Subjective posture in tridimensional space

The internal representation of the body is intimately related to postural orientation. This assists us to correctly identify our position relative to the earth-vertical. The postural orientation about the pitch-Y axis is by default forward tilted while there is no such default about the roll-X axis. We hypothetized that the internal representation of body position would be different between the Y and X axes. We designed a study to assess the accuracy of self-driven whole-body orienting to the horizontal and the vertical about the X (roll) and Y (pitch) axes, with healthy seated subjects in complete darkness. Consistent with our hypothesis, the results showed that when trying to reach the horizontal with the X-axis, subjects remained on the same tilted direction as the initial posture. On the return way to the vertical, the subjects did not cross this (vertical) line. Whereas when reaching the horizontal with the Y-axis, there was an undershoot on the supine direction, and an overshoot on the prone direction: this discrepancy could be due to somatosensory cues to subjects in seated posture. The relevant mechanisms related to the internal representation of the body and graviceptive systems underlying the construction of an head-foot line are discussed.