1.1 Introduction

The practice of intraoperative brain mapping has become a standard of care across multiple subspecialties within neurosurgery. ^{1 ,​ 2 ,​ 3 ,​ 4} It allows for safe delineation of eloquent cortex in awake and asleep patients intraoperatively and provides ways to monitor electrical function and networks in epilepsy patients in and out of the operating room. The multitude of devices available to the contemporary neurosurgeon for brain mapping, including bipolar and unipolar stimulation, and strip and grid electrodes, allow for a broadly stocked armamentarium.

Cartography relies on a general understanding of the environment in question, and brain mapping developed only after centuries of painstaking work elucidating the gross and microanatomic structures of the brain, the role of specialized cells in creating and maintaining neural networks, and the role of functional localization in organizing the brain into interlinked eloquent regions. ²

1.2 Neuroanatomic Basis of Brain Mapping

At the heart of intraoperative neural mapping is the concept of functional localization, which relies on a thorough understanding of the micro- and macrostructure of the human brain. The work of Camille Golgi and Santiago Ramon y Cajal played a pivotal role in delineating individual neural cell characteristics and localizing specific cell types to certain anatomic regions. ⁵ Foster and Sherrington further expanded on this and coined the term “synapse” in describing how certain neurons communicate with one another. ⁶

From the microscopic staining of individual neurons, neuroanatomists began to understand the functional implications of these specialized cell clusters, leading to descriptions of motor and sensory cortices, as well as deep white matter tracts. The presence of white matter tracts was described by Vesalius in his anatomic dissections, and further delineated by Willis, with some of the preeminent dissections done by Josef Klingler. ⁷ The functional implications of these tracts were elucidated with Cajal’s studies on neural connections. ⁸ Clinical observations coupled with intraoperative and postmortem examination of the brain allowed surgeons and researchers to begin the process of functional localization. Paul Broca offered multiple lectures and publications describing the location of injury in patients with deficits in speech production, which have been recently revalidated using magnetic resonance imaging (MRI) of the particular patients’ brains, although the involved area was found to extend beyond the bounds Broca noted in his studies. ⁹ Wernicke published the results of his case series of receptive aphasia, ¹⁰ localizing the sensory component of language function.

Fritsch and Hitzig were credited with the first use of intraoperative electrical stimulation of the cortex, using bipolar electrodes and galvanic current during canine motor mapping experiments. ¹¹ The motor mapping was first used in the human brain a few years later. ^{1 ,​ 12}

Hitzig’s canine experiments led him to theorize that the motor cortex remained anterior to the central sulcus. However, experiments by Horsley and Ferrier demonstrated motor responses with stimulation of the postcentral gyrus, and the division of the cortex into motor and sensory regions was bitterly contested, with Sir Victor Horsley maintaining the motor–sensory cortex was intertwined along the central sulcus, and studies by Sherrington and Cushing demonstrating separate sensory and motor cortices divided by the central sulcus. ^{1 ,​ 13} One of Sherrington’s students, Alfred Campbell, combined information from the cytoarchitecture of the pre- and postcentral gyri with cortical stimulation results, and concluded that the high concentration of motor-associated Betz cells in the precentral region indicated the “motor center” resided solely in this region. ¹⁴ Horsley maintained that his experimental intraoperative mapping deviated from models based on chimpanzee mapping experiments, going so far as to deliver the Linacre Lecture in 1909, titled “The Function of the So-Called Motor Cortex.” ¹⁵ This report described clinical observations in patients without intraoperative mapping, as well as intraoperative findings of a patient undergoing bipolar electrode stimulation for mapping, with subsequent resection of “motor” cortex resulting in combined sensorimotor deficit. These findings placed Horsley in direct opposition to the theories proposed by Sherrington and Cushing, with a narrow motor strip anterior to the central sulcus. This controversy was compounded further in that structures seen in chimpanzee models were notably absent in human brain studies. It was not until Penfield’s later work delineating our contemporary notion of the homunculus, as well as the work of Woolsey regarding the associated supplemental cortices, that the boundaries of motor and sensory cortices were more fully understood. ¹⁶