The role of vestibular cues in postural sway

Abstract

Controlling posture requires continuous sensory feedback about body motion and orientation, including from the vestibular organs. Little is known about the role of tilt vs. translation vs. rotation vestibular cues. We examined whether intersubject differences in vestibular function were correlated with intersubject differences in postural control. Vestibular function was assayed using vestibular direction-recognition perceptual thresholds, which determine the smallest motion that can be reliably perceived by a subject seated on a motorized platform in the dark. In study A, we measured thresholds for lateral translation, vertical translation, yaw rotation, and head-centered roll tilts. In study B, we measured thresholds for roll, pitch, and left anterior-right posterior and right anterior-left posterior tilts. Center-of-pressure (CoP) sway was measured in sensory organization tests (study A) and Romberg tests (study B). We found a strong positive relationship between CoP sway and lateral translation thresholds but not CoP sway and other thresholds. This finding suggests that the vestibular encoding of lateral translation may contribute substantially to balance control. Since thresholds assay sensory noise, our results support the hypothesis that vestibular noise contributes to spontaneous postural sway. Specifically, we found that lateral translation thresholds explained more of the variation in postural sway in postural test conditions with altered proprioceptive cues (vs. a solid surface), consistent with postural sway being more dependent on vestibular noise when the vestibular contribution to balance is higher. These results have potential implications for vestibular implants, balance prostheses, and physical therapy exercises.NEW & NOTEWORTHY Vestibular feedback is important for postural control, but little is known about the role of tilt cues vs. translation cues vs. rotation cues. We studied healthy human subjects with no known vestibular pathology or symptoms. Our findings showed that vestibular encoding of lateral translation correlated with medial-lateral postural sway, consistent with lateral translation cues contributing to balance control. This adds support to the hypothesis that vestibular noise contributes to spontaneous postural sway.

Keywords: feedback; sway; threshold.

Graphical abstract

Figure 1.

Examples of postural sway from 2 subjects with small (A) and large (B) sway. These examples show mediolateral (ML) sway during sensory organization test (SOT) 5, in which subjects have their eyes closed and stand on a support surface that is sway referenced in the anterioposterior (AP) direction. C: coefficients of variation for postural sway across the subjects in study A for each of the SOTs. CoP, center-of-pressure.

Figure 2.

Relationship between the standard deviation of mediolateral (ML) postural center-of-pressure (CoP) sway and vestibular thresholds for sensory organization test (SOT) 5, with each dot representing 1 subject in study A. A: subjects with low lateral translation thresholds tend to have less ML postural sway, whereas subjects with high lateral translation thresholds tend to have larger ML postural sway. Partial slope lines show the results of a multiple variable linear regression between the standard deviation of ML postural CoP and each of the 5 threshold measures simultaneously, with the corresponding P value provided. B–E: there is no obvious intersubject relationship between sway and thresholds for the other 4 thresholds. Some partial slope lines may appear in an unintuitive direction; this occurs because multiple variable regression accounts for interactions between thresholds.

Figure 3.

Results of multiple variable linear regressions between postural sway and the vestibular threshold measures in study A, for each of the 6 sensory organizations (SOTs) and 2 directions [mediolateral (ML) and anterioposterior (AP)]. Each circle shows the estimated coefficient between the corresponding threshold measure and sway, with the 95% confidence intervals shown by black lines.

Figure 4.

Tilt thresholds assayed at 0.2 Hz in study B. Left anterior-right posterior (LARP) tilt motions occur forward-left and backward-right. Right anterior-left posterior (RALP) tilt motions occur forward-right and backward-left. Error bars show 95% confidence intervals.

Figure 5.

Relationship between postural center-of-pressure (CoP) sway in condition 4 of the Romberg test and vestibular thresholds, in which subjects stand on foam with eyes closed. Each dot represents 1 subject in study B. Vestibular migraine (VM) subjects are shown as stars, with 1 subject missing because they did not complete condition 4. Partial slope lines show the results of a multiple variable linear regression between sway and the threshold for all subjects, with the corresponding P value provided. Normal subject results are in Fig. 6.

Figure 6.

Results of multiple variable linear regressions between postural sway and roll and pitch thresholds, for each of the 4 Romberg conditions and 2 directions [mediolateral (ML) and anterioposterior (AP)] in study B. Each circle shows the estimated coefficient between the corresponding threshold measure and sway, with the 95% confidence intervals shown by black lines. Results in gray are for normal subjects only, whereas results in black are normal and vestibular migraine (VM) subjects together.

Figure 7.

Results of multiple variable linear regressions between postural sway aligned with the anterior and posterior canal planes [left anterior-right posterior (LARP) and right anterior-left posterior (RALP)] and LARP and RALP tilt thresholds for each of the 4 Romberg conditions and 2 directions [mediolateral (ML) and anterioposterior (AP)] in study B.