The neural representation of postural control in humans

Abstract

Lesion of the "vestibular cortex" in the human posterior insula leads to a tilted perception of visual vertical but not to tilted body posture and loss of lateral balance. However, some stroke patients show the reverse pattern. Although their processing of visual and vestibular inputs for orientation perception of the visual world is undisturbed, they push away actively from the ipsilesional side (the side of lesion location), leading to a contraversive tilt of the body (tilt toward the side opposite to the lesion) and falling to that side. Recently, the origin of contraversive pushing was identified as an altered perception of the body's orientation in relation to gravity. These patients experience their body as oriented "upright" when actually tilted enormously to the ipsilesional side (18 degrees on average). The findings argued for a separate pathway in humans for sensing body orientation in relation to gravity apart from the one projecting to the vestibular cortex. The present study aimed at identifying this brain area. The infarcted brain regions of 23 consecutively admitted patients with severe contraversive pushing were projected onto a template MRI scan, which had been normalized to Talairach space. The overlapping area of these infarctions centered on the posterolateral thalamus. Our finding necessitates reinterpretation of this area as being only a "relay structure" of the vestibular pathway on its way from the brainstem to the vestibular cortex. The ventral posterior and lateral posterior nuclei of the posterolateral thalamus (and probably its cortical projections) rather seem to be fundamentally involved in the neural representation of a second graviceptive system in humans decisive for our control of upright body posture.

Figure 1

(A) Overlay plots of the infarcted areas of 15 patients with contraversive pushing after unilateral right hemispheric lesions and their 15 controls without pushing. Using MRICRO software (www.psychology.nottingham.ac.uk/staff/crl/mricro.html), the lesions were mapped on slices of a template MRI scan from the Montreal Neurological Institute (www.bic.mni. mcgill.ca/cgi/icbm_view), which was normalized to Talairach space (14). The z-coordinates of each transverse section are given. The number of overlapping lesions is illustrated by different colors coding increasing frequencies from violet (n = 1) to red (n = 15). RBD, right-brain-damaged. (B) Overlay plots of the infarcted areas of 8 patients with contraversive pushing after unilateral left hemispheric lesions and their 8 controls without pushing. LBD, left-brain-damaged.

Figure 2

Exploded view of the center of lesion overlap in the 15 right brain-damaged patients with contraversive pushing. The same conventions are used as for Fig. 1. VPL, ventral posterolateral nucleus; VPM, ventral posteromedial nucleus.