Dependence of the hemodynamic response to functional activation on the CO<sub>2</sub>-induced vasodilation
Bibliographic record
Abstract
Due to its potential to confound the interpretation of BOLD fMRI studies and its significance for the understanding of the biophysical mechanism of BOLD, the effect of the baseline on the activation-induced BOLD response has attracted renewed interest. For small perturbations from rest, published data have supported an additive BOLD signal model, with constant percent changes in BOLD signal irrespective of the resting CBF 1, 2. We investigated the effect of pronounced CO2 induced dilation on both BOLD and CBF responses to functional activation. A 111 mm3 3D T1-weighted GRE sequence (TR/TE of 22/10 ms) was followed by interleaved 6-slice PASL and T2-weighted GRE sequences (445 mm3; TR: 1.5 s, TE: 22/50 ms for CBF/BOLD) on a 1.5 T Siemens Sonata. Twelve volunteers (7F, 5 M; 271 yrs) performed bilateral finger tapping at low (1.5 Hz) or (3 Hz) frequency while presented with a radial yellow/blue checkerboard at low (50%) or (100%) contrast alternating with rest and uniform grey baseline in 0.5/1.5/1 min off/on/off blocks. Three levels of hypercapnia were induced by administering mixtures of CO2 and air, with the inspired CO2 of 5, 7.5 or 10% in 1/3/2 min blocks. Two functional blocks (low/high in randomized order) preceded each hypercapnia block; with another high functional block applied during either first or second half of the hypercapnic period. A reference grey matter region (GRONI) not participating in either motor or visual processing was used to correct the activation induced changes during hypercapnia periods for temporal instability in the hypercapnia induced responses. A typical set of BOLD and CBF time courses, in MC ROI, VC ROI, and GRONI, is shown in Fig. 1a and b. When controlling for inter-subject variability, the effect of hypercapnia on the activation-induced response was significant for both BOLD (p<10-6) and CBF (p<10-4). The linear fits to activation-induced responses as a function of hypercapnia-induced changes are shown in Fig. 2. In view of the slope estimates (-0.320.01 %/% for BOLD MC, VC; -0.180.02 %/% for CBF MC and -0.130.01 %/% for CBF VC), only the effect of hypercapnia on BOLD bears practical significance on this range of basal vasodilation. We observed a significant drop of activation-induced BOLD response magnitude with increasing basal flow levels, in accordance with the BOLD response models, whereby a significant drop in the basal deoxyhemoglobin concentration decreases its sensitivity to CBF increases. A very limited effect of the basal vasodilation on the relative CBF response is consistent with the existing literature 3 and testifies to the nature of CBF regulation following functional activation. These findings characterize the behavior of BOLD response for significantly elevated basal flow and describe the non-linear regime of the deoxyhemoglobin dilution model.
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How this classification was reachedexpand
Full frame machine prediction
Teacher imitationNot calibrated prevalence, not ground truth. Human validation pending. The Gemma side is a direct model label for every work in the frame, read from the title-only record. The Codex side is a classifier learned from the 10,348 direct Codex labels and calibrated to design-weighted sample rates; fields without enough sample support carry no Codex call. Candidate is the union of the two sides; consensus is their intersection. These outputs are machine_predicted_unvalidated and are not human labels.
Distilled classifier scores by category (both heads)
| Category | Codex | Gemma |
|---|---|---|
| Metaresearch | 0.000 | 0.002 |
| Meta-epidemiology (narrow) | 0.000 | 0.000 |
| Meta-epidemiology (broad) | 0.000 | 0.000 |
| Bibliometrics | 0.000 | 0.000 |
| Science and technology studies | 0.000 | 0.000 |
| Scholarly communication | 0.000 | 0.000 |
| Open science | 0.000 | 0.000 |
| Research integrity | 0.000 | 0.000 |
| Insufficient payload (model declined to judge) | 0.002 | 0.000 |
Machine scores (provisional)
The two teacher heads of the student model, read on this work. A score orders the frame for review; it never asserts a category, and the validation status ships verbatim with every row.
Baseline scores from an immature model (maturity gate not passed, 7 training rounds). Scores rank; they never assert a category.
score_only:v0-immature-baseline · verbatim from the scoring run: score_only means the number may rank works, and no category label ships from itClassification
machine, unvalidatedMachine predicted; a candidate call from one source (direct Gemma or distilled Codex), not a consensus.
How this classification was reached, model by model and score by score, is at the end of the page under "How this classification was reached".