Why do our eyes instinctively gravitate toward a bright red apple on a green tree or a flashing light in the dark? The brain's ability to detect and prioritize visually "salient" stimuli—those that stand out from their surroundings—is essential for survival and efficient perception. However, for decades, neuroscientists have debated where in the brain these saliency signals originate.

In a new study published in PLoS Biology on October 14, a research team led by Prof. ZHANG Peng from the Institute of Biophysics of the Chinese Academy of Sciences has uncovered the cortical microcircuit mechanisms underlying the human bottom-up attentional saliency map using vascular space occupancy (VASO)-based laminar functional magnetic resonance imaging (fMRI) with ultra-high spatial specificity at 7 Tesla (VASO laminar fMRI).

The researchers discovered that the bottom-up attention saliency map originates from lateral inhibition within the superficial layers of the primary visual cortex (V1) and is transmitted through feedforward pathways to the intraparietal sulcus (IPS), thereby enabling efficient information selection.

In the experiments, the researchers viewed background bars with a uniform orientation and a small patch of differently oriented foreground bars. They precisely manipulated the visual saliency of the foreground by varying the orientation difference between the foreground and background.

Behavioral results showed that participants exhibited better detection performance for foreground stimuli with higher visual saliency.

Submillimeter-resolution 7T VASO fMRI data revealed that the superficial layers of V1 encoded the visual saliency map in a laminar pattern consistent with lateral connections. This signal was then fed forward to the middle layers of the IPS.

Furthermore, the strength of the saliency signal in the superficial layers of V1 accurately predicted the behavioral detection performance of the participants for the salient target.

These results support the idea that the visual saliency map is computed in the superficial layers of V1 via lateral connections and then sent to higher cortical regions, such as the parietal cortex, to help with bottom-up attentional selection.

This study provides crucial mesoscale functional evidence supporting the "V1 Saliency Hypothesis" of visual attention computation in the human brain and may contribute to the development of brain-inspired visual models.

Generation and transmission mechanisms of the bottom-up attentional saliency map (Image by ZHANG Peng's group)