Direct electrical stimulation during awake surgery: contribution to the understanding of functional anatomy ant its reorganization

Awake brain surgery is a surgical procedure aimed at maximizing lesion resection while preserving brain functions essential for human life. During awake surgery, cortical and subcortical areas are stimulated electrically while the patient performs functional tasks, including language, motor, sensory tasks, and other higher brain functions to assess the functionality of the stimulated area. When positive (impaired) responses are elicited by direct electrical stimulation (DES), the stimulated area is revealed to be functional, whereas normal responses induced by DES indicate that the stimulated area is not functional regarding the function assessed. There are two types of positive responses: functional activation and functional inhibition. Functional activation represents neurological reactions at rest in response to DES in primary areas, including motor, sensory, and visual areas. Functional inhibition means that functions are inhibited during task performance in response to DES in the association areas.
DES during awake surgery provides valuable research opportunities to understand functional neuroanatomy and its reorganization in humans. An advantage of DES during awake surgery is the ability to determine causality between brain areas and map functions with high spatial resolution and sensitivity in real time. Using this technique, previous studies have revealed insights that would be difficult to obtain using noninvasive methods.
When the brain is damaged, functional reorganization, in which brain functions are relocated to regions outside the damaged area, can sometimes occur to restore neurological or cognitive functions. Owing to reorganization, brain functions are often normal despite the invasion of a lesion into areas involved in specific functions. Reorganization is an important phenomenon to consider when performing surgery for brain tumors because if certain areas do not have functions essential for human living, the region can be resected safely. During awake surgery, we can also observe how cortical or subcortical localization changes from the original anatomical localization with high spatial resolution, even in perilesional areas. In this context, we have found using DES during awake surgery that functional reorganization occurs under specific conditions rather than randomly.
Here, I will present some studies on functional neuroanatomy and its reorganization in motor, language, visuospatial cognition, and other higher brain functions, and show how we can use DES for research, as well as clinical purposes.