The sounds of speech, or the pronunciation of words, are an area known technically as phonetics and phonology. Phonetics refers to the physical (acoustic) properties of the actual, individual sound in speech. Phonology refers to the system of sound units in a language, what might be thought of as the abstract set of vowels and consonants of the language. The units are typically represented by a letter (or combination of letters) of the alphabet, such as “s” or “th” or “e” (as in bet).
In some forms of Frontotemporal degeneration (FTD), there may be a disruption of the ability to articulate words properly. A disturbance of phonology might result in a person saying, for example, “myoos” or “boose” for moose. This problem is thought to result from a deterioration of the patient’s memory for the form of a word at some level. Patients sometimes report that they know what they want to say, but they are unable to form the words. Errors in phonology are made by everybody on occasion—leaving out a sound in a word, putting in an extra sound, exchanging two sounds in a word or phrase, or replacing one sound with another. When phonological errors become very numerous in a person’s speech, as can occur in FTD, this interferes with intelligibility and may pose a serious obstacle to communication. This constitutes a form of aphasia or a language disturbance, such as progressive non-fluent aphasia (or the non-fluent/agrammatic variant of primary progressive aphasia).
A disturbance of phonetics, on the other hand, is likely to be caused by a problem in controlling the motor or articulatory apparatus of the vocal tract, including the tongue, lips, velum (which opens and closes the air passage between the mouth and nose), and glottis (the opening formed by the vocal cords). Such a disturbance results in the production of sounds that are not part of the speaker’s language and therefore may be very difficult to understand. This condition may be termed dysarthria, or apraxia of speech. Mistakes in phonetics are rare among healthy speakers. However, apraxia of speech can occur in a large number of conditions, even if FTD is not involved. Individuals with conditions involving poor control over voluntary movements can have prominent apraxia of speech, such as progressive supranuclear palsy and corticobasal degeneration.
Researchers at the Penn FTD Center are studying the mechanisms of speech sound production difficulty in FTD. This will lead to speech therapies that can improve speech intelligibility and potentially reduce the rate of declining speech in FTD.
Very few studies of speech sound perception have been performed in FTD. Researchers at the Penn FTD Center are investigating whether the same linguistic and anatomic factors associated with impaired speech expression also lead to deficits of speech perception.