Computational neurology of gravity perception involving semicircular canal dysfunction in unilateral vestibular...

Unilateral peripheral vestibular lesions not only lead to vertigo, nystagmus and imbalance, but also to a bias in the perception of verticality, which can be measured as tilt of the subjective visual vertical (SVV). Previously, this tilt has been assumed to be caused by a residual otolith bias, for example, because unequal numbers of active haircells on both sides of the utricular striola might result in an imbalance of the firing rates of central otolith neurons. Here we propose that a tilt of the subjective visual vertical might as well be caused by a vertical semicircular canal bias in the roll axis after unilateral peripheral lesions. The canal bias, acting similar to angular velocity stimuli, influences the SVV via the central gravity estimator, which under normal circumstances resolves a perceptual tilt-translation ambiguity. To illustrate our hypothesis, we compare model predictions to data on SVV measurements in patients with unilateral vestibular lesions while being tilted or being rotated eccentrically. We further embed the model of peripheral processing in a neural network that implements the idiotropic bias and represents the direction of gravity as population code in a three dimensional spherical topography.