Multisensory binding: is the contribution of synchrony and semantic congruency obligatory?

•Synchrony perception and semantic congruency in multisensory binding.•Conflicting behavioral evidence on the assumption of unity in multisensory events.•Integration and synchrony may be separate mechanisms.•Semantics and timing produce enhancement or suppression based on incongruency.

We perceive the world as a unified whole with multisensory events being 'aligned' in every possible sense. This 'aligned' sense is a complex orchestration of multiple factors and underlying mechanisms, here we focus on two: synchrony and semantic (or informational) congruency. These factors, the former structural and the latter cognitive, appear to favor the binding of multisensory stimuli, leading in a coherent unified percept. Furthermore, the strong binding of the senses affects our perception of synchrony by making us tolerant to large temporal discrepancies between the input sensory streams. A longstanding debate in the field concerns the contribution of low- and high-level factors in the merging operation (i.e., unity assumption). Recent neuroimaging studies propose the existence of a brain network responsible for multisensory integration, consisting of frontal, temporal, and primary sensory areas, each responding to different stimulus properties. Converging evidence suggests the dissociation of integration and synchrony perception, which is consistent with the view that these processes entail distinct mechanisms, both anatomically and functionally.