Expert-level understanding of social scenes requires early visual experience.

We studied 28 late-sighted Ethiopian children who were born with bilateral cataracts and remained nearly blind for years, recovering pattern-vision only in late childhood. This "natural experiment" offers a rare opportunity to assess the causal effect of early visual experience on later function acquisition. Here, we focus on vision-based understanding of human social interactions. The late-sighted were poorer than typically developing peers (albeit better than chance) in categorizing observed social scenes as friendly or aggressive, irrespective of the display format (i.e., full-body videos, still images, or point-light displays). This deficiency was maintained when retested later. They were also impaired in recognizing single-person attributes, which are useful for human interaction understanding (such as judging heading-direction based on biological-motion cues, or emotional states from body-posture gestures). Thus, the comprehension of visually observed socially relevant actions and body gestures is impaired in the late-sighted. We conclude that early visual experience is necessary for developing the skills required for utilizing visual cues for social scene understanding.