Using a newly released method to analyze functional magnetic resonance imaging (fMRI), Northwestern University researchers have demonstrated that the interconnections between different parts of the brain are dynamic and not static. This and other findings answer longstanding debates about how brain networks operate to solve different cognitive tasks. They are presented in the current (June 1) issue of the Journal of Neuroscience.
Equally important, the researchers discovered that the brain region that performed the integration of information shifted depending on the task their subjects performed. In this study, the subjects were assigned two language tasks. In both, subjects were asked to read individual words and then make a spelling or rhyming judgment.
"We found that one network takes different configurations depending on the goal of the task," said Tali Bitan, primary author of "Shifts of Effective Connectivity Within a Language Network during Rhyming and Spelling."
A post-doctoral fellow in the department of communication sciences and disorders, Bitan worked with Associate Professor James Booth of the same department and M-Marsel Mesulam, director of the Cognitive Neurology and Alzheimer's Disease Center in Northwestern's Feinberg School of Medicine.
Mesulam, who was among the first scientists to predict the existence of convergence zones within interconnected brain networks, said the study presents "the clearest and most convincing evidence to date" of the dynamics in effective connectivity.
To better understand dynamic effective connectivity, Mesulam compares the brain networks to a network of highways connecting different parts of a city. The highway is static. No matter how heavy the traffic load, it always has the same number of lanes. In the brain, there is a dynamic change that allows certain pathways to preferentially facilitate the demands of a given cognitive task. The brain highway in effect "adds lanes" to accommodate the requirements of the particular task.
Depending on the goal of the task -- whether subjects were asked to make an orthographic (spelling) judgment or a phonological (rhyming) judgment – the Northwestern researchers found that different convergence zones in the network were involved in the task.
"The existence and the identity of convergence zones --areas in which information from multiple sources meets in the brain -- have been debated since they were proposed in the late 20th century," said Bitan. "Now, with new techniques to analyze brain imaging data, we can examine the specific role played by different brain regions in the network that are required for any cognitive task. These techniques examining effective connectivity enable us to learn how the brain changes its interconnectivity according to the task at hand."
The Northwestern researchers also propose to explain the role of each brain region as it interacts within a complex network to achieve a specific cognitive goal.
The conventional method for analyzing fMRI data, which can only show which brain regions are active in a given task, showed two brain regions that were specifically active for each of the studied tasks: the lateral temporal cortex (LTC) for the rhyming task and the intraparietal sulcus (IPS) for the spelling task.