title: The roles of physical and physiological simultaneity in audiovisual multisensory facilitation.
abstract: A series of experiments measured the audiovisual stimulus onset asynchrony (SOAAV), yielding facilitative multisensory integration. We evaluated (1) the range of SOAAV over which facilitation occurred when unisensory stimuli were weak; (2) whether the range of SOAAV producing facilitation supported the hypothesis that physiological simultaneity of unisensory activity governs multisensory facilitation; and (3) whether AV multisensory facilitation depended on relative stimulus intensity. We compared response-time distributions to unisensory auditory (A) and visual (V) stimuli with those to AV stimuli over a wide range (300 and 20 ms increments) of SOAAV, across four conditions of varying stimulus intensity. In condition 1, the intensity of unisensory stimuli was adjusted such that d' ≈ 2. In condition 2, V stimulus intensity was increased (d' > 4), while A stimulus intensity was as in condition 1. In condition 3, A stimulus intensity was increased (d' > 4) while V stimulus intensity was as in condition 1. In condition 4, both A and V stimulus intensities were increased to clearly suprathreshold levels (d' > 4). Across all conditions of stimulus intensity, significant multisensory facilitation occurred exclusively for simultaneously presented A and V stimuli. In addition, facilitation increased as stimulus intensity increased, in disagreement with inverse effectiveness. These results indicate that the requirements for facilitative multisensory integration include both physical and physiological simultaneity.
authors.affiliation: Center for Visual and Cognitive Neuroscience, Department of Psychology, NDSU Department 2765, P.O. Box 6050, College of Science and Mathematics, North Dakota State University, Fargo, ND 58105-6050, USA; e-mail: lynnette.leone@ndsu.edu.
authors.forename: Lynnette M
authors.initials: LM
authors.lastname: Leone
authors.affiliation: Center for Visual and Cognitive Neuroscience, Department of Psychology, NDSU Department 2765, P.O. Box 6050, College of Science and Mathematics, North Dakota State University, Fargo, ND 58105-6050, USA; e-mail: mark.mccourt@ndsu.edu.
authors.forename: Mark E
authors.initials: ME
authors.lastname: McCourt
keywords: d′; inverse effectiveness; multisensory integration; neural coactivation; race model; reaction time; simultaneity