Imitation of hand and tool actions is effector-independent

Brain and cognition, 2011

How do humans interact with tools? Gibson (1979) suggested that humans perceive directly what tools afford in terms of meaningful actions. This “affordances” hypothesis implies that visual objects can potentiate motor responses even in the absence of an intention to act. Here we explore the temporal evolution of motor plans afforded by common objects. We presented objects that have a strong significance for action (pinching and grasping) and objects with no such significance. Two experimental tasks involved participants viewing objects presented on a computer screen. For the first task, they were instructed to respond rapidly to changes in background colour by using an apparatus mimicking precision and power grip responses. For the second task, they received stimulation of their primary motor cortex using transcranial magnetic stimulation (TMS) while passively viewing the objects. Muscular responses (motor evoked potentials: MEPs) were recorded from two intrinsic hand muscles (associated with either a precision or power grip). The data showed an interaction between type of response (or muscle) and type of object, with both reaction time and MEP measures implying the generation of a congruent motor plan in the period immediately after object presentation. The results provide further support for the notion that the physical properties of objects automatically activate specific motor codes, but also demonstrate that this influence is rapid and relatively short lived.► How do objects automatically activate specific motor plans known as “affordances”? ► Task-irrelevant pictures shown to activate congruent grip actions. ► Affordance effect evident in both RTs and motor evoked potentials. ► Affordance effect arises rapidly and also dissipates quickly. ► Affordance effect evident for separate hand actions generated in the same hemisphere.

Human Movement Science, 2011

The purpose of this study was to investigate the influence of object function and the observer's action capabilities on grasp facilitation. We used a stimulus-response compatibility (SRC) protocol in which participants were asked to reach and grasp a drinking glass using one of two grasps -the thumb-up or the thumb-down grasp. The reaction time (RT) was used as the index of grasp facilitation. In Experiment 1, we found evidence for the facilitation of ''functionally relevant'' grasps -where the type of grasp facilitated depended on the location of opening but not the shape of the object. However, this effect was found only when attention was directed toward the location of the opening. In Experiments 2 and 3, we found that this facilitation was also affected by whether participants had the ability to functionally interact with the object. These results show that S-R compatibilities are influenced both by the object's function and the actor's action capabilities, and are interpreted in framework of affordances.

Proceedings of The Royal Society B: Biological Sciences, 2007

Recent research in cognitive neuroscience has found that observation of human actions activates the 'mirror system' and provokes automatic imitation to a greater extent than observation of non-biological movements. The present study investigated whether this human bias depends primarily on phylogenetic or ontogenetic factors by examining the effects of sensorimotor experience on automatic imitation of non-biological robotic, stimuli. Automatic imitation of human and robotic action stimuli was assessed before and after training. During these test sessions, participants were required to execute a pre-specified response (e.g. to open their hand) while observing a human or robotic hand making a compatible (opening) or incompatible (closing) movement. During training, participants executed opening and closing hand actions while observing compatible (group CT) or incompatible movements (group IT) of a robotic hand. Compatible, but not incompatible, training increased automatic imitation of robotic stimuli (speed of responding on compatible trials, compared with incompatible trials) and abolished the human bias observed at pre-test. These findings suggest that the development of the mirror system depends on sensorimotor experience, and that, in our species, it is biased in favour of human action stimuli because these are more abundant than non-biological action stimuli in typical developmental environments.

Experimental Brain Research, 2008

Objects can be grasped in several ways due to their physical properties, the context surrounding the object, and the goal of the grasping agent. The aim of the present study was to investigate whether the prior-to-contact grasping kinematics of the same object vary as a result of diVerent goals of the person grasping it. Subjects were requested to reach toward and grasp a bottle Wlled with water, and then complete one of the following tasks: (1) Grasp it without performing any subsequent action; (2) Lift and throw it; (3) Pour the water into a container; (4) Place it accurately on a target area; (5) Pass it to another person. We measured the angular excursions at both metacarpalphalangeal (mcp) and proximal interphalangeal (pip) joints of all digits, and abduction angles of adjacent digit pairs by means of resistive sensors embedded in a glove. The results showed that the presence and the nature of the task to be performed following grasping aVect the positioning of the Wngers during the reaching phase. We contend that a oneto-one association between a sensory stimulus and a motor response does not capture all the aspects involved in grasping. The theoretical approach within which we frame our discussion considers internal models of anticipatory control which may provide a suitable explanation of our results.

Neuropsychologia, 2002

The relations between stimuli triggering a hand grasping movement and the subsequent action were studied in normal human participants. Participants were instructed to prepare to grasp a bar, oriented either clockwise or counterclockwise, and to grasp it as fast as possible on presentation of a visual stimulus with their right hand. The visual stimuli were pictures of the right hand as seen in a mirror. In Experiment 1, they represented the mirror image of the hand final posture as achieved in grasping the bar oriented either clockwise or counterclockwise. In Experiment 2, in addition to the pictures of Experiment 1, another two pictures, obtained rotating the hands represented in the previous ones of 90°, were also used. Both experiments showed that the reaction times were faster when there was a similarity between hand position as depicted in the triggering visual stimulus and the grasping hand final position, the fastest responses being those where this similarity was the closest. In addition, Experiment 2 showed that reaction times to not rotated stimuli were faster than reaction times to the rotated stimuli, thus excluding a simple stimulus-response compatibility explanation of the findings. The data are interpreted as behavioral evidence that there is a close link between specific visual stimuli and specific motor actions. A neurophysiological model for this visuo-motor link is presented.

