7T CBV fMRI reveals cortical microcircuits of bottom-up saliency in the human brain

A visual item in sharp contrast with its neighbors automatically captures attention. Whether bottom-up saliency signals arise initially in the primary visual cortex (V1) or in the parietal cortex is still controversial. To distinguish these two hypotheses, we investigated the cortical microcircuits of bottom-up saliency with cortical layer-dependent CBV fMRI at 7 Tesla. Behavioral experiments measured the contrast detection performance to orientation singletons presented either at low (15 degrees) or high (90 degrees) orientation contrast within uniformly oriented background bars. Contrast sensitivity was higher to singletons with high compared to low orientation contrast. CBV-weighted fMRI results showed that the orientation-saliency signal was strongest in the superficial layers of V1, and peaked in the middle layers of V2/V3 and the intraparietal sulcus (IPS). Contrast sensitivities of the orientation singletons also correlated with CBV signals in the superficial layers of V1. These findings support the hypothesis that bottom-up saliency map is initially created by iso-feature suppression through lateral inhibition in V1 superficial layers, and then projects to the parietal cortex through the feedforward connection.