Postural orientation contributes to modeling the effects of gravity for target interception in humans

Interception of moving targets relies on visual signals and internal models. Less is known about the additional contribution of nonvisual cues about head and body orientation relative to gravity. We took advantage of Galileo’s law of motion along an inclined plane to demonstrate the effects of vestibular and somatosensory cues about body posture on interception timing. We presented a virtual scene without any visual information about gravity direction in a head-mounted display. Participants were asked to hit a ball rolling in a gutter towards the eyes, resulting in image expansion. Participants were tilted backwards in the sagittal plane by 20° or 60°, while ball acceleration was compatible with rolling down a slope of 20° or 60°. At the beginning of the experiment, the timing errors were large and independent of the coherence between acceleration and pitch angle. This is what one would expect if the responses were based exclusively on visual information, since the visual stimuli were identical at both subject tilts. At the end of the experiment, however, the timing errors were systematically smaller in the coherent conditions than the incoherent ones. Moreover, the responses were significantly earlier when participants were pitched by 60° than when they were pitched by 20°.Therefore, practice with the task led to incorporation of postural information about head and body tilt relative to gravity for response timing. Instead, posture did not affect response timing in a control experiment in which participants hit a static target in synchrony with the last of a predictable series of stationary audio-visual stimuli.