Paola Cesari

Motor facilitation of the human cortico-spinal system during observation of bio-mechanically impossible movements

Neurophysiological and neuroimaging studies in the human and the monkey brain indicate that links between action observation and execution are much tighter than previously believed. Indeed, the mere observation of movements performed by other individuals brings about a clear increase in activity in specific fronto-parietal neural networks (mirror system). Here, we report a series of four single-pulse transcranial magnetic stimulation studies of the motor system, which show that observation of index and little finger movements brings about a facilitation of potentials recorded from muscles that would be involved in the actual execution of the observed action. Remarkably, however, a clear representational selectivity was found also during observation of bio-mechanically impossible index or little finger movements. Thus, in movement observation tasks, the human cortico-spinal system reacts similarly to the vision of bio-mechanically possible and impossible movements but it is able to detect which muscle would be involved in the actual execution of the observed movement. Importantly, this system may be more related to coding body part movements than precisely simulating their execution.

Body-goal Variability Mapping in an Aiming Task

Given the number of joints and muscles in the human body, there are typically an infinite number of ways to perform the same action, a feature of directed movements known as equifinality (Bernstein, The coordination and regulation of movements, Oxford, Pergamon, 1967). Here we present a new type of performance analysis based on the idea of a body-goal variability mapping. We show how this mapping arises naturally from the idea of a goal function that theoretically defines a task and, in the presence of equifinality, determines the set of all possible task solution strategies, the goal equivalent manifold (GEM). The approach also yields estimates of the sensitivity of goal-level errors to body-level perturbations, and we derive a general formula expressing the relationship between the two. We apply these ideas to the analysis of redundant kinematic data from subjects performing an aiming task carried out with and without a laser pointer. It is shown that in order to characterize performance one must consider two factors in addition to the body variability: first, the degree of alignment between body variability and the GEM; and second, the sensitivity parameters that control the degree to which goal-relevant body variability is amplified at the target. Both of these factors can be computed using the estimated body-goal mapping. We show that the performance for three conditions involving two different nominal postures and two different sensory conditions (laser/no laser) can be classified by examining the clustering of data in an orientation- sensitivity parameter plane associated with the map.

A common perceptual parameter for stair climbing for children, young and old adults.

In this paper we examine whether a common perceptual parameter is available for guiding old adults, young adults and children in climbing the highest stair mountable in a bipedal fashion. Previous works have shown that the ratio between the height of the stair and the hip height was the body-scaled invariance adopted as information for selecting the highest stair by young adults [Journal of Experimental Psychology: Human Perception and Performance 10 (1984) 683-703] but not by older adults [Journal of Experimental Psychology: Human Perception and Performance 3 (1992) 691-697]. Indeed, for older adults additional bio-mechanical parameters needed to be added to the model due to their decrease in leg strength and flexibility.Up to now, no perceptual invariant has been identified yet for determining the relevant information used for guiding the stair climbing action for normal healthy people. We propose a new parameter as the angle defined by the ratio between the height of the stair and the distance taken from the feet to the top edge of the stair before the initiation of the movement. We show that this angle is the same for children, young adults and older adults despite the different kinematics of the motion, the anthropometrics and the skill ability exhibit by the participants. In summary we show that even when the climbability judgments, based on the simple ratio leg length-stair height, are influenced by differences in age, participants use a common perceptual variable when they are coordinating their stair climbing action.

Expertise with pathological actions modulates a viewer's motor system.

Brain mechanisms for action understanding rely on matching the observed actions into the viewers motor system. Health professionals, who treat patients affected by movement disorders as dystonia, frequently see hyperkinetic action patterns characterized by an overflow of muscle co-contractions. To avert an overload of the motor system during observation of those actions, they might need to look at dystonic motor symptoms in a cool, detached way. To investigate whether visual expertise about atypical movement kinematics influences the viewers motor system, we applied transcranial magnetic stimulation to clinicians and to naive subjects, while they observed handwriting actions performed with two different kinematics: fluent and non-fluent. Crucially, the latter movement pattern was easily recognized by the clinicians as a typical expression of writers cramp, whereas it was unknown to the naive subjects. Results showed that clinicians had similar corticospinal activation during observation of dystonic and healthy writings, whereas naive subjects were hyper-activated during observation of dystonic movements. Hyper-activation was selective for the muscles directly involved in the dystonic co-contractions and inversely correlated with subjective movement fluency scores, hinting at a fine-tuned association between the breakdown of observed movement fluency and corticospinal activation. These findings suggest that observation of unusual pathological actions differently modulates the viewers motor system, depending on knowledge, visual expertise, and ability in recognizing suboptimal movement kinematics.

