No direction-specific bimodal facilitation for audiovisual motion detection.

After several decades of unimodal perceptual research, interest is turning increasingly to cross-modal interactions. At a physiological level, the existence of bimodal cells is well documented and it is known that correlated audiovisual input enhances localisation and orienting behaviours. Audiovisual perceptual interactions have also been demonstrated (e.g., the well-known McGurk effect). The present study explores motion perception and asks whether correlated audiovisual motion signals would be better detected than unimodal motions or bimodal motions in opposing directions. Using a dynamic random-dot field with variable motion coherence as a visual stimulus, together with an auditory motion defined by a stereo noise source smoothly translating along a horizontal trajectory, we find that correlated bimodal motion yields only a slight improvement (approximately a square root of two advantage) in detection threshold relative to unimodal detection. The size of this benefit is consistent with a statistical advantage rather than a bimodal facilitation account. Moreover, anticorrelated bimodal motion showed the same modest improvement, again speaking against linear summation but consistent with statistical combination of visual and auditory signals. These findings were replicated in peripheral as well as in central vision, and with translating visual objects as well as with spatially distributed visual motion. The superadditivity observed neurally (especially in deep-layer superior collicular cells), when weak unimodal signals are combined in bimodal cells does not apply to the detection of linear translational motion.