Inter-trial priming of speed discrimination within modalities: vision vs audition
We investigated whether there were any inter-trial dependencies in our stimulus sequences. Even though our stimulus presentations were brief, a given trial in a sequence is often primed by the preceding one such that current percept shows a bias towards the previous one. This is a priming or ‘positive serial dependence’ effect that occurs for a wide range of stimuli41-44 including motion45. We conducted a sequential dependency analysis which involved binning all trials into two groups based on whether the speed on the preceding trial was slow (20, 40 & 50°/s) or fast (70, 80 & 100°/s). For each group, we then calculated the best-fitting psychometric functions and compared the PSEs. If the previous trial has no influence on the current one, then all trials would be independent and the psychometric functions calculated from the ‘previous slow’ and ‘previous fast’ groups should not differ in PSE.
Figure 3 shows the results of our serial dependence analysis. For both audition and vision, rates of ‘proportion faster’ responses were higher when the previous trial was fast, compared to when the previous trial was slow (Fig. 3A). This pattern is consistent with motion priming, a positive (or ‘attractive’) serial dependence effect often seen when preceding motion stimuli are brief46-48 and contrasts with motion adaptation – a negative (or ‘repulsive’) serial dependence that requires longer adaptation periods and produces the classic motion aftereffect49,50. A bootstrap sign-test confirmed that PSEs were significantly lower (leftward shift of the psychometric function) after ‘previous fast’ trials for both vision and audition (vision: p < .0001; audition: p < .0001), establishing that the previous trial was sufficient to prime the current trial and that vision and audition show the same effect.