---
title: "Vision and the Brain"
book: "High School Biology"
subject: biology
language: en
chapter: 22
exercises: 15
source: https://one-course.com/books/biology/2/en/chapter/22-vision-and-the-brain
---

# Chapter 22 — Vision and the Brain

After a stroke, a patient sees the world perfectly — in black, white and grey. Another sees colours and shapes but no movement: a car is here, then there, with nothing in between, and pouring tea is impossible because the stream never seems to move. A third cannot recognise faces, including her own in a mirror, though she describes every feature. Their eyes are intact. What each has lost is one piece of the brain in which seeing is put together. This chapter follows the optic nerve into the brain, maps what the [visual cortex](#prop-g11-vision-and-brain-pathway) does with the retina’s signals, and shows that the map is not fixed: it is built by experience, altered by learning, and disturbed by drugs.

## 22.1 From the retina to the cortex

**Proposition 22.1 (The visual pathway).**

The million fibres of each optic nerve run back to the *optic chiasm*, where the fibres from the nasal half of each retina cross to the other side while those from the temporal half stay on their own side. Beyond the chiasm, each *optic tract* therefore carries the signals of the left halves of both retinas (right tract) or the right halves (left tract) — that is, everything seen in one half of the visual field. The tracts relay in the *thalamus* and reach the *primary visual cortex*, at the back of each hemisphere: the left hemisphere receives the right half of the visual field, the right hemisphere the left half.

**Evidence.** Cutting one optic nerve blinds one eye; damage to one optic tract, or to one hemisphere’s [visual cortex](#prop-g11-vision-and-brain-pathway), leaves both eyes working but blinds both of them to the same half of the world — the half opposite the lesion. Stimulating a point of the [visual cortex](#prop-g11-vision-and-brain-pathway) of an awake patient during surgery makes them see a flash at a fixed position of the field; neighbouring points give neighbouring flashes: the cortex holds a map of the retina, the centre of the field occupying the largest part. ∎

![The visual pathway seen from above. Light from the left half of the field (red) falls on the right half of each retina; those fibres, crossing at the chiasm where necessary, gather into the right optic tract and reach the right visual cortex. Each hemisphere sees the opposite half of the world with both eyes.](https://one-course.com/images/onecourse/chapters/biology-2/g11-vision-and-brain/fig-cdbb15711464.svg)

*The visual pathway seen from above. Light from the left half of the field (red) falls on the right half of each retina; those fibres, crossing at the chiasm where necessary, gather into the right optic tract and reach the right [visual cortex](#prop-g11-vision-and-brain-pathway). Each hemisphere sees the opposite half of the world with both eyes.*

![The brain from the left side. The primary visual cortex lies at the very back, in the occipital lobe; the further visual areas extend forward from it into the temporal lobe (what things are) and the parietal lobe (where they are and how they move).](https://one-course.com/images/onecourse/chapters/biology-2/g11-vision-and-brain/fig-073ae9d2f5c8.svg)

*The brain from the left side. The [primary visual cortex](#prop-g11-vision-and-brain-pathway) lies at the very back, in the occipital lobe; the further visual areas extend forward from it into the temporal lobe (what things are) and the parietal lobe (where they are and how they move).*

## 22.2 Many areas, one perception

**Proposition 22.2 (Specialised visual areas).**

The [primary visual cortex](#prop-g11-vision-and-brain-pathway) passes its analysis to a dozen further *visual areas*, each specialised: one extracts colour, one movement, others shape, depth, faces, written words. Their outputs are combined — with memory, with the other senses — into the single, coherent perception we experience. The perception is a construction: the brain fills the blind spot, completes hidden contours, holds colours constant under changing light, and can be fooled by illusions that exploit its rules.

