Resting fluctuations in arterial carbon dioxide (a cerebral vasodilator) are believed to be an important source of low-frequency Blood Oxygenation Level Dependent (BOLD) signal fluctuations.In this study the main focus is on the two commonly used resting-states  in functional magnetic resonance imaging experiments, eyes open and eyes closed,and it quantifies the degree to which measured spontaneous fluctuations in the partial pressure of end-tidal carbon dioxide (Pet(Co2)) relate to BOLD signal time series.

A significantly longer latency of Blood Oxygenation Level Dependent signal changes following Pet(co2) fluctuations was found in the eyes closed condition compared to with eyes open, which may reveal different intrinsic vascular response delays in carbon dioxide reactivity or an alteration in the net Blood Oxygenation Level Dependent signal arising from Pet(co2) fluctuations and altered neural activity with eyes closed.

By allowing a spatially varying time delay for the compensation of this temporal difference, a more spatially consistent carbon dioxide correlation map can be obtained.

Finally, Granger-causality analysis demonstrated a “causal” relationship between Pet(co2) and Blood Oxygenation Level Dependent.

The identified dominant Pet(co2)→ Blood Oxygenation Level Dependent directional coupling supports the notion that Pet(co2) fluctuations are indeed a cause of resting Blood Oxygenation Level Dependent variance in the majority of subjects.

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