Monday, August 15, 2011

The Cerebral Cortex and Stuttering, Part 1

We have thus far focused heavily on the basal ganglia as a potential root cause of stuttering. In the next two posts, we discuss the possible role of structures in the cerebral cortex involving speech processes that may either be affected by or directly contribute to disfluency.

The functional location of speech is principally in the left hemisphere of the cerebral cortex for the great majority of right-handed people. For left-handed people, the picture is less clear; some show a specialization for speech in the left hemisphere, while others specialize in the right, and for still others, both hemispheres contribute just about equally. In the following we focus on the vast majority of individuals who specialize chiefly in the left hemisphere.

A number of brain imaging studies have shown that stutterers exhibit a deficient involvement of the left cortical hemisphere in speech activities and greater involvement of the right. An excess of testosterone in newborns due to stress at the time of birth might well be one of the most common causes of slower development in the left hemisphere resulting in greater participation by the right.

Located in the left lateral cortex regions, as shown in the figure below, Wernicke’s and Broca’s areas are involved in the initiation of speech.

There are three sub-areas within Wernicke’s area. The first responds to spoken words (including the individual’s own); the second responds only to words spoken by someone else (but is also activated when the individual recalls a list of words); and the third is associated with producing speech. Basically Wernicke’s area relates to the representation of phonetic sequences, regardless of whether the individual hears them, generates them himself, or recalls them from memory.

Broca’s area is another part of the complex network involved in developing an articulation plan. It is concerned with the meanings of words (i.e., semantics), how words are combined to form phrases and sentences (i.e., syntax), and the specific sounds associated with words (i.e., phonology).

 Broca's area is associated with the serialization of coordinated action of the speech organs.

Various injuries or deficiencies in Broca’s and Wernicke’s areas lead to aphasia which is the partial or total loss of the ability to articulate thoughts and ideas. For example, injuries to Broca’s area may result in agrammatism which typically involves a lack of use of syntax in speech production resulting in laboured speech. Individuals with Wernicke's aphasia have difficulty recalling correct words for the context or they may coin meaningless words.

Being articulate (as opposed to fluent)* depends upon the rich complex of information generated in lateral areas of the cortex, namely Wernicke’s and Broca’s areas. This information is then transferred to medial cortical areas adjacent to Broca’s area such as the motor cortex, which governs the mechanics of speaking. Current theories of stuttering do not implicate the lateral areas as root causes of disfluency. However, stuttering may affect being articulate by virtue of the interruptions of one’s train of thought and the fact that excessive energy and effort is devoted to the mechanics of speaking, which takes away from the focus on content (i.e., what you say).

In addition to the left lateral areas of the cortex, the right hemisphere also comes into play for speech. Prosody refers to the intonation and stress with which the basic units of a language (called phonemes) are pronounced. Feeling and attitude are conveyed through prosody as well as the melody, inflection, and intonation of one's voice and by varying the pitch, inflection, timbre, stress contours, and the rate/amplitude of speech. These aspects of speech enable a speaker to convey and a listener to determine intent, attitude, feeling and meaning. These capacities, both for the speaker (governing the style of presentation, i.e., how you say it) and the listener (interpreting the style), are predominantly mediated by the right half of the cerebral cortex.

Once again the effort/energy spent on stuttering, attempting to speak fluently, or using fluency shaping techniques impairs the ability to convey emotion through one’s speech. As a result, stutterers may speak and behave in a way that seems flat and emotionless.

In the next post, various aspects of the medial cortical areas as relates to speech will be discussed.

* While the aphasias discussed above are regarded by the medical establishment as fluency problems, I prefer to distinguish between being fluent and being articulate.


Anonymous said...

Very good post, as a person who stutters I can say for sure that I speak flat and emotionless a good portion of the time. Even after taking dopamine blocking medication, the production of speech is still very mentally taxing, however slightly more fluent.

Anonymous said...

I have a mild stammer, and for me stammering results through fear of expressing oneself, and after doing work on myself, probably began because of emotional trauma in which the same trauma is constantly replayed when triggered by specific events.