Bimodal sensory discrimination is finer than dual single modality discrimination

Here we show that discriminating between different signal modulation rates can be easier when stimuli are presented in two modalities (vision and audition) rather than just one. This was true even when the single modality signal was repeated. This facilitation did not require simultaneous presentations in both modalities and therefore cannot rely on sensory fusion. Signal detection threshold for bimodal signals and double single modality signals were found to be equivalent indicating that the double single modality signals were not intrinsically noisier. The lack of facilitation in double single modality conditions was not due to inaccessibility of the first sample because there is no performance difference when noise was added to either the first or second samples. We propose that the bimodal signal discrimination advantage arises from fluctuations in the magnitude of sensory noise over time and because observers select the most reliable modality on a trial by trial basis. Noise levels within repeated single modality trials are more likely to be similar than those within signals from different modalities. As a consequence, signal selection would be less effective in the former circumstances. Overall, our findings illustrate the advantage of using separate sensory channels to achieve reliable information processing.