An analysis of the processing of intramodal and intermodal time intervals.

In this 3-experiment study, the Weber fractions in the 300-ms and 900-ms duration ranges are obtained with 9 types of empty intervals resulting from the combinations of three types of signals for marking the beginning and end of the signals: auditory (A), visual (V), or tactile (T). There were three types of intramodal intervals (AA, TT, and VV) and 6 types of intermodal intervals (AT, AV, VA, VT, TA, and TV). The second marker is always the same during Experiments 1 (A), 2 (V), and 3 (T). With an uncertainty strategy where the first marker is 1 of 2 sensory signals being presented randomly from trial to trial, the study provides direct comparisons of the perceived length of the different marker-type intervals. The results reveal that the Weber fraction is nearly constant in the three types of intramodal intervals, but is clearly lower at 900 ms than at 300 ms in intermodal conditions. In several cases, the intramodal intervals are perceived as shorter than intermodal intervals, which is interpreted as an effect of the efficiency in detecting the second marker of an intramodal interval. There were no significant differences between the TA and VA intervals (Experiment 1) and between the AV and TV intervals (Experiment 2), but in Experiment 3, the AT intervals were perceived as longer than the VT intervals. The results are interpreted in terms of the generalized form of Weber's law, using the properties of the signals for explaining the additional nontemporal noise observed in the intermodal conditions.