Vision

Eye wall, outer→inner: sclera (protective, white) → choroid (vascular, nourishes retina, absorbs excess light) → retina (photoreceptors, where vision begins).
Fovea = small cone-packed pit within the macula, sharpest central vision. Optic disc = no photoreceptors → the blind spot.
Front-of-eye focusing: cornea (does most focusing) → aqueous humor (anterior/posterior chambers, pressure) → iris/pupil (light amount) → lens (accommodation via ciliary muscle + suspensory ligaments) → vitreous humor (fills interior, maintains shape, stays in place).
Duplicity theory: cones (~6M, color + acuity, need bright light, 3 photopigments) vs. rods (~120M, night vision, single pigment rhodopsin, no color).
Visual pathway: retina → optic nerve → optic chiasm (nasal retina fibers cross, temporal don't) → optic tract → LGN (thalamus) → optic radiations → primary visual cortex (occipital lobe).
Parvocellular pathway = color + fine detail (cone input). Magnocellular pathway = motion + change detection. Both run in parallel.
Binocular disparity (2 eyes, different angles) drives depth perception; feature detectors (Hubel & Wiesel: simple/complex cells) detect edges, orientation, movement.

Sclera: Outermost eye layer — tough, white, protective connective tissue.
Choroid: Vascular middle eye layer; nourishes retina, absorbs excess light.
Accommodation: The lens changing shape (via ciliary muscle + suspensory ligaments) to focus near vs. far.
Rhodopsin: The single visual pigment in all rods — enables high sensitivity but no color discrimination.
Optic chiasm: Where nasal retina fibers cross to the opposite hemisphere; temporal fibers stay on the same side.
Lateral geniculate nucleus (LGN): Thalamic relay for vision — organizes and preserves spatial arrangement of visual input.

Rods (~120 million)	Cones (~6 million)
High sensitivity, enable night vision	Need bright light to work well
Single pigment (rhodopsin) — no color discrimination	3 photopigments — color vision
Low visual acuity	High visual acuity (fine detail)

Parvocellular	Magnocellular
Fine detail and color	Motion and change detection
Mostly cone input, active in bright light	Tracks movement, rapid environmental changes

1Retinal ganglion cell axons converge to form the optic nerve (one per eye).
2At the optic chiasm, nasal retina fibers cross to the opposite hemisphere; temporal retina fibers stay ipsilateral — reorganizing input by visual field.
3Past the chiasm, fibers form the optic tract (each tract now carries one visual field from both eyes).
4Optic tract → lateral geniculate nucleus (LGN) of the thalamus for initial processing.
5LGN → optic radiations → primary visual cortex (occipital lobe), where color/shape/motion/depth analysis begins.

It's the NASAL retina fibers that cross at the optic chiasm, not the temporal ones — this determines that each hemisphere gets one full visual field, not one full eye.
Optic nerve (before chiasm, one eye's info) vs. optic tract (after chiasm, one visual field from both eyes) — same pathway, different names and different information organization.
Rods vastly outnumber cones, but cones (not rods) provide sharp, colorful vision — more numerous ≠ more important for detail.
Fovea (sharpest vision, high cone density) ≠ optic disc (blind spot, zero photoreceptors) — don't confuse the two.

Which structure produces aqueous humor, and where does it drain?

Why does damage at the optic chiasm cause a different visual field deficit than damage to one optic nerve?

Which pathway (parvocellular or magnocellular) specializes in detecting motion?

What cue does binocular disparity provide?

Biology chapters 1–3 are free. Sign up to unlock every chapter.