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.