Supplementary Motor Area

The supplementary motor area (SMA) is located on the medial surface of the superior frontal gyrus, anterior to the leg representation of primary motor cortex. SMA plays a critical role in planning complex movement sequences, initiating voluntary movements, and coordinating bimanual actions. It is part of the motor cortical hierarchy that transforms abstract movement intentions into specific motor commands.

Function

SMA is involved in:

• Movement sequencing: Planning and executing learned sequences of movements (e.g., typing a familiar word, playing a musical passage)
• Movement initiation: Generating the internal drive to begin a voluntary movement, independent of external triggers
• Bimanual coordination: Coordinating movements of both hands simultaneously
• Speech planning: Pre-articulatory planning of speech sequences (pre-SMA)

Damage to SMA produces characteristic deficits: difficulty initiating voluntary movements (akinetic mutism in severe cases), impaired sequential movements, and reduced spontaneous speech.

BCI Relevance

SMA signals arrive before M1 execution signals, reflecting movement preparation rather than execution. This temporal precedence makes SMA recordings potentially useful for BCI applications that benefit from early prediction of movement intent. Some BCI designs incorporate SMA recordings alongside M1 to capture both planning and execution signals, potentially improving decoder performance for complex sequential tasks like typing or speech.

Pre-SMA

The pre-supplementary motor area (pre-SMA), located anterior to SMA proper, is involved in even higher-level motor planning — deciding which action to perform, inhibiting unwanted actions, and switching between motor plans. Pre-SMA is relevant to cognitive-motor BCIs that decode intention and decision-making rather than specific movement parameters.