White versus gray matter: fMRI hemodynamic responses show similar characteristics, but differ in peak amplitude

Abstract

Background: There is growing evidence for the idea of fMRI activation in white matter. In the current study, we compared hemodynamic response functions (HRF) in white matter and gray matter using 4 T fMRI. White matter fMRI activation was elicited in the isthmus of the corpus callosum at both the group and individual levels (using an established interhemispheric transfer task). Callosal HRFs were compared to HRFs from cingulate and parietal activation.

Results: Examination of the raw HRF revealed similar overall response characteristics. Finite impulse response modeling confirmed that the WM HRF characteristics were comparable to those of the GM HRF, but had significantly decreased peak response amplitudes.

Conclusions: Overall, the results matched a priori expectations of smaller HRF responses in white matter due to the relative drop in cerebral blood flow (CBF) and cerebral blood volume (CBV). Importantly, the findings demonstrate that despite lower CBF and CBV, white matter fMRI activation remained within detectable ranges at 4 T.

Figure 1

Illustration of the mixed fast/slow event-related design.

Figure 2

Group level data showing significant activation in the posterior corpus callosum (arrow) and corresponding GM activation (including both cingulate and parietal activation). Activation results are presented as Z-scores (Z > 3.0, corrected) using radiological format. The white lines indicate the location of the imaging slab. Inset shows group level data processed without smoothing. Green circles identify succinct, robust white matter activation in the corpus callosum (Z > 4.0).

Figure 3

(a) Hemodynamic response function (HRF) data (arbitrary units; AU) for the raw isolates (n = 8 averaged). The horizontal axis depicts time in TRs (2 second repetition time). Data were extracted from ROIs in the corpus callosum, cingulate, and parietal areas. (b) HRF data (AU) for the finite impulse response analysis from all three ROIs. All other details as per Figure 3a.