Imagine the family's consternation when, after a stroke, Grandmother speaks only Croatian, which she spoke for a few years as an adolescent. These cases were especially perplexing to students of language until studies were done using electrical-stimulation mapping during naming in two languages. In bilingual patients, there are some cortical sites where naming is altered in only one language, as well as other sites where stimulation alters both languages. It may be that a large area of language cortex is needed to first learn any language. With continued use, smaller regions may suffice, freeing up other portions of language brain for new tasks, such as learning another language.
Most aphasic patients have difficulty with reading and writing as well as speaking.
Indeed, they tend to make the same kinds of mistakes in both spoken and written language. Impairment of only writing is quite rare, and usually associated with damage to wide areas of brain. But impairment of only reading is not uncommon. There the defect seems to be in tracking the printed words, as that area of brain is also involved in voluntary control of eye movements. Such damage is thought to disconnect the language area of the brain from visual input.