Exposure-based Learning Improved Orientation Discrimination Under Visual Crowding

Visual crowding refers to the impairment of object recognition in the presence of adjacent objects. Perceptual learning reduces peripheral crowded orientation discrimination and learning shows specificity to trained locations. Here we manipulate attention to crowded stimuli, to separate the impacts of top-down attention and bottom-up exposure, on learning with crowded orientation and its transfer to other locations. Observers reported the target orientation (a circular sinusoidal grating centered at 8°-eccentricity, 36°/126°) with two grating flankers with randomized orientation in pre-test and post-test. The orientation discrimination threshold was adjusted by staircase. Four groups of observers underwent five sessions of training or exposure. Results: (1) Baseline-group (N=8): Training improved crowded orientation discrimination, and the reduction of crowding was specific to the trained location. (2) Active-exposure-group (N=8): Crowded contrast discrimination training enabled complete learning transfer to crowded orientation discrimination, and the transfer was specific to the exposed location. (3) Passive-exposure-group (N=8): Observers responded to a central RSVP task while passively exposed to peripheral crowded gratings. Crowded orientation discrimination was substantially improved as continued training produced no further gains, and the reduction of crowding was evident in unexposed locations. (4) Subliminal-exposure-group (N=8): A “continuous-flash-suppression” technique was used to suppress the exposure of crowded gratings into sub-consciousness, meanwhile observers were asked to do a fovea dot color task. Crowded orientation discrimination was mostly improved, and the improvements partially transferred to unexposed locations. (5) A control group (N=9) ruled out the possibility that the improvements were due to the test-retest effect. The results demonstrated the capacity of the visual system to learn to reduce crowding by repeated exposure to crowded stimuli, which provides a complementary of plasticity to practice, attention-based, learning. Releasing spatial attention to crowded stimuli might decrease the location specificity in crowding learning. These findings shed new light on the mechanisms of crowding and learning.