Characteristics of Body Axis Perception under Gravity and Its Neural Basis

Abstract

The ability to perceive the position and orientation of one's body and external objects (spatial orientation) is fundamental to action. Spatial orientation can be classified into two types: 1) egocentric spatial orientation, which estimates the direction of external objects with respect to the body parts such as the body longitudinal axis, and 2) gravity-based spatial orientation, which estimates the orientation of the body and external objects with respect to the gravitational axis. We have focused explicitly on egocentric spatial orientation and investigated its characteristics and neural basis through body tilt experiments. A series of experiments showed that the estimation of visual line orientation relative to the body axis is biased when the body is tilted and that this phenomenon is strongly related to subjective estimates of body tilt. This phenomenon indicates the human characteristic of referring to the gravitational axis when estimating the spatial relationship between one's body and an external object. In addition, a study using voxel-based morphometry and neuropsychological studies in brain-injured patients suggested that the right occipitotemporal cortex may play a vital role in this characteristic. These findings contribute to a better understanding of human spatial perception mechanisms in the gravitational field.