**Evidence.** Localised lesions dissociate the components: a patient with damage to the colour area sees a world in grey with normal acuity; damage to the movement area leaves colour and shape intact but abolishes the perception of motion; damage to a region of the temporal lobe abolishes the recognition of faces alone. Functional imaging of healthy volunteers shows the same areas lighting up, one for coloured patterns, another for moving dots, another for faces, while the primary cortex responds to all. Electrodes in animals find, in each area, neurons responding to that area’s feature and to little else. ∎

![From signals to a scene. The primary cortex distributes its analysis to specialised areas whose results are recombined. A lesion in one area removes one attribute — colour, motion, faces — and leaves the rest.](https://one-course.com/images/onecourse/chapters/biology-2/g11-vision-and-brain/fig-0a9a907ca40f.svg)

*From signals to a scene. The primary cortex distributes its analysis to specialised areas whose results are recombined. A lesion in one area removes one attribute — colour, motion, faces — and leaves the rest.*

**Example 22.3 (Three patients, three areas).**

The patient who sees in grey has a lesion of the colour area on both sides; her retina’s [cones](https://one-course.com/books/biology/2/en/chapter/21-the-eye-and-its-photoreceptors#def-g11-the-eye-photoreceptors) work, and her primary cortex receives their signals, but the stage that turns cone ratios into colour is gone. The patient who sees no movement has lost the movement area: she sees successive snapshots. The patient who cannot recognise faces has a lesion of the face area of the temporal lobe; she recognises people by voice or by a hat. In each case what is lost is precise, and what remains shows that the other areas work on their own.

## 22.3 A cortex built by experience

**Proposition 22.4 (Plasticity).**

The connections of the [visual cortex](#prop-g11-vision-and-brain-pathway) are not fixed at birth. During a *critical period* of early life they are shaped by what the eyes see; in the adult they continue to change, more slowly, with learning and with use. This *cerebral plasticity* is a property of neurons: connections that are used are strengthened and multiplied, connections that are not are weakened and pruned.

**Evidence.** Hubel and Wiesel (1960s) recorded from the [visual cortex](#prop-g11-vision-and-brain-pathway) of cats and monkeys. In a normal animal most neurons respond to both eyes. In an animal whose one eye was kept closed during its first months, almost every neuron responds only to the open eye: the closed eye’s connections have been taken over. Closing the eye of an adult for the same time changes nothing. In humans, a child whose one eye is strongly out of focus or misaligned and not corrected before the age of about seven keeps a permanently weak eye — amblyopia — however good its optics later. In adults, learning to read Braille enlarges the cortical area devoted to the reading finger; learning to juggle enlarges the areas processing motion, and they shrink again if practice stops. ∎

![Hubel and Wiesel’s finding, rounded. In a normal kitten most visual cortex neurons answer to both eyes. After three months with the left eye closed, nine neurons in ten answer only to the right eye: the unused eye’s connections have been lost.](https://one-course.com/images/onecourse/chapters/biology-2/g11-vision-and-brain/fig-0140d2796849.svg)

*Hubel and Wiesel’s finding, rounded. In a normal kitten most [visual cortex](#prop-g11-vision-and-brain-pathway) neurons answer to both eyes. After three months with the left eye closed, nine neurons in ten answer only to the right eye: the unused eye’s connections have been lost.*

**Example 22.5 (Why the squint must be treated early).**

A child of three with one eye turned inward sees double, and the brain suppresses the weaker image. If the eye is not straightened, or the good eye not patched a few hours a day to force the weak one to work, the cortex reorganises around the good eye during the critical period and the weak eye, though optically perfect, stays blind to fine detail for life. Treated at three, the child recovers full vision; treated at twelve, hardly at all.

