When a stroke reaches the language centers of the brain, the words don't disappear — the path to them does. A guided tour of aphasia.
of people have never heard of aphasia — and of those who have, only 8.8% can correctly identify it as a language disorder. It affects around 2 million people in the U.S. By the end of this lesson, you won't be in that 84.5%.
The brain has two hemispheres, and each hemisphere has four lobes. Three of them — the frontal, temporal, and parietal — do the heavy lifting for speech and language.
Damage to language doesn't look the same everywhere. The symptoms depend entirely on where the injury lands. So before we talk about what goes wrong, it's worth knowing what each region is responsible for. Explore the lobes below.
For most people, speech and language are handled primarily by the left hemisphere. The right hemisphere still matters — especially for pragmatics, the social use of language — but the core machinery sits on the left.
of right-hand dominant individuals are left-hemisphere dominant for language.
of left-hand dominant individuals are also left-hemisphere dominant for language.
Zoom in on the left hemisphere and three players come into focus: two regions of gray matter, and the cable that links them.
Aphasia is difficulty with the comprehension and/or production of language. It most often appears after a stroke to the left hemisphere.
Crucially, aphasia is acquired — it isn't something a person is born with, and it isn't a developmental difference. It results from damage to an already-mature language system, through brain injury, stroke, or a neurodegenerative disorder such as dementia. And because language is spread across regions, the specific symptoms trace back to the specific location of the damage.
A stroke happens when the blood supply to part of the brain is interrupted or reduced. Starved of oxygen, that tissue is damaged — and if it's language tissue, aphasia can follow. There are two major types.
A blood clot blocks a vessel in the brain, cutting off flow downstream. This is the more common type.
A blood vessel in the brain bursts, allowing blood to leak into surrounding tissue.
Recognizing a stroke is a race against time. The faster it's treated, the more brain — and language — can be saved. The standard public-health mnemonic is FAST:
Common risk factors
There are several sub-types of aphasia, and clinicians tell them apart by listening along four dimensions. Every sub-type is really just a different profile of strengths and weaknesses across these four.
Does speech flow smoothly, or is it halting and effortful?
Can the person understand what's said to them?
Can they repeat words and phrases back accurately?
Can they retrieve the right word for a thing on demand?
Here's the elegant part: ask just three yes/no questions — Fluent? Comprehends? Repeats? — and you can walk straight to the aphasia type. Try it. Imagine any patient and answer for them.
Below are the five sub-types this course centers on — each with where the damage sits, its profile across the dimensions, and a real glimpse of how it sounds.
Reading a transcript only goes so far. Watch how aphasia actually sounds and unfolds in conversation — then keep its rhythm in mind for the cases that follow.
Press play to watch. The clip pauses automatically if you scroll past it, so the audio won't follow you down the page.
Read each patient sample, weigh it against the four dimensions, and name the aphasia. Five cases. No pressure — well, a little.
These two words get used interchangeably in everyday life, but in this field they are not the same — and the distinction defines two very different disorders.
Think of language as the message and its form: the phonology, the words, the sentences. Think of speech as the acoustics of that language — moving the body's structures, the qualities of the sound, planning and programming sequences of sounds.
Impacts language form, content, and use — the comprehension and/or production of language, in any modality: spoken, written, or gestural.
Impacts the planning and programming of speech-sound production — like sequencing the right sounds for a word — with no physical impairment in producing the sounds themselves.
We'll return to apraxia of speech in the speech-disorders section of the course.
Aphasia is acquired damage to the brain's language network — most often from a left-hemisphere stroke. Where the damage lands shapes the profile across fluency, comprehension, and repetition, and that profile is what gives each sub-type its name.