The eye, the retina & the visual pathway
From a pinhole to the human eye, rods versus cones, why you don't see your blind spot or your own blood vessels, and how to predict exactly what someone sees after damage anywhere from retina to cortex.
1. Vision at Marr's three levels
| Level | Vision |
|---|---|
| Computational | Recognize faces (social interaction), recognize objects (tool use) |
| Algorithmic | Perceptual completion, visual adaptation |
| Implementation | Rods, cones; optic nerve, optic chiasm; thalamus (LGN) and visual cortex |
The L7 study guide asks you to "be able to apply Marr's 3 levels" to vision — this table is the lecture's own answer; every row gets its own section below.
2. From pinhole to eye
We see objects because they reflect light. The problem: light reflects in all directions, so every point on a wall receives light from every point of the object. It all mixes → a blur, not an image.
Ibn al-Haytham (965–1040) worked out the fix: let only one ray from each point through. A camera obscura — light passing through a small hole into a dark space — forms a clear, inverted image. Drag the hole size and watch the tradeoff:
- Dim
- Narrow field of view
- Fixed focus — can't adjust for near vs far
- Bigger hole, letting more light in
- Pupil dilates and contracts to adjust light
- Wide field of view with 2 eyes
- Cornea bends a lot of light into the eye
- Accommodation: the lens changes shape to adjust focus
Darwin admitted that the eye — with its "inimitable contrivances for adjusting the focus to different distances, for admitting different amounts of light" — made natural selection seem "absurd in the highest degree" (he went on to explain how it could evolve anyway).
3. The retina: rods and cones
| Rods | Cones | |
|---|---|---|
| Shape / size | Smaller, rod-shaped | Bigger, cone-shaped |
| Types | 1 | 3 (red, green, blue) |
| Light level | Scotopic — low light | Photopic — bright light, colour |
| Where | Periphery | Fovea (central vision) |
| Opsin | Rhodopsin | Photopsins |
Averted vision
If rods are in the periphery and rods handle dim light, then to see a faint star you should look next to it. That's averted vision — used in astronomy and taught in the U.S. Army field manual: look about 10 degrees above, below or to the side of an object at night.
4. The blind spot and perceptual completion
Where the optic nerve leaves the eye there are no photoreceptors: a blind spot in each eye. Find yours:
Why don't you normally notice it? Two reasons:
- Two eyes cover for each other — each eye's blind spot falls on a part of the scene the other eye can see.
- Perceptual completion — the brain "fills in" gaps by making an educated guess. The brain is always making predictions.
5. Visual adaptation: why you don't see your blood vessels
Blood vessels lie in front of your retina. They should cast shadows on your photoreceptors all day. Why don't you see them? Because they're stationary relative to the photoreceptors, and the brain filters out constant, unchanging stimuli.
6. The visual pathway — and what damage does
The trick to every lesion question: the left visual field goes to the right hemisphere. Light from the left side of the world lands on the right half of each retina (the lens flips it). The fibres from the inner (nasal) half of each retina cross at the optic chiasm; the outer (temporal) fibres don't. After the chiasm, each optic tract carries one half of the visual world from both eyes.
| # | Damage | Deficit |
|---|---|---|
| 1 | Loss of rods | Impaired peripheral vision; difficulty seeing in the dark |
| 2 | Loss of cones | Loss of colour vision; impaired central (foveal) vision; difficulty seeing in daylight |
| 3 | Loss of temporal optic nerve fibres (one eye) | Central scotoma: a blind spot in the affected eye (right eye, affecting the left visual field); overall vision may stay fairly intact because the other eye compensates |
| 4 | Loss of one optic nerve | Monocular blindness: all vision in that eye lost; field reduced on that side |
| 5 | Optic chiasm | Bitemporal hemianopia: peripheral (outer) vision lost on both sides |
| 6, 7, 8 | Right optic tract / LGN / radiation → V1 | Contralateral homonymous hemianopia: the left visual field lost in both eyes |
Sites 6–8 come from the lecture diagram, which says only that "6, 7 and 8 all lead to the same deficit"; which number labels which structure is my reconstruction. The lesion-3 wording follows the slide; its exact extent is a reconstruction — check the original figure.