SHIFTS IN BLOOD-VOLUME ALTER THE PERCEPTION OF POSTURE

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.