Is "circling" behavior in humans related to postural asymmetry?

Abstract

In attempting to walk rectilinearly in the absence of visual landmarks, persons will gradually turn in a circle to eventually become lost. The aim of the present study was to provide insights into the possible underlying mechanisms of this behavior. For each subject (N = 15) six trajectories were monitored during blindfolded walking in a large enclosed area to suppress external cues, and ground irregularities that may elicit unexpected changes in direction. There was a substantial variability from trial to trial for a given subject and between subjects who could either veer very early or relatively late. Of the total number of trials, 50% trajectories terminated on the left side, 39% on the right side and 11% were defined as "straight". For each subject, we established a "turning score" that reflected his/her preferential side of veering. The turning score was found to be unrelated to any evident biomechanical asymmetry or functional dominance (eye, hand...). Posturographic analysis, used to assess if there was a relationship between functional postural asymmetry and veering revealed that the mean position of the center of foot pressure during balance tests was correlated with the turning score. Finally, we established that the mean position of the center of pressure was correlated with perceived verticality assessed by a subjective verticality test. Together, our results suggest that veering is related to a "sense of straight ahead" that could be shaped by vestibular inputs.

Figure 1. Trajectories during blindfold walking.

A1: Graphical representation in the horizontal plane of all recorded trajectories (n = 88) in the whole area. Due to technical reasons, one of the participants only performed 4 trials instead of the 6. The x-axis represents the straight ahead direction. X-2SD (vertical dotted line) is the mean position value on the x-axis at which the deviation occurs for all trajectories. A2: Enlarged view of the first 20 m for all trajectories. Y-2SD₁ and X-2SD₁ are the positions on the y-axis and on the x-axis respectively at which the deviation occurs for one example trajectory (black curve). X-2SD (vertical dotted line) is the mean position on the x-axis at which the deviation occurs for all trajectories. B1: Walking trajectories of two subjects; the top graph shows a consistent subject that veers on the same side (left side), the bottom graph shows an inconsistent subject that veers on both sides. The dotted line indicates the straight ahead direction. B2: “Turning score”: number of trials that veer to the left, the right and straight ahead (black, white and grey) for all subjects (n = 15). The turning score is annotated on the bottom line; subjects who always turned on the left side scored −1 while subjects who always turned on the right side scored +1. Subjects who had an equal number of trials on both sides scored 0. The grey area overlaps consistent subjects.

Figure 2. Speed and walking trajectories.

A: One representative subject showing instantaneous velocity for the 6 trials performed with the eyes closed and the control trial performed with open eyes (A1). A2: An enlarged view of the speed profile during the first 25 m (same subject shown in A1). X-2SD (vertical dotted line) indicates the mean position value on the x-axis at which the deviation occurs (see Figure 1A). B1: Plot of the final position on the x-axis (X final) versus the position value on the x-axis at which deviation occurs (X-2SD) for all trajectories (n = 88). B2: Plot of the final position on the x-axis (X final) versus the mean speed.

Figure 3. Radius circle of the trajectory.

A: Circles fitted for all trajectories (A) and enlarged view for the circles with radius <300 m (B). Over 80% of the trials are displayed in this view. C: Distribution of trials according to their radius circle. Seven classes are represented: <100 m, 100–200 m, 200–300 m, 300–400 m, 400–500 m, 500–600 m and 600–700 m. Approximately 80% of the trials belong to the first three classes.

Figure 4. Center of pressure (COP) during posturographic tests.

A: Stabilograms for two representative subjects. The vertical dotted line represents the midline of the base of support that is determined from the position of the feet. The black curves represent the COP position during a 1-minute balance test. The white diamond indicates the mean position of the COP trajectory. B. Relationships between posturographic parameters and walking trajectories: correlation analysis of mean COP position versus turning score for all subjects. C. Percentage of subjects with the COP on the left side and the COP on the right side during the one minute balance test. D. Relationships between posturographic parameters and subjective visual vertical.

Figure 5. Kinematic analysis during static posture.

A: Schematic frontal view of the subject (A1). Relative position of the pelvis and the trunk in the frontal plane (α1), the angle between the pelvis and the horizontal axis (α3), the angle between the trunk and the horizontal axis (α4), the height position between right and left shoulders (Δ1) and the height position between the right and left ASIS (Δ2); Schematic horizontal view of the trunk (A2). Relative position of the pelvis and the trunk in the horizontal plane (α2). B: Correlation analysis between kinematic parameters and mean position of the COP during a 1-minute balance test.