Yasuomi Ouchi,1 Hiroyuki Okada,2 Etsuji Yoshikawa,2 Shuji Nobezawa1 and Masami Futatsubashi2. 1Positron Medical Center, Hamamatsu Medical Center and
2Central Research Laboratory, Hamamatsu Photonics K.K., Hamakita, Japan
Correspondence to: Yasuomi Ouchi MD, Positron Medical
Center, Hamamatsu Medical Center, 5000 Hirakuchi, Hamakita 434-0041, Japan.
The regulatory mechanism of bipedal standing in humans remains to be elucidated. We investigated neural substrates for maintaining standing postures in humans using PET with our mobile gantry PET system. Normal volunteers were instructed to adopt several postures: supine with eyes open toward a target; standing with feet together and eyes open or eyes closed; and standing on one foot or with two feet in a tandem relationship with eyes open toward the target. Compared with the supine posture, standing with feet together activated the cerebellar anterior lobe and the right visual cortex (Brodmann area 18/19), and standing on one foot increased cerebral blood flow in the cerebellar anterior vermis and the posterior lobe lateral cortex ipsilateral to the weight-bearing side. Standing in tandem was accompanied by activation within the visual association cortex, the anterior and posterior vermis as well as within the midbrain. Standing with eyes closed activated the prefrontal cortex (Brodmann area 8/9). Our findings confirmed that the cerebellar vermis efferent system plays an important role in maintenance of standing posture and suggested that the visual association cortex may subserve regulating postural equilibrium while standing.
The mapping of human brain regions responsible for maintenance of standing posture is an unprecedented study, which may be comparable with the first report, in which the investigators used single photon emission tomography (SPECT) and described a focal increase in regional cerebral blood flow (rCBF) in the hand somatosensory area by simple finger movement (Lassen et al., 1977; Roland et al., 1980). Standing itself may be classified as simple behaviour, but maintenance of the postural balance requires rapid processing of signals from the visual, vestibular and somatosensory systems (Nashner, 1976). Alterations in these sensory functions relating to standing postures in human brain could never be studied with ordinary tomographic scanners. To date, the role of the sensory information for postural control has been investigated with a movement kinematic method by measuring body sway in conditions in which a certain sensory input was changed or limited. It is common clinically that patients with vestibular deficits can show gait ataxia, abnormal head and body righting reactions, and difficulties in balancing on one leg and in heel-to-toe stance (Horak et al., 1988). The movement kinetic studies stressed that somatosensory as well as vestibular information was important in selection of postural movement adjustment according to the environmental context (Diener et al., 1984; Horak et al., 1990). It was also reported that vision ameliorated the fluctuation of the body position caused by standing with a narrow stance width or with eyes closed (Day et al., 1993). For elderly people, disequilibrium can be a common problem, as revealed by posturography (Fife and Baloh, 1993). Their instability can be triggered easily by malfunctioning responses to visual cues (Nashner et al., 1982), vestibular (Norre et al., 1987) and proprioceptive reflexes (Lord et al., 1991). Previous clinicopathological studies suggested that the cerebellum plays a central role in controlling postural balance (Holmes, 1922a, b) and that both the spinocerebellum and vestibulocerebellum participate in this sensory processing (Parent, 1996). Despite these lines of evidence, no conventional radiological scanners are suitable for mapping functional topography in the cerebellum involved in the postural equilibrium system for standing in humans. Previous animal studies on cerebellar functions for body balance stressed that the cerebellar vermis located in the medial longitudinal cortical zone (spinocerebellum), considered important for co-ordinating body movement with respect to gravity and head movement in space, played a role in integrating the senses such as tactile proprioception from the feet (Chambers and Sprague, 1955; Leicht and Schmidt, 1977; Leicht et al., 1977). Similarly, the human cerebellar vermis seems to co-ordinate the timing in keeping the centre of gravity within the limits required for stable upright standing (Diener et al., 1989), and this co-ordination deteriorates with the age-related shrinkage of the vermis (Koller et al., 1983).