Interrupting the "stream of consciousness": an fMRI investigation.

In functional neuroimaging, a local decrease in blood flow during an active task, relative to a "resting" baseline, is referred to as task-induced deactivation (TID). TID may occur when resources shift from ongoing, internally generated processing typical of "resting" states to processing required by an exogenous task. We previously found specific brain regions in which TID increased as task processing demands increased. When engaged in an exogenous cognitive task, reallocation of resources from areas involved in internal processing should result in suspension of that processing. Self-reported thought content has been used as an indicator of the extent of internal processing activity. We investigated the relationship between TID and task-unrelated thought (TUT) frequency using an auditory target detection task with seven levels of task difficulty. At varied intervals during task performance, subjects indicated whether they were experiencing a TUT. We expected TUT frequency to decrease as task demands increased and for this pattern to correlate with TID magnitude across conditions. Generally, fewer TUTs were reported during difficult task conditions than during easier conditions. As TID magnitude increased across task conditions, the frequency of TUTs declined (r = 0.90, P = 0.005). Four left hemisphere regions (posterior parieto-occipital cortex, anterior cingulate gyrus, fusiform gyrus, and middle frontal gyrus) showed strong relationships between TUTs and TID (r > 0.79, P < 0.05 corrected). As these regions have been implicated in semantic processing and self-referential thought, the findings support the suspension of internal cognitive processing as one mechanism for TID.