Data_Sheet_1_Effects of Optokinetic Stimulation on Verticality Perception Are Much Larger for Vision-Based Paradigms Than for Vision-Independent Paradigms.pdf

Introduction<p>Verticality perception as assessed by the subjective visual vertical (SVV) is significantly biased by a rotating optokinetic stimulus. The underlying mechanisms of this effect remain open. Potentially, the optokinetic stimulus induces a shift of the internal estimate of the direction of gravity. This hypothesis predicts a shift of perceived vertical using other, non-vision dependent, paradigms as well. Alternatively, an optokinetic stimulus may only induce a shift of visual orientation, and so would be task specific.</p>Methods<p>To test this prediction, both vision-dependent SVV and vision-independent [subjective haptic vertical (SHV)] paradigms were applied. In 12 healthy human subjects, perceived vertical was measured in different whole-body roll positions (up to ±120°, steps = 30°) while watching a clockwise or counterclockwise rotating optokinetic stimulus. For comparison, baseline trials were collected in darkness. A generalized linear model was applied for statistical analysis.</p>Results<p>A significant main effect for optokinetic stimulation was noted both for the SVV paradigm (p < 0.001) and the SHV paradigm (p = 0.013). However, while pairwise comparisons demonstrated significant optokinetic-induced shifts (p ≤ 0.035) compared to baseline in all roll-tilted orientations except 30° and 60° left-ear-down position and counterclockwise optokinetic stimulation for the SVV paradigm, significant shifts were found in only 1 of the 18 test conditions (120° left-ear-down roll orientation, counterclockwise optokinetic stimulation) for the SHV paradigm. Compared to the SHV, the SVV showed significantly (p < 0.001) larger shifts of perceived vertical when presenting a clockwise (15.3 ± 16.0° vs. 1.1 ± 5.2°, mean ± 1 SD) or counterclockwise (−12.6 ± 7.7° vs. −2.6 ± 5.4°) rotating optokinetic stimulus.</p>Conclusion<p>Comparing the effect of optokinetic stimulation on verticality perception in both vision-dependent and vision-independent paradigms, we demonstrated distinct patterns. While significant large and roll-angle dependent shifts were noted for the SVV, offsets were minor and reached significance only in one test condition for the SHV. These results suggest that optokinetic stimulation predominately affects vision-related mechanisms, possibly due to induced torsional eye displacements, and that any shifts of the internal estimate of the direction of gravity are relatively minor.</p

Single trial SVV adjustment errors are plotted against time for all head-roll orientations separately in a typical subject (DH) for both the control condition (no visual feedback, in grey) and the test condition (with visual feedback, in black).

<p>Compared to the control condition, adjustment errors relative to true earth-vertical were significantly reduced in the test condition at 90, 105 and 120° RED, while at the other roll angles no clear difference between the two conditions was noticeable.</p

Grand average (±1 SD) trial-to-trial variability (pooled from all 12 subjects) is plotted against head-roll orientation both for the control (grey) and the test (black) condition.

<p>Grand average (±1 SD) trial-to-trial variability (pooled from all 12 subjects) is plotted against head-roll orientation both for the control (grey) and the test (black) condition.</p

Grand average SVV adjustment errors (±1 SD) are plotted against head-roll for the control (in grey) and the test conditions (in black).

<p>Grand average SVV adjustment errors (±1 SD) are plotted against head-roll for the control (in grey) and the test conditions (in black).</p

Individual average SVV adjustment errors for both the control condition (no visual feedback, in grey) and the test condition (with visual feedback, in black) are plotted against head-roll orientation in all subjects.

<p>The dashed horizontal lines refer to perfect SVV adjustments. While subjects in the first two rows all show a clear decrease in adjustment errors in the visual feedback condition compared to the control condition, subjects in the bottom row had either no A-effect in the control condition or showed no improvement by providing visual feedback.</p

Illustration of a single SVV trial while the subject is tilted right-ear down (RED) by 75°, as indicated by angle α.

<p>At the beginning of each trial (A) the luminous arrow (in grey) is offset by angle δ. The subject then rotates the arrow towards perceived direction of vertical and confirms the adjustment when no further change is intended (illustrated by the arrow in black) (B). Then the arrow disappears (C) and either the next trial is started (control condition) or visual feedback of the adjustment is provided (D, test condition). For visual feedback, the room lights are turned on and both the arrow as adjusted by the subject and a grid oriented along earth-vertical and earth-horizontal become visible.</p

Comparison of adjustment errors obtained with distinct feedback conditions, split up in three different blocks (first without visual feedback, second with visual feedback, and third, immediately after the previous block without pause, again without visual feedback) are plotted against time for head-roll orientations of 90°RED, 105°RED and 120°RED.

<p>While the left column shows single subject data (subject GB), the right column illustrates the pooled individual trial data from all subjects (n = 7). Trials without visual feedback are in light grey, trials with visual feedback in dark grey. A running median (solid black line, window size: 50 samples) is also depicted. Note that the first block (without visual feedback) originates from the control session (session 1), while the second and third blocks were obtained in session 3.</p

Comparison of changes in adjustment errors over the 5-minute recording periods during (panels A and B) and immediately after (panels C and D) prolonged roll-tilt (±45° or ±90°).

<p>Grey filled circles refer to runs with significant drift for both RED and LED while empty circles indicate runs with non-significant drift in at least one of the two conditions. Each panel is split up in 4 areas separated by dashed horizontal and vertical lines along zero: while the grey-shaded areas indicate runs where drift was not symmetric (e.g. error was increasing at 45° RED and decreasing at 45°LED or vice versa), trials with symmetric drift for LED and RED (for a given roll angle) will fall in the white areas, either in the lower left corner (if adjustment errors increased over time) or in the upper right corner (if adjustment errors decreased over time). </p

Individual (in grey) and median (in black) adjustment errors while roll-tilted are plotted against time.

<p>Traces with significant (p < 0.05) CW, significant CCW and non-significant drift while roll-tilted and in upright position are shown in separate columns, always indicating the number of subjects (n) that met the criteria for a given trial type. Note that for baseline upright trials the scaling along the y-axis differs from the roll-tilted positions. For a more detailed description see figure legend of <a href="http://www.plosone.org/article/info:doi/10.1371/journal.pone.0078079#pone-0078079-g002" target="_blank">Figure 2</a>.</p

Characteristics of the post-tilt drift amplitude: correlation with initial offset and drift during prolonged roll-tilt.

<p>Panel A: Correlation analysis between the post-tilt drift amplitude and the initial post-tilt bias when returning to upright position using principal components analysis (PCA). The diamonds refer to single runs, the solid line indicates the fit obtained. In an inset, goodness of fit (R²-value), the slope and the 95% CI of the slope are provided. Panel B: Comparison of the individual drift amplitudes during prolonged roll-tilt and immediately after returning back upright using principal components analysis (PCA). The diamonds refer to single runs, the solid line indicates the fit obtained. In an inset, goodness of fit (R²-value), the slope and the 95% CI of the slope are provided.</p