PLoS ONE, 2013

Understanding the interactions of visual and proprioceptive information in tool use is important as it is the basis for learning of the tool's kinematic transformation and thus skilled performance. This study investigated how the CNS combines seen cursor positions and felt hand positions under a visuo-motor rotation paradigm. Young and older adult participants performed aiming movements on a digitizer while looking at rotated visual feedback on a monitor. After each movement, they judged either the proprioceptively sensed hand direction or the visually sensed cursor direction. We identified asymmetric mutual biases with a strong visual dominance. Furthermore, we found a number of differences between explicit and implicit judgments of hand directions. The explicit judgments had considerably larger variability than the implicit judgments. The bias toward the cursor direction for the explicit judgments was about twice as strong as for the implicit judgments. The individual biases of explicit and implicit judgments were uncorrelated. Biases of these judgments exhibited opposite sequential effects. Moreover, age-related changes were also different between these judgments. The judgment variability was decreased and the bias toward the cursor direction was increased with increasing age only for the explicit judgments. These results indicate distinct explicit and implicit neural representations of hand direction, similar to the notion of distinct visual systems.

Human dexterity with tools is believed to stem from our ability to incorporate and use tools as parts of our body. However tool incorporation, evident as extensions in our body representation and peri-personal space, has been observed predominantly after extended tool exposures and does not explain our immediate motor behaviours when we change tools. Here we utilize two novel experiments to elucidate the presence of additional immediate tool incorporation effects that determine motor planning with tools. Interestingly, tools were observed to immediately induce a trial-by-trial, tool length dependent shortening of the perceived limb lengths, opposite to observations of elongations after extended tool use. Our results thus exhibit that tools induce a dual effect on our body representation; an immediate shortening that critically affects motor planning with a new tool, and the slow elongation, probably a consequence of skill related changes in sensory-motor mappings with the repeated use of the tool.

Brain and Cognition, 2000

Intuitively, one can assume that imitating a movement is an easier task than responding to a symbolic stimulus like a verbal instruction. Support for this suggestion can be found in neuropsychological research as well as in research on stimulusresponse compatibility. However controlled experimental evidence for this assumption is still lacking. We used a stimulus-response compatibility paradigm to test the assumption. In a series of experiments, it was tested whether observed finger movements have a stronger influence on finger movement execution than a symbolic or spatial cue. In the first experiment, we compared symbolic cues with observed finger movements using an interference paradigm. Observing finger movements strongly influenced movement execution, irrespective of whether the finger movement was the relevant or the irrelevant stimulus dimension. In the second experiment, effects of observed finger movements and spatial finger cues were compared. The observed finger movement dominated the spatial finger cue. A reduction in the similarity of observed and executed action in the third experiment led to a decrease of the influence of observed finger movement, which demonstrates the crucial role of the imitative relation of observed and executed action for the described effects. The results are discussed in relation to recent models of stimulus-response compatibility. Neurocognitive support for the strong relationship between movement observation and movement execution is reported.

The ability to manipulate objects and use them as tools is a fundamental feature of human behaviour. It represents a challenge for sensorimotor control because grasping an object changes the dynamics of the arm. As such, each time an object is grasped, the motor commands controlling the arm must rapidly adapt to the particular dynamics of the object. In order to examine this process, we developed a virtual object manipulation task which was simulated using a novel robotic manipulandum. The task required subjects to rotate a small hammer-like tool while keeping the grasp point stationary. To successfully perform the task, subjects had to generate a torque to rotate the object as well as a force to stabilise the handle. Because the object was simulated, we could vary its dynamics and visual orientation from trial to trial. We also recorded the performance of subjects during the task. Results showed that object manipulation is mediated by multiple internal representations of dynamics which can be appropriately recalled by visual feedback of the object. Each representation is associated with a particular dynamic context (the orientation of the object), such that adaptation to the dynamics in one context leads to limited generalization to other contexts. To capture this context-dependent behaviour, we developed a novel state-space model. The model includes multiple states which are associated with different dynamic contexts of the object. It reproduced both the timecourse of adaptation and de-adaptation, as well as the context-dependent behaviour observed during a range of experiments. In addition, the model made predictions which were confirmed in further experiments. Together, the experimental and theoretical results presented in the current study provide important details regarding the representations of dynamics which mediate sensorimotor control during object manipulation.

Experimental brain research, 2014

The goal of this study was to elucidate the underlying mechanisms of hand and tool grasping control. We assumed that there is a single principle-governing grasping control irrespective of its effectors and that the degree of prior experience of the effector determines the smoothness of aperture control. Eight participants performed a reach-to-grasp task with four different effectors: index finger and thumb, middle finger and thumb, chopsticks, and a scissor-like tool. Although we employed different effectors with large mechanical variations and different degrees of prior use, maximum grip aperture was scaled as a function of object size and appeared at almost the same timing in all four types of grasping movements. Moreover, reaching time did not substantially differ among grasping conditions. However, plateau duration of the aperture profile differed by effector. Plateau duration was the longest in the unfamiliar scissor-like tool grasping condition. There was no difference between...