Search

Studies on multisensory perception often focus on simplistic conditions where one signal is presented per modality. Yet, in everyday life we encounter multiple signals per modality that may be associated with each other. Using the spatial ventriloquism paradigm as model, we tested how the presence of two visual stimuli shapes sound location judgements. Across three experiments we systematically paired one sound with either one or two visual stimuli that differed in relative timing and spatial offset to the sound. We focused on two biases typically studied in this paradigm, the ventriloquism effect and the trial-wise aftereffect. Our results show that these are not shaped by a single visual stimulus in a winner-takes-all manner but rather by their collective influence. In particular, the contribution of individual visual stimuli to the ventriloquism bias depends on their own relative timing to the sound and the temporal context provided by the other visual signal. Depending on this context, two visual signals influence the collective bias with comparable or asymmetric weights, which we attribute to the temporal evolvement of evidence for sensory causal relations during the individual trials. Concerning the aftereffect, we show that this emerges following both auditory and visual judgements in the preceding AV trials and following asynchronous AV stimulus pairs, confirming the notion that this reflects an automatic adjustment to spatial discrepancies.

Any series of sensorimotor actions shows fluctuations in speed and accuracy from repetition to repetition, even when the sensory input and the motor output requirements remain identical over time. Such fluctuations are particularly prominent in reaction time (RT) series from laboratory neurocognitive tasks. Despite their omnipresent nature, trial-to-trial fluctuations remain poorly understood. Here, we systematically analysed RT series from various neurocognitive tasks, quantifying how much of the total trial-to-trial RT variance can be explained by three sources of variability that are frequently investigated in behavioural and neuroscientific research: 1) the experimental conditions, which are commonly employed to induce systematic patterns in variability, 2) short-term temporal dependencies such as the autocorrelation between subsequent trials, and 3) long-term temporal trends over entire experimental blocks and sessions. Furthermore, we examined to what extent the explained variances by these sources are shared or unique. In total, we analysed 1913 unique RT series from 30 different cognitive control and perception-based tasks. On average, the three sources together explained only about 8-17% of the total variance. The experimental conditions explained on average about 2.5-3.5%, but did not share explained variance with temporal dependencies. Thus, the largest part of the trial-to-trial fluctuations in RT remained unexplained by the three variability sources that are typically analysed here. These may be attributable to observable endogenous factors, such as ongoing fluctuations in brain activity and bodily states, though some extent of randomness may be a feature of behaviour rather than just nuisance.