Multisensory Interactions in Head and Body Centered Perception of Verticality

Percepts of verticality are thought to be constructed as a weighted average of multisensory inputs, but the observed weights differ considerably between studies. In the present study, we evaluate whether this can be explained by differences in how visual, somatosensory and proprioceptive cues contribute to representations of the Head In Space (HIS) and Body In Space (BIS). Participants (10) were standing on a force plate on top of a motion platform while wearing a visualization device that allowed us to artificially tilt their visual surroundings. They were presented with (in)congruent combinations of visual, platform, and head tilt, and performed Rod & Frame Test (RFT) and Subjective Postural Vertical (SPV) tasks. We also recorded postural responses to evaluate the relation between perception and balance. The perception data shows that body tilt, head tilt, and visual tilt affect the HIS and BIS in both experimental tasks. For the RFT task, visual tilt induced considerable biases (≈ 10° for 36° visual tilt) in the direction of the vertical expressed in the visual scene; for the SPV task, participants also adjusted platform tilt to correct for illusory body tilt induced by the visual stimuli, but effects were much smaller (≈ 0.25°). Likewise, postural data from the SPV task indicate participants slightly shifted their weight to counteract visual tilt (0.3° for 36° visual tilt). The data reveal a striking dissociation of visual effects between the two tasks. We find that the data can be explained well using a model where percepts of the HIS and BIS are constructed from direct signals from head and body sensors, respectively, and indirect signals based on body and head signals but corrected for perceived neck tilt. These findings show that perception of the HIS and BIS derive from the same sensory signals, but see profoundly different weighting factors. We conclude that observations of different weightings between studies likely result from querying of distinct latent constructs referenced to the body or head in space.


Instructions for participants
Dear sir/madam, Thank you for your interest in participating in this study. The purpose of this text is to provide you with information regarding the goals and design of the experiment 'Interactions between the Subjective Visual and Postural representations of Verticality'. This information should allow you to make a well-informed decision regarding your willingness to participate in this study.
The present study is designed to determine how our brain constructs percepts of upright, how these percepts affect balance, and how this process may change as we age. Resolving this question is of importance to neuroscience and may ultimately have applications in rehabilitation after stroke. It is known that visual and physical information contribute to perception of an upright. To determine exactly how this information is combined, we will manipulate what you see and feel as upright while you stand on a motion platform. These manipulations will be achieved by using a Head-Mounted Display (HMD) and a motion platform, on a series of experimental trials. On each trial, the platform and HMD will independently tilt to particular target angles. It is your task to estimate the true direction of upright (that is, gravity) in two different tasks:  Subjective Visual Vertical: a rod is shown in the image visible in the HMD. Your task is to adjust the orientation of the rod until it aligns with your feeling of upright using a game controller. Imagine that the rod reflects your body and put it upright to the real world (in other words: such that it aligns with the direction of gravity).
 Subjective Postural Vertical: when the platform and HMD have reached the target tilt angles, use the game controller to adjust the platform orientation until the platform is level, and you are upright.
Both tasks will be performed with the head aligned with the body, as well as with the head tilted to the right, making a total of four experimental blocks.
Try to perform the tasks intuitively: when the motions have stopped, orient the rod (SVV) or platform (SPV) to align with your first impression. Do not close your eyes or sway your body intentionally. Focus your gaze on the pink post-it note that is attached to the middle of the hand rail on the stairs, and do not look down to your feet. Confirm your responses and advance to new trials by button press. The experimenter will show you which buttons to use to perform each task. For each block with the head upright, there will be 63 trials; for each block with the head tilted, there will be 30 trials. After each block there will be a break. The experimenter will tell you the conditions of a block before you start. Before the experiment begins there will be a small number of training trials to get used to the simulator and the tasks.
Including instructions, training trials, and (compulsory) breaks, each session will take approximately two to three hours to complete. You will be compensated for your time at a rate of € 8, -per hour. We aim to collect data from 20 participants; one group of younger and one group of older participants.
There are no known (dis-)advantages for you personally as a result of participating in this study, although there is a risk of motion sickness.
Motion sickness can be recognized from the following symptoms: Vertigo is the feeling that either you or the surroundings are spinning If you experience symptoms of motion sickness at any point in the experiment or if at any time you wish to have an extra break, then please do not hesitate to communicate this immediately to the researcher. The experiment will then be paused or stopped.
Note that your participation is entirely voluntary and you may withdraw from the study at any time without penalty.