Abstract
High-resolution functional magnetic resonance imaging (fMRI) is becoming increasingly popular because of the growing availability of ultra-high magnetic fields which are capable of improving sensitivity and spatial resolution. However, it is debatable whether increased spatial resolutions for haemodynamic-based techniques, like fMRI, can accurately detect the true location of neuronal activity. We have addressed this issue in functional columns and layers of animals with haemoglobin-based optical imaging and different fMRI contrasts, such as blood oxygenation level-dependent, cerebral blood flow and cerebral blood volume fMRI. In this review, we describe empirical evidence primarily from our own studies on how well these fMRI signals are spatially specific to the neuronally active site and discuss insights into neurovascular coupling at the mesoscale.This article is part of the theme issue ‘Key relationships between non-invasive functional neuroimaging and the underlying neuronal activity’.
Highlights
Increased neuronal activity in the brain is accompanied by changes in oxygen metabolism, blood flow, and blood volume, which can be captured with functional magnetic resonance imaging as blood oxygenation leveldependent (BOLD), cerebral blood flow (CBF) or cerebral blood volume (CBV) changes
While maximal BOLD responses always appeared in the same superficial layer regardless of which layer was stimulated, the maximal contrast-enhanced plasma blood volumeweighted (CE-CBV) responses consistently peaked in the elicited layer independent of the baseline blood volume
As technology improves and submillimetre human functional magnetic resonance imaging (fMRI) becomes practical, animal models become increasingly important to validate its limits on neural specificity
Summary
Increased neuronal activity in the brain is accompanied by changes in oxygen metabolism, blood flow, and blood volume, which can be captured with functional magnetic resonance imaging (fMRI) as blood oxygenation leveldependent (BOLD), cerebral blood flow (CBF) or cerebral blood volume (CBV) changes. A location mismatch between the site of increased neuronal activity and the site of vascular responses measured by fMRI varies depending on how far these sites are separated and which vascular compartment the fMRI technique is sensitive to. One of the outstanding issues of high-resolution fMRI is how accurately the signal changes mark the true site of increased neuronal activity. To examine this spatial specificity of fMRI, activation must be measured within well-defined neuronal circuits, like functional columns or layers, and validated with other well-established complementary techniques, including optical imaging and electrophysiology, in animal models.
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Schedule a callMore From: Philosophical transactions of the Royal Society of London. Series B, Biological sciences
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