Causal Inference in the Perception of Verticality
Authors: Heinrich H B√ºlthoff
Ksander N de Winkel
Mikhail Katliar
Daniel Diers
Journal: Scientific Reports (2018)
Discipline: Multidisciplinary
Keywords: perceptual upright; central nervous system; vector sum; visual system; body's inertial sensors
Affiliations: Department of Human Perception, Cognition, and Action, Max Planck Institute for Biological Cybernetics, Max-Planck-Ring 8, 72076, T√ºbingen, Germany. ., Department of Human Perception, Cognition, and Action, Max Planck Institute for Biological Cybernetics, Max-Planck-Ring 8, 72076, T√ºbingen, Germany.

Abstract:  The perceptual upright is thought to be constructed by the central nervous system (CNS) as a vector sum; by combining estimates on the upright provided by the visual system and the body's inertial sensors with prior knowledge that upright is usually above the head. Recent findings furthermore show that the weighting of the respective sensory signals is proportional to their reliability, consistent with a Bayesian interpretation of a vector sum (Forced Fusion, FF). However, violations of FF have also been reported, suggesting that the CNS may rely on a single sensory system (Cue Capture, CC), or choose to process sensory signals based on inferred signal causality (Causal Inference, CI). We developed a novel alternative-reality system to manipulate visual and physical tilt independently. We tasked participants (n = 36) to indicate the perceived upright for various (in-)congruent combinations of visual-inertial stimuli, and compared models based on their agreement with the data. The results favor the CI model over FF, although this effect became unambiguous only for large discrepancies (¬±60¬∞). We conclude that the notion of a vector sum does not provide a comprehensive explanation of the perception of the upright, and that CI offers a better alternative. 

Retracted: No

PubPeer comments:
 u201cModels that can account for different behaviors depending on inferred causality are Causal Inference (CI) models22,23. Put simply, these models state that the CNS constructs intermediate estimates of environmental properties consistent with different interpretations of their causes (i.e., a common cause or separate causes) in tandem, and combines these into final estimates, **taking into account a-priori beliefs on the probability of alternative causal structures.** We hypothesized that CI models provide a better explanation of the perceptual upright than the vector-sum approach. In an experiment, we independently manipulated participantsu2019 physical and visual orientation with respect to the true vertical, and tasked them to provide estimates of what they thought was the true, physical, upright. We developed statistical versions of prominent models of multisensory perception and compared their ability to explain the participantsu2019 responses.u201d u201cWe conclude that the notion of a vector sum does not provide a comprehensive explanation of the perception of the upright, and that **CI offers a better alternative.**u201d I wonder if the authors have experienced the tilted room phenomenon. It is quite extraordinary. You walk into a space that you know is tilted u2013 walls, floor, etc., your organs of balance are all in accord that you are upright, yet you keep almost falling down because you keep trying to  right yourself, because your visual percept indicates that you are tilted (erroneously, in this case). How does a model of u201ccausal inferenceu201d deal with the fact that in this and many other cases, definite knowledge, not mere belief, as well as organs of balance and proprioception seem to have *no* influence on correcting the sense of body tilt experienced? The spinning room is another example of the ease with which we may create conditions that cause us to experience effects (in this case, that we, not the room, are spinning wildly) that violate what we know, and what our bodily sense organs are telling us. (A related phenomenon is the experience of feeling our car rolling backward when in fact the vehicle next to us is rolling forward). It is a well-known and well-studied fact that our visual perceptual environment dominates our sense of the relationship of our bodies to our environment, including the gravitational vertical. These all seem to violate idea that, with respect to our perception of this relationship, eitheru201cprior beliefsu201d or body senses play a significant role. ,