||When we change our orientation with respect to gravity, we perceive a stable and upright surrounding. The information on the orientation of the gravity vector, mainly provided by the otolith system, is not only used to assess
the orientation of our body relative to gravity, but also to judge the orientation of visual objects with respect to the gravity vector. To produce a stable percept of an upright visual world (the subjective visual vertical=SVV), the brain has to use the information on the orientation of the gravity vector in order to reinterpret the orientation of retinal images such as to keep the percept of the visual scene upright. For human subjects, it is well established that this updating of the visual percept is not perfect, leading to a characteristic deviation of the SVV from the
gravitational vertical. For small roll-tilt angles (<60°), subjects typically overestimate the tilt (Müller- or E-effect), whereas for larger angles they underestimate it (Aubert- or A-effect). As these effects might be exploited to identify the neural networks underlying the approximately tilt-independent perceptual representation of the orientation of the visual
scene, we wanted to know if nonhuman primates, the model system of choice for invasive studies of visual perception, are subject to similar
tilt-induced distortions of the SVV.