Study topic · about 8 minutes

Language and the brain

In most people, language depends on a network in the left hemisphere. Broca's area, in the frontal lobe, helps you speak and build sentences, and Wernicke's area, in the temporal lobe, helps you understand words. A curved bundle of fibers, the arcuate fasciculus, links the two. Damage to this network, most often from a stroke, causes aphasia, a loss of language.

See Broca's area in 3D

Which side of the brain controls language?

Language is lateralized: one hemisphere does most of the work, and in most people it is the left. This is true for about 95 percent of right-handed people and roughly 70 percent of left-handed people. In the rest, language sits mainly on the right or is shared by both sides.

Before epilepsy surgery, doctors can check which side handles language with the Wada test. A drug injected into one carotid artery puts that hemisphere to sleep for a few minutes. Meanwhile, the doctors check whether the person can still speak. Today, functional MRI is often used instead. For more on how the two halves differ, see left brain and right brain.

Broca's area, Wernicke's area and the arcuate fasciculus

In 1861, the French doctor Paul Broca examined a man who could say little but the syllable 'tan'. Yet the man understood much of what was said to him. After the man died, Broca found damage in the left frontal lobe. Broca's area lies in the lower frontal lobe, just in front of the face area of the motor strip. It has two parts: the pars triangularis in front (Brodmann area 45) and the pars opercularis behind it (area 44). It helps plan the sounds and mouth movements of speech, find words and put them in grammatical order.

In 1874, the German doctor Carl Wernicke described patients with the opposite problem. They spoke fluently, but their words made little sense, and they could not understand what others said. Their damage was farther back, in the left temporal lobe. Wernicke's area is usually placed at the back of the left superior temporal gyrus (Brodmann area 22), beside the primary auditory cortex. It links the sounds of words to their meanings, though its exact borders are still debated.

A curved bundle of white matter, the arcuate fasciculus, connects the two areas. It arches around the back end of the lateral sulcus, the deep groove on the side of the brain. Wernicke predicted that damage to the link itself would cause a separate kind of aphasia, now called conduction aphasia.

Follow the language pathway in 3D

From the classic model to a wider network

In the 1960s, the American neurologist Norman Geschwind built on Wernicke's ideas. The result, the Wernicke-Geschwind model, treats language as a chain of steps. To repeat a word you hear:

1. The primary auditory cortex receives the sound.
2. Wernicke's area recognizes it as a word and links it to its meaning.
3. The arcuate fasciculus carries it forward to Broca's area.
4. Broca's area plans how to say it.
5. The face area of the primary motor cortex moves the lips, tongue and larynx.

To read a word aloud, the chain starts in the primary visual cortex. It then passes through the angular gyrus, which links the written word to its sound, and on to Wernicke's area. The model is still taught because it predicts the classic aphasias well.

What modern research adds

The real system is bigger. Damage to Broca's area alone usually causes only mild, short-lived problems; lasting Broca's aphasia needs wider damage, including the insula and the white matter beneath. In 2007, scans of the preserved brain of Broca's first patient showed damage reaching much deeper than the area Broca described. Today, language is seen as a network with two main streams, plus help from the right hemisphere.

Damage to the right hemisphere can cause aprosodia. The person may speak in a flat voice, or miss the anger or sarcasm in other people's voices, even though grammar and vocabulary are fine.

Hearing, reading and signing

Sound signals travel from the inner ear, through the brainstem and the medial geniculate nucleus of the thalamus, to the primary auditory cortex. This strip lies on top of the temporal lobe, hidden inside the lateral sulcus, and each side receives sound from both ears. Nearby parts of the superior temporal gyrus and the superior temporal sulcus pick out speech sounds and voices. Both sides help with this step, but linking sounds to words and meanings leans to the left.

Writing was invented only about 5,000 years ago, too recently for evolution to build a reading area. Instead, learning to read trains a patch of the left fusiform gyrus, on the underside of the brain, to recognize letters and words. This visual word form area takes the words that the primary visual cortex sees and passes them on to the language network. A stroke in the left posterior cerebral artery can cut this route. The result is alexia without agraphia: the person can still write, but cannot read what they just wrote.

Sign languages, such as American Sign Language, have their own full grammar, and they depend on the same left-hemisphere areas as speech. Deaf signers with damage near Broca's area struggle to make signs, and those with damage near Wernicke's area struggle to understand them. Damage to the right hemisphere can upset their sense of space but usually leaves signing intact. So the left hemisphere is specialized for language itself, not just for sound.

Dyslexia, also called developmental reading disorder, is a difficulty with reading that is not caused by poor vision or low intelligence. Most people with dyslexia have normal or above-average intelligence, and it often runs in families. The core problem is linking the sounds of language to letters. Brain scans often show less activity in the left-hemisphere reading areas, and specialized teaching can improve reading.

Types of aphasia

Aphasia is a loss of language caused by brain damage. Doctors sort it with three questions. Is speech fluent? Does the person understand? Can they repeat a sentence? The answers point to where the damage is.

All four are usually caused by a stroke in the left middle cerebral artery (MCA), which feeds the whole language network. Its upper branch also feeds the face and arm areas of the motor strip. So Broca's aphasia often comes with weakness of the right face and arm, while Wernicke's aphasia usually comes without weakness. Global aphasia usually comes with a weak right side. For the other arteries, see blood supply of the brain.

People with Broca's aphasia usually know what they want to say and are frustrated by their speech. People with Wernicke's aphasia are often unaware of their mistakes, so their speech can be mistaken for confusion. In the rarer transcortical aphasias, the loop between the two areas is spared, so people can repeat well even though speaking or understanding is poor.

Aphasia is a language problem. It is different from dysarthria, slurred speech from weak or poorly coordinated speech muscles, in which language itself is intact.

Getting language back

About a third of people who survive a stroke have aphasia. Many improve a great deal in the first few months, and language can keep improving for years. Speech-language therapy helps people relearn words and find other ways to get their message across.

Recovery relies on the brain's ability to rewire itself, called neuroplasticity. In adults, the best recovery usually comes from surviving areas near the damage in the left hemisphere. The matching areas on the right can also help. In young children, the right hemisphere can take over much of language.

Some people who cannot speak fluently can still sing the words of familiar songs. A method called melodic intonation therapy uses melody and rhythm to help them find their words again.

Memory tricks

In these study guides

• MCATSleep, drugs, memory, language and reward
• Intro psychologyLanguage (section 7.2)
• NursingStroke
• Medical schoolCortex, language and lateralization
• A-level PsychologyLocalisation of function

Take it further in 3D

• LanguageHearing, understanding, speaking, reading. A short guided lesson in the 3D app.

Sources

1. Anatomy and Physiology 2e, 16.2 The Mental Status Exam, OpenStax
2. Aphasia, National Institute on Deafness and Other Communication Disorders (NIH)
3. Developmental reading disorder, MedlinePlus Medical Encyclopedia, National Library of Medicine (NIH)

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