Localized, task-induced decreases in cerebral blood flow are a frequent finding in functional brain imaging research but remain poorly understood. One account of these phenomena postulates processes ongoing during conscious, resting states that are interrupted or inhibited by task performance. Psychological evidence suggests that conscious humans are engaged almost continuously in adaptive processes involving semantic knowledge retrieval, representation in awareness, and directed manipulation of represented knowledge for organization, problem-solving, and planning. If interruption of such "conceptual" processes accounts for task-induced deactivation, tasks that also engage these conceptual processes should not cause deactivation. Furthermore, comparisons between conceptual and nonconceptual tasks should show activation during conceptual tasks of the same brain areas that are "deactivated" relative to rest.
To test this model, functional magnetic resonance imaging data were
acquired during a resting state, a perceptual task, and a semantic
retrieval task. A network of left-hemisphere polymodal cortical
regions showed higher signal values during the resting state than
during the perceptual task but equal values during the resting and
semantic conditions. This result is consistent with the proposal that
perceptual tasks interrupt processes ongoing during rest that involve
many of the same brain areas engaged during semantic retrieval. As
further evidence for this model, the same network of brain areas was
activated in two direct comparisons between semantic and perceptual
processing tasks. This same "conceptual processing" network was also
identified in several previous studies that contrasted semantic and
perceptual tasks or resting and active states. The model proposed here
offers a unified account of these findings and may help to explain
several unanticipated results from prior studies of semantic
processing.