http://journal.frontiersin.org/Journal/10.3389/fnins.2014.00239/full?
- 1Division of Medical Sciences, Department of Psychology, University of Victoria, Victoria, BC, Canada
- 2Department of Radiology, Faculty of Medicine, University of Calgary, Calgary, AB, Canada
- 3Applied Sciences, Simon Fraser University, Burnaby, BC, Canada
- 4Fraser Health Authority, Surrey Memorial Hospital, Surrey, BC, Canada
Motivation to Investigate White Matter fMRI
Functional magnetic resonance imaging (fMRI) is used to
visualize the neuroanatomical regions associated with brain function.
The most commonly used technique for fMRI, blood oxygenation level
dependent (BOLD) contrast, was first demonstrated in the early 1990s (Ogawa et al., 1992).
Since then, fMRI has broadened our understanding of how the brain
functions under both healthy and diseased conditions (e.g., Rosen et al., 1998; Dolan, 2008; Haller and Bartsch, 2009; Rosen and Savoy, 2012).
Although fMRI continues to grow in popularity in both research and
clinical settings, the full potential of this technique remains untapped
because fMRI activity has historically not been considered to be
detectable in white matter tissue (Logothetis and Wandell, 2004).
In spite of this, fMRI studies often produce activation in white matter
and consequently there has been much debate over whether this
activation is a true or false representation of underlying neural
activity. There are two main reasons that white matter fMRI is
controversial. First, BOLD signal relies on cerebral blood volume and
flow, which are three to seven times lower in white matter (Rostrup et al., 2000; Preibisch and Haase, 2001; Helenius et al., 2003). However, the vasculature and perfusion of white matter (Figure 1)
are capable of supporting hemodynamic changes that are detectable with
BOLD fMRI [see Section White Matter Vasculature, Cerebral Blood Flow
(CBF), and Cerebral Blood Volume (CBV)]. Second, the primary source of
fMRI signal is thought to arise from post-synaptic potentials (which
occur mainly in gray matter) as opposed to action potentials (e.g., Logothetis et al., 2001; but see e.g., Smith et al., 2002).
However, neither of these statements exclude the possibility, and there
is no direct evidence against the possibility of measuring fMRI
activation in white matter.
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