Tutorial 7: BOLD & neural activity


The interpretation of functional MRI relies on the relationship between the Blood Oxygen Level Dependent response and neuronal activity. This topic has been the focus of much debate. The articles that are listed on this web-page provide an up to date view on how to interpret brain signals based on the hemodynamic response.


What does fMRI tell us about neuronal activity?

Authors: Heeger DJ, Ress D.

Abstract | In recent years, cognitive neuroscientists have taken great advantage of functional magnetic resonance imaging (fMRI) as a non-invasive method of measuring neuronal activity in the human brain. But what exactly does fMRI tell us? We know that its signals arise from changes in local haemodynamics that, in turn, result from alterations in neuronal activity, but exactly how neuronal activity, haemodynamics and fMRI signals are related is unclear. It has been assumed that the fMRI signal is proportional to the local average neuronal activity, but many factors can influence the relationship between the two. A clearer understanding of how neuronal activity influences the fMRI signal is needed if we are correctly to interpret functional imaging data.
>> Read this review (requires subscription).

Cerebral Blood Flow Regulation



Authors: N.P. Mitagvaria, H.I. Bicher

Studies of the mechanisms relating blood supply to the brain appeared to be, in some sense, at a deadlock. Despite extensive application of different methodical approaches, no qualitative progress has been observed in these studies at the present time. This is perhaps due to the traditional, but not understandable, separation of neurophysiological and 'circulatory' studies. It may seem very paradoxical, but the study of cerebral blood circulation proceeds almost in complete isolation from the knowledge about brain functions and does not take into account the specificity of the working brain as a part of the whole body.This book comprehensively addresses the issues of blood flow regulation. It is well known that the brain belongs to the group of organs having a high level of oxygen consumption. Oxygen consumption by the brain is an average 4.6 ml per 100 g of tissue per minute. In humans, the level of oxygen consumption by the whole brain attains 46 ml/min. This makes up approximately 20 per cent of the total oxygen volume consumed by the organism. Consequently, the cerebral tissue is characterised by highly energetic processes. There is evidence indicating that even in functionally resting conditions, 18 per cent of the entire energy expenditure of the body is utilised by the brain Calculations made by Rushmer indicate that the intensity of energy consumption by the human brain appears to be on average 20 Watt. Download the table of contents of this book.

Metabolic and hemodynamic events following changes in neuronal activity: current hypotheses, theoretical predictions and in vivo NMR experimental findings



Authors:
S. Mangia et al

Abstract | Unraveling the energy metabolism and the hemodynamic outcomes of excitatory and inhibitory neuronal activity is critical not only for our basic understanding of overall brain function, but also for the understanding of many brain disorders. Methodologies of magnetic resonance spectroscopy (MRS) and magnetic resonance imaging (MRI) are powerful tools for the noninvasive investigation of brain metabolism and physiology. However, the temporal and spatial resolution of in vivo MRS and MRI is not suitable to provide direct evidence for hypotheses that involve metabolic compartmentalization between different cell types, or to untangle the complex neuronal microcircuitry, which results in changes of electrical activity. This review aims at describing how the current models of brain metabolism, mainly built on the basis of in vitro evidence, relate to experimental findings recently obtained in vivo by (1)H MRS, (13)C MRS, and MRI. The hypotheses related to the role of different metabolic substrates, the metabolic neuron-glia interactions, along with the available theoretical predictions of the energy budget of neurotransmission will be discussed. In addition, the cellular and network mechanisms that characterize different types of increased and suppressed neuronal activity will be considered within the sensitivity-constraints of MRS and MRI.


What we can do and what we cannot do with fMRI.

Author: N. Logothetis

Abstract | Functional magnetic resonance imaging (fMRI) is currently the mainstay of neuroimaging in cognitive neuroscience. Advances in scanner technology, image acquisition protocols, experimental design, and analysis methods promise to push forward fMRI from mere cartography to the true study of brain organization. However, fundamental questions concerning the interpretation of fMRI data abound, as the conclusions drawn often ignore the actual limitations of the methodology. Here I give an overview of the current state of fMRI, and draw on neuroimaging and physiological data to present the current understanding of the haemodynamic signals and the constraints they impose on neuroimaging data interpretation.
>> Read this review (requires subscription).

Brain Energetics and Neuronal Activity: Applications to fMRI and Medicine


This book is unique in linking in vivo 13C NMR measurements of neuronal activity and energetics with applications to functional imaging and certain disease states It provides a fundamental neurochemical explanation of brain activity applicable to functional imaging, theories of neuronal activity and disease states, e.g. epilepsy, psychiatric diseases and developmental disorders. Download a sample chapter of this book.
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brain_energetics_sample.pdf
(2453k)
Niels van Strien,
Apr 9, 2011, 4:51 AM
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