What is Visual Parallax and how does it affect gait analysis? : Is your video gait analysis really telling you what you think it is telling you ?
(*2 pictures above, toggle between)
Have you ever watched someone’s gait, only to reach for your camera to capture a gait deficit digitally, and then later rewatch the video and have a difficult time seeing the same deficit? There is a logical answer.
Vision exists with both eyes open (binocular), or with only one open eye (monocular). Our visual system uses all available depth cues to determine distances between objects, called physiological cues (actual or perceived differences), and psychological cues (experiential; or derived from past experience, or logical deduction).
Our 2 eyes see the world from slightly different locations (or different lines of sight), so the images transmitted to the visual cortex (in the occipital lobe) by the eyes are slightly different (see left picture above, compliments of Wikipedia). This difference in the perceived images is called binocular parallax. The amount or angle can be measured by the angle or semi-angle of inclination between those two lines. The term is derived from the Greek parallaxis, meaning “alteration”.
Our visual system is very sensitive to these “differences”, and binocular parallax is the most important depth cue for medium viewing distances (see right picture above, compliments of www.cns.nyu.edu). The sense of depth can be achieved using binocular parallax even if all other depth cues are removed.
Nearby objects have a larger parallax than more distant objects when observed from different positions, so parallax can be used to determine distances. Usually video provides us with confirmation of we are actually seeing in 3D. Beware, visual parallax may be playing tricks with you, as there is a discrepancy when translating 3D to 2D (cameras have one lens and are therefor monocular). Yes, binocular effect is lost in video; there is little depth perception with 2D. A different vantage point (ie multiple camera angles: front, back and side) often offer a different perspective which is why we always suggest 3 views.
Remember, what you see (actually or on video) IS their compensation, NOT the problem, but it can often lead you to the problem. Pelvic drift to right during stance often means weak Gluteus medius on that side. Is that the problem? Maybe. However, “Why” is the bigger question. Is it from the foot? The Knee? the hip? Or maybe central and involving the vestibular apparatus? We examine, try to make a change, and see if it sticks.
So, in the future, keep in mind some of these limitations of what you are diagnosing off of video analysis because what you are seeing is a monocular interpretation of the real thing. Some information has been lost in the process of monocular motion capture. We are sure that in time video analysis will reach the 3D realm, and solve this problem.
Binocular Parallax. 2 different views of the same thing. Kind of like us…The Gait Guys…Ivo and Shawn
