Combining information across modalities can affect sensory performance. We studied how co-occurring sounds modulate behavioral visual detection sensitivity (d), and neural responses, for visual stimuli of higher or lower intensity. Co-occurrence of a sound enhanced human detection sensitivity for lower-but not higher-intensity visual targets. Functional magnetic resonance imaging (fMRI) linked this to boosts in activity-levels for sensory-specific visual and auditory cortex, plus multisensory superior temporal sulcus (STS), specifically for a lower-intensity visual event when paired with a sound. Thalamic structures in visual and auditory pathways, the lateral and medial geniculate bodies, respectively (LGB, MGB), showed a similar pattern. Subject-by-subject psychophysical benefits correlated with corresponding fMRI signals in visual, auditory, and multisensory regions. We also analyzed differential "coupling" patterns of LGB and MGB with other regions in the different experimental conditions. Effective-connectivity analyses showed enhanced coupling of sensoryspecific thalamic bodies with the affected cortical sites during enhanced detection of lower-intensity visual events paired with sounds. Coupling strength between visual and auditory thalamus with cortical regions, including STS, covaried parametrically with the psychophysical benefit for this specific multisensory context. Our results indicate that multisensory enhancement of detection sensitivity for low-contrast visual stimuli by co-occurring sounds reflects a brain network involving not only established multisensory STS and sensoryspecific cortex but also visual and auditory thalamus.

In everyday life our brain often receives information about events and objects in the real world via several sensory modalities, because natural objects often stimulate more than one sense. These different types of information are processed in our brain along different sensory-specific pathways, but are finally integrated into a unified percept. During the last years, studies provided compelling evidence that the neural basis of multisensory integration is not restricted to higher association areas of the cortex, but can already occur at low-level stages of sensory cortical processing and even in subcortical structures. In this article we will review the potential role of several thalamic structures in multisensory interplay and discuss their extensive anatomical connections with sensory-specific and multisensory cortical structures. We conclude that sensory-specific thalamic structures may act as a crucial processing node of multisensory interplay in addition to their traditional role as sensory relaying structure.

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