Causal relationships affect audio-visual asynchrony detection : opposite trends for different stimuli

Thresholds for discriminating physically asynchronous from physically synchronous audio-visual stimuli are asymmetric: thresholds are smaller when the auditory component leads than when the visual component leads. Here we show that this asymmetry can be reversed by changing the apparent causal relationship between the components. We used a simulation of Newton's Cradle, showing a left-to-right pendulum movement, presented in three visual conditions: (1) the entire stimulus, (2) the left half only, or (3) the right half only. In conditions 1 and 2, the visual movement appears to cause the collision and thus the auditory component, whereas in condition 3 the movement appears to result from the collision. Audio-visual discrimination thresholds were measured using a 3-interval, 2-alternative forced-choice procedure with a variable (adaptive) asynchronous target delay. Conditions 1 and 2 resulted in smaller thresholds when the auditory component led, whereas condition 3 showed a trend in the opposite direction; thresholds were smaller when the visual component led. Relating these thresholds to perceptual synchrony judgments shows that the thresholds tend to be smaller near the boundaries of the synchrony judgment range. Both threshold measurements and the distribution of synchrony judgments show the same types of shifts in response to changes in implied causality.