Image motion with color contrast suffices to elicit an optokinetic reflex in Xenopus laevis tadpoles.

The optokinetic reflex is a closed-loop gaze-stabilizing ocular motor reaction that minimizes residual retinal image slip during vestibulo-ocular reflexes. In experimental isolation, the reflex is usually activated by motion of an achromatic large-field visual background with strong influence of radiance contrast on visual motion estimation and behavioral performance. The presence of color in natural environments, however, suggests that chromatic cues of visual scenes provide additional parameters for image motion detection. Here, we employed Xenopus laevis tadpoles to study the influence of color cues on the performance of the optokinetic reflex and multi-unit optic nerve discharge during motion of a large-field visual scene. Even though the amplitude of the optokinetic reflex decreases with smaller radiance contrast, considerable residual eye movements persist at the 'point of equiluminance' of the colored stimuli. Given the color motion preferences of individual optic nerve fibers, the underlying computation potentially originates in retinal circuits. Differential retinal ganglion cell projections and associated ocular motor signal transformation might further reinforce the color dependency in conceptual correspondence with head/body optomotor signaling. Optokinetic reflex performance under natural light conditions is accordingly influenced by radiance contrast as well as by the color composition of the moving visual scene.

I spy with my little eye: a simple behavioral assay to test color sensitivity on digital displays.

Passive and interactive virtual reality (VR) environments are becoming increasingly popular in the field of behavioral neuroscience. While the technique was originally developed for human observers, corresponding applications have been adopted for the research of visual-driven behavior and neural circuits in animals. RGB color reproduction using red, green and blue primary color pixels is generally calibrated for humans, questioning if the distinct parameters are also readily transferable to other species. In particular, a visual image in the RGB color space has a clearly defined contrast pattern for humans, but this may not necessarily be the case for other mammals or even non-mammalian species, thereby impairing any interpretation of color-related behavioral or neuronal results. Here, we present a simple method to estimate the sensitivity of animals to the three primary colors of digital display devices based on the performance of object motion-driven visuo-motor reflexes and demonstrate differences in the color sensitivity between Xenopus laevis and Ambystoma mexicanum (Axolotl).This article has an associated First Person interview with the first author of the paper.

Head-Movement-Emphasized Rehabilitation in Bilateral Vestibulopathy.

Objective: Although there is evidence that vestibular rehabilitation is useful for treating chronic bilateral vestibular hypofunction (BVH), the mechanisms for improvement, and the reasons why only some patients improve are still unclear. Clinical rehabilitation results and evidence fromeye-head control in vestibular deficiency suggest that headmovement is a crucial element of vestibular rehabilitation. In this study, we assess the effects of a specifically designed head-movement-based rehabilitation program on dynamic vision, and explore underlying mechanisms. Methods: Two adult patients (patients 1 and 2) with chronic BVH underwent two 4-week interventions: (1) head-movement-emphasized rehabilitation (HME) with exercises based on active head movements, and (2) eye-movement-only rehabilitation (EMO), a control intervention with sham exercises without head movement. In a double-blind crossover design, the patients were randomized to first undergo EMO (patient 1) and-after a 4-week washout-HME, and vice-versa (patient 2). Before each intervention and after a 4-week follow-up patients' dynamic vision, vestibulo-ocular reflex (VOR) gain, as well as re-fixation saccade behavior during passive headmotion were assessed with the head impulse testing device-functional test (HITD-FT). Results: HME, not EMO, markedly improved perception with dynamic vision during passive head motion (HITD-FT score) increasing from 0 to 60% (patient 1) and 75% (patient 2). There was a combination of enhanced VOR, as well as improved saccadic compensation. Conclusion: Head movement seems to be an important element of rehabilitation for BVH. It improves dynamic vision with a combined VOR and compensatory saccade enhancement.

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.

It's not all black and white: visual scene parameters influence optokinetic reflex performance in Xenopus laevis tadpoles.

The maintenance of visual acuity during active and passive body motion is ensured by gaze-stabilizing reflexes that aim at minimizing retinal image slip. For the optokinetic reflex (OKR), large-field visual motion of the surround forms the essential stimulus that activates eye movements. Properties of the moving visual world influence cognitive motion perception and the estimation of visual image velocity. Therefore, the performance of brainstem-mediated visuo-motor behaviors might also depend on image scene characteristics. Employing semi-intact preparations of mid-larval stages of Xenopus laevis tadpoles, we studied the influence of contrast polarity, intensity, contour shape and different motion stimulus patterns on the performance of the OKR and multi-unit optic nerve discharge during motion of a large-field visual scene. At high contrast intensities, the OKR amplitude was significantly larger for visual scenes with a positive contrast (bright dots on a dark background) compared with those with a negative contrast. This effect persisted for luminance-matched pairs of stimuli, and was independent of contour shape. The relative biases of OKR performance along with the independence of the responses from contour shape were closely matched by the optic nerve discharge evoked by the same visual stimuli. However, the multi-unit activity of retinal ganglion cells in response to a small single moving vertical edge was strongly influenced by the light intensity in the vertical neighborhood. This suggests that the underlying mechanism of OKR biases related to contrast polarity directly derives from visual motion-processing properties of the retinal circuitry.

Influence of person- and situation-specific characteristics on collision avoidance behavior in human locomotion.

In everyday situations, pedestrians deploy successful strategies to avoid collisions with other persons crossing their paths. In this study, 2 experiments were conducted to investigate to what extent personal or situational characteristics affect role attribution and contribution to successful collision avoidance in human locomotion. Pairs of subjects walked at their natural speed from a start to a goal point. Walking paths were defined in such a way that subjects would collide halfway on their trajectory, if they did not actively avoid colliding by speed or path adjustments. In the first experiment, we investigated whether crossing order, path, and speed adjustments correlate with subject-specific parameters, such as gender, height, and personality traits. It is interesting that individuals' collision avoidance behavior was not correlated with any of these factors. In the second experiment, initial walking speed and heading were used to predict the crossing order. It was found that these 2 parameters are sufficient to estimate future role attribution with 95% confidence already 2.5 m before the crossing; that is, even before any collision avoidance behavior is initiated. In sum, this suggests that collision avoidance strategies in human locomotion are based on situational rather than on personal characteristics. These situational characteristics result in role attributions, which are highly predictable within and across pairs of pedestrians, whereby the role-dependent contribution of the pedestrian giving way is of greater relevance for successful collision avoidance. (PsycINFO Database Record