Influence of stimulus duration on the neurovascular coupling response

The neurovascular coupling mechanism adapts cerebral blood flow in accordance with cortical activity. It reacts rapidly, reliably and in a finely tuned manner. Therefore, the question emerged at to whether this might be a linear mechanism. Recently, non-linearity was concluded from different time curves of flow regulation, resulting from stimulation of varying duration. Short lasting stimulation resulted in a peak response, whereas longer stimulation led to a peak and plateau characteristic of flow response. We suggest the different responses may be caused by dynamic properties of the coupling.

A functional transcranial Doppler test was performed using a visual stimulation paradigm in 10 healthy volunteers (age 25.8 +/- 0.5 years, 7 men). Two tests with different stimulus duration were used (20 s versus 40 s). The short test was known to result in the dynamic pattern, and the long test in the static phase of flow regulation. The data from the long test were evaluated according to a control system approach. The resultant model was used to calculate data of an assumed 20 s stimulation. A paired t-test was used to compare the measured and calculated flow data for the short stimulation test.

The flow velocity responses were identical when comparing the first 20 s of stimulation. At the termination of stimulation in the short test, flow velocity returned to baseline leading to a peak response, whereas flow velocity stabilised at a constant level above baseline in the long stimulation paradigm, resulting in the peak and plateau characteristic.

The different flow responses due to different stimulus duration are caused by dynamic properties of the coupling mechanism and are not indicative of non-linearity.