16-2. Action Controllers in the Premotor Cortex

As discussed in Chapter 1, “Neuronal Circuitry: The Key to Unlocking the Brain,” a controller of a motor action (popularly now called an “action controller”) is located in the premotor cortex. It receives instructions from the anterior cingulate gyrus and supplementary motor cortex and, in turn, acts on a controlled object, which nests the primary motor cortex and the segmental motor system including its peripheral effectors (Figure 12C). Anatomically, the premotor cortex is linked to the cerebellar hemisphere and dentate nucleus forming a cerebrocerebellar loop (Figure 49). It is also linked to the parietal association cortex (see Figure 50 and following text). In an fMRI study on normal subjects, a sequential key-press task of increasing length and complexity was imposed, without changing the frequency, force, and number of single finger movements. This task involved activation of a subset of cortical areas, notably the contralateral ventral and dorsal premotor cortices, in addition to the bilateral superior parietal cortices, left inferior frontal gyrus/Broca’s area, right dentate nucleus, and left visual association cortex (Haslinger et al., 2002). The results of this study suggested the importance of premotor-cerebellar-parietal circuits for the studied movements. Another fMRI study showed that during musical performance, musically naïve control subjects displayed stronger activation than did pianists in the anterior cingulate cortex, right dorsal premotor cortex, both cerebellar hemispheres, and the right basal ganglia. Also while playing “parallel” and “mirror” bimanual movements (five fingers of two hands were paired in-phase and anti-phase, respectively), control subjects exhibited stronger signal increases than pianists within the supplementary motor cortex, bilateral cerebellar hemispheres and vermis, bilateral prefrontal cortex, left ventral premotor cortex, right anterior....Haslinger et al., 2004). These findings suggested an increased efficiency in musicians of neuronal processing within the cortical and subcortical systems controlling bimanual piano-playing movements. An fMRI study during cyclical hand tasks of spatiotemporal complexity and varying frequency also demonstrated that the dorsal premotor cortex and the cerebellum are critical sites for bimanual coordination. Subjects performed four such tasks of increasing complexity (unimanual left-right hand-, bimanual in-phase-, bimanual anti-phase-, and bimanual 90° out-of-phase movements) at four frequencies (0.9, 1.2, 1.5, 1.8 Hz). Activation in the supplementary motor area and superior parietal cortex were correlated mainly with increasing spatiotemporal complexity of the lim....Debaerea et al., 2004). The results are in accordance with the idea that the premotor cortex acts as the action controller for several types of highly skilled movements.