New functional neuroimaging techniques are changing our understanding of the human brain, and there is now convincing evidence to move away from the classic and clinical static concepts of functional topography. In a modern neurocognitive view, functions are thought to be represented in dynamic large-scale networks. The authors review the current (limited) role of functional MR imaging in brain surgery and the possibilities of new functional MR imaging techniques for research and neurosurgical practice. A critique of current clinical gold standard techniques (electrocortical stimulation and the Wada test) is given.
Geert-Jan Rutten and Nick F. Ramsey
Kuan H. Kho, Frans S. S. Leijten, Geert-Jan Rutten, Jan Vermeulen, Peter van Rijen, and Nick F. Ramsey
✓ The Wada test is still considered the gold standard for determining the language-dominant hemisphere prior to brain surgery. The authors report on a 34-year-old right-handed woman whose Wada test results indicated that the right hemisphere was dominant for language. In contrast, functional magnetic resonance (fMR) imaging was indicative of bilaterally represented language functions. Activation in the left hemisphere demonstrated on fMR imaging was most pronounced in the Broca area. Importantly, fMR imaging results in this area were confirmed on electrocortical stimulation mapping. These contradictory findings indicated that a right hemispherre dominance for language according to the Wada test should be questioned and verified using electrocortical stimulation. Nonetheless, the question remains whether involvement of these areas in the left frontal hemisphere is critical for language, as these were spared during surgery.
Kuan H. Kho, Geert-Jan M. Rutten, Frans S. S. Leijten, Arjen van der Schaaf, Peter C. van Rijen, and Nick F. Ramsey
✓Electrocortical stimulation mapping (ESM) is the clinical standard for localizing critical sensorimotor and language functions, but other functions can be assessed with this technique as well. The authors describe an 8-year-old girl with a left frontal desmoplastic gangliocytoma and medically intractable epilepsy who underwent a chronic invasive recording using electrode grids. Prior to electrode implantation, functional magnetic resonance (fMR) imaging was performed using a research protocol that included a working memory task. The ESM procedure interfered with working memory at a dorsolateral prefrontal site as predicted by fMR imaging, but because this site was part of the epileptogenic region, it was included in the resection. Since the operation the patient has been seizure free and her overall cognitive performance has improved. Yet she shows a selective impairment in working memory tasks that has persisted for over two years, indicating that the area identified using fMR imaging and ESM was critically involved in working memory. Her performance did improve, however, suggesting that compensatory mechanisms took place. This case reveals an important and perhaps critical function of the dorsolateral prefrontal cortex. Work continues to assess the specific cognitive functions subserved by the region identified with fMR imaging and ESM.
David P. McMullen, Tessy M. Thomas, Matthew S. Fifer, Daniel N. Candrea, Francesco V. Tenore, Robert W. Nickl, Eric A. Pohlmeyer, Christopher Coogan, Luke E. Osborn, Adam Schiavi, Teresa Wojtasiewicz, Chad R. Gordon, Adam B. Cohen, Nick F. Ramsey, Wouter Schellekens, Sliman J. Bensmaia, Gabriela L. Cantarero, Pablo A. Celnik, Brock A. Wester, William S. Anderson, and Nathan E. Crone
Defining eloquent cortex intraoperatively, traditionally performed by neurosurgeons to preserve patient function, can now help target electrode implantation for restoring function. Brain-machine interfaces (BMIs) have the potential to restore upper-limb motor control to paralyzed patients but require accurate placement of recording and stimulating electrodes to enable functional control of a prosthetic limb. Beyond motor decoding from recording arrays, precise placement of stimulating electrodes in cortical areas associated with finger and fingertip sensations allows for the delivery of sensory feedback that could improve dexterous control of prosthetic hands. In this study, the authors demonstrated the use of a novel intraoperative online functional mapping (OFM) technique with high-density electrocorticography to localize finger representations in human primary somatosensory cortex. In conjunction with traditional pre- and intraoperative targeting approaches, this technique enabled accurate implantation of stimulating microelectrodes, which was confirmed by postimplantation intracortical stimulation of finger and fingertip sensations. This work demonstrates the utility of intraoperative OFM and will inform future studies of closed-loop BMIs in humans.