The defining characteristic of the Starks' loyal servant Hodor is that he can say only that single word, "hodor." But a real 19th-century patient who could utter just one syllable over and over gave us great insight into the workings of our brains.
Mother Jones points us to the case of Louis Victor Leborgne, a patient living in the care of Paris' Bicêtre Hospital during the mid-19th century. In 1861, famed French physician Paul Broca learned of Leborgne, who was suffering from a very Hodor-like condition. Broca wrote of him:
He could no longer produce but a single syllable, which he usually repeated twice in succession; regardless of the question asked him, he always responded: tan, tan, combined with varied expressive gestures. This is why, throughout the hospital, he is known only by the name Tan.
By the time Broca learned of Leborgne's condition, Leborgne had been in the hospital for quite some time and suffered various ailments that rendered communication difficult. But in interacting with Leborgne, Broca realized that some of the patient's mental faculties remained sharp, such as his ability to handle numbers and tell time, and he could understand what was said to him.
After Leborgne passed away, an autopsy revealed that he had lesions on his brain in what is now known as "Broca's area," a part of the brain linked to speech production. (However, other regions of the brain have been implicated in speech issues as well.) Broca soon encountered a second patient, a man named Lelong, who could speak only five words following a stroke. After Lelong's death, Broca found lesions in roughly the same part of the brain where he found Leborgne's lesions. Broca dubbed the condition "aphemia," and after the word "aphasia" came to describe disruptions in comprehension and expression of language, the inability to express oneself through speech or writing became known as "expressive aphasia" or "Broca's aphasia."
So, while Hodor may encounter plenty of magical elements on his adventures through Westeros, what's going on with his speech may, in fact, be entirely natural.