The interactions between intrinsic grouping principles (by proximity or by similarity in shape or luminance) and extrinsic grouping by common region were examined by presenting both principles acting alone or conjoined with another principle in visual patterns. The procedure used in our study closely mirrored that of Quinlan and Wilton (1998 Perception 27 417 – 430). However, in the present experiment, the intrinsic and extrinsic grouping principles involved had similar relative salience in order to avoid their possible effects on interactions. The results showed that the grouping effect of conjoined cooperating principles was greater than that of either principle acting alone, and the grouping effect of conjoined competing principles was lower than that of either principle operating alone. Compatibility of data with additive effects of grouping principles has been examined.
