A Tale of Two Aphasias

It’s 1861, and the French neurologist Paul Broca is examining a new patient. Dr. Broca is puzzled because all the patient can say is “tan.” When Dr. Broca asks him questions, Tan cannot seem to form the words. However, it is clear that Tan can understand language because, when he asked to whistle or sing a melody, he can do so without a problem. Something is wrong with his ability to speak! When he is asked to speak grammatically or create complete sentences, he cannot do it – not even in writing! Dr. Broca doesn’t know what to do for Tan, since he knows that Tan must have brain damage… but where?

After Tan died, Dr. Broca got his answer: there was a lesion in the posterior region of the frontal lobe of Tan’s brain. Over the next few years, Dr. Broca examined eight more patients with similar symptoms and found that each of them had a similar lesion in the left cerebral hemisphere. This discovery intrigued Dr. Broca so much that he exclaimed, “Nous parlons avec l’hemisphere gauche!” or “We speak with the left hemisphere!” And thus began the search for other cortical sites of specific behavioral functions.

Inspired by Dr. Broca’s work, the German neurologist Carl Wernicke began pursuing his own research on the effects of brain disease on speech and language. At the age of 26, Dr. Wernicke published a paper describing a patient who could speak but could not understand language. This patient produced lots of language, stringing together random words in a way that sounded like a sentence, but it actually made no sense! Sometimes the patient would make up completely new words, or neologisms, altogether. Worst of all, the patient didn’t seem to realize that he wasn’t making sense! The symptoms of Wernicke’s patient were almost opposite to Broca’s patient. Dr. Wernicke wondered if that meant there was another language center in the brain.

Postmortem examination of the brain of Wernicke’s patient revealed lesions in the posterior part of the temporal lobe, where it joins the parietal and occipital lobes. These findings led Wernicke to propose that language involves separate motor and sensory programs, each governed by separate cortical regions. He proposed that the motor program of language, which governs the mouth movements for speech, is located in Broca’s area. This makes sense because Broca’s area is situated just in front of the motor area that controls the mouth, tongue, palate, and vocal cords. He assigned the sensory program of language, which governs word perception, to the temporal lobe area he discovered, now called Wernicke’s area. This also makes sense because it is surrounded by the auditory cortex and the association cortex, areas that integrate auditory, visual, and somatic sensation into complex perceptions.

These two language programs have to work together to create coherent speech. This communication occurs via the arcuate fasciculus, the nerve bundle that connects these Broca’s and Wernicke’s areas. When this connection is damaged, patients can understand words they hear and read and have no motor difficulties when speaking (because both Broca’s and Wernicke’s areas are intact), but they cannot speak coherently. Patients with conduction aphasia cannot repeat sentences back and often omit parts of words or substitute incorrect sounds within words.

Broca’s and Wernicke’s discoveries marked some of the earliest structure-function relationships of the brain. Their work inspired future neuroscientists to search for discrete areas of the cortex with specialized roles in behavior, and this work continues today!

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References:

Price C.J. (2000). The anatomy of language: contributions from functional neuroimaging, Journal of Anatomy, 197 (3) 335-359. DOI: http://dx.doi.org/10.1046/j.1469-7580.2000.19730335.x

Kertesz A. (1993). Clinical Forms of Aphasia, Acta Neurochirurgica Supplementum, 52-58. DOI: http://dx.doi.org/10.1007/978-3-7091-9239-9_9

Damasio A. (1984). The Neural Basis of Language, Annual Review of Neuroscience, 7 (1) 127-147. DOI: http://dx.doi.org/10.1146/annurev.neuro.7.1.127

Images via Wikipedia (Paul Broca and Carl Wernicke) and made by Jooyeun Lee.