Recent experiments have shown that somatic graviceptors exist in human
s. Traditionally, extravestibular gravity information has been thought
to originate from mechanoreceptors in the joints, muscles and skin. E
xperiments with normal, paraplegic and nephrectomized subjects reveale
d that the kidneys and the cardiovascular system are involved in provi
ding truncal gravity information. The present study intends to determi
ne the influence of shifts in body fluid, especially of the distributi
on of blood along the subjects' spinal (Z-) axis, on the perception of
posture. To this end, the distribution of body fluids was altered by
means of the technique of lower body negative and positive pressure (L
BNP and LBPP). LBNP leads to venous pooling of blood in the legs, wher
eas LBPP prevents venous blood from pooling, increasing central volume
. Changes in blood distribution were measured by segmental impedance c
ardiography for four body segments: the upper torso (thoracic cavity),
lower torso (abdominal and pelvic region), thigh and calf. Seventeen
healthy subjects (mean age: 27.3 years) participated in the experiment
. They were positioned on the side (right-ear-down head position) on a
tilt table which the subjects and the experimenter could tilt via rem
ote control around an axis parallel to the subjects' visual (X-) axis.
The experimenter set the initial tilt in total darkness to arbitrary
angles while strictly alternating between head-up and head-down tilts.
Subjects were then asked to rotate the board until they felt they wer
e in a horizontal posture. Means and variances of eight pairs of setti
ngs were taken as a measure of the subjective horizontal posture (SHP)
. During LBNP (-30 mmHg), subjects perceived being tilted head-up, whe
reas LBPP (+30 mmHg) led them to feel tilted head-down. The results co
rroborate the hypothesis of an effect of the blood's mass on gravicept
ion and also indicate supplementary contributions of other visceral af
ferences. (C) 1997 Elsevier Science B.V.