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Ca2+-dependent pathways in neurons and astrocyte endfeet initiate changes in arteriole diameter to regulate local brain blood flow. Whether there exists a threshold of synaptic activity in which arteriole diameter is controlled independent of astrocyte endfeet Ca2+ remains unclear. We used two-photon fluorescence microscopy to examine synaptically evoked synthetic or genetic Ca2+ indicator signals around penetrating arterioles in acute slices of the rat neocortex. We discovered a threshold below which vasodilation occurred in the absence of endfeet Ca2+ signals but with consistent neuronal Ca2+ transients, suggesting endfoot Ca2+ is not necessary for activity-dependent vasodilation under subtle degrees of brain activation. [ABSTRACT FROM AUTHOR]