Abstract
Two lines of evidence indicate that there exists a reciprocal inhibitory relationship between opposed brain networks.
First, most attention-demanding cognitive tasks activate a stereotypical set of brain areas, known as the
task-positive network and simultaneously deactivate a different set of brain regions, commonly referred to as the
task negative or defaultmode network. Second, functional connectivity analyses show that these same opposed
networks are anti-correlated in the resting state. Wehypothesize that these reciprocally inhibitory effects reflect
two incompatible cognitive modes, each of which may be directed towards understanding the external world.
Thus, engaging onemode activates one set of regions and suppresses activity in the other.Wetest this hypothesis
by identifying two types of problem-solving task which, on the basis of prior work, have been consistently associated
with the task positive and task negative regions: tasks requiring social cognition, i.e., reasoning about the
mental states of other persons, and tasks requiring physical cognition, i.e., reasoning about the causal/mechanical
properties of inanimate objects. Social and mechanical reasoning tasks were presented to neurologically normal
participants during fMRI. Each task type was presented using both text and video clips. Regardless of presentation
modality, we observed clear evidence of reciprocal suppression: social tasks deactivated regions associated
with mechanical reasoning and mechanical tasks deactivated regions associated with social reasoning. These
findings are not explained by self-referential processes, task engagement, mental simulation,mental time travel
or external vs. internal attention, all factors previously hypothesized to explain default mode network activity.
Analyses of resting state data revealed a close match between the regions our tasks identified as reciprocally
inhibitory and regions of maximal anti-correlation in the resting state. These results indicate the reciprocal inhibition
is not attributable to constraints inherent in the tasks, but is neural in origin. Hence, there is a physiological
constraint on our ability to simultaneously engage two distinct cognitive modes. Furtherwork is needed tomore
precisely characterize these opposing cognitive domains.