Is it me or the train moving? Humans resolve sensory conflicts with a nonlinear feedback mechanism in balance control

Humans use multiple sensory systems to estimate body orientation in space. A predominant concept for the underlying multisensory integration (MSI) is the linear summation of weighted inputs from individual sensory systems. The sensory contributions change depending on context and environmental conditions. These changes are typically attributed to ‘reweighting’ by some higher order mechanism. We provide evidence for a conceptually different mechanism that combines 1) sensory inputs with fixed weights and 2) multisensory corrections, if the reference of the sensory inputs move in space and are therefore unreliable. This reconstruction of the sensory reference frame motion (RFM) allows humans to use vision and automatically counteract erroneous inputs, e.g. when looking at a moving train. The proposed RFM estimator contains a nonlinear dead-zone that blocks corrections at slow velocities. We first demonstrate that this mechanism accounts for changes in sensory contributions. Secondly, we hypothesized that such a nonlinearity would distort balance responses to perturbations in a very specific way. We predicted such distortions in body sway using a balance control model. Experiments using visual scene movements as a specific implementation of RFMs confirmed the predictions. The findings indicate that the central nervous system resolves sensory conflicts by an internal reconstruction of the cause of the conflict. Thus, the mechanism links the concept of causal inference to shifts in sensory contributions, providing a cohesive picture of MSI for the estimation of body orientation in space.