Dear Bernardus, samplig rate is a difficult matter, sometimes difficult to manage. I would like to suggest to read preliminarly some tutorial on "Sampling Theorem" and/or "ADC" (Analog-to-Digital Conversion). Shannon theorem states that you MUST adquire the signal at twice the maximum frequency of your signal. Now, your question tries to relate sampling rate to capillaries and stationarity. The question is not very well posed.

I know about sampling rate, ADC conversion, the nyquist theorem, and data acquisition. I merely used sampling rate to explain why I was interested in the maximum blood velocity capilaries in the pre-frontal cortex.

I am interested in the local change in oxygenation separate from the effect of the vascular response.

Dear Bernardus, in my opinion the separation of one effect from the other (e.g., local change in oxygenation from vascular response) is a very hard issue. And this also complicated by the choice of the site of the investigation: the pre-frontal cortex. We made some successfull trials by using NIRS (Near InfraRed Spectroscopy) in order to assess blood oxygenation of the arm. If needed, I can send you some more information ([email protected] )

Functional near infra red spectroscopy (fNIRS) is exactly the reason I am asking for the capillary blood flow velocity. It seems most of the work has looked at the BOLD vascular response. There is about a 4-8 seconds lag between an event and the vascular response. Consequently, the fNIRS signal

Is low pass filtered at 0.14 Hz.

There is however a local response as well. Without the vascular response the brain really does not have a whole lot of oxygen surplus. So, if the blood flow is slow enough and the sampling rate fast enough, one may be able to pick up the local response of the oxygen level to the use of oxygen by brain cells that are using it to burn fuel. It turns out that the capillary blood velocity is in the order if 0.8 mm pet second. So, at 800 samples per second a volume of. Blood in the capilaries would only have moved about a micrometer. Pretty stationary if we can sample that fast.