Dysprosody, which is also known as pseudo-foreign dialect syndrome, refers to a disorder in which one or more of the prosodic functions are either compromised or eliminated completely.
Prosody refers to the variations in melody, intonation, pauses, stresses, intensity, vocal quality and accents of speech. As a result, prosody has a wide array of functions, including expression on linguistic, attitudinal, pragmatic, affective and personal levels of speech. People diagnosed with dysprosody most commonly experience difficulties in pitch or timing control. Essentially, a person diagnosed with the disease can comprehend language and vocalize what he intends to say, however, he is not able to control the way in which the words come out of his mouth. Since dysprosody is the rarest neurological speech disorder discovered, not much is conclusively known or understood about the disorder. The most obvious expression of dysprosody is when a person starts speaking in an accent which is not their own. It is important to note, however, that speaking in a foreign accent is only one type of dysprosody, as the disease can also manifest itself in other ways, such as changes in pitch, volume, and rhythm of speech. It is still very unclear as to how damage to the brain causes the disruption of prosodic function. The only form of effective treatment developed for dysprosody is speech therapy.
(Note: The information above is courtesy of Wikipedia)