Relative judgment seems to be the key: Revisiting the beck effect

In multiple-stimulus presentation, orientation disparity has been known to be more discriminable than disparity in line arrangement (e.g., J. Beck, 19 72). The source of the effect and its locus were studied in 7 experiments. In different experiments a discrimination between an upright T and either a tilted T or an L, or a discrimination between a tilted T and an L, was req uired, either in a single stimulus presentation or in the context of uprigh t Ts. Number of stimuli, location uncertainty, and adjacency between stimul i were manipulated. The results indicated that the effect is insensitive to these factors, which is incommensurate with predictions from several accou nts of the effect. All the effect requires is that disparate stimuli are si multaneously presented, suggesting that relative judgment is a necessary co ndition for its manifestation. The effect surfaces when the task calls for procedures based on perception of homogeneity or salience.