Improved differentiation of tactile activations in human secondary somatosensory cortex and thalamus using cardiac-triggered fMRI.

Functional magnetic resonance imaging (fMRI) can reveal human brain activations with high precision. The accuracy may, however, be impaired by movement and deformation of brain tissue associated with cardiac pulsations. Here we corrected for such artifacts by time-locking the fMRI data acquisition to the cardiac cycle in ten subjects who received tactile stimuli to their lips, fingers, and toes. The imaged brain areas covered the parietal operculum and the thalamus, including the secondary somatosensory cortex (SII) bilaterally. Variance of the blood-oxygen-level-dependent signal decreased on average by 38-40% in the SII cortex and by 26% in the thalamus during cardiac triggering compared with conventional imaging. Consequently, statistically significant responses were seen both in the SII cortex and in the ventroposterior thalamus in a larger number of subjects. At the cortical level, the activation pattern revealed two distinct representations for both fingers and toes in the SII region, and the more medial representations were detected with enhanced clarity during cardiac-triggered imaging. In the group-level analysis, the thalamic response to finger stimulation was seen with cardiac triggering, only.