Optimising the sound-induced flash illusion paradigm: Determining the minimum number of trials required for robust multisensory integration measurement

Psychophysical tasks often require substantial time to complete, increasing dropout rates and limiting overall participation. The sound-induced flash illusion is a prime example in which participants are typically required to complete a large number of trials, resulting in lengthy sessions. To address this issue, the present study aimed to identify the minimum number of trials needed to obtain robust audiovisual integration measurements in the SIFI paradigm across young (18-30 years old) and older (50+ years) adults. More specifically, participants completed 20 “blocks” of trials, each requiring them to judge whether they saw one or two flashes across all possible trial conditions (different combinations of visual and auditory stimuli) once. This allowed us to evaluate how many blocks (i.e., trial repetitions) are needed to obtain SIFI results equivalent to the full 20-block version using a Region of Practical Equivalence analysis. Results showed that younger participants required 10 blocks to achieve group-level equivalence and 16 blocks to achieve individual-level equivalence in their SIFI results. Similarly, older adults required 11 blocks of the SIFI for group-level equivalence and 15 blocks for individual-level equivalence. Taken together, these findings indicate that robust group-level SIFI measurements can be achieved in approximately 10-11 trial repetitions, whereas individual analyses require 15-16 repetitions across both age groups. Thus, our results demonstrate that the SIFI paradigm can be substantially shortened while retaining reliable measurements of audiovisual integration for both young and older adults, offering a more time-efficient and accessible version of the SIFI task.