Functional heterogeneity of the supplementary motor area

Abstract

Background and purpose: The purpose of this study was to assess supplementary motor area (SMA) activation during motor, sensory, word generation, listening comprehension, and working memory tasks by using functional MR imaging (fMRI). Human supplementary motor area (SMA) has been shown to play roles in motor control and other various functions such as sensory, speech expression, and memory. However, topographical localizations of these functions in the SMA remain unclear. The purpose of this study was to assess SMA activation during motor, sensory, word generation, listening comprehension, and working memory tasks by using functional MR imaging (fMRI).

Methods: Sixteen healthy right-handed subjects (nine men and seven women) were imaged on a Siemens 1.5T system. Whole-brain functional maps were acquired by using blood oxygenation level-dependent echo-planar imaging sequences in the axial plane. Each paradigm consisted of five epochs of activation versus the control condition. The activation tasks consisted of left-finger complex movement, heat sensory stimulation of the left hand, word generation, listening comprehension, and working memory. The reference function was a boxcar waveform. Activation maps were thresholded at an uncorrected P = .0001. The thresholded activation maps were placed into MNI (Montreal Neurologic Institute) stereotactic coordinates, and the anatomic localization of activation within the SMA was compared across tasks.

Results: SMA activation was observed in 16 volunteers for the motor task, 11 for the sensory task, 15 for the word generation task, five for the listening comprehension task, and 15 for the working memory task. Although not statistically significant, qualitative differences in the location of activation within the SMA were present by task. The rostral aspects of the SMA tended to activate during word generation and working memory tasks, and the caudal aspect of the SMA tended to activate during the motor and sensory tasks. Right (contralateral) SMA activation was observed during the motor and sensory tasks, and left SMA activation during the word generation and memory tasks.

Conclusion: Our results suggest that SMA is involved in a variety of functional tasks, including motor, sensory, word generation, and working memory. Some are tasks that are traditionally associated with this area (such as motor and sensory), and others are not (such as word generation and working memory). Qualitatively, the anterior and posterior portions of the SMA appeared to be engaged by different types of tasks.

Fig 1.

Functional MR images showing activation in the supplementary motor area during motor (A), sensory (B), word generation (C), working memory (D), and listening comprehension (E) tasks in volunteer 13. Volume-rendered functional MR images showing activation in the SMA for the entire group are illustrated for the motor (F), sensory (G), word generation (H), working memory (I), and listening comprehension (J) tasks.

Fig 2.

Sagittal functional MR group maps of complex motor, hot sensory, listening comprehension, word generation, and working memory tasks. Functional MR images showing SMA activation during complex motor (red), heat sensory (yellow), word generation (green), and working memory (blue) tasks. The rostral aspect of the SMA tended to activate during word generation and working memory tasks, and the caudal aspect of the SMA during motor and sensory tasks. These differences, however, did not reach statistical significance.

Fig 3.

Axial functional MR group maps of complex motor (red), heat sensory (yellow), listening comprehension (magenta), word generation (green), and working memory (blue) tasks. Right (contralateral) SMA activation was observed during motor, and sensory tasks, and left SMA activation during word generation and memory tasks.