In Darkness Visual-Auditory Fusion Variations in Space Depend on Relation between Unimodal Egocentric Reference Frames

The present experiment investigates the frame of reference involved in Visual-Auditory (VA) fusion over space. This multisensory phenomenon refers to the perception of unity (Welch & Warren, 1986) resulting from visual and auditory stimuli despite their potential spatial disparity. The extent of this illusion depends on the eccentricity in azimuth of the bimodal stimulus (Godfroy, Roumes, & Dauchy, 2003). A recent study, performed in a luminous environment, has shown that variation of VA fusion is gaze dependent (Roumes, Hartnagel, & Godfroy, 2004). The present experiment questions the contribution of egoor allocentric visual cues by repeating the experiment in total darkness. Auditory and visual stimuli were displayed in synchrony sustaining various spatial disparities. Subjects had to judge about their unity (“fusion” or “non fusion”). Results showed that VA fusion in darkness remains gaze-dependent despite the lack of any allocentric cues and reinforced the assumption that the reference frame of the bimodal space is neither head-centered nor eye-centered but results from an integration phenomenon.