The influence of obstacles on grasp planning.

When reaching to grasp for an object in the absence of obstacles, the choice of contact points is highly consistent within and between healthy humans, suggesting a preplanning of grasping movements (Gilster et al. in Exp Brain Res 217:137-151, 2012). In real life, objects may obstruct the favored contact points at a target object, requiring adjustments to avoid collision. In the present study, we investigated how an obstacle that directly obstructs the favored contact points for two-digit grasping changes the planning and execution of reach-to-grasp movements. Furthermore, we elucidated to what extent an obstacle placed at various angular positions around the target object (thereby not directly obstructing the favored contact points) still influences trajectories, contact points, and time-related parameters. When obstacles directly obstructed favored contact points participants either chose a completely new contact point or grasped the object only slightly away from the favored contact point. Obstacles located near the favored contact points but not directly obstructing them still resulted in a repulsive effect, meaning that contact points were shifted away from the obstacle to ensure sufficient distance to the obstacle. We found that the position of an obstacle even influences the direction in which the fingers set off. This leads to a deviation of the trajectory very early in the time course, yielding longer movement times if the main contact points are obstructed. Taken together, the early significant influence of obstacles on the grasping movement supports the assumption that grasping movements are preplanned.

Insufficient compensation for self-motion during perception of object speed: The vestibular Aubert-Fleischl phenomenon.

To estimate object speed with respect to the self, retinal signals must be summed with extraretinal signals that encode the speed of eye and head movement. Prior work has shown that differences in perceptual estimates of object speed based on retinal and oculomotor signals lead to biased percepts such as the Aubert-Fleischl phenomenon (AF), in which moving targets appear slower when pursued. During whole-body movement, additional extraretinal signals, such as those from the vestibular system, may be used to transform object speed estimates from a head-centered to a world-centered reference frame. Here we demonstrate that whole-body pursuit in the form of passive yaw rotation, which stimulates the semicircular canals of the vestibular system, leads to a slowing of perceived object speed similar to the classic oculomotor AF. We find that the magnitude of the vestibular and oculomotor AF is comparable across a range of speeds, despite the different types of input signal involved. This covariation might hint at a common modality-independent mechanism underlying the AF in both cases.

Towards dynamic modeling of visual-vestibular conflict detection.

Visual-vestibular mismatch is a common occurrence, with causes ranging from vehicular travel, to vestibular dysfunction, to virtual reality displays. Behavioral and physiological consequences of this mismatch include adaptation of reflexive eye movements, oscillopsia, vertigo, and nausea. Despite this significance, we still do not have a good understanding of how the nervous system evaluates visual-vestibular conflict. Here we review research that quantifies perceptual sensitivity to visual-vestibular conflict and factors that mediate this sensitivity, such as noise on visual and vestibular sensory estimates. We emphasize that dynamic modeling methods are necessary to investigate how the nervous system monitors conflict between time-varying visual and vestibular signals, and we present a simple example of a drift-diffusion model for visual-vestibular conflict detection. The model makes predictions for detection of conflict arising from changes in both visual gain and latency. We conclude with discussion of topics for future research.

Visual-Vestibular Conflict Detection Depends on Fixation.

Visual and vestibular signals are the primary sources of sensory information for self-motion. Conflict among these signals can be seriously debilitating, resulting in vertigo [1], inappropriate postural responses [2], and motion, simulator, or cyber sickness [3-8]. Despite this significance, the mechanisms mediating conflict detection are poorly understood. Here we model conflict detection simply as crossmodal discrimination with benchmark performance limited by variabilities of the signals being compared. In a series of psychophysical experiments conducted in a virtual reality motion simulator, we measure these variabilities and assess conflict detection relative to this benchmark. We also examine the impact of eye movements on visual-vestibular conflict detection. In one condition, observers fixate a point that is stationary in the simulated visual environment by rotating the eyes opposite head rotation, thereby nulling retinal image motion. In another condition, eye movement is artificially minimized via fixation of a head-fixed fixation point, thereby maximizing retinal image motion. Visual-vestibular integration performance is also measured, similar to previous studies [9-12]. We observe that there is a tradeoff between integration and conflict detection that is mediated by eye movements. Minimizing eye movements by fixating a head-fixed target leads to optimal integration but highly impaired conflict detection. Minimizing retinal motion by fixating a scene-fixed target improves conflict detection at the cost of impaired integration performance. The common tendency to fixate scene-fixed targets during self-motion [13] may indicate that conflict detection is typically a higher priority than the increase in precision of self-motion estimation that is obtained through integration.