Guest post written by multiple neurosurgeons from the Department of Neurological Surgery at UCSF.
Philosophers, theologians, and scientists have been interested in the mechanisms responsible for human thought and behavior since the earliest days of recorded history. It’s believed that the Roman physician Galen in the second century AD was one of the first to suggest the brain as a source of mental activity. Debates about the origins of thought continued over the centuries including many false assumptions and ideas which have since been disproven. Over time, some came to adopt a localizationist view which assigned mental abilities to specific areas of the brain. It was French neurologist Paul Broca, MD and German neurologist Carl Wernicke, MD who first observed specific patterns of altered language following brain injury and stroke to the left frontal, parietal, and temporal lobes.
The human brain is comprised of nearly one billion neurons and over one trillion inter-neuronal connections. Presently the neural connections underlying many mental and cognitive processes remain largely a mystery. The truth, however, remains that central nervous system diseases such as epilepsy, gliomas, psychiatric disorders, Alzheimer’s, Parkinson’s disease, traumatic brain injury (TBI), and mood disorders alter these connections and take a tremendous toll on patients, families, and society at large. Therefore, efforts to improve our collective understanding of neural network connections in healthy and disease states will increase our understanding of multiple disease processes leading to reduced patient suffering and promoting brain health for the entire population.
The notion of brain mapping began in the 1870s when German physicians Eduard Hitzig, MD and Gustav Fritsch, MD published their findings that electrical stimulation to distinct cortical areas in dogs led to muscle contractions. Human brain mapping in the setting of neurosurgery was popularized by Wilder Penfield, MD in the 1930s. In the present day, brain mapping research encompasses the following:
- Non-invasive imaging (e.g., structural magnetic resonance imaging (MRI); voxel lesion mapping; diffusion tensor imaging (DTI); MRI, magnetoencephalography (MEG); positron emission tomography (PET); task-based functional magnetic resonance imaging (fMRI); and resting-state fMRI);
- Neurostimulation (e.g., transcranial magnetic stimulation (TMS), transcranial direct-current stimulation (tDCS); and
- Direct human brain recordings from clinical patient populations during and following neurosurgery via electrocorticography and direct electrical stimulation.
Each of these human brain mapping strategies has distinct strengths and weaknesses.
Non-invasive imaging studies uncover regions of activation within the brain while a volunteer engages in a behavioral task. This allows us to observe patterns of association for assigning function to brain regions at the broadest level.
In neurostimulation studies, researchers can utilize TMS to deliberately influence (turn on or off) groups of neurons, to observe changing patterns in behavior. Similarly, scientists can study disease states in which groups of neurons have been destroyed (for example, using voxel lesion mapping following stroke), to observe how damage to specific brain regions impacts cognitive function. While these studies offer more direct causation between specific functions and related brain regions, the nature of these methods limits interpretation of results to large regions of the brain rather than specific subpopulations of neurons.
Direct human brain recordings build on the unparalleled access neurosurgeons have to patients who at times require neural recordings of distinct brain regions for clinical purposes can, therefore, be collected in parallel for both clinical and scientific purposes. Neurosurgeon-scientists have a unique role to play in human brain mapping studies.
- In the intraoperative setting, direct electrical stimulation (DES) brain mapping during awake craniotomies permits surgeons to customize removal of brain lesions (including brain tumors, epilepsy lesions, vascular malformations) for the precise language, motor, and cognitive make-up of an individual patient. DES in clinical brain mapping, therefore, offers another perspective of correlative studies of cognition and behavior at a higher resolution than possible with non-invasive methods (such as MRI) that do not involve surgery. This is particularly true for speech and language processing.
- Electrocorticography (ECoG) and depth electrode electrophysiology recordings in the setting of epilepsy, movement disorders, and brain tumors also permit direct neural recordings with excellent resolution of both time (while a cognitive task is being performed) and space (recordings from specifically targeted populations of neurons).
Over the past several decades, neurosurgeons have contributed considerably to human brain mapping neuroscience research. Ongoing scientific training opportunities for both clinicians and neuroscientists will ensure that the next generation of investigators continues to contribute to this important field.
Editor’s Note: We encourage everyone to join the conversation online by using the hashtags #NeurosurgeryMonth and #NeurosurgeryAwarenessMonth.