Stiffness judgments are affected by visual occlusion

Objective: The current protocol for judging posteroanterior (PA) spinal stiffness has been shown to provide unreliable estimates of PA stiffness. It is possible that a failure to standardize therapists' use of vision in the test protocol partly contributes to the disagreement between raters. This study sought to establish whether vision affects stiffness judgments and so needs to be standardized when judging PA stiffness. Design: Perceptual study using a mechanical device to provide stiffness stimuli with physiotherapists and lay people an judges. Setting: University psychophysics laboratory. Interventions: Occlusion of vision via opaque goggles. Main outcome measures: Measures of inter-stimulus discriminability and bias. Results: Occluding vision had no effect on judges' ability to discriminate between stiffness stimuli; however, the name stimuli were judged as significantly stiffer under the visual occlusion condition. Conclusion: The data from this study suggest that vision needs to be controlled when using manual tests to judge PA spinal stiffness.