Vestibular, visual, and somatosensory contributions to human control of upright stance C. Maurer a , T. Mergner a, * , B. Bolha b , F. Hlavacka b a Neurologische Klinik, Neurozentrum, Universita Èt Freiburg, Breisacher Strasse 64, D-79106 Freiburg, Germany b Institute of Normal and Pathological Physiology SAS, Sienkiewiczova 1, 81371 Bratislava, Slovakia Received 4 November 1999; received in revised form 6 January 2000; accepted 7 January 2000 Abstract We investigated the changes of human posture control of upright stance which occur when vestibular cues (VEST) are absent and visual and somatosensory orientation cues (VIS, SOM) are removed. Postural responses to sinusoidal tilts of a motion platform in the sagittal plane (^28,f  0:05, 0.1, 0.2 and 0.4 Hz) were studied in normal subjects (Ns) and patients with bilateral vestibular loss (Ps). We found that absence of VEST (Ps, visual reference) and removal of VIS (Ns, no visual reference) had little effect on stabilization of upright body posture in space. In the absence of both VEST and VIS (Ps, no visual reference) somatosensory graviception still provided some information on body orientation in space at 0.05 and 0.1 Hz. However, at the higher frequencies Ps qualitatively changed their behavior; they then tended to actively align their bodies with respect to the motion platform. The ®ndings con®rm predictions of a novel postural control model. q 2000 Elsevier Science Ireland Ltd. All rights reserved. Keywords: Posture control; Upright stance; Vestibular; Visual; Somatosensory; Intersensory interaction; Vestibular loss The role of vestibular input and of its interaction with visual and somatosensory cues for human postural control is still not well understood. Learning the interplay between these cues may provide the key to understanding the complex function. The necessity for vestibular-propriocep- tive interaction was anticipated already by Holst and Mittel- staedt [9] who pointed out that the vestibular signal from the head is to be transferred via the neck to the trunk (where the center of mass (COM), is located). In this concept, called the `classical concept' in the following, a vestibular-derived signal of COM motion in space is fed into the postural control mechanism in a negative feedback way. However, this concept was recently challenged by psychophysical studies [5]. They suggest that the transformation of the vestibular signal actually proceeds further (compare [3]), reaching those sites of the body support which create the most relevant reaction forces in a dynamic situation. This vestibular-proprioceptive interaction yields an internal notion of support motion in space. The notion of body motion is obtained only indirectly. It is based primarily on a purely proprioceptive signal of body motion relative to the support, which then is transformed into space co-ordinates by combining it with the vestibular-proprioceptive notion of support-in-space motion. These ®ndings led to a `new concept' of postural control, which postulates that the vestibular signal is used in a feed forward way, while the body-on-support adjustments are handled mainly by proprioceptive control mechanisms [4]. The difference between the classical and the new concept may be appreciated if one considers an imposed body lean on stationary support (e.g. ®rm `ground'). The classical concept would predict a vestibular-driven postural reaction, whereas the new concept predicts a purely proprioceptive reaction (vestibular input would not come into play unless the support is perceived as moving). Another prediction concerns the situation where the body support is moving, without subjects perceiving this motion (e.g. because they are deprived of the relevant sensory cues). According to the classical concept there would be no longer a vestibular driv- ing signal for the postural reaction, so that the subject would be at the mercy of the physical forces (gravity, inertia, etc.: note that this concept does not specify default mechanisms that then take over). In contrast, the new concept predicts that, as a default, the body is stabilized by the proprioceptive body-on-support mechanism. Neuroscience Letters 281 (2000) 99±102 0304-3940/00/$ - see front matter q 2000 Elsevier Science Ireland Ltd. All rights reserved. PII: S0304-3940(00)00814-4 www.elsevier.com/locate/neulet * Corresponding author. Tel.: 149-761-270-5313 ext. 5306; fax: 149-761-270-5310. E-mail address: mergner@sun1.ruf.uni-freiburg.de (T. Mergner)