Internal versus Sensorimotor Mapping

The idea that lateral nonprimary motor areas (such as PM) underlie external control of action, whereas medial areas (such as SMA) subserve internal control, has a link to related ideas concerning the motor functions of the basal ganglia and the cerebellum. It has been proposed that the cerebellum functions preferentially in externally guided action, whereas the basal ganglia controls mainly internally guided action. However, the functions of these structures are more complex than can be captured by such a simple dichotomy. The cerebellum participates in movements based on internal as well as external cues, and the basal ganglia plays a role in movements modulated by sensory as well as nonsensory information. Furthermore, contrary to earlier views, influences from both cerebellum and basal ganglia converge on both medial and lateral nonprimary motor areas.

How can these differing views be reconciled? Some progress can be made by recognizing functional subdivisions with both the basal ganglia and the cerebellum and by eschewing all-or-none dichotomies. Cells in caudal parts of one deep cerebellar nucleus, the dentate nucleus, have a preference for movements based on visual inputs (externally guided action) compared to kinematically similar movements generated from memory, whereas rostral parts have fewer cells with such preferences (i.e., they are relatively nonselective). Likewise, cells in the dorsal parts of the GPi have a strong preference for memorized sequences (internally guided action), whereas the ventral parts of GPi lack such selectivity. M1 and SMA receive their largest inputs (via the thalamus) from the less selective parts of the basal ganglia and cerebellum. Accordingly, M1 and SMA play a fairly general role in motor control and lack strong specializations for either internal or external control.

In contrast, PM receives its predominant inputs from the part of the dentate nucleus more selective for externally guided movements. Accordingly, PM has a large role in externally (often visually) guided action. For example, lesions to or inactivation of dorsal PM prevent monkeys from using color (or other nonspatial visual information) to choose an action.

The internally selective (dorsal) part of GPi preferentially influences the pre-SMA, which likewise appears to be specialized for internally guided action. Cells in pre-SMA have their greatest activity for internally guided movements, including movement sequences, and show other sequence- or order-specific patterns of activity. Temporary inactivation of the pre-SMA disrupts the ability to produce a memorized (internally guided) sequence but not a visually triggered one. Brain imaging and brain lesion research also suggests the involvement of medial nonprimary motor areas in the learning of both limb- and eye-movement sequences and the "spontaneous" generation of action. In particular, pre-SMA, has been proposed to function in changing or updating motor plans based primarily on signals internal to the CNS.

Therefore, both medial and lateral parts of the motor cortex contain generalized as well as specialized components. Generalized parts include SMA (medially), and Ml (laterally). Specialized elements include the laterally situated PM, which maps sensory information onto motor outputs in a highly flexible manner, and the medially situated pre-SMA, which along with SMA maps a vast array of memorized and other nonsensory information onto motor output. A medial specialization for internal information accords with the role recently proposed for the rostral cingulate motor area in linking incentive magnitude to the selection of action, as occurs when one must choose an action based on some estimation of preferred outcome.

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