Cognition and the Brain

Abstract

Abstract : Magnetic fields associated with spontaneous neuronal activity of cerebral cortex are shown to be locally suppressed when an area of the brain engages in a cognitive function. Suppression occurs in visual cortex when the image of an object is compared with a memory set of objects previously seen, or with the same object rotated. Suppression occurs in auditory cortex when memory of a tone is compared with a memory set of tones. Suppression occurs first over a visual cortex and subsequently over the anterior temporal area when a subject responds to a displayed word by seeking a word that rhymes with it. Significant correlations are found between the timing of cortical suppression and classic behavioral studies of reaction times. It is concluded that regional changes in cortical spontaneous activity are meaningfully related to memory scanning, image transformations, and silent speech. A computational procedure, called the minimum-norm least-square (MNLS) estimate, has been developed to provide a unique solution for the magnetic inverse problem. With this algorithm, the distribution of intracellular current across the surface of cerebral cortex can be deduced from the magnetic field pattern that it produces across the scalp. This approach has been generalized to provide a unique estimate for the distribution of time-average current power, obtained from the average field power. It can also be applied to determine the pattern of current power suppression when the subject is engaged in a cognitive task.