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Abstract

Objective: 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 T. Methods: Behavioral experiments measured the contrast sensitivity and attention cueing effect of orientation singletons presented either at low (15 deg) or high (90 deg) orientation contrast with uniformly oriented background bars. In the 7T fMRI experiment, we studied layer-dependent CBV fMRI activity to the orientation singletons in early visual and parietal cortices. Results The more salient singleton shows higher contrast sensitivity, and stronger cueing effect in both visible and invisible conditions. The orientation-saliency signal is strongest in the superficial layers of V1, and peaks in the middle layers of V2 and V3 and intraparietal sulcus. Contrast sensitivities of the orientation singletons also correlate with CBV signals in superficial V1. Conclusion Our finding supports the hypothesis that bottom-up saliency map is initially created by iso-feature suppressions through lateral inhibitions in V1 superficial layers, and then projecting to the parietal cortex through the corticocortical feedforward pathway.