Stuttering and the Dual Premotor System

Although we discussed the medial and lateral premotor systems separately, they are part of an integrated motor function system known as the dual premotor system as shown in Figure 1. Loop 1 characterizes the medial premotor system, while loop 2 represents the lateral system.

The planning and initiation stages of speech originate in the cerebral cortex and the signals then pass through the basal ganglia back to the cerebral cortex via the supplementary motor area (SMA; not shown). The thalamus regulates the messaging to the SMA. This is the upstream loop for self-initiated, internally cued speaking situations.

For non-stutterers, the segments of motor activity (i.e., syllables), then pass unimpeded through the SMA and various other premotor areas, eventually reaching the cerebellum, which is a part of the downstream loop. On the other hand, stutterers experience impaired signaling in the area of the brain associated with the basal ganglia/SMA and neuronal signals are impeded from reaching the cerebellum.

Note that the inputs to the cerebellum from various regions of the cerebral cortex are more limited than the cerebral inputs to the basal ganglia as indicated by the smaller box within the larger one that denotes the cerebral cortex. The cerebellum promotes coordination and fine motor control of movement by influencing the output of brain motor systems to the peripheral nervous system (not shown in Fig. 1). To achieve this fine motor control, the cerebellum may be engaged in feedback control, going through several iterations in loop 2, modulated by sensory or other input.

As we indicated in the previous post, for certain activities such as chorus speaking, singing, altered auditory feedback, etc., loop 1 may be preempted, allowing the speaker to utilize only loop 2 which does not have the impairments associated with loop 1. Finally, note that there are limbic inputs (related to emotions) to loop 1, implying that emotional factors may further influence (perhaps negatively) the activity of this loop.