Here's the full passage in The Language Instinct about Denyse:

Here is another interview, this one between a fourteen-year-old girl called Denyse and the late psycholinguist Richard Cromer; the interview was transcribed and analyzed by Cromer's colleague Sigrid Lipka.

> I like opening cards. I had a pile of post this morning and not one of them was a Christmas card. A bank statement I got this morning!

< [A bank statement? I hope it was good news.]

> No it wasn't good news.

< [Sounds like mine.]

> I hate . . . , My mum works over at the, over on the ward and she said "not another bank statement." I said "it's the second one in two days." And she said "Do you want me to go to the bank for you at lunchtime?" and I went "No, I'll go this time and explain it myself." I tell you what, my bank are awful. They've lost my bank book, you see, and I can't find it anywhere. I belong to the TSB Bank and I'm thinking of changing my bank 'cause they're so awful. They keep, they keep losing . . . [someone comes in to bring some tea] Oh, isn't that nice.

< [Uhm. Very good.]

> They've got the habit of doing that. They lose, they've lost my bank book twice, in a month, and I think I'll scream. My mum went yesterday to the bank for me. She said "They've lost your bank book again." I went "Can I scream?" and I went, she went "Yes, go on." So I hollered. But it is annoying when they do things like that. TSB, Trustees aren't. . . uh the best ones to be with actually. They're hopeless.

I have seen Denyse on videotape, and she comes across as a loquacious, sophisticated conversationalist—all the more so, to American ears, because of her refined British accent. (My bank are awful, by the way, is grammatical in British, though not American, English.) It comes as a surprise to learn that the events she relates so earnestly are figments of her imagination. Denyse has no bank account, so she could not have received any statements in the mail, nor could her bank have lost her bankbook. Though she would talk about a joint bank account she shared with her boyfriend, she had no boyfriend, and obviously had only the most tenuous grasp of the concept "joint bank account" because she complained about the boyfriend taking money out of her side of the account. In other conversations Denyse would engage her listeners with lively tales about the wedding of her sister, her holiday in Scotland with a boy named Danny, and a happy airport reunion with a long-estranged father. But Denyse's sister is unmarried, Denyse has never been to Scotland, she does not know anyone named Danny, and her father has never been away for any length of time. In fact, Denyse is severely retarded. She never learned to read or write and cannot handle money or any of the other demands of everyday functioning.

Denyse was born with spina bifida ("split spine"), a malformation of the vertebrae that leaves the spinal cord unprotected. Spina bifida often results in hydrocephalus, an increase in pressure in the cerebrospinal fluid filling the ventricles (large cavities) of the brain, distending the brain from within. For reasons no one understands, hydrocephalic children occasionally end up like Denyse, significantly retarded but with unimpaired—indeed, overdeveloped—language skills. (Perhaps the ballooning ventricles crush much of the brain tissue necessary for everyday intelligence but leave intact some other portions that can develop language circuitry.) The various technical terms for the condition include "cocktail party conversation," "chatterbox syndrome," and "blathering."

Source: http://f.javier.io/rep/books/The-Language-Instinct-How-the-M...