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White matter fMRI : Exploring functional differentiation in the corpus callosum

Introduction White matter fMRI is a controversial idea. To date, the majority of fMRI studies have focused on gray matter, largely for two reasons: 1) cerebral blood flow/volume is reduced in white matter relative to gray matter (e.g., [1, 2]); and 2) post-synaptic potentials are thought to be the primary source of the signal measured in fMRI (e.g., [3]). Despite this, a growing number of studies have observed white matter activation (e.g., [4-9]). Indeed, we have recently reported the first two prospective white matter 4T fMRI studies using interhemispheric transfer tasks [8,9]. The initial study examined interhemispheric transfer during a visual word/face task, which elicited activation in the isthmus of the corpus callosum (Figure 1) [8]. White matter activation was present in 20% of individuals and detected at the group level using a liberal threshold (p<0.005, uncorrected). A subsequent study examined interhemispheric transfer using a visual-motor checkerboard task, which elicited activation in the anterior corpus callosum (Figure 2) [9]. White matter activation was observed in 100% of subjects as well as in the group averaged data (p<0.05, corrected). This study used a specialized asymmetric spin echo (ASE) spiral sequence [10], which collected three images (per slice/per volume) of equal BOLD contrast weighing and increasing T2 weighting. The results showed increased sensitivity to white matter activation with increased T2 weighting. It is noteworthy that these two studies elicited activation in different regions of the corpus callosum, suggesting that task type may be useful for mapping functional differentiation in white matter. To test this, the current study examined whether varying task type differentially activates white matter within the same subjects.