Evidence from Damage
Early brain science relied on damage to understand language.
In 1861, Paul Broca described a patient who understood speech but could not produce it. Broca linked this to left frontal damage. In 1874, Carl Wernicke described fluent speech with poor comprehension, linked to temporal damage.
| Broca's aphasia | Wernicke's aphasia | |
|---|---|---|
| Fluency | Halting | Fluent |
| Content | Meaningful words | Semantically empty |
| Comprehension | Good | Poor |
| Awareness | Aware | Unaware |
In conduction aphasia, speech and comprehension remain intact, but repetition fails.
Lateralisation
Language is left-lateralised in 95% of right-handers and 70% of left-handers. The classic source for these numbers is the Wada test, which anaesthetises one hemisphere.
Beyond the Classical Model
The classic model is oversimplified:
- Damage in Broca's patients extended well beyond Broca's area.
- Language relies on broad, distributed networks rather than two isolated centres.
- Broca's patients struggle to understand complex syntax.
The Critical Period
First-language acquisition declines with age. The case of Genie, isolated until age thirteen, showed vocabulary growth without normal syntax. Studies of deaf individuals learning sign language at different ages confirm this decline.
Common pitfall: Broca and Wernicke areas are not simple functional modules for production and comprehension. The actual neural processing relies on wide networks.