When the world is moving, we have to determine whether it is the environment or ourselves that is moving in order to recognize our orientation in space. To do this, we must use the sensory information linked to the context of the movement and determine whether there is a mismatch between the visual world motion and our vestibular and somatosensory afference. If we believe that the environment around us is stationary, it is relatively easy to identify our physical motion. However, when the world is also moving, we need to shape our reactions to accurately match the demands of the environment. The ability to orient ourselves in space is a multisensory process [28
], and the impairment of any one of the relevant pathways (i.e., proprioceptive, vestibular, and visual) will impact postural stability.
Whole body sway responses of subjects exposed to visual rotation stimuli in our environment were qualitatively similar to those observed and published in the literature that was available prior to our initial experiments [20
]. The novelty in our approach was to explore how each body segment acted to maintain posture during visual disturbances rather than looking at postural sway as a single output variable. We chose to examine the body segments individually because of previous studies suggesting differential control mechanisms in the upper and lower body [24
]. In our first study with a VE [33
], subjects stood in quiet stance while observing either random dots or a realistic visual scene that moved sinusoidally or at constant velocity about the pitch or roll axes (). Segmental displacements, Fast Fourier Transforms (FFT), and root mean square (RMS) values were calculated for the head, trunk, and lower limb. We found that with scene motion in either the pitch and roll planes, subjects exhibited greater magnitudes of motion in the head and trunk than at the lower limb. Additionally, the frequency or velocity content of the head and trunk motion was equivalent to that of the visual input, but this was not the case in the lower limb. Smaller amplitudes and frequent phase reversals observed at the ankle suggested that control at the ankle was directed toward keeping the body over the base of support (the foot) rather than responding to changes in the visual environment. These results suggested to us that the lower limb postural controller was setting a limit of motion for postural stabilization while posture of the head and trunk may have been governed by a perception of the visual vertical driven by the visual scene.
Figure 3 (Left) A subject standing within a field of random dots projected in the VE. The subject is tethered to three flock-of-birds sensors that are recording 6 axes of motion of the head, trunk, and lower limb. (Right) Graphs of two subjects (A and B) showing (more ...)
When our subjects were asked to walk while the visual environment rolled counterclockwise, all of the subjects compensated for the visual field motion by exhibiting one of two locomotion strategies. Some subjects exhibited a normal step length, taking only two or three steps to cover the seven-foot distance which would be a normal gait for this distance. However, a lateral shift took place so that they walked sideways in the direction of the rolling scene (). In each case, the subject’s first step was straight ahead and the second step was to the left regardless of which foot was placed first. For example, one subject who made the first step with the left foot then made the second step by crossing the right leg over the left leg when responding to the visual stimulus [in order to move to the left]. When queried about the amount of translation produced during the walking trials, subjects responded that they recognized they were moving off center. In fact, these subjects were three feet to the left of center at the end of their trial but were unable to counteract the destabilizing force.
The other subjects walked with short, vertically projected stamping, taking approximately seven or eight steps in the seven feet traveled (). These subjects exhibited an increased frequency of medial-lateral sway of the head and trunk as though they were rocking over each foot as they stepped forward. These subjects reported that they were only focused on “not falling over”. Shortened step lengths and increased flexion at the knee and ankle implied that these subjects were exerting cognitive decisions about their locomotion that focused on increasing their awareness of both the somatosensory feedback and their motor output. This locomotion pattern was reminiscent of the gait observed in elderly fallers [34
] or subjects that have been walking with reversing prisms [35
From these results we concluded that subjects could only counteract the effects of the destabilizing visual stimulus by altering their normal locomotion pattern and, correspondingly, the altered perception of vertical. Interestingly, the content of the visual scene did not determine response strategy selection (subjects receiving the random dot pattern also exhibited the different strategies), thus this paradigm can be used in laboratories with less advanced technologies than those reported here.