Decoding Aphasia: Separating Language From Thought
What aphasia tells us about cognition.
Updated April 10, 2026 | Reviewed by Kaja Perina
Imagine you’re a physician and you are called in to evaluate a patient who has had a sudden change in his neurological status, likely a stroke. You find him alert, mobile, and talking. But when you ask him how he is feeling he says, “We’re out with them. Other people are working with them and them. I’m very happy with them. This girl with verly good. And happy and I play golf and hit up trees. We play out with the hands. We save a lot of hand on hold for peoples for us. Other hands. I don’t know what you get, but I talk with a lot of hand fram. Sometime. Am I talk of anymore to saying.” You don’t detect any other neurological defects, like weakness or paralysis on one side of the body. What’s going on? Why does he seem so scatterbrained?
If you’re a modern neurologist, you’ll recognize this as a type of aphasia, a common symptom of stroke. You’ll also know that aphasia is a language problem, not an intelligence problem; the patient may be able to think quite clearly but just can’t translate those thoughts into coherent words and sentences. But it took many decades and a great deal of scientific effort to arrive at this modern understanding. Indeed, many physicians practicing a hundred or more years ago would have agreed with French neurologist, Armand Trousseau, who coined the term “aphasia” in the 1860s and claimed that in sufferers of the condition, “intelligence is always lamed.” Many other prominent clinician-scientists of that era agreed, including the likes of Hughlings Jackson, a leader in early neurology. The view persisted into the 1930s.
A game-changing study
The talent and effort of a young graduate student, Katharine McBride, coupled with the resources of an established neurologist, Theodore Weisenburg, challenged the dogma of the day. Weisenburg had secured funding to put the Trousseau-Jackson theory to the test. The problem was that there was no widely accepted and comprehensive tool for the assessment of aphasia and precious little in the way of testing non-verbal intelligence in neurological populations. He recruited McBride to solve these problems, which she did.
McBride crafted an impressive assessment battery that would be familiar to clinicians today, comprising a range of language tasks involving spontaneous speaking, naming, repetition, comprehension, reading, and writing, to name a few. She also used a range of nonverbal tasks drawn from some of the early IQ assessment tests. It took an average of nineteen hours to administer the whole thing to each patient!
Beyond the assessment tool itself, a major advance of the team’s research was the inclusion of a control group to provide a performance baseline from neurologically healthy individuals. The final study included an impressive 234 individuals: 60 people with aphasia, 38 people with lesions but no aphasia, 25 cases with miscellaneous neurological conditions, and 85 neurological healthy controls. At the time, it was the most comprehensive study of aphasia ever undertaken and ranks highly even by modern standards.
Weisenburg and McBride launched their study with a decided sympathy for the idea that aphasia and intelligence were inseparable. What they found was something quite different. After noting that nonlinguistic deficits are often present in aphasia, they concluded that “it is evident that purposeful and effective thinking may be carried through when language is extremely inadequate or confused, and must therefore depend largely on nonlinguistic symbols such as visual or kinaesthetic images.” Intelligence is not always lamed in aphasia. Thinking and language are separable abilities.
Subsequent studies confirmed the dissociability of language and thought, including relatively recent experiments using today’s assessment tools, such as the Western Aphasia Battery and the Raven’s Progressive Matrices test for fluid intelligence (see an example item in the figure here). For example, a recent study tested 200 people with aphasia and found similar results to Weisenburg and McBride: overall aphasia severity and nonverbal intelligence (score on the Raven’s task) were only weakly related with many individuals showing dissociations on the two types of measures.
My colleagues and I have looked at this question in our own aphasia database, and we find the same results. The chart below shows you graphically where each person falls with respect to their aphasia severity score (vertical axis) and their nonverbal intelligence score (horizontal axis). What you see is that a number people are impaired on both language and intelligence measures (bottom left points) or normal on both (top right points). Enough lie along the diagonal in between to give rise to a statistical correlation (shown by the slanted line). But what hits you in the face is the variability, the number of people who deviate from the correlation line showing dissociations between language and thought.
Language as a mental scratch pad
Despite the clear dissociability of language and intelligence, large-scale group studies, including my own, do show a statistical correlation, even if it is weak. Some researchers, too, have zoomed in on more specific linguistic and non-linguistic abilities, suggesting that some aspects of language may contribute more substantially to some aspects of our intellect. I don’t disagree in general, but I do think language ability contributes to our intellect more like a crutch than a muscle; it’s augmentative rather than inherent.
Consider the example item from the Raven’s task above. I find it helpful to talk the problem through, even though it’s not necessary to solve the problem: “Left column: three gray bars in front, three gray bars in back: three front, three back. Right column: Two back, two … front. Look for two gray bars in front.” Perhaps if I took this test without my language ability, I would make a couple of more errors, not because I lack the intellect to appreciate the relation between the puzzle elements, but rather because it’s a little easier to slip up, like making “stupid mistakes” on a math problem.
The idea is that once we evolved language—the ability to convert our thoughts into speech, and speech into thoughts—we can use that system as a mental scratch pad to augment our intellectual ability. Just like our ability to work out a math problem or cook a meal isn’t inherent to the scratch pad where we jot the elements and steps involved, so too our ability to think and reason isn’t inherent to our capacity to translate those thoughts to words and sentences. In fact, if language was at the core of thought, we’d all probably be able to communicate our ideas a lot more clearly than we sometimes do!
Excerpted and adapted from Wired for Words: The Neural Architecture of Language by Gregory Hickok. Published by MIT Press.
Baldo, J. V., Bunge, S. A., Wilson, S. M., & Dronkers, N. F. (2010). Is relational reasoning dependent on language? A voxel-based lesion symptom mapping study. Brain Lang, 113 (2), 59-64.
Gonzalez, R., Rojas, M., & Ardila, A. (2020). Non-linguistic abilities in aphasia. Journal of Neurolinguistics, 56 , 100916.
Hickok, G. (2025) Wired for words: the neural architecture of language. Cambridge, MA: MIT Press.
Kertesz, A. (2012). The Western Aphasia Battery-Revised . San Antonio, TX: Pearson.
Weisenburg, T., & McBride, K. E. (1935). Aphasia: A clinical and psychological study . New York: The Commonwealth Fund.
Speech sample from https://www.youtube.com/watch?v=3oef68YabD0
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Greg Hickok, Ph.D., is Distinguished Professor at the University of California, Irvine, and the author of the forthcoming book Wired for Words.
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