Dynamic backgrounds enhance the confusion effect

Natural environments are generally dynamic; examples of background motion include changes in illumination and moving physical features, such as wind-blown foliage or flowing water. Experimental evidence suggests dynamic backgrounds can impede predation by masking the movement of a target, but the interaction between background motion and the movement of prey in groups has not been investigated. Dynamic backgrounds could potentially enhance the confusion effect, the phenomenon whereby locating and/or tracking a single target among similar individuals ('distractors') becomes less successful owing to difficulty in discriminating this target from its conspecifics. We investigated whether dynamic backgrounds contribute to the confusion effect by impeding accurate tracking of a target among distractors. We hypothesize that any effect would be greatest when the temporal properties of the background's movement match those of the moving objects. In two experiments, human surrogate 'predators' tracked a single target among distractors, moving unpredictably, on backgrounds that drifted coherently in a given direction. Tracking was generally poorer when backgrounds and objects moved at more closely matched speeds, probably owing to temporary masking of prey target movement and subsequent difficulty in re-locating the target among distractors. Our findings identify a role for dynamic backgrounds in defending against predation.