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Abstract:
Crossmodal stimuli can be perceived as being simultaneous even if they are not physically synchronous. This phenomenon has been attributed to different conduction delays. In this work we tested whether time in different modalities is processed independently or if crossmodal interaction influence the perception of synchrony. (1) If unimodal timing is processed independently, perceived simultaneity across modality pairs should be Transitive. For example, if modality A has to be presented 20ms before modality B to appear simultaneous and modality B 10ms before modality C, then A should be presented 30ms before C to appear simultaneous. Subjects made Temporal Order Judgments (TOJ) of asynchronous signals in three modality pairs (audio-visual, audio-tactile, visual-tactile). The Point of Subjective Simultaneity (PSS) calculated for each modality pair are not transitive, indicating that perceived time is not processed independently in each modality. (2) It has been shown that PSS of audio-visual signals can be recalibrated by the repeated presentation of asynchronous stimuli. It is not clear whether this effect is the result of an adaptation mechanism specific to the audio-visual modality pair or whether it is due to a common crossmodal mechanism. Using the same type of measurements, we show that PSS following presentation of an asynchronous audio-visual stimulus is not constant in the audio-tactile modality pair. Hence, crossmodal timing is also affected by a common adaptation mechanism. Since PSS for visual-tactile stimuli was not affected, audio-visual adaptation effects are likely the result of a phenomenal shift of the auditory events in time. Our results indicate that perceived timing in one modality depends on which other modality this is paired with and that perceived simultaneity changes also for non adapted modality pairs. These results are not consistent with independent-channels models of crossmodal timing, but they rather indicate that time perception is affected by crossmodal interactions.
