4.1.4 Intermodal tasks
Psychophysical studies have attempted to address the issue of centralized versus distributed timing by comparing performance on intraversus intermodal tasks. In the intermodal tasks a standard interval may be demarcated by a tone at 0 ms and a flash of light at 100 ms. Performance on the intermodal condition is then compared to pure auditory and visual discrimination. The first observation that comes from these studies is that interval discrimination in the auditory modality is better then that in the visual modality (Rousseau et al. 1983, Grondin&Rosseau 1991). Additionally, these studies show that interval discrimination between modalities is significantly worse than that within modalities (Rousseau et al. 1983, Grondin & Rousseau 1991,Westheimer 1999).
These data are consistent with the notion of distributed timers. Specifically, because the stimulus features that delimit the interval in a cross-modality task are arriving at different timers, performance is decreased. However, an alternative explanation is that timing is still centralized, but intermodal timing is simply a more difficult task because it requires a shift of attention from one modality to the other.