The motor homunculus of the primary motor cortex was defined in the 1930s by the pioneering work of Penfield and colleagues. They demonstrated that the direct cortical stimulation of the precentral gyrus elicited specific motor responses organized in a continuous representation, from head to toe.
Since then, Penfield’s motor homunculus has been a hallmark in neuroscience, strongly influencing clinical and conceptual neurological thinking.
In this article, Gordon and colleagues present strong evidence suggesting that the motor homunculus representation shall be redefined.
They combined large samples of multimodal functional MRI data, namely tasks and resting-state functional connectivity, and demonstrated that inter-effector regions intermingle the effector-specific (foot, hand and mouth) areas. These inter-effector regions are not activated by the motor activation of discrete body parts but in whole-body action planning and axial body movements. They have distinct functional connectivity with brain regions whose activity precedes voluntary movements, such as the dorsal anterior cingulate cortex and the supplementary motor area.
This work also proposes that a concentric model is more appropriate than the linear representation of Penfield’s homunculus. The analyzed MRI data showed a distal-proximal organization of the motor areas, for example, with toes in the center surrounded by ankle-knee-hip instead of toes to face disposition.
The solid update of the motor homunculus proposed in this article may impact our understanding of cortical mapping, motor integration, and recovery after brain lesions.
Key Points:
- The specific effector areas of the motor homunculus are intermingled by inter-effector regions, activated in whole-body action planning and axial body movements.
- The inter-effector regions have a particular pattern of functional connectivity with the cingulo-opercular network, which is involved in motor planning.
- The motor homunculus has a concentric organization rather than a linear one.
References:
- Penfield, W. & Boldrey, E. Somatic motor and sensory representation in the cerebral cortex of man as studied by electrical stimulation. Brain 60, 389-443 (1937).
- Dosenbach, N. U. F. et al. A core system for the implementation of task sets. Neuron 50, 799–812 (2006).
- Mitra, A., Snyder, A. Z., Hacker, C. D. & Raichle, M. E. Lag structure in resting-state fMRI. J. Neurophysiol. 111, 2374–2391 (2014).
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