Broca's aphasia, from left inferior frontal damage, causes agrammatism: impaired production of function words and complex syntax, while lexical meaning and comprehension remain relatively preserved. This dissociation reveals distinct neural systems for grammar and lexicon, highlighting frontal regions' importance for syntactic structure building and explaining why grammatical versus semantic deficits can diverge in neural disease.
From your prerequisite on language and the brain, you know that the left hemisphere dominates language processing for most right-handed people, and that different brain regions contribute differently to language. Broca's area — the left inferior frontal gyrus, specifically Brodmann areas 44 and 45 — became the first brain region systematically linked to a specific aspect of language. Paul Broca's 1861 report on a patient called "Tan" (because "tan" was almost the only syllable he could produce) established that damage to this region caused profound impairment to speech production while leaving comprehension relatively intact. This was neurological evidence that language was not a unified faculty but a collection of dissociable systems.
The defining symptom of Broca's aphasia is agrammatism: the systematic loss or impairment of grammatical elements in production. In practice, this means speech becomes telegraphic — content words survive but function words (the, a, is, was, that, with) are dropped, verb inflections (-ed, -ing, -s) are omitted or regularized, and syntactically complex structures become impossible. A patient trying to say "The boy was chased by the dog" might produce "Boy... dog... chase." The meaning is partially recoverable from the content words, but the grammatical structure that encodes who did what to whom has been stripped away.
Why does this pattern occur? The leading explanation is that Broca's area is specifically involved in syntactic structure building — the computational process of combining words into hierarchically organized phrases according to grammatical rules. Content words (nouns, main verbs, adjectives) are stored in long-term lexical memory distributed across the cortex and can be retrieved even when frontal regions are damaged. Function words and morphology — the grammatical glue of sentences — require active assembly in working memory, and this assembly process depends heavily on the left inferior frontal region. When that region is damaged, the assembly process degrades even though the lexical pieces remain available.
An important nuance: comprehension in Broca's aphasia is "relatively" preserved, not fully intact. For simple sentences, especially active constructions where word order makes meaning clear ("The dog chased the cat"), comprehension is adequate. But for syntactically complex sentences where grammatical structure carries the meaning — passives, object relatives ("The dog that the cat chased was brown") — Broca's aphasics often fail. They tend to default to canonical role assignment: interpreting the first noun as the agent, regardless of whether the syntax supports that reading. This tells us that their comprehension, too, relies partly on pragmatic heuristics rather than grammatical parsing.
The broader significance is what Broca's aphasia reveals about the architecture of language. The double dissociation between Broca's and Wernicke's aphasia — Broca's patients produce agrammatic speech but understand reasonably; Wernicke's patients produce fluent but semantically incoherent speech and comprehend poorly — suggests that grammar and lexical semantics are not the same thing neurally. This is consistent with linguistic theories that distinguish syntactic computation from semantic interpretation, and it means that neurolinguistics and formal linguistics have been engaged in a productive mutual conversation: brain lesion data constrains theories of language structure, and linguistic theory provides precise vocabulary for describing what is impaired. Broca's aphasia was not just a medical curiosity; it was the first experimental evidence that the brain treats grammar as a separable, localizable function.