Cerebral stiffness changes during visual stimulation: Differential physiological mechanisms characterized by opposing mechanical effects.

We performed functional intrinsic Magnetic Resonance Elastography (fiMRE) as well as Time of Flight angiography and BOLD fMRI on 7 healthy human subjects to monitor shear stiffness and arterial dilation during periods of prolonged visual stimulation. FiMRE activation with increased stiffness was observed to occur almost equally within brain white and gray matter ( 49.7 ± 14.0 % and 40.9 ± 12.2 % , respectively), while activation with decreased stiffness was significantly ( p = 0.018 ) more likely to occur in white matter than gray matter ( 50.8 ± 11.4 % and 37.0 ± 4.5 % , respectively). At the low mechanical activation and block design frequencies used in this intrinsic MRE (iMRE) approach, the aggregate stiffness change across the entire BOLD activation region was not significant. However, we observed significant reduction in shear stiffness (1.40 ± 0.15 to 0.68 ± 0.22 [kPa], p < 0.001) in areas adjacent to the Posterior Cerebral Artery, where vasodilation is evident, in the V1 region. In addition, we observed significant shear stiffness increase (1.29 ± 0.12 to 0.62 ± 0.16 [kPa], p < 0.001) in areas adjacent to the Middle Temporal or V5 region of the visual cortex. These results show that iMRE can measure intrinsic cerebro-mechanical reactions due to visual stimulation as well as the differential physiological response detected in distinct regions of the visual cortex.