## 22.4 Drugs and the seeing brain

**Proposition 22.6 (Neurons, messengers, and molecules that mimic them).**

Neurons communicate at *synapses* by releasing chemical messengers, *neurotransmitters*, onto the receptors of the next [cell](https://one-course.com/books/biology/2/en/chapter/2-cells-the-common-unit-of-life#def-g10-cells-common-unit-cell); the final year describes the mechanism. Several neurotransmitters, among them *serotonin*, regulate the flow of signals through the visual areas. Molecules whose shape resembles a neurotransmitter can bind its receptors and disturb that flow: lysergic acid diethylamide (LSD) binds serotonin receptors in the [visual cortex](#prop-g11-vision-and-brain-pathway) and produces hallucinations — colours, shapes and movements seen with no corresponding light. Such drugs can leave lasting changes: episodes recurring months later, and in some people persistent visual disturbances.

**Proof.** *Admitted at this level.* ∎

**Example 22.7 (What a hallucination shows).**

Under LSD the retina is unchanged and the primary cortex still maps what the eyes see; the disturbance is in the later areas and in how they are combined. Patterns appear that look like the cortex’s own building blocks — grids, spirals, tunnels — because the drug sets the machinery of perception running without its input. The hallucination is a demonstration, by breakdown, that seeing is made by the brain and that a molecule can change it.

**Method 22.8 (Locating a visual deficit).**

1. One eye blind, the other normal: the eye or its optic nerve, before the chiasm.
2. Both eyes blind to the same half of the field: the opposite optic tract, thalamus or primary cortex.
3. Both eyes blind to the outer halves of the field: the chiasm, where the crossing fibres are cut.
4. Vision intact but one attribute lost (colour, motion, faces): the corresponding specialised area.
5. Perception disturbed with normal eyes and cortex, transient: a drug or a metabolic cause.

**Remark 22.9 (What this year has shown).**

The eye of [Chapter 21](https://one-course.com/books/biology/2/en/chapter/21-the-eye-and-its-photoreceptors#ch-g11-the-eye) was a matter of [genes](https://one-course.com/books/biology/2/en/chapter/3-dna-a-universal-genetic-molecule#def-g10-universal-dna-gene) and [proteins](https://one-course.com/books/biology/2/en/chapter/14-from-gene-to-protein#def-g11-gene-expression-protein): [opsins](https://one-course.com/books/biology/2/en/chapter/21-the-eye-and-its-photoreceptors#def-g11-the-eye-photoreceptors), their duplications, a colour-blindness inherited on the X. The brain of this chapter is a matter of connections, shaped by use. Between them lies the whole subject of the year — from the sequence of a [gene](https://one-course.com/books/biology/2/en/chapter/3-dna-a-universal-genetic-molecule#def-g10-universal-dna-gene) to a trait, and from the trait to what a person can do — and the lesson that at every step the [genotype](https://one-course.com/books/biology/2/en/chapter/15-enzymes-and-the-phenotype#def-g11-enzymes-and-phenotype-phenotype) proposes and the environment, from the temperature of a cat’s ears to the light a kitten’s eye receives, disposes.

## 22.5 Exercises

**Exercise 22.1 ★.**

Trace the path of a signal from the left half of the visual field to the cortex.

**Solution of Exercise 22.1.**

Light from the left field falls on the right half of each retina; the right-half fibres of the left eye cross at the chiasm, those of the right eye stay on the right; together they form the right optic tract, relay in the thalamus and reach the right [primary visual cortex](#prop-g11-vision-and-brain-pathway).

**Exercise 22.2 ★.**

What happens at the optic chiasm, and which fibres cross?

**Solution of Exercise 22.2.**

The two optic nerves meet; the fibres from the nasal half of each retina cross to the other side, those from the temporal half do not. Beyond it each tract carries one half of the visual field.

**Exercise 22.3 ★.**

Name three specialised visual areas and the deficit produced by a lesion of each.

**Solution of Exercise 22.3.**

Colour area (world seen in grey), movement area (no perception of motion), face area (faces not recognised).

**Exercise 22.4 ★.**

Define [cerebral plasticity](#prop-g11-vision-and-brain-plasticity) and the critical period.

