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1 Cognitive Neuroscience Section, IRCCS San Giovanni di Dio Fatebenefratelli, Brescia, Italy 2 Department of Biomedical Sciences and Biotechnology, National Institute of Neuroscience, University of Brescia, Brescia, Italy 3 Centre for Cognitive Neuroimaging (CCNi), Institute of Neuroscience and Psychology, University of Glasgow, Glasgow, UK 4 Wellcome Trust Centre for Neuroimaging at UCL, Institute of Neurology, University College London, London, UK 5 UCL Institute of Cognitive Neuroscience, University College London, London, UK

Multisensory interactions are a fundamental feature of brain organization. Principles governing multisensory processing have been established by varying stimulus location, timing and efficacy independently. Determining whether and how such principles operate when stimuli vary dynamically in their perceived distance (as when looming/receding) provides an assay for synergy among the above principles and also means for linking multisensory interactions between rudimentary stimuli with higher-order signals used for communication and motor planning.Humanparticipants indicated movement of looming or receding versus static stimuli that were visual, auditory, or multisensory combinations while 160-channel EEG was recorded. Multivariate EEG analyses and distributed source estimations were performed. Nonlinear interactions between looming signals were observed at early poststimulus latencies (~75 ms) in analyses of voltage waveforms, global field power, and source estimations. These looming-specific interactions positively correlated with reaction time facilitation, providing direct links between neural and performance metrics of multisensory integration. Statistical analyses of source estimations identified looming-specific interactions within the right claustrum/insula extending inferiorly into the amygdala and also within the bilateral cuneus extending into the inferior and lateral occipital cortices. Multisensory effects common to all conditions, regardless of perceived distance and congruity, followed (~115 ms) and manifested as faster transition between temporally stable brain networks (vs summed responses to unisensory conditions). We demonstrate the early-latency, synergistic interplay between existing principles of multisensory interactions. Such findings change the manner in which to model multisensory interactions at neural and behavioral/perceptual levels. We also provide neurophysiologic backing for the notion that looming signals receive preferential treatment during perception. © 2012 the authors.

Study Objectives: The aim of the study is to assess, in humans, transcallosal inhibition upon awakening from rapid eye movement (REM) and non-REM sleep, by paired-pulse transcranial magnetic stimulation (TMS). Design: During the daytime, a baseline session of motor evoked potentials (MEPs) was recorded. During the nighttime, the TMS sessions were administered just before sleep onset and upon awakenings from REM and stage 2 sleep, both in the early and final part of night. Setting: The sleep research laboratory at the University of Rome "La Sapienza." Participants: Ten right-handed subjects participated in the experiment for 4 consecutive sleep-recording nights. Interventions: N/A. Measurements and Results: During the daytime, a robust transcallosal inhibition was found; the MEP amplitude reduction ranged from 35% to 40%. During the nighttime, a decrease of transcallosal inhibition from right-to-left motor cortex, as compared to that from left-to-right motor cortex, was observed. The direct assessment of MEP changes, as a function of sleep stage and of the time of night, pointed to a drop of transcallosal inhibition after awakening from REM sleep. Therefore, the inhibitory activity of transcallosal fibers observed after non-REM awakening almost disappeared after REM sleep awakenings. Conclusions: The drastic drop of transcallosal inhibition after awakenings from REM sleep represents the first evidence in humans of a change of interhemispheric connectivity mediated by the corpus callosum during this sleep stage and may open new avenues for a better understanding of some aspects of sleep mechanisms (ie, dreaming function and dream mentation). Abbreviations: REM, rapid eye movement; MEP, motor evoked potentials; TMS, transcranial magnetic stimulation; EEG, electroencephalogram; ADM muscle, Abductor Digiti Minimi muscle; ISI, interstimulus intervals; MT, motor threshold; IHPP paradigm, interhemispheric paired pulse paradigm; ANOVA, analysis of variance.