Background
Functional neuroimaging studies generally discard the control state in comparisons with an activation state. The difference is said to be the work associated with an incremental cognitive task. To ignore the activity of a control state is then meaningful only if the brain truly is at rest in that condition. However, in principle, the restful baseline state is equal neither to a default state of absent experimentation, nor to an arbitrary control state of a given experimental comparison. Although blood flow (CBF) is the conventional marker of brain activity, oxygen consumption (CMRO2) is a better index of energy turnover. Raichle et al (PNAS, 2001) define the default mode as the state consistent with the globally average oxygen extraction (EO2). To assign labels of baseline, control or default state to experimental conditions of awake volunteers, we contrasted CBF, CMRO2 and EO2 in two PET studies.
Materials and Methods
In the first study, 12 volunteers underwent PET motor activation studies with control CBF as the first scan of the first session and control CMRO2 as the first scan of the second session. Eyes were closed with an eye-patch. In the second study, 12 subjects fixated a cross-hair during all scans. Absolute magnitudes of CMRO2 and CBF were determined with two-compartment weighted-integration and mean subtracted image volumes were converted to z-statistic volume.
Results
Subtraction of the state of eyes-closed from the state of eyes-open yielded significant bilateral CBF increases in medial frontal gyrus, insula, thalamus, anterior cingulate, superior parietal lobe, lateral front-orbital cortex, right precentral gyrus, lateral occipitotemporal gyrus, cuneus, and extrastriate visual cortex, while CMRO2 increased significantly at sites in right lingual gyrus (BA 17/18), precentral gyrus, and superior parietal lobe that did not coincide with the sites of CBF increase.
Discussions
The direction of change of EO2 from 40% is said to be the index of a functional state's relation to the default state. In the present study, however, EO2 declined at the sites of CBF increase, and increased at the separate sites of CMRO2 increase. Thus, during the control state of eyes-closed, neurons in middle frontal gyrus, occipitotemporal area, and cingulate cortex went to rest in terms of CBF, while other neurons in striate cortex and precentral gyrus went to rest in terms of CMRO2 when not in the state of eyes-open. Therefore, the condition of having closed eyes is likely to be a more inactive state and hence a baseline in most brain regions involved. In contrast, the default state of striate cortex and precentral gyrus appears to be the condition of having open eyes, while the default state of the middle frontal gyrus, occipitotemporal area, and cingulate cortex is likely to be the condition of closed eyes. We interpret this to mean that little information is transmitted from primary visual cortex to these regions in the default mode.