**Solution of Exercise 22.4.**

Plasticity: the ability of neuronal connections to be strengthened, weakened, formed or pruned according to use. Critical period: the early phase of life during which experience shapes the cortex decisively and irreversibly.

**Exercise 22.5 ★.**

How does LSD act on the visual system?

**Solution of Exercise 22.5.**

It binds the serotonin receptors of neurons in the visual areas, disturbing the flow of signals and producing hallucinations without any corresponding light.

**Exercise 22.6 ★★.**

Using [Method 22.8](#met-g11-vision-and-brain-locate), locate the lesion of a patient blind in the right half of the field with both eyes.

**Solution of Exercise 22.6.**

After the chiasm, on the left: the left optic tract, the left thalamic relay or the left [primary visual cortex](#prop-g11-vision-and-brain-pathway).

**Exercise 22.7 ★★.**

A [tumour](https://one-course.com/books/biology/2/en/chapter/17-genome-damage-and-cancer#def-g11-cancer-cancer) presses on the optic chiasm from below, cutting the crossing fibres. Which parts of the field does the patient lose? Explain with the figure.

**Solution of Exercise 22.7.**

The crossing fibres come from the nasal halves of both retinas, which see the outer (temporal) halves of the field: the patient loses the left half of the left eye’s field and the right half of the right eye’s — tunnel vision on both sides.

**Exercise 22.8 ★★.**

From the Hubel–Wiesel figure, what fraction of neurons respond to the left eye at all in the normal kitten and in the deprived one?

**Solution of Exercise 22.8.**

Normal: $12 + 20 + 36 + 20 = 88\%$. Deprived: $2 + 3 + 5 + 30 = 40\%$, of which only 10% mainly.

**Exercise 22.9 ★★.**

Why does patching the good eye of a squinting child for a few hours a day treat amblyopia? Why does the same treatment fail in an adult?

**Solution of Exercise 22.9.**

Patching forces the cortex to use the weak eye’s input, so its connections are kept and strengthened during the critical period instead of being pruned. In an adult the period is over: connections no longer reorganise on that scale, so the weak eye cannot regain its territory.

**Exercise 22.10 ★★.**

Explain why the patient who sees in grey is not colour-blind in the sense of [Chapter 21](https://one-course.com/books/biology/2/en/chapter/21-the-eye-and-its-photoreceptors#ch-g11-the-eye), and how a test could distinguish the two conditions.

**Solution of Exercise 22.10.**

Her [cones](https://one-course.com/books/biology/2/en/chapter/21-the-eye-and-its-photoreceptors#def-g11-the-eye-photoreceptors) and their [opsins](https://one-course.com/books/biology/2/en/chapter/21-the-eye-and-its-photoreceptors#def-g11-the-eye-photoreceptors) are normal; what is lost is the cortical stage that interprets their ratios. An electrical recording of the retina’s response to coloured lights would be normal in her and abnormal in a colour-blind person; and her deficit is total and acquired, theirs partial and inborn.

**Exercise 22.11 ★★.**

A pianist’s cortical area for the fingers is larger than average. Explain with plasticity, and say what you predict if she stops playing for years.

**Solution of Exercise 22.11.**

Years of practice have strengthened and multiplied the connections serving her fingers, enlarging their cortical territory. Without practice the area would shrink back over years, as the juggling studies show.

**Exercise 22.12 ★★★.**

A person born blind from cataracts has them removed at 30. Predict what she sees in the first weeks and what she will and will not recover, using the critical period.

**Solution of Exercise 22.12.**

At first she sees light, colours and moving blobs but cannot recognise shapes, faces or depth: the retina works, but the cortex never built, during the critical period, the connections that interpret its signals. She will learn to use vision partly, slowly, but never with the fluency of someone who saw from birth.

