Blood oxygenation level-dependent functional magnetic resonance imaging of bilateral but asymmetrical responses to gustatory stimulation in the rat insular cortex.

Evidence has suggested asymmetrical processing of taste in the human insular cortex, but this phenomenon has not been demonstrated in the rodent brain. Functional magnetic resonance imaging (fMRI) is a powerful tool for studying the functional organization of the brain. In this study, we established a blood oxygenation level-dependent (BOLD) fMRI method at 7 T to investigate the responses to gustatory stimulation in the insular cortex of anesthetized rats (220-310 g, n=15). BOLD signals were observed in the insular cortex in response to 0.5 M sucrose solution as the tastant but not observed in response to distilled water as the control. The reproducibility of the BOLD signals in response to the tastant was confirmed between fMRI runs in the same animal and across animals. The signals were mainly located between 2.3 mm and 0.0 mm anterior to the bregma in the insular cortex. Interestingly, the signals were observed in the insular cortex of both hemispheres, but they were asymmetrical: the anterior and posterior regions to the intersection of the middle cerebral artery and the rhinal fissure as the landmark of the gustatory cortex were dominant in the left and right hemispheres of the insular cortex, respectively. These results suggest that activity in both hemispheres of the insular cortex should be considered to analyze taste processing. We think that BOLD fMRI of taste function in rodents will improve our understanding of taste information processing.