Cross-modal bias and perceptual fusion with auditory-visual spatial discordance

Investigations of situations involving spatial discordance between auditory and visual data which can otherwise be attributed to a common origin have revealed two main phenomena:cross-modal bias andperceptual fusion (or ventriloquism). The focus of the present study is the relationship between these two. The question asked was whether bias occurred only with fusion, as is predicted by some accounts of reactions to discordance, among them those based on cuesubstitution. The approach consisted of having subjects, on each trial, both point to signals in one modality in the presence of conflicting signals in the other modality and produce same-different origin judgments. To avoid the confounding of immediate effects with cumulative adaptation, which was allowed in most previous studies, the direction and amplitude of discordance was varied randomly from trial to trial. Experiment 1, which was a pilot study, showed that both visual bias of auditory localization and auditory bias of visual localization can be observed under such conditions. Experiment 2, which addressed the main question, used a method which controls for the selection involved in separating fusion from no-fusion trials and showed that the attraction of auditory localization by conflicting visual inputs occurs even when fusion is not reported. This result is inconsistent with purely postperceptual views of cross-modal interactions. The question could not be answered for auditory bias of visual localization, which, although significant, was very small in Experiment 1 and fell below significance under the conditions of Experiment 2.