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Microstimulation of the supplementary motor area (SMA) in the awake monkey
Abstract:
The supplementary motor area of three Macaca fascicularis was mapped using intracortical microstimulation (ICMS). Both forelimb and hindlimb movements were evoked using currents of 30 microA or less. However, thresholds for evoking movements were higher than those in the primary motor cortex. Proximal motor effects predominated, but distal joint movements were also elicited. Forelimb points were clustered in mesial cortex of area 6, anterior to the precentral hindlimb and tail region. Distal joint effects were located deep in the cortex, intermingled with proximal effects. Hindlimb response which were less spatially localized, were found both ventral to the forelimb area, in the dorsal bank of the cingulate sulcus, and in mesial cortex, well anterior to area 4. No movements of facial muscles were elicited. Injections of HRP were made into the spinal cord at the cervical level in two animals and the lumbar level in the third one. An area of labelled cells was seen in mesial area 6 which corresponded closely to the region from which ICMS effects were elicited. No movements were evoked from the anterior portions of the fundal region of the cingulate sulcus which were also labelled.
Insights
This study mapped the supplementary motor area in macaques using intracortical microstimulation. Findings reveal specific forelimb and hindlimb motor representations, distinct from primary motor cortex.
Area of Science:
- Neuroscience
- Motor Control
- Primate Neurophysiology
Background:
- The supplementary motor area (SMA) is crucial for motor planning and execution.
- Understanding SMA organization in primates is essential for comparative neuroanatomy.
- Previous studies have provided limited detailed mapping of the SMA in Macaca fascicularis.
Purpose of the Study:
- To precisely map the functional organization of the supplementary motor area in Macaca fascicularis.
- To correlate microstimulation-evoked movements with anatomical locations within the SMA.
- To investigate the somatotopic organization of motor representations within the SMA.
Main Methods:
- Intracortical microstimulation (ICMS) was employed to evoke movements in three Macaca fascicularis.
- Horseradish peroxidase (HRP) injections into the spinal cord were used for anatomical tracing.
- Analysis focused on movement thresholds, evoked muscle groups, and anatomical localization of responsive cortical sites.
Main Results:
- Forelimb and hindlimb movements were evoked with low currents (≤30 microA), but thresholds were higher than in the primary motor cortex.
- Proximal movements predominated, with distal joint movements also elicited, particularly deep within the cortex.
- Forelimb representation was found in the mesial cortex of area 6, anterior to hindlimb/tail regions; hindlimb responses were less localized.
Conclusions:
- The study successfully mapped the SMA in Macaca fascicularis, demonstrating distinct, albeit less localized, forelimb and hindlimb representations.
- Anatomical tracing confirmed the correspondence between ICMS-evoked motor regions and spinal cord projection areas within mesial area 6.
- No facial movements were elicited, and certain cingulate sulcus regions, despite HRP labeling, did not yield motor responses.