Mental Work and Brain Work

The notion of mental workload as a reflection of how hard one's mind is working at any given moment is intuitively appealing. Given that the mind is a function of the brain, it follows that mental workload should be associated with brain work. How can brain work be assessed? Various candidate measures have been proposed over the years. The work of Sir Charles Sherrington, the great nineteenth century physiologist, suggested that an answer could be found in the regulation of the blood supply of the brain. Sherring-ton demonstrated that there is a close coupling between the electrical activity of neuronal cells, the energy demands of the associated cellular processes, and regional blood flow in the brain. His pioneering work suggested that, if mental activity results in increased neuronal response in localized regions of the brain, then in principle it should be possible to measure mental workload by assessing regional cerebral metabolism and blood flow.

Autoradiographic studies conducted in animals have confirmed Sherrington's principle for the regulation of brain blood flow and its coupling to neuronal activity and energy usage. But it would take several years before sensitive techniques were developed for measuring regional brain blood flow in humans. An early development was the invention of the xenon-133 (Xe-133) method for assessing regional cortical changes in brain blood flow and glucose metabolism. Injection of the radioactively tagged xenon gas, which passes freely across the blood-brain barrier, into human patient volunteers showed that the performance of various mental tasks (such as mental arithmetic of the type mentioned at the beginning of this article) led to increased metabolic activity in specific cortical regions.

This was the first demonstration of a link between mental work and brain work in humans. However, the Xe-133 technique was too invasive to be used routinely in normal human subjects. The development of positron emission tomography (PET) paved the way for less invasive measurement of regional cerebral metabolism and blood flow. PET is an adaptation of autoradiographic techniques originally developed for measuring blood flow in animals. Regional cerebral glucose metabolism can be determined noninvasively using PET and radioactively labeled glucose (18-fluorodeoxyglucose), whereas regional cerebral blood flow may be assessed with PET and radioactively labeled oxygen (O-15) in water. PET was also more accurate than the older methods in localizing the specific cortical regions activated by cognitive task demands. Nevertheless, the spatial resolution of PET, particularly in individual subjects, had room for improvement. Furthermore, the need for ionizing radiation, although safe when used within exposure limits, was an impediment against frequent use in studies with normal human subjects. The development of functional magnetic resonance imaging (fMRI) overcame both of these limitations. fMRI provides noninvasive, high-resolution assessment of regional cerebral blood flow. PET and fMRI studies of mental workload are discussed further later.

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