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Abstract

When signals in different modalities are physically asynchronous, observers may still perceive them as being simultaneous due to differential physical transmission and physiological conduction delays.
If sensory signals in different modalities are processed independently of each other as assumed by independent-channels models, then the relative timings that lead to perceived simultaneity should be transitive across several modality pairs. For instance, if modality A has to be presented 20 ms before modality B to seem simultaneous with it and modality B 10 ms before modality C, then A should be presented 30 ms before C to seem simultaneous with it.
Using temporal order judgments we measured the point of subjective simultaneity (PSS) in three different modality pairs (visual-auditory, tactile-auditory, visual-tactile). Our results indicate that PSS are not transitive. Thus, we infer that signals are not processed independently from each other. Perceived signal timing in one modality depends on which other modality it is paired with. Therefore, independent-channels models cannot account for processes underlying decisions about simultaneity of signals in different modalities.