Language and the Brain

From Broca and Wernicke to modern networks: how the brain produces and understands speech, reading, sign language and multiple languages — and what aphasia, dyslexia and FOXP2 reveal.

Language is one of the most distinctive human abilities. The classic model, from Broca (1861) and Wernicke (1874), placed speech production in the left frontal lobe and comprehension in the left temporal lobe, linked by the arcuate fasciculus. Later, the neurologist Norman Geschwind extended it into the Wernicke–Geschwind model.

A modern view: networks and streams

Brain imaging and lesion studies show language is more distributed. A widely used model by Gregory Hickok and David Poeppel proposes two streams, as in vision: a ventral stream in the temporal lobes that maps sounds to meaning, and a dorsal stream linking temporal and frontal regions via parietal cortex that maps sounds to the movements of speech (useful for repeating words and learning new ones). Grammar, word meaning, and the melody of speech involve different overlapping regions.

Lateralisation and sign language

In about 95% of right-handers and roughly 70% of left-handers, language depends mainly on the left hemisphere. The right hemisphere contributes to tone of voice, humour and understanding non-literal language. Deaf signers who have left-hemisphere strokes develop sign-language aphasias resembling spoken ones — showing these regions serve language itself, not just sound.

Reading: a cultural recycling

Writing was invented only about 5,000 years ago — too recent for brains to have evolved a reading organ. Instead, learning to read recruits part of the left ventral visual cortex, which Stanislas Dehaene named the visual word form area, "recycling" circuitry used for recognising objects. Dyslexia, a common difficulty in learning to read despite normal intelligence, is often linked to problems processing speech sounds (phonological processing).

Genes and language

In the 1990s researchers studied a British family (known as KE) in which about half the members had severe speech and language difficulties. The cause was a mutation in a gene called FOXP2. FOXP2 is not a "language gene" — it is a regulator of many genes, also found in birds and mice — but it is needed for the development of circuits for learned vocal sequences.

Bilingual brains

Languages learned in childhood share largely overlapping networks. Claims that bilingualism gives broad cognitive advantages have been hotly debated, with mixed results.