Integration of visual and inertial cues in the perception of angular self-motion.

The brain is able to determine angular self-motion from visual, vestibular, and kinesthetic information. There is compelling evidence that both humans and non-human primates integrate visual and inertial (i.e., vestibular and kinesthetic) information in a statistically optimal fashion when discriminating heading direction. In the present study, we investigated whether the brain also integrates information about angular self-motion in a similar manner. Eight participants performed a 2IFC task in which they discriminated yaw-rotations (2-s sinusoidal acceleration) on peak velocity. Just-noticeable differences (JNDs) were determined as a measure of precision in unimodal inertial-only and visual-only trials, as well as in bimodal visual-inertial trials. The visual stimulus was a moving stripe pattern, synchronized with the inertial motion. Peak velocity of comparison stimuli was varied relative to the standard stimulus. Individual analyses showed that data of three participants showed an increase in bimodal precision, consistent with the optimal integration model; while data from the other participants did not conform to maximum-likelihood integration schemes. We suggest that either the sensory cues were not perceived as congruent, that integration might be achieved with fixed weights, or that estimates of visual precision obtained from non-moving observers do not accurately reflect visual precision during self-motion.

Multisensory determinants of orientation perception: task-specific sex differences.

Females have been reported to be more 'visually dependent' than males. When aligning a rod in a tilted frame to vertical, females are more influenced by the frame than are males, who align the rod closer to gravity. Do females rely more on visual information at the cost of other sensory information? We compared the subjective visual vertical and the perceptual upright in 29 females and 24 males. The orientation of visual cues presented on a shrouded laptop screen and of the observer's posture were varied. When upright, females' subjective visual vertical was more influenced by visual cues and their responses were more variable than were males'. However, there were no differences between the sexes in the perceptual upright task. Individual variance in subjective visual vertical judgments and in the perceptual upright predicted the level of visual dependence across both sexes. When lying right-side down, there were no reliable differences between the sexes in either measure. We conclude that heightened 'visual dependence' in females does not generalize to all aspects of spatial processing but is probably attributable to task-specific differences in the mechanisms of sensory processing in the brains of females and males. The higher variability and lower accuracy in females for some spatial tasks is not due to their having qualitatively worse access to information concerning either the gravity axis or corporeal representation: it is only when gravity and the long body axis align that females have a performance disadvantage.

Multisensory determinants of orientation perception in Parkinson's disease.

Perception of the relative orientation of the self and objects in the environment requires integration of visual and vestibular sensory information, and an internal representation of the body's orientation. Parkinson's disease (PD) patients are more visually dependent than controls, implicating the basal ganglia in using visual orientation cues. We examined the relative roles of visual and non-visual cues to orientation in PD using two different measures: the subjective visual vertical (SVV) and the perceptual upright (PU). We tested twelve PD patients (nine both on- and off-medication), and thirteen age-matched controls. Visual, vestibular and body cues were manipulated using a polarized visual room presented in various orientations while observers were upright or lying right-side-down. Relative to age-matched controls, patients with PD showed more influence of visual cues for the SVV but were more influenced by the direction of gravity for the PU. Increased SVV visual dependence corresponded with equal decreases of the contributions of body sense and gravity. Increased PU gravitational dependence corresponded mainly with a decreased contribution of body sense. Curiously however, both of these effects were significant only when patients were medicated. Increased SVV visual dependence was highest for PD patients with left-side initial motor symptoms. PD patients when on and off medication were more variable than controls when making judgments. Our results suggest that (i) PD patients are not more visually dependent in general, rather increased visual dependence is task specific and varies with initial onset side, (ii) PD patients may rely more on vestibular information for some perceptual tasks which is reflected in relying less on the internal representation of the body, and (iii) these effects are only present when PD patients are taking dopaminergic medication.

Does handedness influence the strength of phantom limb illusions in the virtual reality box?

Subjects had to judge the size of a tactile stimulus held in the unseen hand, while a visible phantom hand representing that unseen hand held a tactile stimulus of same or different size. No asymmetries in interference effects were found that could be related to hand or handedness. The method lends itself to quantification of virtual reality box illusions and can be used to evaluate the role of experience and sources of variability in the strength of the illusion across subjects.