**Exercise 22.13 ★★★.**

Explain why the cortical map devotes more area to the centre of the field than to the periphery, using [Chapter 21](https://one-course.com/books/biology/2/en/chapter/21-the-eye-and-its-photoreceptors#ch-g11-the-eye)’s distribution of receptors and ganglion [cells](https://one-course.com/books/biology/2/en/chapter/2-cells-the-common-unit-of-life#def-g10-cells-common-unit-cell).

**Solution of Exercise 22.13.**

At the fovea each cone has its own ganglion [cell](https://one-course.com/books/biology/2/en/chapter/2-cells-the-common-unit-of-life#def-g10-cells-common-unit-cell), so the centre of the field sends far more fibres per degree than the periphery, where a hundred [rods](https://one-course.com/books/biology/2/en/chapter/21-the-eye-and-its-photoreceptors#def-g11-the-eye-photoreceptors) share one; the cortex allots area in proportion to fibres, hence to detail, not to degrees of field.

**Exercise 22.14 ★★★.**

Functional imaging shows the colour area active when a subject imagines a red apple with closed eyes. What does this say about the relation between the specialised areas and perception, and about hallucinations?

**Solution of Exercise 22.14.**

The area’s activity is the perception of colour, whether the signal comes from the eye or from memory; perception is what the areas do, not what the eye delivers. A hallucination is that activity started by a drug instead of by intention or light.

**Exercise 22.15 ★★★.**

"Seeing is done by the eye." Rewrite this statement correctly in a paragraph, using the three patients, the kittens and the drug.

**Solution of Exercise 22.15.**

The eye converts light into signals; seeing is done by the brain. A patient with intact eyes can lose colour, motion or faces from a cortical lesion; a kitten with intact eyes goes functionally blind in one of them if the cortex is not allowed to connect it; and a drug that touches neither eye nor cortex’s structure makes a person see what is not there. The eye supplies; the brain sees.

## 22.6 Problem: Four Patients and a Kitten

**Problem 22.1.**

Weekend problem — visual deficits located along the pathway, a cortex mapped by stimulation, a kitten’s deprivation counted, and a child’s squint treated in time

Four patients are examined. A: blind in the left eye, right eye normal. B: both eyes blind to the left half of the field. C: both eyes blind to the outer half of their field (left eye to the left, right eye to the right). D: normal fields, normal acuity, but cannot perceive movement.

**Part I — Locating.**

1. Locate A’s lesion. Which structure, and before or after the chiasm?
2. Locate B’s lesion, and say why it must lie after the chiasm.
3. Locate C’s lesion precisely, explaining which fibres are cut.
4. Locate D’s lesion. What does the preservation of fields and acuity tell you about the primary cortex?
5. For each patient, say whether the retina is intact, and how you would check.

**Part II — The map.** During surgery, stimulating points of a patient’s right [visual cortex](#prop-g11-vision-and-brain-pathway) produces flashes. Stimulating the tip of the occipital lobe gives a flash at the centre of the field; points further forward give flashes further out in the left field; $1\,\mathrm{cm}$ of cortex at the tip covers about $2^\circ$ of the field, while $1\,\mathrm{cm}$ further forward covers $20^\circ$.

6. Why are all the flashes in the left field?
7. How much cortex represents the central $2^\circ$ compared with a $2^\circ$ band at $40^\circ$ from centre? Compute the ratio.
8. Relate this ratio to the retina’s fovea and periphery.
9. A small lesion at the tip of the right occipital lobe: what does the patient lose? A lesion of the same size further forward?
10. Why can the patient with the small central lesion still read, slowly, by moving the eyes?

**Part III — The kitten.** From the Hubel–Wiesel figure.

11. In the normal kitten, what percentage of neurons respond to both eyes to some degree?
12. In the deprived kitten, what percentage respond to the closed (left) eye at all?
13. The closed eye’s retina and optic nerve are intact. Where has its input gone, and what has taken the space?
14. The same closure in an adult cat changes nothing. What does this establish, and what is its human counterpart?
15. A kitten has both eyes closed for three months. Predict the histogram and the animal’s vision afterwards.

