Human Exteroception during Object Handling with an Upper Limb Exoskeleton.

Upper limb exoskeletons may confer significant mechanical advantages across a range of tasks. The potential consequences of the exoskeleton upon the user's sensorimotor capacities however, remain poorly understood. The purpose of this study was to examine how the physical coupling of the user's arm to an upper limb exoskeleton influenced the perception of handheld objects. In the experimental protocol, participants were required to estimate the length of a series of bars held in their dominant right hand, in the absence of visual feedback. Their performance in conditions with an exoskeleton fixed to the forearm and upper arm was compared to conditions without the upper limb exoskeleton. Experiment 1 was designed to verify the effects of attaching an exoskeleton to the upper limb, with object handling limited to rotations of the wrist only. Experiment 2 was designed to verify the effects of the structure, and its mass, with combined movements of the wrist, elbow, and shoulder. Statistical analysis indicated that movements performed with the exoskeleton did not significantly affect perception of the handheld object in experiment 1 (BF01 = 2.3) or experiment 2 (BF01 = 4.3). These findings suggest that while the integration of an exoskeleton complexifies the architecture of the upper limb effector, this does not necessarily impede transmission of the mechanical information required for human exteroception.

Evidence for an internal model of friction when controlling kinetic energy at impact to slide an object along a surface toward a target.

Although the role of an internal model of gravity for the predictive control of the upper limbs is quite well established, evidence is lacking regarding an internal model of friction. In this study, 33 male and female human participants performed a striking movement (with the index finger) to slide a plastic cube-like object to a given target distance. The surface material (aluminum or balsa wood) on which the object slides, the surface slope (-10°, 0, or +10°) and the target distance (25 cm or 50 cm) varied across conditions, with ten successive trials in each condition. Analysis of the object speed at impact and spatial error suggests that: 1) the participants chose to impart a similar speed to the object in the first trial regardless of the surface material to facilitate the estimation of the coefficient of friction; 2) the movement is parameterized across repetitions to reduce spatial error; 3) an internal model of friction can be generalized when the slope changes. Biomechanical analysis showed interindividual variability in the recruitment of the upper limb segments and in the adjustment of finger speed at impact in order to transmit the kinetic energy required to slide the object to the target distance. In short, we provide evidence that the brain builds an internal model of friction that makes it possible to parametrically control a striking movement in order to regulate the amount of kinetic energy required to impart the appropriate initial speed to the object.

Social Contact Acts as Appetitive Reinforcement and Supports Associative Learning in Honeybees.

Social learning is taxonomically widespread in the animal kingdom [1], and although it is long thought to be a hallmark of vertebrates, recent studies revealed that it also exists in insects [2-5]. The adaptive functions of social learning are well known, but its underlying mechanisms remain debated [2, 5, 6]. Social insects critically depend on the social transmission of information for successful food search and their colonies' fitness [7] and are tractable models for studying the social cues and cognitive mechanisms involved [2-5]. Besides the well-known dance language allowing them to communicate the location of food sources among nestmates [8], honeybees also learn chemosensory information about these sources both outside and within the hive [9, 10]. In the latter case, they associate the floral scent carried by returning foragers on their body with the nectar provided through mouth-to-mouth trophallaxis, similar to the manner in which foragers directly learn odorant-nectar reward associations at the foraging patch [9-11]. Strikingly, however, neither the dance nor trophallaxis is strictly necessary for foragers recruited within the hive to find the right floral source, and simple body contact between foragers may be sufficient [12]. What is the reinforcing agent in this case? We show here that simple social contact acts as appetitive reinforcement and can be used in associative olfactory learning. We demonstrate that this social reinforcement is mediated by bees' antennal movements and modulated by bees' behavioral development. These results unveil a social learning mechanism that may play a facilitating role in resource exploitation by social groups.