Ascending neuromodulatory projections from deep brain nuclei generate internal brain states that differentially engage specific neuronal cell types. Because neurovascular coupling is cell-type specific and neuromodulatory transmitters have vasoactive properties, we hypothesized that the impulse response function (IRF) linking spontaneous neuronal activity with hemodynamics would depend on brain state.

To test this hypothesis, we used optical imaging to measure (1) release of neuromodulatory transmitters norepinephrine (NE) or acetylcholine (ACh), (2) Ca²⁺ activity of local cortical neurons, and (3) changes in hemoglobin concentration and oxygenation across the dorsal surface of cerebral cortex during spontaneous neuronal activity in awake mice.

Fluctuations in total hemoglobin (HbT), reflective of dilation dynamics, were well predicted by a weighted sum of positive Ca²⁺ and negative NE contributions, while ACh signals were largely redundant with Ca²⁺. IRF varied in time and depended on the arousal (dilation of the eye pupil, whisking) captured by NE but not ACh. During high arousal, the dynamic nature of IRF resulted in the loss of hemodynamic coherence between cortical regions (known as “functional connectivity” in BOLD fMRI studies) despite coherent behavior of the underlying neuronal Ca²⁺ activity.

We conclude that neurovascular coupling is a dynamic phenomenon that reflects NE neuromodulation and behavior. Dynamics of IRF challenges the metric of functional connectivity because the loss of hemodynamic coherence can be falsely interpreted as neuronal desynchronizations.