The roles of physical and physiological simultaneity in audiovisual multisensory facilitation.

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.