4 DISCUSSION
The spatial orientation is formed from the vestibular, visual and somatosensory inputs as mentioned above.1 In this study, we covered the patients’ eyes with goggles to remove the visual input. Therefore, all of the patients had to rely on their vestibular and somatosensory inputs when they tried to reproduce 0°, and right and left 5°in this study.
There were no significant differences in any of the tasks during the procedure between the bilateral group and the control group. There were also no differences in the right and left direction of tilt within each of the groups. The bilateral group do not receive vestibular input. But there were no differences. Therefore, the somatosensory input is sufficient to provide information about the spatial orientation. Fitzpatrick et al and Horak et al reported that the contribution from visual and somatosensory inputs are the most important, and that the vestibular input is primarily used for balance control of normal humans.7,8 Bronstein et al. reported that patients with bilateral vestibular disorders are “visually dependent” soon after the vestibular insult, but that they gradually learn to ignore conflicting visual stimuli as compensation develops.1Nashner et al, Bles et al, and Peterka et al reported that somatosensory input increases for balance control in patients with bilateral vestibular disorders.9-12 Nandapalan et al used the Sway Weigh balance platform to determine the efficacy of a walking stick in 25 patients with peripheral vestibular balance disorders.13 In their study, the patients were tested with their eyes open and closed while they were standing on a flat surface and on an air-filled bed (to alter limb proprioception) on the Sway Weigh balance platform. All the tests were carried out with and without a walking stick. They found that body sway decreased when the subjects used a walking stick when they were standing on an air-filled bed with their eyes either opened or closed, although there were no differences with or, without the walking stick when they were standing on a flat surface.
We suppose that humans form spatial orientation by effectively using remaining information and control their balance according to this information. Judging from these reports, and the tilt perception in this study, it appears that while the contribution of vestibular input is important, somatosensory input is more important (the source of somatosensory input was the hip, because we studied subjects under the condition that they had to sit in their chair without leaning against a backrest).
Although the Jumbling phenomenon is able to occur when bilateral vestibular function is disordered, there were no significant differences between patients with and without it. There were also no differences when we covered subjects’ eyes with goggles to remove the visual input. Therefore, we concluded that the somatosensory input is more important than the other types of input.
Furthermore, we also tested a patient with multiple sclerosis (MS) who had disequilibrium and dysbasia due to anesthesia of the hip and bilateral lower legs and a lesion with contrastable effect in the spinal cord under Th4 magnetic resonance imaging (MRI). The result was that the patient was unable to correctly perceive a tilt over the mean tilt angle ± 2 SD of control group in most of the task in the procedure (Table 1, Figure 3, 4). The importance of somatosensory input was further supported by this result.
Although we studied tilt perception immediately, and 1, 3, 5 and 10 minutes after changing positions in order to study the memory of tilt perception, there were no significant differences in the both groups.
There have been reports showing that the hippocampus is involved in memory and recognition of space by experiments on the rat hippocampus.14, 15 Furthermore, it has been known that there are neurons involved in movement and memory of space in the monkey hippocampus.16, 17 In addition, there is a neuron group which fires to synchronize a rat’s movement that was demonstrating by simultaneously recording freely moving rats’ hippocampus, including nearly a hundred neurons. These neuron groups react to differences in position, and they are distributed like a belt in the hippocampus.18 Based on these reports, it appears that the information processing in the hippocampus facilitates memorization of space and position, via groupable and cooperative neurons, and dynamic change of many neuron circuits are likely involved in the memory of space and position with action.
With regard to the spatial orientation and posture control, groupable and
cooperative information processing of many neural circuits including the
hippocampus, vestibules, visual system, somatosensory system, cerebellum,
limbic system and so on are done unconsciously. It is suspected that many
cooperatively and functionally connected neuron circuits dynamically change in
response to input of information for that purpose.
There were no abnormalities in tilt perception in the control and bilateral
groups in the present study, because the vestibular and somatosensory input
that are necessary to integrate spatial orientation function adequately.
Therefore, it is suspected that groupable and cooperative information
processing of the above-mentioned neuron circuits, as well as the
hippocampus-related memory of tilt perception, can be maintained for at least
10 minutes.