**Part IV — The child.** A child of four has a left eye turned inward; its optics are normal. Untreated, the cortex takes over the right eye’s input.

16. Explain, with the kitten, what will happen to the left eye’s vision if nothing is done, and why glasses alone will not prevent it.
17. The treatment is to patch the *right* eye several hours a day for months. Explain why the good eye is covered.
18. Why must this be done before about seven, and why does the same treatment at fifteen fail?
19. An adult who lost an eye at thirty does not lose the cortex devoted to it in the same way. Reconcile this with the kitten.
20. State the result: the four lesions located from front to back of the pathway, and the one property of the cortex — named — that makes the child’s treatment work only in time.

**Solution of Problem 22.1.**

**1.** The left eye or its optic nerve, before the chiasm: only one eye’s fibres are affected.

**2.** The right optic tract, thalamus or [visual cortex](#prop-g11-vision-and-brain-pathway): a deficit shared by both eyes for the same half-field can only lie where the two eyes’ fibres for that half have been gathered together, i.e. after the chiasm.

**3.** The chiasm itself: the crossing fibres, from the nasal halves of both retinas, which see the temporal halves of the field, are cut.

**4.** The movement area, beyond the primary cortex. Intact fields and acuity show the primary cortex and everything before it are working.

**5.** B, C and D have intact retinas; A’s may or may not be. Check by examining the retina with an ophthalmoscope and recording its electrical response to flashes.

**6.** The right cortex receives the left half of the field.

**7.** Central $2^\circ$: $1\,\mathrm{cm}$; a $2^\circ$ band at $40^\circ$: $0.1\,\mathrm{cm}$. Ratio 10.

**8.** The fovea packs its [cones](https://one-course.com/books/biology/2/en/chapter/21-the-eye-and-its-photoreceptors#def-g11-the-eye-photoreceptors) densely, each with its own ganglion [cell](https://one-course.com/books/biology/2/en/chapter/2-cells-the-common-unit-of-life#def-g10-cells-common-unit-cell), so the central degrees send many more fibres than the periphery; the cortex gives them proportionally more area.

**9.** A small blind spot at the centre of the left field, which is crippling for reading; further forward, a larger blind region in the periphery of the left field, less noticed.

**10.** By moving the eyes the patient brings each word onto a part of the retina whose cortex is intact, using the border of the lesion; slow, but possible.

**11.** $20 + 36 + 20 = 76\%$.

**12.** $2 + 3 + 5 + 30 = 40\%$, and only 10% with the left eye as their main or equal input.

**13.** Its input reaches the cortex but its connections were weakened and pruned for lack of use; the right eye’s connections expanded into the space.

**14.** That the reorganisation is confined to a critical period of early life; the human counterpart is amblyopia, which develops only in childhood and is untreatable after it.

**15.** With neither eye favoured, no takeover: the histogram stays roughly symmetric but with fewer responsive neurons in all, and the kitten’s vision is poor in both eyes — deprivation without competition weakens both.

**16.** The brain suppresses the turned eye’s image to avoid double vision; unused, its connections are pruned during the critical period and the eye becomes permanently weak. Glasses correct optics, but the eye is not used, so the pruning proceeds.

**17.** Covering the good eye forces the cortex to use the weak eye’s signals, keeping and strengthening its connections.

**18.** The critical period ends around seven; after it the cortex no longer reallocates territory on that scale, so the weak eye’s lost connections cannot be rebuilt.

**19.** In the adult the connections are already established and stable; the takeover by the other eye is a phenomenon of the developing cortex, during the critical period, not of the mature one.

**20.** A: eye or optic nerve; C: chiasm; B: right tract, thalamus or primary cortex; D: the movement area beyond it. The cortex’s plasticity, confined to a critical period, makes the child’s patching work at four and fail at fifteen.