Overshoot and undershoot: control system analysis of haemodynamics in a functional transcranial Doppler test

Local cerebral blood flow is adjusted to cortical activity in a rapid and fine-tuned manner. The responsible mechanism, called neurovascular coupling, is suggested to be governed by a complex biological control system. The main characteristics of the regulative process can be modelled in terms of a control system of low order. We aimed to test the hypothesis that the overshoot and undershoot in haemodynamics are part of the regulative processes of the neurovascular coupling mechanism.

Using a visual stimulation paradigm, the evoked blood flow velocity in the posterior cerebral artery was measured in 20 healthy volunteers (aged 24.7 +/- 0.4 years, 12 males) with transcranial Doppler. The dynamic flow velocity response was evaluated according to a 5-parameter control system model.

The parameter time delay (1.2 +/- 1; 2.2 +/- 1.3; p < 0.05) and natural frequency (0.23 +/- 0.06; 0.47 +/- 0.18; p < 0.001) were significantly smaller, the parameter rate time (4.9 +/- 2.3; 2.2 +/- 1.6; p < 0.01) greater in conditions of stimulation.

The finding of an identical control system model for both regulative conditions supports the hypothesis that the mechanism is mainly functionally governed and that the underlying biological elements do not appear to be limiting factors. The difference in parameters is suggested to reduce the time span of an initial hypoperfusion in conditions of cortical activation.