[Influence of auditory object formation by multimodal interaction.]
BACKGROUND: The task of assigning concurrent sounds to different auditory objects is known to depend on temporal and spectral cues. When tones of high and low frequencies are presented in alternation, they can be perceived as a single (integrated) melody, or as two parallel (segregated) melodic lines, according to the presentation rate and frequency distance between the sounds. At an intermediate distance or stimulation rate, the percept is ambiguous and alternates between segregated and integrated. This work studied whether an ambiguous sound organization could be modulated towards a robust integrated or a segregated percept by the synchronous presentation of visual cues. METHODS: Two interleaved sets of sounds, one high frequency and one low frequency set were presented with concurrent visual stimuli, synchronized to either a within-set frequency pattern or to the across-set intensity pattern. Elicitation of the mismatch negativity (MMN) component of event-related brain potentials served as indices for the segregated organization, when no task was performed with the sounds. As a result, MMN was elicited only when the visual pattern promoted the segregation of the sounds. RESULTS: By spatial analysis of the distribution of electromagnetic potentials, four separated neuronal sources underlying the obtained MMN response were identified. One pair was located bilaterally in temporal cortical structures and another pair in occipital areas, representing the auditory and visual origin of the MMN response, evoked by inverted triplets as used in this study. Thus, the results demonstrate cross-modal effects of visual information on auditory object perception.