Wednesday, August 31, 2011

The Cerebral Cortex and Stuttering, Part 2

Per Alm has recently published a chapter in a book focusing on cluttering. As he points out, since stuttering and cluttering are overlapping but contrasting disorders, understanding cluttering may help in understanding stuttering. A lot of the presentation below, representing a theoretical framework for the production of speech, owes much to this chapter. However, I take full credit for any misrepresentations that I may have made.

In this post, we focus on the medial cortical areas that are involved in the production of speech. Specifically, these are the motor cortical areas, which are typically divided into three regions having different functional roles:

• pre-motor area (PMA)
• supplementary motor area
• primary motor cortex(M1)

The premotor cortex guides movements of the vocal apparatus by integrating sensory information. It helps to regulate motion by dictating an optimal position to the motor cortex for any given movement.

The supplementary motor area lies just in front of the primary motor cortex. What had previously been regarded as the supplementary motor area is actually composed of two anatomically and functionally distinct parts: the supplementary motor area proper (called the SMA) and the pre-SMA (see figure*).

The Pre-SMA and the PMA function in processes that are preparatory to speech. The pre-SMA is involved in acquiring new motor sequences, being more active when the sequence is novel as opposed to one that had already been learned. Higher level planning and sequencing takes place in the anterior pre-SMA, while the posterior pre-SMA is involved in the sequencing of sounds and syllables.

The SMA, along with M1, functions in the initiation and execution of movement. The SMA is responsible for the coordination of sequences of motor movements such as those involved in speech. It is concerned with actions under internal control, for example the retrieval of a motor sequence from memory involving the pronunciation of a word.

SMA neurons are more active when the task requires the arrangement of multiple movements in the correct sequence and correct temporal order. Specifically, the timing of an articulation and the speech rate is controlled by the SMA with support from both the basal ganglia and the cerebellum. And it is this timing mechanism that has been implicated in stuttering.

The purpose of the primary motor cortex (M1) is to connect the brain to the lower motor neurons via the spinal cord.

Aside from the motor cortical areas mentioned above, the anterior cingulate cortex (ACC)** plays the role of a central executive in the production of speech The ACC (as well as the preSMA) are involved in assembling the phrase, from the selection of words and word forms to the sequencing of the words.

In addition, the ACC is involved in the high-level monitoring of speech errors (along with the SMA) through auditory connections. There seems to be a lack of such monitoring among clutterers who seem to be unaware of their disfluency as pointed out in Per Alm’s chapter.

Stutterers, on the other hand, may not have this problem. Instead they seem to be able to monitor their speech errors and are aware of their disfluency to such an extent that they ultimately regress into secondary stutttering patterns such blocking, etc. However for stutterers, the auditory aspects, feedback, and timing associated with this monitoring may be deficient as shown in a number brain imaging studies.

The ACC, the pre-SMA, and the SMA represent an assembly center for spontaneous speech that retrieves the linguistic components from the left lateral cortex regions such as Broaca’s and Wernicke’s areas. And as discussed in a previous post, the selection of a single word from a number of competing alternatives is further facilitated by the basal ganglia through a winner-take-all function.

Brain imaging studies have shown that stutterers exhibit deficiencies in the left cortical areas discussed above, while the right hemisphere tends to be overactive. However, the greater right hemisphere activity may merely reflect a compensatory mechanism due to the left hemisphere deficiencies. Similarly, it is unclear at this point as to whether: (1) the activity level deficits in these left cortical regions are a secondary effect of dysregulation of the basal ganglia circuits, (2) the left cortical activity level deficits cause dysregulation of the basal ganglia, or (3) deficiencies in both these areas of the brain simultaneously contribute to disfluency.

* This figure, taken from Per Alm’s chapter, represents the medial portion of the brain’s left hemisphere, i.e., split in half from front to back; the figure in the Part 1 post represents the exterior of the left hemisphere of the brain.

** More specifically, that part of the ACC identified as the cognitive ACC.

No comments: