Here you see the principal axes and reference frames of the body as depicted in Volume 1 of Perceiving in Depth by the incomparable Professor Ian Howard, who I referenced last week in my Tribute to Al. Normal binocular vision is a head to toe experience, and the center of gravity (c.g.) pictured at the intersection of the hips, representing the egocenter, normally transects the oculocenter. A mismatch between the egocenter and oculocenter results in a midline shift, which can translate into the x/y/z axes as roll/pitch/yaw, and serves as a basis for visual midline shifts either fore (toe walk), aft (heel walk) or laterally. It also helps explains why lenses and prisms can exert a visceral experience, particularly in adaptive phases.
The terms roll/pitch/yaw and their respective axes may be familiar from airplane flight, in which the center of mass of the plane represents the center of gravity of the body. I’ve made a fuss elsewhere about the importance of the simple looking pointer-in-straw procedure in which we work on attaining accuracy in x/y/z axes, ensuring that calibration between the two eyes is relatively similar, and that binocular localization represents a good match between eyes and hand. I also fussed about it when showing how the eccentric circle procedure should be done in x/y/z planes.
Here is a nice composite model of the the x/y/z planes translating through the EOMs and Semicircular Canals, embedded in a nice discussion about adaptive control in these systems from a chapter in a book co-edited by Alain Berthoz.
I’ve used the analogy before, regarding visual-vestibular interactions, that the fluid in the semicircular canals functions much like the fluid in the bubble of a level. By that I mean that the brain reflexively makes computations to match the planes of rotation of the head/neck with the planes of rotation of the eyes so that the fluid stays level in the bubble. Think of the each pair of yoked muscles as the left and right sides of the bubble, and think of the hatch marks as the visual feedback that informs the brain that the fluid in the proper orientation.
The 3-D transformation of x/y/z planes therefore works as a composite of a three level/bubble system, and as far as the EOM/Semicircular Canal/VOR coupling looks more like this:
Or mapped out diagrammatically like the bottom center image here:
This should make it easier to conceptualize how visual centers of gravity undergo adaptive or maladaptive shifts. My buddy Bob Sanet is fond of citing Myron “Mickey” Weinstein’s maxim that vision writes spatial equations for muscles to solve, and what I’m emphasizing here is that a variety of conditions can result in dyscalculations. Or perhaps it is just as accurate to state that a variety of dyscalculations can result in a range of conditions that are either adaptive or maladaptive in nature.