Diametrical modulation of tactile and visual perceptual thresholds during the rubber hand illusion: a predictive coding account.

During the rubber hand illusion (RHI), the synchronous stroking of the participants' concealed hand and a visible rubber hand induces a conflict among visuo-tactile inputs, leading healthy subjects to perceive the illusion of being touched on the rubber hand, as if it were part of their body. The predictive coding theory suggests that the RHI emerges to settle the conflict, attenuating somatosensory inputs in favour of visual ones, which "capture" tactile sensations. Here, we employed the psychophysical measure of perceptual threshold to measure a behavioural correlate of the somatosensory and visual modulations, to better understand the mechanisms underpinning the illusion. Before and after the RHI, participants underwent a tactile (Experiment 1) and a visual (Experiment 2) task, wherein they had to detect stimuli slightly above the perceptual threshold. According to the predictive coding framework, we found a significant decrease of tactile detection (i.e. increased tactile perceptual threshold) and a significant increase of visual detection (i.e.  decreased visual perceptual threshold), suggesting a diametrical modulation of somatosensory and visual perceptual processes. These findings provide evidence of how our system plastically adapts to uncertainty, attributing different weights to sensory inputs to restore a coherent representation of the own body.