The split homunculus: How in vivo brain parcellation provides new perspectives on brain health

Body perception and movement control are altered in most neurological, neurodegenerative, and psychiatric diseases. The homuncular organization of the sensorimotor cortex, referring to the regular organization of neurons responsive to body movement and body touch, is therefore of fundamental relevance for brain health. Developmental changes of homuncular readouts, such as during aging and neurodegeneration, relate to mechanisms of resilience against cognitive, emotional, and behavioural decline. Recent advances in ultra-high field MRI allow the in-vivo parcellation of cortical areas. Based on mesoscale profiles, this method has been used to evidence the existence of distinct cortical fields within the homuncular map, each representing a major body part and, and borders between these fields. This topographic "split" defines new units of homuncular network organization and questions the idea that mechanisms of neurodegeneration or plasticity detected in one area of the sensorimotor cortex transfer to other areas within the topographic map. In-vivo brain parcellation that allows the definition of these units in individual patients provides a novel and unique perspective on brain health that support an individualized investigation of affected circuits.