Review Article Multisensory Integration and Internal Models for Sensing Gravity Effects in Primates Francesco Lacquaniti, 1,2,3 Gianfranco Bosco, 1,2,3 Silvio Gravano, 1,3 Iole Indovina, 1,3 Barbara La Scaleia, 3 Vincenzo Maffei, 3 and Myrka Zago 3 1 Centre of Space Bio-Medicine, University of Rome Tor Vergata, Via Montpellier 1, 00133 Rome, Italy 2 Department of Systems Medicine, University of Rome Tor Vergata, Via Montpellier 1, 00133 Rome, Italy 3 Laboratory of Neuromotor Physiology, IRCCS Santa Lucia Foundation, Via Ardeatina 306, 00179 Rome, Italy Correspondence should be addressed to Francesco Lacquaniti; lacquaniti@med.uniroma2.it Received 2 May 2014; Accepted 26 May 2014; Published 1 July 2014 Academic Editor: Mariano Bizzarri Copyright © 2014 Francesco Lacquaniti et al. his is an open access article distributed under the Creative Commons Attribution License, which permits unrestricted use, distribution, and reproduction in any medium, provided the original work is properly cited. Gravity is crucial for spatial perception, postural equilibrium, and movement generation. he vestibular apparatus is the main sensory system involved in monitoring gravity. Hair cells in the vestibular maculae respond to gravitoinertial forces, but they cannot distinguish between linear accelerations and changes of head orientation relative to gravity. he brain deals with this sensory ambiguity (which can cause some lethal airplane accidents) by combining several cues with the otolith signals: angular velocity signals provided by the semicircular canals, proprioceptive signals from muscles and tendons, visceral signals related to gravity, and visual signals. In particular, vision provides both static and dynamic signals about body orientation relative to the vertical, but it poorly discriminates arbitrary accelerations of moving objects. However, we are able to visually detect the speciic acceleration of gravity since early infancy. his ability depends on the fact that gravity efects are stored in brain regions which integrate visual, vestibular, and neck proprioceptive signals and combine this information with an internal model of gravity efects. 1. Introduction Intuitively, sensing gravity efects should be a trivial problem for a complex nervous system such as our own. On the one hand, direction and magnitude of gravity are quasi-constant on Earth. hus, gravitational acceleration varies by <1% by changing latitude or altitude, while the vertical delection is <0.05 ∘ . On the other hand, our nervous system is compu- tationally high-powered, being endowed with ≈10 11 neurons interconnected via ≈10 15 synapses. All axons pieced together would cover the distance between the Earth and the Moon (about 400.000 km). One would assume that we are able to monitor gravity directly by means of our sensory systems, but this is not the case. As we shall review in this paper, gravity efects are only extrapolated indirectly by the brain by combining multisensory information with internal models, that is, with neural processes which mimic a physical event. Sensing and coping with gravity is crucial for space perception, control of upright posture, and generation of movements. Indeed, gravity provides a unique reference axis to which we can anchor body orientation and monitor orien- tation changes. Gravity efects on limb and body movements are two-sided, insofar as gravity acts both as a perturbing force that must be counteracted to avoid falling down and as a facilitating force which allows walking and running via the ground contact forces. 2. Vestibular Information he vestibular receptors lie inside the labyrinth of the temporal bone. Somewhat similar sensors evolved irst in invertebrates and then in vertebrates about 500 Myrs ago [1]. he vestibular apparatus acts as an inertial navigation system, including in each ear three semicircular canals oriented roughly orthogonal to each other and two otolithic organs, the sacculus and utriculus with sensory epithelia oriented roughly vertically and horizontally, respectively [2]. Hindawi Publishing Corporation BioMed Research International Volume 2014, Article ID 615854, 10 pages http://dx.doi.org/10.1155/2014/615854