Interaction of posture and conscious perception of gravitational vertical and surface horizontal

The extent to which postural orientation and perception of orientation share common central processes is unknown. If central resources are shared, automatic postural orientation in space and conscious perception of gravitational vertical (GV) or surface horizontal (SH) orientation should vary similarly with changing frequencies of surface tilts. Furthermore, shared central processing may result in altered postural orientation depending on the focus of conscious perception. Blindfolded, healthy subjects indicated perception of gravitational vertical and surface horizontal while standing on a tilting surface or sitting in an earth-fixed chair with only the feet on the tilting surface. Subjects oriented a hand-held rod to GV or perpendicular to the tilting support surface to indicate SH during anterior-posterior (A/P) sinusoidal surface tilt oscillation (+/-5 degrees /0.025-0.4 Hz). The gain and phase relationships of rod and postural tilt with respect to surface tilt were analyzed in all conditions. Average rod-tilt during the GV task was below 1.5 degrees and decreased with increasing frequency of surface oscillation, similar to the decrease in trunk tilt. Average rod-tilt when indicating SH was three to four times that of actual surface tilt during both sitting and standing postures and did not change across frequencies, unlike trunk tilt. Measures of postural stability were less variable during the GV than the SH task. This increased postural stability during the GV task and similar gain/phase patterns of postural and GV perceptual orientation across frequencies is consistent with shared central processing of automatic postural and conscious perception of orientation to gravity. In contrast, different gain/phase patterns of posture and conscious perception of SH orientation are consistent with separate processing of postural responses of changing surface orientation and perception of it. Conscious perception of sensory inputs related to verticality share central processing with postural control, which when accessed can have a stabilizing effect, whereas consciously perceiving a moving support surface can be destabilizing.