The perception of cross-modal simultaneity (or “the Greenwich Observatory Problem” revisited)

Abstract

One of the oldest questions in experimental psychology concerns the perception of simultaneous events, particularly when input arrives through two different sensory channels (such as sight and sound or touch and sound). How far apart in time do two events have to be before they are perceived as sequential? It was this problem in the Greenwich Observatory in 1795 that led to the first experiments in perceptual thresholds in the 1830s, and the development of psychophysics. Laboratory experiments in this century have yielded contradictory results about the magnitude of temporal offset (or asynchrony) that gives rise to the perception of successiveness, perhaps (we argue) because of the artificial nature of the tasks involved. Accurate successiveness and asynchrony judgments require the human organism to invoke precise neural anticipatory mechanisms, comparative mechanisms, as well as feedback and recursion modules in order to correctly perceive the actual order of events in the real world. Among many problems in modeling the neural apprehension of such anticipations and synchronization judgments is the fat that the time it takes for sensory signals to arrive at the brain is heavily dependent on the sensory modality stimulated. This paper reports data from new experiments in which the authors developed a cross-modal simultaneity task to have greater ecological validity. The results indicate that the threshold for successiveness is smaller than that found in previous experiments, suggesting that people are more accurate at discriminating temporal asynchrony than has been previously thought. The present findings are relevant to theories of time and order perception (1, 2, 3) and consciousness (4, 5).