Recent Logothetis Comment on fMRI and Local Fields

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Alfredo Pereira Jr (group admin)

Date:

05 October 2007

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I am sending the first paragraphs of the Logothetis comment on a recent finding.

Alfredo Pereira Jr.

Nature Neuroscience – 10, 1230 – 1232 (2007)
doi:10.1038/nn1007-1230

The ins and outs of fMRI signals
Nikos K Logothetis

Perhaps because a picture is worth a thousand words, images from functional imaging studies seem particularly revealing of the neural mechanisms behind our thoughts and actions. Despite this perception, neuroimaging techniques, such as invasive quantitative autoradiography, positron emission tomography, optical imaging of intrinsic signals and functional magnetic resonance imaging (fMRI), use hemodynamic responses as surrogates for neural function. The blood oxygen level–dependent (BOLD) contrast in fMRI depends primarily on blood oxygenation, which in turn reflects metabolic activity in the tissues. Although BOLD fMRI has helped to address important questions about our cognitive capacities and their relationships to cortical activity, we still lack a clear understanding of the neurometabolic and neurovascular coupling underlying BOLD signals. This information is necessary for a sound interpretation of fMRI signals, as BOLD fMRI reflects a complex interplay of changes in cerebral blood flow, cerebral blood volume and blood oxygenation. There are many open questions. What kind of neural activity draws most of the metabolic energy? Which cell types generate it? What is the dynamic link between such activity and energy demands? What are the exact processes coupling the supply of and demand for energy in the brain tissues? A study in this issue addresses the first of these questions.

In their study, Viswanathan and Freeman used a dual microelectrode to conduct simultaneous and colocalized measurements of oxygen partial pressure and electrical activity at a high spatio-temporal resolution in the cat visual cortex. Their electrical measurements assessed both local field potentials (LFPs), which represent the integrated local dendritic events in an area, and action potentials, which depend on the activity of projection neurons. Their experiments revealed a strong coupling between LFPs and changes in tissue oxygen concentration. This important finding suggests that perisynaptic activity places the greatest demands on metabolic energy. These results also imply that fMRI signals most likely reflect the input and intracortical processing in the mapped brain site, rather than its output instantiated in the firing of the projection neurons. In this context, ‘perisynaptic’ captures the classical events of synaptic transmission, with its respective population of excitatory or inhibitory postsynaptic potentials, as well as a number of integrative processes, including somatic and dendritic spikes with their ensuing after-potentials and voltage-dependent membrane oscillations.

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