Target occlusion and the confusion effect

Abstract One benefit for prey species living in groups is that predators find it harder to track and target individuals as aggregation size (or density) increases. While this ‘confusion effect’ is known to increase with factors such as object similarity and motion unpredictability, occlusion has received little empirical attention. This is despite the likelihood of a predator’s view of a target being obstructed by other group members moving in three-dimensional space. Using humans as surrogate predators in a computer-based visual search task, we tested how occlusion interacts with group size, target–distractor similarity and motion speed to affect the time to locate a target. Occlusion significantly increased the confusion effect, but only when the target was difficult to distinguish from distractors. When targets ‘popped out’ visually, occlusion had little or no effect, and search efficiency remained high. Surprisingly, motion did not modulate the confusion effect, at least at the tested speeds, and confusion was evident even in static objects. Overall, this implies that occlusion enhances confusion primarily by disrupting target re-identification when visual similarity is high, highlighting occlusion as an important but context-dependent factor in predator–prey interactions.