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Abstract:
Accurate perception of the direction of earth vertical can be achieved by sensing the direction of gravity
in body coordinates. This is equivalent to knowing body orientation in world coordinates. There are a number
of visual and non-visual cues we can use to estimate earth vertical relative to the body. Non-visual cues include
the sensation of gravity and forces due to acceleration, and they can be measured by the somatosensory and
vestibular systems. These systems cannot always tell us directly about the direction of gravity because they signal
gravito-inertial (GI) force, which is the sum of all forces acting on the body at a given time. For example, if one is
accelerating, the GI force is the sum of the force due to acceleration and the force due to gravity. In these situations, the direction of GI force does not indicate the direction of earth vertical, but visual cues may be used to resolve the ambiguity. We conducted an experiment in which the direction of GI force was manipulated by pitching observers (rotation about the body’s x-axis) on a motion platform. Their task was to indicate the direction of earth vertical using a pointing device. In some conditions, no visual stimulus was presented. In other conditions, observers were presented with a visual scene depicting acceleration over a flat, textured ground plane. Two cues in the visual display contained information relevant to judging the direction of earth vertical: 1) the location and orientation of the horizon and 2) the rate of acceleration over the ground plane. We present a model of how these visual and non-visual cues might be used to generate an estimate of the direction of earth vertical. Observer responses are compared with the predictions of this model. Results suggest that under the conditions of the present experiment, visual cues had very little effect, and perception of earth vertical was estimated primarily on the basis of vestibular and somatosensory cues.
