Magnetic Resonance Imaging (Functional). Fig. 2. BOLD fMRI of ocular dominance (OD) columns in the human visual cortex: vertical neuronal columns that respond preferentially to visual stimuli presented to one eye rather than to stimuli presented to the other eye. fMRI studies of OD are often presented to showcase the level of spatial specificity that can be achieved with advanced fMRI techniques. In humans, OD columns are separated by approximately 1 mm. Left panel a: The imaging slice from a single subject selected in a study by Yacoub et al. (2007) that permitted a resolution of 0.25 x 0.25 mm2 in-plane for a slice thickness of 3 mm. Right panel b: Differential functional OD maps depicting increased activity for left eye stimulation (blue) and right eye stimulation (red) for this subject across distinct sessions (A, B, C & F, G, H) and different filtered averages (D, E and I, J). The upper and lower rows show maps obtained using gradient-echo (GE) and hahn spin-echo (HSE) fMRI, respectively. Both approaches reproduced the expected OD columns, although with increased specificity seen with the HSE method due to its enhanced ability to suppress the influence of large vessels. Pos posterior; RH right hemisphere; IHF inter-hemispheric fissure. (Reproduced with permission from Yacoub et al. 2007 © 2007 Elsevier Inc.)
and the cerebral metabolic rate of oxygen consumption (CMRO2). Signal increases reported in BOLD fMRI experiments are related to the fact that neuronal activity increases regional CBF and glucose utilization (CMRglu) to a larger extent than CMRO2. The net effect of neuronal excitation is therefore to decrease the concentration of deoxygenated hemoglobin ("deoxyhemoglobin washout''; Brown et al 2007), which in turn increases BOLD signal strength. It is now understood that the characteristic BOLD signal changes observed in fMRI studies reflects the summation of these competing events (CBF, CMRO2, and CBV), resulting in a complex response function that is controlled by several parameters (Buxton 2002). In other words, the BOLD signal does not reflect a single physiological process, but rather represents the combined effects of CBF, CBV, and CMRO2 (Fig. 3).
The BOLD response to a short duration event or single stimulus has a canonical hemodynamic waveform shape, which is often described as consisting of (1) a fast response lasting 1-2 s ("initial dip'') in which there is a small decrease in BOLD signal amplitude, (2) a larger amplitude hyperemia associated with the inflow of oxygenated blood, which peaks at approximately 4-6 s after stimulus presentation, and (3) a refractory period lasting 6-12 s where the signal undershoots the baseline due to the combination of reduced regional CBF and increased CBV ("post-stimulus undershoot"). The second phase of hyperemia (or hyper-oxic phase) is the common focus for detecting increases in brain activity measured by BOLD fMRI. The so-called "initial dip'' is suspected to result from early oxygenation changes (oxygen extraction) localized to capillaries, and has been argued as more closely related to neuronal activity than the ensuing hyperemia. However, this observation remains controversial and is not reliably detected in BOLD fMRI studies. Like the BOLD response to a single event, multiple repetitions of the same stimuli in blocks (see below), will see the BOLD response rise to a steady plateau and decline once the block ends, although there are variations to this rule, such as an initial overshoot, slow increasing and decreasing ramps, or an undershoot at the end of the stimulus.
Neurovascular Coupling and Neuronal Correlates of BOLD
The process by which neural activity influences the hemodynamic properties of the surrounding vasculature (principally arterioles) is referred to as neurovascular coupling,
Magnetic Resonance Imaging (Functional). Fig. 3. Left panel a: Measuring signal changes in fMRI experiments depicted as a complex multistage process, beginning with neuronal activity and ending with BOLD signal measurement as a property of the MRI scanner and pulse sequence. This figure presents a schematic illustration of interactions in the formation of the BOLD signal. Positive/negative arrows indicate positive/negative correlations between the parameters. The right pathway (bold arrows) is the most significant effect in most BOLD fMRI. Right panels b and c: Simplified schematic representation of the BOLD hemodynamic response waveform to a short duration stimulus (b), and to a block of multiple consecutive stimuli (c). (Parts of this figure reproduced with permission from http://www.eecs.umich.edu/~dnoll.)
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