Crowding, the identification difficulty for a target in the presence of nearby flankers, is ubiquitous in spatial vision and is considered a bottleneck of object recognition and visual awareness. Despite its significance, the neural mechanisms of crowding are still unclear.

Fang Fang’s group performed ERP and fMRI experiments to measure the cortical interaction between the target and flankers in human subjects. They found that the magnitude of the crowding effect was closely associated with an early suppressive cortical interaction. The cortical suppression was reflected in the earliest ERP component (C1), which originated in V1, and in the blood-oxygenation-level-dependent (BOLD) signal in V1, but not other higher cortical areas. Intriguingly, spatial attention played a critical role in the manifestation of the suppression. These findings provide direct and converging evidence that attention-dependent V1 suppression contributes to crowding at a very early stage of visual processing.

Juan Chen, Yingchen He, Ziyun Zhu, Tiangang Zhou, Yujia Peng, Xilin Zhang, and Fang Fang. Attention-dependent early cortical suppression contributes to crowding.  The Journal of Neuroscience , 6 August 2014, 34(32):10465-10474;