Evidence against independence of peripheral latencies and central threshold in temporal-order perception

Independent-channels models of temporal-order and simultaneity perception assume that a central (amodal) timing mechanism compares the arrival times of incoming sensory signals independently of peripheral (modal) events. This implies that peripheral latencies and central threshold are additive. The present study tested whether the central threshold is invariant to intramodal versus intermodal stimulation (Experiment 1) and strong versus weak intermodal stimulation (Experiment 2). The central threshold was isolated via the distance between the two psychometric functions in the ternary-response task, which does not depend on peripheral latencies under the additivity assumption. In Experiment 1, the central threshold was estimated to be significantly higher for intermodal stimuli (light and sound) than for intramodal stimuli (two lights) for all subjects across the entire practice curve (20 one-hour sessions) and also at the group level. In Experiment 2, the estimates of the central threshold were significantly higher for weak intermodal stimuli (dim light and soft sound) than for strong ones (bright light and loud sound) for most subjects and at the group level. These violations of threshold invariance in both experiments provide converging evidence against the independence of peripheral latencies and central threshold, and thus challenge the dominant class of theoretical models of temporal-order and simultaneity perception.