It is well known that under certain conditions crossmodal interactions alter our perception of multisensory events, especially in conflicting/ambiguous situations, e.g. presenting physically separated (incongruent) audiovisual cues. In case of the ventriloquist illusion, subjects shift the location of an acoustic signal to a spatially discordant visual cue (perceptual fusion), i.e. subjects perceptually unify visual and acoustic stimuli as coming from the same source although they are spatially separated.
In a behavioral experiment applying a simple unification task we presented audiovisual stimuli in the freefield either spatially discordant or spatially concordant. Participants were required to judge spatial relation of co-occurring events (separation/unity). The focus of the present study was to quantify participants’ response behavior using a recently developed nonparametric adaptive sequential sampling procedure (Poppe et al., 2012). We tested four acoustic reference positions in central and para-central space (±8° and ±25° corresponding to left (–) and right (+) hemispace). Deviating visual signals were presented between 30° left and right of acoustic signals. Acquired psychometric functions allowed a detailed analysis of individual response behavior across all sampled conditions. Additionally, estimations of 50% correct response thresholds were used as guideline for the parameter setting of audiovisual stimuli in subsequent EEG experiments. We tested the hypothesis according to which the prestimulus cortical activity modulates the perceptual organization of a given stimulus and therefore determines the perceptual outcome of ambiguous audiovisual stimuli. For that EEG was recorded while presenting physically identical stimuli which, however, generate a perception that varies between two possible alternatives.

audiovisual; ambiguity; spatial processing; EEG