Alerts: Sign up at www.eneuro.org/alerts to receive customized email alerts when the fully formatted version of this article is published.1. Title: Multivariate analysis of evoked responses during the rubber hand illusion suggests a temporal parcellation into manipulation and illusion-specific correlates 2. Abbreviated title: Multivariate analysis of EEG during the rubber hand illusion suggests a temporal parcellation into manipulation and illusion-specific correlates
Previous studies have reported correlates of bodily self-illusions such as the rubber hand in signatures of rhythmic brain activity. However, individual studies focused on specific variations of the rubber hand paradigm, used different experimental setups to induce this, or used different control conditions to isolate the neurophysiological signatures related to the illusory state, leaving the specificity of the reported illusion-signatures unclear. We here quantified correlates of the rubber hand illusion in EEG-derived oscillatory brain activity and asked two questions: which of the observed correlates are robust to the precise nature of the control conditions used as contrast for the illusory state, and whether such correlates emerge directly around the subjective illusion onset. To address these questions, we relied on two experimental configurations to induce the illusion, on different non-illusion conditions to isolate neurophysiological signatures of the illusory state, and we implemented an analysis directly focusing on the immediate moment of the illusion onset. Our results reveal a widespread suppression of alpha and beta-band activity associated with the illusory state in general, whereby the reduction of beta power prevailed around the immediate illusion onset. These results confirm previous reports of a suppression of alpha and beta rhythms during body illusions, but also highlight the difficulties to directly pinpoint the precise neurophysiological correlates of the illusory state.
Crossmodal correspondences describe our tendency to associate sensory features from different modalities with each other, such as the pitch of a sound with a specific size of a visual object. While such crossmodal correspondences (or associations) have been described in many behavioural studies their neurophysiological correlates remain debated. Under the current working model of multisensory perception both a low- and a high-level account seem plausible. That is, the neurophysiological processes shaping these associations could commence in early sensory regions, or may rather emerge only later and as a result from high-level multisensory integration processes or in semantic and object identification networks. We implemented a paradigm based on steady-state visual evoked potentials (SSVEP) to directly test the hypothesis that the associations between acoustic pitch and the visual features of size, hue or chromatic saturation are visible in brain activity from early visual cortices. Our results reveal a pitch-size congruency effect in the SSVEP over occipital electrodes and in the behavioural data, supporting a low-level account of this crossmodal association. For the associations between pitch and chromatic features we found no significant effects. We speculate that this signature of the pitch size association in early visual cortices reflects the successful pairing of congruent visual and acoustic object properties and may contribute to establishing causal relations between multisensory objects.
The neurophysiological processes reflecting the illusory ownership over an artificial limb remain debated. We used multivariate (cross-)classification of evoked EEG responses to probe for signatures of the illusion that robustly generalize across a number of confounding factors identified based on previous studies: the spatial arrangement of limbs, controls involving either a misaligned artificial object or participant’s own hand, and which provide evidence of illusory ownership directly within an experimental trial. Our results show that sensory-evoked responses differ between illusion and non-illusion epochs from early latencies on. While these responses exhibit distinct sensitivity to the experimental factors at distinct times, around 140 ms the evoked activity reflects the illusory state robustly across experimental manipulations. This neurophysiological signature of illusory ownership was not correlated with increases in skin conductance accompanying the illusion, suggesting that neurophysiological and bodily signals reflect distinct processes related to the embodiment of an artificial limb.
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