Interpretations of Brain Activity Based on Cognitive Theories Fail to Recognize Background Neuronal Firing
When the brain is stimulated, functional imaging results are misinterpreted by neglecting the resting brain neurotransmitter activity, a study by a Yale researcher concludes.
“There is an assumption made in the use of PET scans and functional magnetic resonance imaging (fMRI) that the brain works only when you give it a task to do,” said Robert Shulman, Sterling Professor of Molecular Biophysics and Biochemistry. “What I show here is that the brain works all the time. The brain at rest is doing the same sort of neuronal firing as it does when stimulated by a task. Brain activity slightly increases when a task is performed and those increases are generally assumed to measure activity.”
Shulman, in a study published in the January issue of The American Journal of Psychiatry, said what this means can be seen by considering that the brain’s signal at rest is, for example, 100. Once it undertakes a task, the brain’s activity level rises by a small amount, say, from 100 to 101. “When we look at it pixel by pixel and subtract the activity of the brain at rest from that during a task as is presently done, you would get this increment of one in certain areas of the brain,” he said.
“The localized nature of imaging increments is accommodated readily by a conception of the brain, based on cognitive psychology or cognitive neuroscience, in which individual regions respond, like computer modules, to components of tasks selectively stimulated,” Shulman said. “In this way, images are interpreted and experiments are planned in terms of a theory of mind, and are designed to extend the theory rather than to test it.”
In his article, Shulman reviewed recent research done with colleagues at Yale which enabled imaging results to be interpreted in terms of a specific neuronal activity, the release of the neurotransmitted glutamate. The glutamate fluxes showed that the resting brain, in the absence of explicit external activity, was actively transmitting information.
Shulman said that his study offers hope of bridging two major divisions in psychiatry – those scientists who have a psychiatric view of the mind and those with a neuroscientific view.
“The ability to quantitate neurotransmitter activity both in the presence and absence of stimulation highlights and provides a criticism of the psychological assumptions behind the standard interpretation of images,” he said. “Instead of allowing resting activity to be disregarded, as it is when the brain is considered as a set of localized computers, it shows that resting activity is required for function and suggests ways in which more holistic theories of mind are supported by the imaging experiments.”