the light stimulus is unilateral, this is called a consensual light response. Meiosis also occurs when the eyes adjust for near vision (near-vision response ^ p. 360).

The corneal reflex protects the eye. An object touching the cornea (afferent: trigeminal nerve) or approaching the eye (afferent: optic nerve) results in reflex closure of the eyelids.

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