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Searching for a baseline: Functional imaging and the resting human brain

Abstract

Functional brain imaging in humans has revealed task-specific increases in brain activity that are associated with various mental activities. In the same studies, mysterious, task-independent decreases have also frequently been encountered, especially when the tasks of interest have been compared with a passive state, such as simple fixation or eyes closed. These decreases have raised the possibility that there might be a baseline or resting state of brain function involving a specific set of mental operations. We explore this possibility, including the manner in which we might define a baseline and the implications of such a baseline for our understanding of brain function.

Key Points

  • A fundamental aspect of scientific experimentation is the identification of a control or baseline against which the condition of interest can be compared. This has been a significant issue in cognitive neuroscience — a field of research in which functional brain-imaging techniques are used to measure changes in brain activity that are associated with specific mental operations. Many believe that, left unconstrained, brain activity would vary unpredictably. As a result, elaborately designed control tasks have often been used in functional brain-imaging experiments.

  • Evidence for the existence of a baseline level of activity in the human brain, defined in terms of its circulation and metabolism, derives from studies of the function of the normal human brain using positron emission tomography (PET) and functional magnetic resonance imaging (fMRI). Studies in normal human subjects using these techniques have consistently revealed expected task-induced increases in regional brain activity during goal-directed behaviours. These changes are commonly referred to as 'activations'.

  • From functional imaging work, it has become clear that activations represent transient, local departures from a baseline of very tight coupling between oxygen delivery to the brain (that is, blood flow) and oxygen utilization by the brain that exists uniformly across the brain. So, during a local increase in activity in the brain, oxygen consumption does not increase commensurate with the increase in blood flow. The baseline is defined as the absence of activation, which is specified in terms of the relationship between blood flow and oxygen utilization.

  • This baseline is achieved when normal subjects lie quietly, but awake, with their eyes closed, or passively view a fixation point or other simple visual stimulus.

  • Possible functions that might be present in the baseline are revealed in areas of the brain in which activity is regularly attenuated during the performance of various goal-directed actions that cause the brain to depart from its baseline.

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Acknowledgements

The National Institutes of Health and the Charles A. Dana Foundation supported this work.

Author information

Correspondence to Marcus E. Raichle.

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Glossary

LOCAL FIELD POTENTIAL

A weighted average of dendro-somatic pre- and postsynaptic currents, which might include dendritic spikes or activity of small interneurons. So, it predominantly reflects the input to and the local processing in an area, rather than output from the area. The local field potential is an important determinant of the fMRI and PET signals.

BRODMANN AREAS

(BA). Korbinian Brodmann (1868–1918) was an anatomist who divided the cerebral cortex into numbered subdivisions based on cell arrangements, types and staining properties (for example, the dorsolateral prefrontal cortex contains subdivisions including BA 44, BA 45, BA 47 and others). Modern derivatives of his maps are commonly used as the reference system for discussion of brain-imaging findings.

VEGETATIVE STATE

A disorder of consciousness wherein arousal, sleep–wake cycles, ventilation and autonomic control persist but external awareness, including all cognitive function and emotion, is abolished. It can result from a variety of causes including carbon monoxide intoxication, cardio-respiratory arrest, traumatic head injury and drug overdose.

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Figure 1: Task-independent decreases observed in functional imaging experiments.
Figure 2: Resting measurements of brain blood flow, oxygen consumption and oxygen extraction fraction.
Figure 3: Parietal cortices and conscious awareness.
Figure 4: Dynamic range of dorsal medial prefrontal cortex activity.