Fig 1.
A simplified version of the model presented in Clemens et al [3], with several additions.
The original model posits two final position estimates used to determine the direction of gravity, one based on the head’s position (with indirect input from body-based sensors, converted via neck proprioception into head-centric coordinates) and the other on the body (with indirect input from the head). The authors suggest SVV uses the head-based gravity estimate; here we argue that SHV accesses the body-based estimate.
Fig 2.
A. Experiment 1. Mean PSEs for SHV (black) and SVV (grey) for each of three body tilts (0°, 30° and 45°, shown as insets on the left of histogram) where 0° corresponds to gravitational vertical. B. Experiment 2. Mean PSEs for SHV and SVV for head and body tilt (see insets on the left). C. Experiment 3. Mean PSEs for SHV and SVV for head and body tilt with disruptive GVS. Asterisks indicate mean was significantly different from 0° at p<0.05. Error bars are 95% confidence intervals, which make clear which conditions included an accurate perception of gravitational vertical.
Fig 3.
Results for experiments 4 and 5.
A. Mean PSEs for SVV (with a blurry visual stimulus), SHV and bimodal measures for experiment 4 (dark grey bars), and experiment 5 (with neck vibration, light grey bars). PSEs predicted by the maximum likelihood estimate (MLE) model for each experiment are bolded. 0° corresponds to gravitational vertical. Asterisks indicate mean was significantly different from 0° at p<0.05. Error bars are 95% confidence intervals. B. Mean variance for all three modalities, with and without neck vibration. Error bars are 95% confidence intervals. MLE-predicted values are outlined in bold. C. Observed bimodal PSEs versus MLE predicted bimodal PSEs for each participant, for experiment 4 (open circles) and experiment 5 (filled circles). Solid line indicates a slope of 1. D. Observed variances of the bimodal measure versus predicted variances, for experiment 4 (open circles) and experiment 5 (filled circles). Solid line indicates a slope of 1.
Fig 4.
An image of the vibration apparatus.
Bilateral vibration of the dorsal neck muscles was applied using two handheld vibrators embedded in foam blocks that were secured with a tensor bandage. The individual photographed has given written informed consent (as outlined in PLOS consent form) to publish this image.