Distinguishing excitatory and inhibitory transcallosal interactions.

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The human brain is divided into two halves, or hemispheres, which interact with each other to process information and control the body. A large amount of research has characterized the ways in which the two hemispheres are specialized. For example, the left hemisphere is dominant for language functions, while the right is superior at spatial judgments. However, most human cognition requires contributions from both hemispheres, and they communicate with each other in ways that are partially cooperative and partially competitive. Understanding how this communication happens between hemispheres is critical for the development of new methods to modulate brain activity in beneficial ways, such as through noninvasive brain stimulation. Recently, many new methods have been developed to detect the brain's electrical activity with a machine called magnetoencephalography (MEG). MEG records the electromagnetic waves generated by neurons with high spatial and temporal precision. By analyzing the activity in two brain regions simultaneously, researchers can examine the degree to which information is transferred between them at different times, and in which direction. Our laboratory's studies have suggested that it may be possible to distinguish cooperative and competitive interactions, which may manifest in different brain signals. To validate the analysis of these signals, we will take advantage of some well-known asymmetries of the brain, in which information is transferred from brain hemisphere to the other (a cooperative interaction), and in which one hemisphere suppresses the other (competition). For example, the left motor cortex controls the right side of the body, and the right motor cortex controls the left side of the body. When someone moves their right hand, their left motor cortex sends an excitatory signal to the right hand via the spinal cord, but also sends an inhibitory signal to the right motor cortex to prevent it from interfering. By measuring electrical activity during hand movements, and comparing it with other known measures of competition, we will validate the use of new techniques to detect competitive interactions. Another asymmetry occurs with peripheral vision. The left side of the brain receives visual information from the right visual field, and vice versa. Because the left hemisphere is dominant for language, information about words presented in the left visual field (to the right hemisphere) must be transferred to the left hemisphere, but the reverse occurs much less. On the other hand, the right hemisphere is dominant for making decisions about faces, so information about faces presented to the right visual field (left hemisphere) is differentially routed to the right hemisphere. By comparing interactions for words and faces, we will validate the use of new techniques to detect asymmetrical transfer of information between the hemispheres.