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RESISTANCE VESSELS Arterioles and small arteries . ISCHAEMIA A condition of insufficient oxygen delivery to a tissue

Functional MRI has rapidly emerged as a powerful non-invasive technique for studying brain function with superb spatial and temporal resolution. It is an essential tool for studies that seek to determine the mechanisms of sensorimotor processes or high-level complex behaviour. It also has great potential to provide insight into the neural mechanisms that underlie normal development, ageing and neurological and psychiatric disorders. However, despite the exponential increase in published papers that have used fMRI to explore brain–behaviour relationships in normal or diseased individuals, little is known about the physiological basis of its basic unit of measurement, the BOLD signal. The BOLD signal depends on the blood-flow mediated relationship between neural activity and the concentration of deoxyhaemoglobin in the surrounding microvasculature.When a neural event occurs anywhere in the brain, there is a subsequent increase in local blood flow resulting in a decrease in the concentration of deoxygenated haemoglobin in the microvasculature that surrounds the activated region. This change leads to an increase in the BOLD signal that reflects the ratio of non-paramagnetic oxygenated haemoglobin to paramagnetic deoxygenated haemoglobin. Neural activity alters this ratio by influencing several factors including cerebral blood flow (CBF), cerebral blood volume (CBV) and cerebral blood oxygen consumption (CMRO 2 ) (FIG. 1). So, the BOLD signal is an indirect measure of neural activity. The process by which neural activity influences the haemodynamic properties of the surrounding vasculature is referred to as neurovascular coupling. Recently, there has been much interest in the relationship between neural activity and the BOLD signal. The few studies that have examined this have generally supported a roughly linear relationship (for a review on the topic, see REF. 11). These studies showed that, up to some limit, the BOLD response to successive neural events could be predicted from the arithmetic addition of the responses to single neural events, considering the time delay between the events. It is to be hoped that similar studies in monkeys that simultaneously record neural activity and the BOLD signal will provide even deeper insight into the boundary conditions of this linearity. Notwithstanding, the parameters of this relationship might vary between brain regions and between populations. As most fMRI studies are performed on healthy young individuals, little attention has focused on the effects of changes in the cerebrovascular system on the BOLD signal. However, direct comparisons of the BOLD signal made between groups of individuals rely heavily on the assumption of ALTERATIONS IN THE BOLD FMRI SIGNAL WITH AGEING AND DISEASE: A CHALLENGE FOR NEUROIMAGING