In everyday life, our brains decide about the relevance of huge amounts of sensory input. Further complicating this situation, this input is distributed over different modalities. This raises the question of how different sources of information interact for the control of overt attention during free exploration of the environment under natural conditions. Different modalities may work independently or interact to determine the consequent overt behavior. To answer this question, we presented natural images and lateralized natural sounds in a variety of conditions and we measured the eye movements of human subjects. We show that, in multimodal conditions, fixation probabilities increase on the side of the image where the sound originates showing that, at a coarser scale, lateralized auditory stimulation topographically increases the salience of the visual field. However, this shift of attention is specific because the probability of fixation of a given location on the side of the sound scales with the saliency of the visual stimulus, meaning that the selection of fixation points during multimodal conditions is dependent on the saliencies of both auditory and visual stimuli. Further analysis shows that a linear combination of both unimodal saliencies provides a good model for this integration process, which is optimal according to information-theoretical criteria. Our results support a functional joint saliency map, which integrates different unimodal saliencies before any decision is taken about the subsequent fixation point. These results provide guidelines for the performance and architecture of any model of overt attention that deals with more than one modality.