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

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

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

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

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

Differential effects of visual feedback on subjective visual vertical accuracy and precision

The brain constructs an internal estimate of the gravitational vertical by integrating multiple sensory signals. In darkness, systematic head-roll dependent errors in verticality estimates, as measured by the subjective visual vertical (SVV), occur. We hypothesized that visual feedback after each trial results in increased accuracy, as physiological adjustment errors (A-/E-effect) are likely based on central computational mechanisms and investigated whether such improvements were related to adaptational shifts of perceived vertical or to a higher cognitive strategy. We asked 12 healthy human subjects to adjust a luminous arrow to vertical in various head-roll positions (0 to 120deg right-ear down, 15deg steps). After each adjustment visual feedback was provided (lights on, display of previous adjustment and of an earth-vertical cross). Control trials consisted of SVV adjustments without feedback. At head-roll angles with the largest A-effect (90, 105, and 120deg), errors were reduced significantly (p0.05) influenced. In seven subjects an additional session with two consecutive blocks (first with, then without visual feedback) was completed at 90, 105 and 120deg head-roll. In these positions the error-reduction by the previous visual feedback block remained significant over the consecutive 18-24 min (post-feedback block), i.e., was still significantly (p<0.002) different from the control trials. Eleven out of 12 subjects reported having consciously added a bias to their perceived vertical based on visual feedback in order to minimize errors. We conclude that improvements of SVV accuracy by visual feedback, which remained effective after removal of feedback for ≥18 min, rather resulted from a cognitive strategy than by adapting the internal estimate of the gravitational vertical. The mechanisms behind the SVV therefore, remained stable, which is also supported by the fact that SVV precision - depending mostly on otolith input - was not affected by visual feedback