We conducted behavioral experiments in which we investigated the influence of visual stimuli on auditory apparent motion (AAM) perception. Successive presentation of discrete auditory stimuli can induce the impression of a single moving stimulus. The quality of the perceived motion depends primarily on the Interstimulus Onset Interval (ISOI): Short ISOIs usually lead to the perception of a continuous motion at constant velocity, whereas longer ISOIs are perceived as broken, interrupted motion or even as a succession of discrete auditory events.
We investigated whether, and to which extent, the temporal threshold (i.e., the ISOI that determines the quality of AAM perception), is influenced by the presentation of visual stimuli which were temporally and spatially aligned between the auditory stimuli. In other words, visual stimuli were systematically added to fill the temporal and spatial gaps of the discretized auditory trajectory. The results of this coherent-bimodal condition were compared to a unimodal control condition in which only auditory stimuli were presented without any additional visual stimuli. In a second noncoherent-bimodal control condition, visual stimuli at random positions were added to each AAM sequence in order to assess whether the mere presence of visual stimuli may cause an alteration of the threshold due to attentional mechanisms.
The experimental setup consisted of 31 loudspeakers (Sennheiser HD201) arranged in a semicircle (radius 115 cm) with 31 light emitting diodes (LEDs) mounted in the middle of each speaker cone. AAM sequences were presented in four variations using different numbers of speakers or nodes of the apparent trajectory. During the experiment the ISOI was varied from 0 – 400ms. The participants’ task was to listen to the auditory stimuli (amplitude modulated white-noise) and categorize their perception of each stimulus sequence into one of four categories: (1) simultaneous presentation, (2) continuous motion, (3) broken motion and (4) succession. Each of the 17 subjects participated in the experimental condition (coherent-bimodal), control condition 1 (auditory), as well as control condition 2 (noncoherent-bimodal).
We found a facilitation of auditory apparent motion perception by systematically adding visual stimuli to the discretized auditory trajectory. Coherent-bimodal presentation led to a significant increase of the temporal threshold up to which continuous motion was perceived in comparison to the unimodal control condition, whereas no change of the threshold ISOI was found under the noncoherent-bimodal control condition. We interpret these results as evidence for audio-visual integration mechanisms in movement perception and assume a common bimodal representation of movement. This interpretation is corroborated by the assumption that audiovisual stimuli are mapped onto a common spatial reference frame within the neural system, as indicated by neurophysiological research and is consistent with findings of our earlier investigations on audio-visual integration mechanisms[1].

[1] Hofbauer, M., Wuerger, M., Meyer, G., Roehrbein, Florian, Schill, Kerstin and Zetzsche, Christoph, Catching audiovisual mice: Predicting the arrival time of audio-visual motion signals (2004), in: Cognitive, Affective and Behavioral Neuroscience, 4:2(241--250)