Marc Dax (27 December 1770 – 3 June 1837) was a French neurologist, sometimes credited for discovering the link between neurological damage to the left hemisphere, right-sided hemiplegia, and a loss of the ability to produce speech (aphasia). He submitted his discovery, based on the observations of three patients in Montpellier, to the French Academy of Sciences and two previous notes were published in 1836, 25 years before Paul Broca's more famous description. His papers were titled Observations tending to prove the constant coincidence of disturbances of speech with a lesion of the left hemisphere of the brain, and Lesions of the left half of the encephalon coincident with the forgetting of signs of thinking. He died one year later and thus his discovery remained obscure.
In 1863, Gustave Dax, the son of Marc Dax, published his father's work on the subject, two years after Paul Broca's presentation of the same phenomenon to Société d’Anthropologie [1] (Broca's original French communication, plus link to English translation here http://psychclassics.yorku.ca/Broca/perte.htm). The publication included the 1836 memoir of Marc Dax, his deceased father, and additional clinical observations of his own on 140 patients. His contribution received a negative appraisal by the Academy, however, and Gustave Dax was overlooked by the scientific establishment of the time. In consequence, today the discovery of the link between the left hemisphere and speech is typically credited to Paul Broca.
According to authors Cubelli and Montagna, the Broca's theory should be renamed:
"Probably, Broca was aware of the paper prior to 1865, but he never acknowledged Dax's original theoretical contribution. On the contrary, he always claimed to be the first to espouse the theory of left hemisphere dominance for language and never quoted Marc Dax (Broca, 1877 p 536), I do not like dealing with the questions of priority concerning myself. That is the reason why I did not mention the name of Dax in my paper. In our opinion, the weight of evidence reported here suggests that the theory of the left hemisphere dominance for speech must be attributed equally to Dax and Broca, and henceforth should be called the theory of Dax-Broca."
The medical historians S. Finger, M. Crichtley and A. L. Benton were responsible for bringing to light the role and importance of the Daxes for neurolinguistics, in a number of papers. Citing Finger on Gustave Dax:
"Gustave Dax's own unique contribution, however, has been almost completely overlooked. Although his theory lacked specificity, he preceded Meynert, Schmidt, and Wernicke in suggesting that the left temporal lobe may be especially important for speech."