Prof. Fang Fang’s lab used the population receptive field (pRF) technique to reveal the neural mechanisms of visual crowding effect


On July 8th, 2019, a paper entitled “The critical role of V2 population receptive fields in visual orientation crowding” was published in Current Biology by Prof. Fang Fang’s research group at the School of Psychological and Cognitive Sciences at Peking University, Peking-Tsinghua Center for Life Sciences and the PKU-IDG/McGovern Institute for Brain Research. This paper reports important advances in investigating the neural mechanisms of visual crowding effect by using the fMRI-based population receptive field (pRF) technique. The corresponding author is Prof. Fang Fang and the first author is Dr. Dongjun He, former postdoctoral fellow in Fang Fang’s group.


Visual crowding effect is the identification difficulty for a target in the presence of nearby flankers (Figure 1). Crowding is an essential bottleneck for object recognition and visual awareness. It occurs with almost all kinds of visual stimuli and impacts many everyday tasks, including object recognition, eye and hand movements, visual search, reading, and so on. Crowding also has important clinical implications for patients with amblyopia, macular degeneration, dyslexia or apperceptive agnosia. Therefore, understanding the mechanisms of crowding is important both theoretically and practically. In the past one hundred years, numerous behavioral studies suggest that crowding is due to that the visual system lacks the necessary resolution to isolate the target from flankers and therefore integrate them mistakenly. To date, this idea has rarely been tested with neuroscience methods directly.


Figure 1. Visual crowding effect. A) The visual crowding effect of letter recognition. The small black dot on the left is the fixation point of a subject. When the target letter ‘R’ is presented in the peripheral vision of the subject, the subject can easily recognize the target letter (the upper part of panel A). But when two irrelevant flanker letters are added to each side of the target letter, it becomes difficult to recognize the target letter "R" (the lower part of panel A). B) Visual crowding in natural scenes. When fixating the bull 's eye in the center of the scene, it is difficult or impossible for us to identify the child on the left side of the road due to the signs near the child on the left side of the road, but we can easily identify the child on the right side of the road.


Here we used the population receptive field (pRF) technique to uncover the bottleneck of visual orientation crowding. The pRF mapping is a widely used technique to measure aggregate human visual receptive field properties by recording non-invasive signals using functional magnetic resonance imaging (fMRI) (Dumoulin and Wandell, 2008; Mo, He and Fang, 2018). This technique not only estimates the visual field position preferred by each voxel but also its spatial selectivity, the range of visual field locations where a stimulus can evoke a response, indicated by the size of the pRF. We measured the average pRF size of the voxels in V1-V4 that responded to the target and hypothesized that the pRF sizes in some visual area(s) are positively associated with the magnitude of the crowding effect. Our logic is that smaller pRF sizes would help the visual system to isolate and access the target, therefore reducing interference from nearby flankers. We performed a series of experiments to test this hypothesis and yielded several interesting findings (Figure 2).


Figure 2. Experimental results. A) Correlations between the behavioral crowding index and the average pRF size of the target voxels in V1-V4 across individual subjects (Experiment 1). Asterisks indicate a statistically significant correlation (** p < 0.01). B) Percent changes of the pRF sizes in V1-V4 from the parallel condition to the perpendicular condition (Experiment 2). The asterisk indicates that the percent change of the pRF size was significantly below zero (*p < 0.05). C) Percent changes of the pRF sizes in V1-V4 from the pre-training test to the post-training test (upper) and correlations between the percent improvement in orientation discrimination performance with the crowded target and the percent change of the pRF size in V2 (lower) (Experiment 3). Asterisks indicate that the percent change of the pRF size was significantly below zero (*** p < 0.001) or a significant correlation (*** p < 0.001). Error bars denote 1 SEM calculated across subjects.


First, even when the magnitude of the orientation crowding effect and the pRF size in V2 were measured independently, they were positively correlated across subjects (Figure 2A). Second, within subjects, the pRF size in V2 in the strong crowding condition was larger than that in the weak crowding condition (Figure 2B). Third, perceptual training alleviated the crowding effect. Meanwhile, it reduced the pRF size in V2. These two changes were remarkably correlated (Figure 2C). Taken together, these findings provide converging and causal evidence that the orientation crowding effect is closely associated with the pRF size in V2 (the smaller the pRF, the weaker the crowding effect) and suggest that the pRF in V2 is a potential bottleneck for visual orientation crowding.


This study was supported by the National Natural Scientific Foundation of China (NNSFC), the Ministry of Science and Technology of China (MOST), Beijing Municipal Science and Technology Commission (BMSTC) and the Peking-Tsinghua Center of Life Sciences (CLS).




He D., Wang Y. and Fang F. (2019). The critical role of V2 population receptive fields in visual orientation crowding. Current Biology. (in press)

Dumoulin, S.O., and Wandell, B.A. (2008). Population receptive field estimates in human visual cortex. Neuroimage 39, 647-660.

Mo C.*, He D. * and Fang F. (2018). Attention priority map of face images in human early visual cortex. Journal of Neuroscience. 38(1), 149-157.( *co-first authors)