Simultaneous presentation of functionally identical signals in two sensory modalities may theoretically yield a gain of information from increased input redundancy. The potential gain from probability summation depends, however, on the strategy adopted in integrating information, and may be further complicated by sensory interaction. Review of the literature on intersensory effects suggests that these in turn may be due to several distinct mechanisms. The difficulties of designing experiments to distinguish the various theoretical models are discussed, and the results of four experiments are presented.