To interpret our environment, we integrate information from all our senses. For moving objects, auditory and visual motion signals are correlated and provide information about the speed and the direction of the moving object. We investigated at what level the auditory and the visual modalities interact and whether the human brain integrates only motion signals that are ecologically valid. We found that the sensitivity for identifying motion was improved when motion signals were provided in both modalities. This improvement in sensitivity can be explained by probability summation. That is, auditory and visual stimuli are combined at a decision level, after the stimuli have been processed independently in the auditory and the visual pathways. Furthermore, this integration is direction blind and is not restricted to ecologically valid motion signals.