Does multisensory integration improve temporal numerosity judgements?

Abstract Multisensory integration happens automatically and creates a single, coherent and unambiguosly percept about the events in the world. Sometimes integration happens when sensory channels yield information about different events. A consistant body of literature seems to suggest that for incongruent sensory channels integration takes place based on the reliability of the channels combined with a prior knowledge about the signals. Since this theory has not yet been tested on congruent stimuli, we tested this with a temporal numerosity judgement task. Subjects where ordered to count series of pulses of different numbers which where congruently presented in different combinations of three modalities. One would predict that multimodal perception would yield a variance equal or smaller than the lowest variance of each unimodal percept. Our experimental data show that this is not the case. In some cases the variance of the trimodal stimuli was even higher than for each unimodal stimulus. Further we found some interesting characteristics in performance: -the estimated number of pulses is nearly linear with the presented number, but the smaller ISI, the smaller the gradient. -variance increases with the number of pulses and decreases with ISI. -trimodal estimates are usually better than bi- and unimodal.