Abstract
Recent BOLD-fMRI studies have revealed spatial distinction between variability- and mean-based between-condition differences, suggesting that BOLD variability could offer complementary and even orthogonal views of brain function with traditional activation. However, these findings were mainly observed in block-designed fMRI studies. As block design may not be appreciate for characterizing the low-frequency dynamics of BOLD signal, the evidences suggesting the distinction between BOLD variability and mean are less convincing. Based on the high reproducibility of signal variability modulation between continuous eyes-open (EO) and eyes-closed (EC) states, here we employed EO/EC paradigm and BOLD-fMRI to compare variability- and mean-based EO/EC differences while the subjects were in light. The comparisons were made both on block-designed and continuous EO/EC data. Our results demonstrated that the spatial patterns of variability- and mean-based EO/EC differences were largely distinct with each other, both for block-designed and continuous data. For continuous data, increases of BOLD variability were found in secondary visual cortex and decreases were mainly in primary auditory cortex, primary sensorimotor cortex and medial nuclei of thalamus, whereas no significant mean-based differences were observed. For the block-designed data, the pattern of increased variability resembled that of continuous data and the negative regions were restricted to medial thalamus and a few clusters in auditory and sensorimotor networks, whereas activation regions were mainly located in primary visual cortex and lateral nuclei of thalamus. Furthermore, with the expanding window analyses we found variability results of continuous data exhibited a rather slower dynamical process than typically considered for task activation, suggesting block design is less optimal than continuous design in characterizing BOLD variability. In sum, we provided more solid evidences that variability-based modulation could represent orthogonal views of brain function with traditional mean-based activation.
Highlights
In traditional task-based functional magnetic resonance imaging studies, activation regions were typically identified by comparing the temporal mean of a task related time course with baseline
For the block-designed data, the activation regions were mainly located in PVC, lateral thalamus and a few voxels in right primary sensorimotor cortex (PSMC) and primary auditory cortex (PAC) (p < 0.05, corrected, Figure 1A)
We focused on the comparisons between blood oxygenation level dependent (BOLD) derived variability- and mean-based EO/EC differences, within the same subjects and under the same experimental environment
Summary
In traditional task-based functional magnetic resonance imaging (fMRI) studies, activation regions were typically identified by comparing the temporal mean of a task related time course with baseline. A few studies have revealed that the betweencondition differences in signal variability have distinct spatial patterns from the traditional activation regions (Protzner et al, 2013; Garrett et al, 2014; Guitart-Masip et al, 2015). These findings implied that traditional activation could not represent a complete description of human brain response, and brain variability may provide complementary and even orthogonal views (Garrett et al, 2013b)
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