Temporal interval discrimination thresholds depend on perceived synchrony for audio-visual stimulus pairs

Audio-visual stimulus pairs presented at various relative delays, are commonly judged as being "synchronous" over a range of delays from about -50 ms (audio leading) to +150 ms (video leading). The center of this range is an estimate of the point of subjective simultaneity (PSS). The judgment boundaries, where "synchronous" judgments yield to a predominance of "asynchronous" judgments, usually include physical synchrony, or the point of objective simultaneity (POS). When the POS is used as a standard in a delay discrimination paradigm, thresholds tend to be asymmetric; that is, smaller on the audio-leading side than on the video-leading side of the standard. We replicated this finding using different stimulus types, and standards distributed within and beyond the synchrony judgment boundaries. Thresholds were symmetric near the center of the synchrony range (PSS), but showed opposite patterns of asymmetry as the standard approached either of the two synchrony judgment boundaries. That is, apparent synchrony shows a type of categorical perception in that discriminations within the synchrony category are difficult, but those near the category boundary are easy.