Functional role of the supplementary and pre-supplementary motor areas

Key Points The dorsomedial frontal cortex contains a cluster of areas that are designated the supplementary motor area (SMA), the supplementary eye field (SEF) and the pre-supplementary motor area (pre-SMA). The defining functional feature of the members of this supplementary motor complex (SMC) is a marked sensitivity to various aspects of action. The anatomical features of the SMC are not homogeneous: there is a gradient of morphological and connectional change where affinity with the prefrontal and primary motor cortices changes reciprocally in the rostro–caudal plane. More-rostral regions show greater kinship with the prefrontal cortex than with the primary motor cortex; for more-caudal regions the reverse is true. The SMC is also heterogeneous neurophysiologically. The subregions of the SMC show different patterns of effector predilection and exhibit relative differences in the preponderance of cells that are sensitive to more-complex aspects of action. Compared with primary motor areas, the SMC exhibits greater sensitivity to tasks in which action contingencies are broader in range and not unambiguously specified by the immediate external environment. This contrast is illustrated by differences in SMC activity between 'self-initiated' and 'externally triggered' actions, by movement sequences, by well-learnt and poorly learnt actions, and by switching between action possibilities. Damage to the SMC disrupts behaviour in complex ways, affecting not just the commission but also the omission of actions, and the effects are broadly reflective of the neurophysiological properties of the region. This plurality of functional responses has traditionally been taken as implying a plurality of neural functions, including implementing intentions, learning and performing temporally organized actions, switching between actions, and inhibiting unwanted actions. Traditional accounts of the role of the SMC in voluntary action assume a fundamentally discrete modular architecture, with different functions assigned to macroscopically defined and functionally homogeneous regions. Thus, one function could be assigned to the SMA (for example, performing sequences) and another could be assigned to the pre-SMA (for example, changing between sequences). To explain the full range of behaviours, these discrete units must be able to switch between different functions depending on task demands — a feature we term functional pleomorphism. Functional pleomorphism is conceptually pro