Exploring the role of visual perceptual grouping on the audiovisual integration of motion Daniel Sanabria, 1,CA Salvador Soto-Faraco 1,2 and Charles Spence 1 1 Department of Experimental Psychology, University of Oxford, South Parks Road, Oxford, OX13UD, UK; 2 Parc Cient|¤ ¢c de Barcelona, Universitat de Barcelona, Spain CA Corresponding Author: daniel.sanabria@psy.ox.ac.uk Received 23 July 2004; accepted 18 October 2004 In the present study, we explored the role of visual perceptual grouping on audiovisual motion integration, using an adaptation of the crossmodal dynamic capture task developed by Soto-Faraco et al. The principles of perceptual grouping were used to vary the per- ceived direction (horizontal vs vertical) and extent of apparent mo- tion within the visual modality. When the critical visual stimuli, giving rise to horizontal local motion, were embedded within a larger array of lights, giving rise to the perception of global motion vertically, the in£uence of visual motion information on the perception of auditory apparent motion (moving horizontally) was reduced signi¢cantly. These results highlight the need to consider intramodal perceptual grouping when investigating cross- modal perceptual grouping. NeuroReport 15:2745^2749  c 2004 Lippincott Williams & Wilkins. Key words: Auditory; Motion perception; Multisensory integration; Perceptual grouping; Visual INTRODUCTION The traditional approach to the study of perception involved examining each of the sensory modalities in isolation. By contrast, current research has more often considered perception as a consequence of multisensory integration [1]. The study of motion processing is no exception to this, and there is now a wealth of literature demonstrating that the perceptual system benefits from convergent motion information available to different sen- sory modalities. Neuroimaging studies have highlighted the multisensory convergence of motion information in a number of different brain areas in humans. For example, using fMRI, researchers have identified a number of sites of activation common to motion in the visual, auditory and tactile modalities [2,3]. Other studies have reported the activation of what have traditionally been thought of as unimodal auditory areas when investigating the pattern of brain activity during different types of unimodal visual motion [4]. Consistent with these neurophysiological data, recent behavioral studies have documented the existence of robust crossmodal links in motion perception [5]. However, despite the growing evidence for crossmodal interactions in the perception of audiovisual motion, little is known about the influence of processes occurring at the level of unimodal perceptual grouping. For instance, to what extent changes in the intramodal grouping of visual stimuli will modulate the crossmodal grouping of audiovisual stimuli [5,6]. In the present experiment, we explored the role of intramodal perceptual grouping on multisensory integra- tion [6]. Motion integration in humans was examined using an adaptation of Soto-Faraco et al.’s crossmodal dynamic capture task [7]. To date, the crossmodal dynamic capture task has most frequently involved participants responding to the direction of an auditory apparent motion stream (elicited by the sequential presentation of two sounds) while trying to ignore a synchronously-presented visual apparent motion stream (elicited by the sequential presentation of two visual stimuli). Typically, an impairment in perfor- mance is observed in directionally incongruent trials (i.e., when the visual and auditory motion streams move in opposite directions) as compared to directionally congruent trials (i.e., when the visual and auditory motion streams move in the same direction). In order to explore the influence of visual perceptual grouping on audiovisual integration, we manipulated the number and spatial distribution of visual stimuli presented. In the crucial experimental condition we presented a visual apparent motion stream with two potential perceptual interpretations that had orthogonal directions: One consisting of two lights moving in the horizontal plane, and the other consisting of four lights moving vertically. According to the classic grouping principles, we expected the local horizontal visual motion to be subsumed within the larger global vertical visual stream [8]. Because of the different direction of the visual (vertical) and auditory (horizontal) motion streams we therefore expected to find a reduced effect of the visual motion information on the perception of the direction of the auditory motion in the critical experimental condition (i.e., when more visual stimuli were presented) compared to the typical 2 lights–2 sounds condition. We therefore hoped to demonstrate a modulation of audiovisual motion integra- tion as a consequence of changes in perceptual grouping taking place within vision [6,9,10]. AUDITORYAND VESTIBULAR SYSTEMS NEUROREPORT 0959-4965  c Lippincott Williams & Wilkins Vol 15 No 18 22 December 2004 2745 Copyright © Lippincott Williams & Wilkins. Unauthorized reproduction of this article is prohibited.