Assessing the perception of trunk movements in military personnel with chronic non-specific low back pain using a virtual mirror

Abstract

Chronic pain, including chronic non-specific low back pain (CNSLBP), is often associated with body perception disturbances, but these have generally been assessed under static conditions. The objective of this study was to use a "virtual mirror" that scaled visual movement feedback to assess body perception during active movement in military personnel with CNSLBP (n = 15) as compared to military healthy control subjects (n = 15). Subjects performed a trunk flexion task while sitting and standing in front of a large screen displaying a full-body virtual mirror-image (avatar) in real-time. Avatar movements were scaled to appear greater, identical, or smaller than the subjects' actual movements. A total of 126 trials with 11 different scaling factors were pseudo-randomized across 6 blocks. After each trial, subjects had to decide whether the avatar's movements were "greater" or "smaller" than their own movements. Based on this two-alternative forced choice paradigm, a psychophysical curve was fitted to the data for each subject, and several metrics were derived from this curve. In addition, task adherence (kinematics) and virtual reality immersion were assessed. Groups displayed a similar ability to discriminate between different levels of movement scaling. Still, subjects with CNSLBP showed an abnormal performance and tended to overestimate their own movements (a right-shifted psychophysical curve). Subjects showed adequate task adherence, and on average virtual reality immersion was reported to be very good. In conclusion, these results extend previous work in patients with CNSLBP, and denote an important relationship between body perception, movement and pain. As such, the assessment of body perception during active movement can offer new avenues for understanding and managing body perception disturbances and abnormal movement patterns in patients with pain.

Fig 1. Grand average psychophysical curves for subjects with CNSLBP (black line) and HC subjects (black interrupted line).

Colored lines indicate the curve metrics derived: X_Y0.25 (green), X_Y0.50 = point of subjective equality (PSE) (red), and X_Y0.75 (blue). CNSLBP: chronic non-specific low-back pain, HC: healthy control. A PSE higher than 0 indicates that subjects with CNSLBP tended to overestimate their own movements. The two screenshots and arrows illustrate the effect of negative (left) and positive (right) scaling factors on avatar movements based on actual movements (white skeleton).