“In 1882, Mr. L, a teacher and journalist, suffered a stroke, his second. His first attack, five years earlier, had been mild, causing some language problems involving paraphasias, difficulty selecting the correct words or speech sounds while speaking. Happily, he made a near-complete recovery. He wasn’t so lucky following his second stroke, though. Immediately following the event, his paraphasias worsened again and he could not read or write, but these symptoms later subsided. Mr. L might have escaped serious disability once again, except for one glaring change in his neurological status: while he could hear, he behaved as if he were completely deaf. His doctor, the now-famous German aphasiologist Ludwig Lichtheim (1845-1928), later described Mr. L’s case in a seminal 1885 report: “Patient gives the impression of an absolutely deaf man… One cannot have the least communication with him except through writing… I should probably have taken him for really deaf had I not been assured that he was acute of hearing, and could perceive all noises.”

Mr. L was the first reported case of word deafness, a severe inability to perceive and understand spoken words despite retaining the ability to hear, speak, read, and write. According to modern reports, the experience of word deafness is similar to listening to foreign speech: words sound jumbled, run together, or just as meaningless clangs. It is a rare condition—less than 100 cases have been reported—but word deafness has figured prominently in the development of our understanding of how the brain converts clangs into words we can understand.

The key observation about the neuroanatomy of word deafness is that it is most often caused by damage involving the same brain areas in both hemispheres, specifically, auditory areas in the temporal lobe. What’s instructive about this pattern is that two-sided damage to similar areas is rare compared to just the one side, meaning that destruction of just one isn’t typically enough to cause the disorder. The logical conclusion is that the speech perception network is distributed over both hemispheres, roughly equally, unlike other language functions. Yet, many researchers in the field of brain and language still cling to the idea that speech perception is left-dominant. Why?”

Continue reading: https://www.psychologytoday.com/us/blog/wired-for-words/202508/we-are-still-under-the-spell-of-split-brain-research