FUNCTIONAL BRAIN: MAPPING AND IT’S GROWING IMPACT ON NEUROSURGERY￼
-Sai Lavanya Patnala, Intern, Apollo Institute of Medical Sciences and Research, Hyderabad
“The human brain is an incredible pattern-matching machine”-Jeff Bezos
Defining functional brain activity and how the brain implements given functions are both arduous tasks.
Function can be defined as the ability to perform a given cognitive or physiological task. Insofar as individuals’ behavioral performance results from brain properties, functional activity refers to both behavior and neural structures reflecting two complementary goals: understanding how brain anatomical structure and dynamics control function, and how task performance’s action produces functional brain subdivisions.
WHAT IS THE FUNCTIONAL BRAIN?
The human brain continues to develop for some time after birth, providing an opportunity for experience to influence neural development. In the first few years after birth, both brain volume and cognitive function increase markedly.
Although most neurons are in place by birth, synaptogenesis occurs at a high rate during the first year of life, and the number of synapses peaks during this period at ∼150% of adult levels. Brain activity patterns change during this time and most myelination is postnatal. Such changes occur at different times for different brain areas.
Three theories of functional brain development are proposed. The maturational perspective states that cognitive abilities develop as the cortical areas mediating them mature. The interactive specialization approach suggests that cognitive abilities develop as the networks of cortical areas that mediate them develop appropriate interactions. The skill-learning hypothesis proposes that certain regions will be active during the development of skills in infants, but that other regions will be active once the skill has been learned (as in adult motor learning).
HOW IS THE FUNCTIONAL BRAIN STUDIED?
Functional brain mapping has evolved from the idea that the brain consists of functionally specialized macroscopic regions. In early neuroimaging experiments using positron emission tomography, brain activity was measured at a spatial resolution in the centimeter range. In the classical approach to functional brain mapping, therefore, the experiment is designed to activate a functional region as a whole.
Direct cortical stimulation (DCS), and the intracarotid amytal (IAT) or Wada test are often considered the gold standard procedures for functional mapping and language lateralization respectively. Both procedures involve deactivation, in which a brain region, either localized in the case of DCS, or nearly hemispheric in the case of IAT, is taken off line, and the patient’s neurologic function tested. Although these procedures have proven to be effective, they are highly invasive.
Presently, the most commonly used functional mapping procedure for surgical planning is fMRI, a functional mapping technique that measures the relative changes in oxygenated and deoxygenated hemoglobin, and thus blood flow, as a surrogate for neuronal activity. Being completely non-invasive and deployable on many clinical MRI scanners, its clinical role has rapidly expanded.
Physicians perform fMRI to:
- examine the functional anatomy of the brain.
- determine which part of the brain is handling critical functions such as thought, speech, movement and sensation, which is called brain mapping.
- help assess the effects of stroke, trauma, or degenerative disease (such as Alzheimer’s) on brain function.
- monitor the growth and function of brain tumors.
- guide the planning of surgery, radiation therapy, or other invasive treatments for the brain.
During functional MRI (fMRI) or magneto-encephalography (MEG), patients are asked to perform a task such as a language, visual, or movement paradigm, while changes in blood flow, metabolism, or electric activity in activated brain regions is measured. Although this general approach can demonstrate all functional brain regions that are involved in the execution of a particular task, it cannot differentiate between brain regions that are essential for execution of the task and those that are merely playing a supportive role.
Thus, the recent development of less invasive mapping techniques has provided an appealing alternative or adjunct for many neurosurgeons.
SURGERY AND FUNCTIONAL BRAIN MAPPING
Functional neurosurgery involves precise surgical targeting of anatomic structures in order to modulate neurologic function. The ultimate aim is to improve the symptoms and quality of life of patients suffering from chronic neurologic disorders; this demands minimal risk of inflicting morbidity and mortality. The technologic advances of magnetic resonance imaging (MRI) and deep brain stimulation (DBS) are driving the resurgence of functional neurosurgery, making it one of the most rapidly expanding neurosurgical fields.
The use of deep brain stimulation (DBS) to intervene directly in pathological neural circuits has changed the way that brain disorders are treated and understood. DBS is a neurosurgical procedure that involves the implantation of electrodes into specific targets within the brain and the delivery of constant or intermittent electricity from an implanted battery source. As a scientific tool, DBS can be used to investigate the physiological underpinnings of brain dysfunction, which enables identification and correction of pathological neuronal signatures and helps to drive technological innovation and enhance safety and clinical outcomes.
The development of numerous functional mapping techniques now gives neurosurgeons many options for pre-operative planning. Integrating functional and anatomical data can inform patient selection and surgical planning and makes functional mapping much more accessible. Successful implementation of functional image guided procedures requires efficient interactions between neurosurgeon, neurologist, radiologist, neuropsychologist, and others but promises to enhance the care of our patients.
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