Is there a timeline in the human brain? or The spatial coding of audiovisual timing information in the human brain

Participants in the present study had to judge whether audiovisual speech streams (~20 sec in duration) appeared to be synchronous or asynchronous (either auditory or visual leading) while fMRI-images were acquired. Individual delay-thresholds were determined prior to the scanning procedure. Statistical analysis of the imaging data differentiated participants’ stable perceptual states from their decisions. Focusing on the STS (an area known to be involved in the audiovisual integration of speech), we observed that distinct regions within this region showed an enhanced BOLD-response for the different experimental (a)synchrony conditions. These regions were further modulated by the participant’s perceptual state; a pattern of results that was observed in eachandevery participant. By calculating the distances between the modulated brain areas we highlight the fact that the ‘auditory’ and ‘visual leading areas’ lie closer to ‘synchrony areas’ than to each other. Taken together, these results highlight the presence of a putative ‘timeline’ within the STS (at least when processing audiovisual speech) that is dependent upon both the physical properties of the stimuli used and the participant’s perception of (a)synchrony.