Visual crowding is the disruptive effect of clutter on object recognition. Although most prominent in adult peripheral vision, crowding also disrupts foveal vision in typically-developing children and those with strabismic amblyopia. Do these crowding effects share the same mechanism? Here we exploit observations that crowded errors in peripheral vision are not random: target objects appear either averaged with the flankers (assimilation), or replaced by them (substitution). If amblyopic and

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... elopmental crowding share the same mechanism then their errors should be similarly systematic. We tested foveal vision in children aged 3-9 years with typical vision or strabismic amblyopia, and peripheral vision in adults. The perceptual effects of crowding were measured by requiring observers to adjust a reference stimulus to match the perceived orientation of a target "Vac-Man" element. When the target was surrounded by flankers that differed by 30deg., adults and children reported orientations between the target and flankers (assimilation). Errors were reduced with 90deg. differences, but primarily matched the flanker orientation (substitution) when they did occur. A population pooling model of crowding successfully simulated this pattern of errors in all three groups. We conclude that the perceptual effects of amblyopic and developing crowding are systematic and resemble the near periphery in adults, suggesting a common underlying mechanism.