Bi-articular muscles and the accuracy of motor control

In a model study, the behaviour of two sets of muscles in controlling multi-joint arm movements is compared. Both the sensory and the motor accuracy of the set containing bi-articular muscles were in general better than those of the set containing only mono-articular muscles. Accuracy considerations can explain differences in strategies for the control of redundant muscle sets between situations which do not differ biomechanically from each other. Furthermore, the role of bi-articular muscles for the robustness of motor programming is discussed

How people achieve their amazing temporal precision in interception

People can hit rapidly moving balls with amazing precision. To determine how they manage to do so, we explored how various factors that we could manipulate influenced people's precision when intercepting virtual targets. We found that temporal precision was highest for fast targets that subjects were free to intercept wherever they wished. Temporal precision was much poorer when the point of interception was specified in advance. Examining responses to abrupt perturbations of the target's motion revealed that people adjusted where rather than when they would hit the target if given the choice. A model that combines judging how long it will take to reach the target's path with estimating the target's position at that time from its visually perceived position and velocity could account for the observed precision with reasonable values for all the parameters. The model considers all relevant sources of errors, together with the delays with which the various aspects can be adjusted. Our analysis provides a biologically plausible explanation for how light falling on the eye can guide the hand to intercept a moving ball with such high precision

Mass Is All That Matters in the Size–Weight Illusion

An object in outer space is weightless due to the absence of gravity, but astronauts can still judge whether one object is heavier than another one by accelerating the object. How heavy an object feels depends on the exploration mode: an object is perceived as heavier when holding it against the pull of gravity than when accelerating it. At the same time, perceiving an object’s size influences the percept: small objects feel heavier than large objects with the same mass (size– weight illusion). Does this effect depend on perception of the pull of gravity? To answer this question, objects were suspended from a long wire and participants were asked to push an object and rate its heaviness. This way the contribution of gravitational forces on the percept was minimised. Our results show that weight is not at all necessary for the illusion because the size–weight illusion occurred without perception of weight. The magnitude of the illusion was independent of whether inertial or gravitational forces were perceived. We conclude that the size–weight illusion does not depend on prior knowledge about weights of object, but instead on a more general knowledge about the mass of objects, independent of the contribution of gravity. Consequently, the size–weight illusion will have the same magnitude on Earth as it should have on the Moon or even under conditions of weightlessness

Quickly 'learning' to move optimally.

People take account of the variability in their movements in a near-optimal manner in various visuo-motor tasks. Is knowledge of one’s variability needed for such near-optimal performance, or could it arise from responding to one’s success in previous attempts in some simple manner? We asked subjects to move a pen back and forth across a tablet to make a cursor move as quickly as possible between two targets. The cursor had to stop within the targets. Task difficulty was varied between blocks. Part of the variation in difficulty was explicit (three target sizes) whereas the rest had to be discovered during the movements (two mappings between the movements of pen and cursor). In all cases, subjects sped up after stopping within a target and slowed down after failing to do so. We interpret this as evidence that explicit knowledge of one’s variability is not necessary for performing close to optimally

The influence of previously seen objects' sizes in distance judgments

An object's retinal image size is determined by a combination of its physical size and its distance, so judgments of an object's size and distance from its retinal image size are coupled. Since one does not have direct access to information about the object's physical size, people may make assumptions about how large it is likely to be. Here we investigated whether the sizes of similar, previously encountered objects influence the assumptions about the physical size of an object and therefore the interpretation of its retinal image size in terms of its distance. Subjects moved their unseen index finger to the positions of binocular simulations of red cubes. For identical target cubes at the same position, they indicated a nearer position of the cube when the preceding cube was small than when it was big. This is in agreement with a tendency to expect the cube to be the same size as that on the previous trial. However, if the expectation were simply adjusted slightly on each trial, the cube would be judged to be nearer when preceded by two consecutive smaller cubes than when preceded by only one smaller cube. It was not, so there must be a more direct influence of the size in the previous trial on distance judgments

The effect of variability in other objects’ sizes on the extent to which people rely on retinal image size as a cue for judging distance

Retinal image size can be used to judge objects’ distances because for any object one can assume that some sizes are more likely than others. It has been shown that an increased variability in the size of otherwise identical target objects over trials reduces the weight given to retinal image size as a distance cue. Here, we examined whether an increased variability in the size of objects of a different color, orientation, or shape reduces the weight given to retinal image size when judging distance. Subjects had to indicate the 3D position of a simulated target object. Retinal image size was given significantly less weight as a cue for judging the target cube’s distance when differently colored and differently oriented target objects appeared in many simulated sizes but not when differently shaped objects had many simulated sizes. We also examined whether increasing the variability in the size of cubes in the surroundings reduces the weight given to retinal image size when judging distance. It does not. We conclude that variability in surrounding or dissimilar objects’ sizes has a negligible influence on the extent to which people rely on retinal image size as a cue for judging distance

Judging an unfamiliar object’s distance from its retinal image size

How do we know how far an object is? If an object's size is known, its retinal image size can be used to judge its distance. To some extent, the retinal image size of an unfamiliar object can also be used to judge its distance, because some object sizes are more likely than others. To examine whether assumptions about object size are used to judge distance, we had subjects indicate the distance of virtual cubes in complete darkness. In separate sessions, the simulated cube size either varied slightly or considerably across presentations. Most subjects indicated a further distance when the simulated cube was smaller, showing that they used retinal image size to judge distance. The cube size that was considered to be most likely depended on the simulated cubes on previous trials. Moreover, subjects relied twice as strongly on retinal image size when the range of simulated cube sizes was small. We conclude that the variability in the perceived cube sizes on previous trials influences the range of sizes that are considered to be likely. © ARVO