Steady-state evoked potentials as an index of multisensory temporal binding Sylvie Nozaradan a, b , Isabelle Peretz b , André Mouraux a, ⁎ a Institute of Neuroscience (Ions), Université catholique de Louvain (UCL), Belgium b International Laboratory for Brain, Music and Sound Research (BRAMS), Université de Montréal (UdeM), Canada abstract article info Article history: Received 6 July 2011 Revised 10 November 2011 Accepted 22 November 2011 Available online 1 December 2011 Keywords: EEG Multisensory integration Rhythm perception Neuronal entrainment Steady-state evoked potentials Temporal congruency promotes perceptual binding of multisensory inputs. Here, we used EEG frequency- tagging to track cortical activities elicited by auditory and visual inputs separately, in the form of steady- state evoked potentials (SS-EPs). We tested whether SS-EPs could reveal a dynamic coupling of cortical activ- ities related to the binding of auditory and visual inputs conveying synchronous vs. non-synchronous tempo- ral periodicities, or beats. The temporally congruent audiovisual condition elicited markedly enhanced auditory and visual SS-EPs, as compared to the incongruent condition. Furthermore, an increased inter-trial phase coherence of both SS-EPs was observed in that condition. Taken together, these observations indicate that temporal congruency enhances the processing of multisensory inputs at sensory-specific stages of corti- cal processing, possibly through a dynamic binding by synchrony of the elicited activities and/or improved dynamic attending. Moreover, we show that EEG frequency-tagging with SS-EPs constitutes an effective tool to explore the neural dynamics of multisensory integration in the human brain. © 2011 Elsevier Inc. All rights reserved. Introduction Building coherent representations of the external world requires integrating and merging information concurrently sampled through our different senses (Gibson, 1966; Spence and Driver, 2004). Most events occurring in the environment concomitantly activate afferents from different sensory modalities. For example, the perception of an explosion simultaneously emitting light, noise, vibrations and heat requires the integration of combined visual, auditory and somatosen- sory inputs. Because the information conveyed by these different sen- sory modalities is often complementary, cross-modal integration of these inputs may provide information about the environment that is absent in any one modality presented in isolation and, hence, cross- modal integration may improve behavior (Adrian, 1949; Elliott et al., 2010; Stein and Meredith, 1993). Temporal congruency facilitates cross-modal integration (Bertelson, 1999; Fujisaki and Nishida, 2005; Petrini et al., 2009; Sekuler et al., 1997; Vatakis and Spence, 2006; Vroomen and Keetels, 2010; Welch and Warren, 1980; Zampini et al., 2003). Multisensory perception may result from a process of binding by synchrony of the cortical responses to sensory inputs sharing similar temporal dynamics (Kayser, 2009; Luo et al., 2010; Schroeder et al., 2008; Senkowski et al., 2008). Support for this hypothesis can be found in the electrophysiological recordings per- formed in the sensory cortices of monkeys where congruent multisenso- ry inputs elicit an increased phase coherence of neuronal oscillatory activity within the activated sensory cortices, as compared to incongru- ent multisensory inputs (Kayser and Logothetis, 2007; Kayser et al., 2008; Senkowski et al., 2007). Similarly, in humans, electroencephalo- graphic (EEG) recordings reveal that the congruency of combined audi- tory and visual stimulation enhances the magnitude of stimulus- induced EEG oscillations across both auditory and visual cortices (Luo et al., 2010; Schall et al., 2009; Schroeder et al., 2008). However, because of the unavoidable temporal overlap between the neural responses to concurrent streams of sensory input, disentangling the neural activities related to each sensory stream, although critical to study multisensory integration, is difficult (Besle et al., 2009). Hence, current knowledge of how the human brain extracts, integrates and exploits the temporal dy- namics of sensory input remains, at present, poorly understood. Frequency-tagging using EEG steady-state evoked potentials (SS- EPs) could overcome this limitation, and thus may constitute a mean to study, non-invasively, multisensory integration in humans (Regan, 1989; Regan and Heron, 1969). SS-EPs are elicited by the con- tinuous presentation of a sensory stimulus in which a given feature is modulated periodically at a given frequency. SS-EPs appear as an in- crease in the EEG frequency spectrum peaking specifically at the fre- quency of stimulation (Regan, 1989). Therefore, different SS-EP frequencies can be used to tag the different sensory inputs constitut- ing a multimodal stimulus and, thereby, isolate the neural activity re- lated specifically to each stream of sensory input (Morgan et al., 1996; Regan, 1989; Tononi et al., 1998). This frequency-tagging approach has already been used to characterize the neural activity triggered by intermodal interactions of selective attention, using simultaneous auditory and visual inputs and comparing the magnitude of SS-EPs obtained in unisensory vs. multisensory conditions, according to the NeuroImage 60 (2012) 21–28 ⁎ Corresponding author at: Institute of Neurosciences, Université catholique de Lou- vain, 53, Avenue Mounier – UCL 53.75, B-1200 Bruxelles, Belgium. E-mail address: andre.mouraux@uclouvain.be (A. Mouraux). 1053-8119/$ – see front matter © 2011 Elsevier Inc. All rights reserved. doi:10.1016/j.neuroimage.2011.11.065 Contents lists available at SciVerse ScienceDirect NeuroImage journal homepage: www.elsevier.com/locate/ynimg