{'fraction_non_alphanumeric': 0.02631578947368421, 'fraction_numerical': 0.018259935553168637, 'mean_word_length': 5.38013698630137, 'pattern_counts': {'":': 0, '<': 0, '<?xml version=': 0, '>': 0, 'https://': 0, 'lorem ipsum': 0, 'www.': 0, 'xml': 0}, 'pii_count': 0, 'substrings_counts': 0, 'word_list_counts': {'cursed_substrings.json': 0, 'profanity_word_list.json': 0, 'sexual_word_list.json': 0, 'zh_pornsignals.json': 0}}
Oscillations in cerebral haemodynamics. Implications for functional activation studies.

The blood oxygenation level dependent (BOLD) contrast signal in functional magnetic resonance imaging (fMRI) provides highly spatially resolved deoxygenation maps of the activated cortex (Turner 1995). In common with many other activation measurement techniques, (including optical measurements) fMRI relies upon multiple repetition of the stimulus followed by signal averaging, baseline subtraction and extensive thresholding techniques to provide difference images of activation. One of the roles of this type of data manipulation is to account for non activation related changes in the measured cerebral parameters. It is well known that the "resting" brain is in a constant state of haemodynamic redistribution (Halsey & McFarland 1974, Dora & Kovach 1980) and hence may not be considered to provide a static baseline for activation studies. An important aspect of the status of the resting brain is the phenomena of vasomotion-the name given to rhythmical oscillatory behavior of capillary blood flow of different frequencies caused by the contraction and relaxation of the precapillary vessels (Berne & Levy 1997). The phenomenon of vasomotion has been known for some time and was in fact first described in 1854 (Schniff 1854). Several studies have been designed to elucidate its origin however many of the techniques employed have the disadvantage of requiring exposed cortex preparations and anaesthesia. An example of one such study on the exposed cortex of the rat has revealed slow (typically 0.1 Hz) oscillations in optical attenuation signal, which have been ascribed to the phenomenon of vasomotion (Mayhew et a!., 1996). Laser doppler studies have also demonstrated the presence of "spontaneous" oscillations in flow (Hudetz et aI., 1992). The advent of non