Evolving the keys to visual crowding

Peripheral vision can be severely impaired by nearby clutter. Decades of research using sparse displays have established that this phenomenon, known as visual crowding, follows Bouma's rule: Interference occurs for target-distractor separations up to half the target's eccentricity. Although considered a fundamental constraint on human vision, it is unclear whether Bouma's rule holds in dense heterogeneous visual environments. Using a genetic algorithm we investigated crowding in densely cluttered displays. Participants were instructed to identify the orientation of a target line (6° eccentricity) among 284 distractor lines. Displays supporting highest accuracy were selected ("survival of the fittest") and combined to create new displays. Performance improved over generations, predominantly driven by the emergence of horizontal flankers within 1° of the near-vertical target, but with no evidence of interference beyond this radius. We conclude that Bouma's rule does not necessarily hold in densely cluttered displays. Instead, a nearest-neighbor segmentation rule provides a better account.