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Abstract

Sensory processing times can greatly differ between the senses. Hence, signals from different modalities that are presented with a delay corresponding to the processing time difference between these modalities appear simultaneous to the observer.
We hypothesized that if processing is independent for each modality and if there is a common mechanism for the perception of simultaneity across modalities then subjective simultaneity should be transitive. For example, if modality A has to be presented 20ms before B and modality B 10ms before C to be perceived as synchronous, then modality A should be presented 30ms before C to seem synchronous with it.
Observers judged the temporal order of three different modality pairs (visual-tactile, tactile-auditory, and visual-auditory) for eleven stimulus onset asynchronies. Stimuli from the three conditions were not blocked but presented randomly to prevent attentional prior-entry effects that might lead to artifactual intransitivity.
From the responses, we determined the presentation delay leading to subjective simultaneity. To appear synchronous the visual signal has to be presented 34ms before the tactile, the tactile 55ms before the auditory, and the visual 28ms before the auditory. These results deviate significantly from transitivity. We conclude that either stimulus processing time in one modality depends on which other modality it is paired with, or the notion of a common mechanism for crossmodal simultaneity has to be rejected.