The neural correlate of the polarity advantage effect in crowding
43.408, Monday, 19-May, 8:30 am - 12:30 pm, Banyan Breezeway
Ziyun Zhu1, Fang Fang1,2,3; 1Department of Psychology and Key Laboratory of Machine Perception (Ministry of Education), Peking University, Beijing 100871, China, 2Peking-Tsinghua Center for Life Sciences, Peking University, Beijing 100871, China, 3PKU-IDG/McGovern Institute for Brain Research, Peking University, Beijing 100871, China
If the target in a crowding display differs from the flankers in its contrast polarity, the extent of crowding is reduced compared to the condition where the target and flankers have the same polarity. This phenomenon is referred to as the polarity advantage effect. Here, we investigated its neural mechanisms using event-related potentials (ERPs). A target was centered at 8° eccentricity in the upper-left visual quadrant, either alone or with two adjacent flankers positioned radially. The target and flankers were a circular patch of a sine-wave grating and were presented in a uniform gray background. They were rendered in black or white. They could have the same or opposite polarities. Only in the same polarity condition, there was a significant crowding effect as manifested by orientation discrimination impairment with the target. We measured the earliest ERP component (C1) evoked by five stimulus configurations, including the target only, the target with the flankers of same polarity, the target with the flankers of opposite polarity, the flankers of same polarity only, and the flankers of opposite polarity only. The C1 had a peak latency of about 80 ms and is believed to be generated in early visual cortical areas (i.e. V1 and V2). We found that, the sum of the C1 amplitudes evoked by the target and the flankers was smaller than the C1 amplitude by the target with the flankers, suggesting a mutual cortical suppression between the target and flankers. Importantly, the suppression was significantly stronger when the target and flankers had the same polarity than when they had opposite polarities. We also found that the suppression depended on subjects spatial attention to the stimuli. These results suggest that the early cortical suppression enabled by spatial attention might contribute to the polarity advantage effect in crowding.