Audiovisual estimation of Time-to-contact.

Time-to-contact (TTC) is the remaining time for a moving object to reach its observer. A good estimation of TTC is essential in everyday situations such as crossing a road or catching a ball. So far, most studies have only looked at estimation of TTC at constant speed, and in visual condition, whereas it is a multisensory task by essence. In our study, we investigated TTC estimation at constant or accelerated speed for three modalities: Auditory, Visual and Audiovisual. At constant speed, it has been demonstrated that TTC estimation performance is already accurate so the addition of auditory cues would not lead to significant performance changes. However, at accelerated speed, visual estimation of TTC becomes more impaired. In this context, auditory cues were expected to play a more prominent role in improving performance. For this reason, we hypothesized that V and AV performance would be identical at constant speed, whereas at accelerated speed, auditory cues would allow a better performance in AV compared to V. Our results show that observers do use both modalities in the AV condition and therefore demonstrate a multisensory integration, but for both levels of acceleration and better performance is not always observed in the multimodal condition. Specifically, auditory cues lead to an underestimation of TTC, which compensates for the visual overestimation. Therefore, whether multisensory integration has a beneficial or detrimental effect on the performances of TTC estimation depends on the level of baseline error in the visual-only and auditory-only conditions.