Upright Perception and Ocular Torsion Change Independently during Head Tilt

The otoliths are stimulated in the same fashion by gravitational and inertial forces, so otolith signals are ambiguous indicators of self-orientation. The ambiguity can be resolved with added visual information indicating orientation and acceleration with respect to the earth. Here we present a Bayesian model of the statistically optimal combination of noisy vestibular and visual signals. Likelihoods associated with sensory measurements are represented in an orientation/acceleration space. The likelihood function associated with the otolith signal illustrates the ambiguity; there is no unique solution for self-orientation or acceleration. Likelihood functions associated with other sensory signals can resolve this ambiguity. In addition, we propose two priors, each acting on a dimension in the orientation/acceleration space: the idiotropic prior and the no-acceleration prior. We conducted experiments using a motion platform and attached visual display to examine the influence of visual signals on the interpretation of the otolith signal. Subjects made pitch and acceleration judgments as the vestibular and visual signals were manipulated independently. Predictions of the model were confirmed: (1) visual signals affected the interpretation of the otolith signal, (2) less variable signals had more influence on perceived orientation and acceleration than more variable ones, and (3) combined estimates were more precise than single-cue estimates. We also show that the model can explain some well-known phenomena including the perception of upright in zero gravity, the Aubert effect, and the somatogravic illusion. Show more

Most evidence that the brain uses Bayesian inference to integrate noisy sensory signals optimally has been obtained by showing that the noise levels in each modality separately can predict performance in combined conditions. Such a forward approach is difficult to implement when the various signals cannot be measured in isolation, as in spatial orientation, which involves the processing of visual, somatosensory, and vestibular cues. Instead, we applied an inverse probabilistic approach, based on optimal observer theory. Our goal was to investigate whether the perceptual differences found when probing two different states--body-in-space and head-in-space orientation--can be reconciled by a shared scheme using all available sensory signals. Using a psychometric approach, seven human subjects were tested on two orientation estimates at tilts < 120°: perception of body tilt [subjective body tilt (SBT)] and perception of visual vertical [subjective visual vertical (SVV)]. In all subjects, the SBT was more accurate than the SVV, which showed substantial systematic errors for tilt angles beyond 60°. Variability increased with tilt angle in both tasks, but was consistently lower in the SVV. The sensory integration model fitted both datasets very nicely. A further experiment, in which supine subjects judged their head orientation relative to the body, independently confirmed the predicted head-on-body noise by the model. Model predictions based on the derived noise properties from the various modalities were also consistent with previously published deficits in vestibular and somatosensory patients. We conclude that Bayesian computations can account for the typical differences in spatial orientation judgments associated with different task requirements. Show more

One of the key questions in spatial perception is whether the brain has a common representation of gravity that is generally accessible for various perceptual orientation tasks. To evaluate this idea, we compared the ability of six tilted subjects to indicate earth-centric directions in the dark with a visual and an oculomotor paradigm and to estimate their body tilt relative to gravity. Subjective earth-horizontal and -vertical data were collected, either by adjusting a visual line or by making saccades, at 37 roll-tilt angles across the entire range. These spatial perception responses and the associated body-tilt estimates were subjected to a principal-component analysis to describe their tilt dependence. This analysis allowed us to separate systematic and random errors in performance, to disentangle the effects of task (horizontal vs. vertical) and paradigm (visual vs. oculomotor) in the space-perception data, and to compare the veridicality of space perception and the sense of self-tilt. In all spatial-orientation tests, whether involving space-perception or body-tilt judgments, subjects made considerable systematic errors which mostly betrayed tilt underestimation [Aubert effect (A effect)] and peaked near 130 degrees tilt. However, the A effect was much smaller in body-tilt estimates than in spatial pointing, implying that the underlying signal processing must have been different. Pointing results obtained with the visual and the oculomotor paradigm were not identical either, but these differences, which were task-related (horizontal vs. vertical), were subtle in comparison. The tilt-dependent pattern of random errors (noisy scatter) was almost identical in visual and oculomotor pointing results, showing a steep monotonic increase with tilt angle, but was again clearly different in the body-tilt estimates. These findings are discussed in the context of a conceptual model in an attempt to explain how the different patterns of systematic and random errors in external-space and self-tilt perception may come about. The scheme proposes that basically similar computational mechanisms, working with different settings, may be responsible. Show more