Shotgun approaches to gait analysis: insights & limitations.

BACKGROUND: Identifying features for gait classification is a formidable problem. The number of candidate measures is legion. This calls for proper, objective criteria when ranking their relevance. METHODS: Following a shotgun approach we determined a plenitude of kinematic and physiological gait measures and ranked their relevance using conventional analysis of variance (ANOVA) supplemented by logistic and partial least squares (PLS) regressions. We illustrated this approach using data from two studies involving stroke patients, amputees, and healthy controls. RESULTS: Only a handful of measures turned out significant in the ANOVAs. The logistic regressions, by contrast, revealed various measures that clearly discriminated between experimental groups and conditions. The PLS regression also identified several discriminating measures, but they did not always agree with those of the logistic regression. DISCUSSION & CONCLUSION: Extracting a measure's classification capacity cannot solely rely on its statistical validity but typically requires proper post-hoc analysis. However, choosing the latter inevitably introduces some arbitrariness, which may affect outcome in general. We hence advocate the use of generic expert systems, possibly based on machine-learning.

Canal and otolith contributions to visual orientation constancy during sinusoidal roll rotation.

Using vestibular sensors to maintain visual stability during changes in head tilt, crucial when panoramic cues are not available, presents a computational challenge. Reliance on the otoliths requires a neural strategy for resolving their tilt/translation ambiguity, such as canal-otolith interaction or frequency segregation. The canal signal is subject to bandwidth limitations. In this study, we assessed the relative contribution of canal and otolith signals and investigated how they might be processed and combined. The experimental approach was to explore conditions with and without otolith contributions in a frequency range with various degrees of canal activation. We tested the perceptual stability of visual line orientation in six human subjects during passive sinusoidal roll tilt in the dark at frequencies from 0.05 to 0.4 Hz (30 degrees peak to peak). Because subjects were constantly monitoring spatial motion of a visual line in the frontal plane, the paradigm required moment-to-moment updating for ongoing ego motion. Their task was to judge the total spatial sway of the line when it rotated sinusoidally at various amplitudes. From the responses we determined how the line had to be rotated to be perceived as stable in space. Tests were taken both with (subject upright) and without (subject supine) gravity cues. Analysis of these data showed that the compensation for body rotation in the computation of line orientation in space, although always incomplete, depended on vestibular rotation frequency and on the availability of gravity cues. In the supine condition, the compensation for ego motion showed a steep increase with frequency, compatible with an integrated canal signal. The improvement of performance in the upright condition, afforded by graviceptive cues from the otoliths, showed low-pass characteristics. Simulations showed that a linear combination of an integrated canal signal and a gravity-based signal can account for these results.

Nature of the transition between two modes of external space perception in tilted subjects.

A striking feature of visual verticality estimates in the dark is undercompensation for lateral body tilt. Earlier studies and models suggest that this so-called Aubert (A) effect increases gradually to around 130 degrees tilt and then decays smoothly on approaching the inverted position. By contrast, we recently found an abrupt transition toward errors of opposite sign (E effect) when body tilt exceeded 135 degrees . The present study was undertaken to clarify the nature of this transition. We tested the subjective visual vertical in stationary roll-tilted human subjects using various rotation paradigms and testing methods. Cluster analysis identified two clearly separate response modes (A or E effect), present in all conditions, which dominated in different but overlapping tilt ranges. Within the overlap zone, the subjective vertical appeared bistable on repeated testing with responses in both categories. The tilt range where bistability occurred depended on the direction of the preceding rotation (hysteresis). The overlap zone shifted to a smaller tilt angle when testing was preceded by a rotation through the inverted position, compared with short opposite rotations from upright. We discuss the possibility that the A-E transition reflects a reference shift from compensating line settings for the head deviation from upright to basing them on the tilt deviation of the feet from upright. In this scenario, both the A and the E effect reflect tilt undercompensation. To explain the hysteresis and the bistability, we propose that the transition is triggered when perceived body tilt, a signal with known noise and hysteresis properties, crosses a fixed threshold.

Interpretation of a discontinuity in the sense of verticality at large body tilt.

Results of earlier spatial-orientation studies focusing on the sense of verticality have emphasized an intriguing paradox. Despite evidence that nearly veridical signals for gravicentric head orientation and egocentric visual stimulus orientation are available, roll-tilted subjects err in the direction of the long body axis when adjusting a visual line to vertical in darkness (Aubert effect). This has led to the suggestion that a central egocentric bias signal with fixed strength and direction acts to pull the perceived vertical to the subjects' zenith (M-model). In the present study, the subjective visual vertical (SVV) was tested in six human subjects, across the entire 360 degrees range. For comparison, body-tilt estimates from four subjects where collected in a separate series of experiments. For absolute tilts up to approximately 135 degrees, SVV responses showed a gradually increasing Aubert effect that could not be attributed to errors in perceived body tilt but was nicely in line with the M-model. At larger absolute tilts, SVV errors abruptly reversed sign, now showing a pattern concordant with errors in body-tilt estimates but incompatible with the M-model. These results suggest that, in the normal working range, the perception of external space and the perception of body posture are based on different processing of body-tilt signals. Beyond this range, both spatial-orientation tasks seem to rely mainly on a common tilt signal.