Might as Well Jump: Sound Affects Muscle Activation in Skateboarding

The aim of the study is to reveal the role of sound in action anticipation and performance, and to test whether the level of precision in action planning and execution is related to the level of sensorimotor skills and experience that listeners possess about a specific action. Individuals ranging from 18 to 75 years of age - some of them without any skills in skateboarding and others experts in this sport - were compared in their ability to anticipate and simulate a skateboarding jump by listening to the sound it produces. Only skaters were able to modulate the forces underfoot and to apply muscle synergies that closely resembled the ones that a skater would use if actually jumping on a skateboard. More importantly we showed that only skaters were able to plan the action by activating anticipatory postural adjustments about 200 ms after the jump event. We conclude that expert patterns are guided by auditory events that trigger proper anticipations of the corresponding patterns of movements.

Excitability of Motor Cortices as a Function of Emotional Sounds

We used transcranial magnetic stimulation (TMS) to clarify how non-verbal emotionally-characterized sounds modulate the excitability of the corticospinal motor tract (CST). While subjects were listening to sounds (monaurally and binaurally), single TMS pulses were delivered to either left or right primary motor cortex (M1), and electromyographic activities were recorded from the contralateral abductor pollicis brevis muscle. We found a significant increase in CST excitability in response to unpleasant as compared to neutral sounds. The increased excitability was lateralized as a function of stimulus valence: Unpleasant stimuli resulted in a significantly higher facilitation of motor potentials evoked in the left hemisphere, while pleasant stimuli yielded a greater CST excitability in the right one. Furthermore, TMS induced higher motor evoked potentials when listening to unpleasant sounds with the left than with the right ear. Taken together, our findings provide compelling evidence for an asymmetric modulation of CST excitability as a function of emotional sounds along with ear laterality.

Motor imagery beyond the joint limits: a transcranial magnetic stimulation study.

The processes and neural bases used for motor imagery are also used for the actual execution of correspondent movements. Humans, however, can imagine movements they cannot perform. Here we explored whether plausibility of movements is mapped on the corticospinal motor system and whether the process is influenced by visuomotor vs. kinesthetic-motor first person imagery strategy. Healthy subjects imagined performing possible or biomechanically impossible right index finger movements during single pulse TMS of the left motor cortex. We found an increase of corticospinal excitability during motor imagery which was higher for impossible than possible movements and specific for the muscle involved in the actual execution of the imagined movement. We expand our previous action observation studies, suggesting that the plausibility of a movement is computed in regions upstream the primary motor cortex, and that motor imagery is a higher-order process not fully constrained by the rules that govern motor execution.

Expert players accurately detect an opponent's movement intentions through sound alone

Sounds offer a rich source of information about events taking place in our physical and social environment. However, outside the domains of speech and music, little is known about whether humans can recognize and act upon the intentions of another agent’s actions detected through auditory information alone. In this study we assessed whether intention can be inferred from the sound an action makes, and in turn, whether this information can be used to prospectively guide movement. In 2 experiments experienced and novice basketball players had to virtually intercept an attacker by listening to audio recordings of that player’s movements. In the first experiment participants had to move a slider, while in the second one their body, to block the perceived passage of the attacker as they would in a real basketball game. Combinations of deceptive and nondeceptive movements were used to see if novice and/or experienced listeners could perceive the attacker’s intentions through sound alone. We showed that basketball players were able to more accurately predict final running direction compared to nonplayers, particularly in the second experiment when the interceptive action was more basketball specific. We suggest that athletes present better action anticipation by being able to pick up and use the relevant kinematic features of deceptive movement from event-related sounds alone. This result suggests that action intention can be perceived through the sound a movement makes and that the ability to determine another person’s action intention from the information conveyed through sound is honed through practice.

Dimensional analysis and ground reaction forces for stair climbing: Effects of age and task difficulty

Altered perception-action capability is often associated with falls and diminished self-efficacy in older people. This study evaluated and compared perception-action capability in stair-climbing performance of 18 healthy volunteers assigned to two age groups (mean age, 26.3+/-4.3 years and 66.4+/-4.7 years, respectively). The experimental set-up included 14 stairs (50 cm wide, 60 cm deep, riser height 35-90 cm) positioned at the edge of a force platform. The task was to climb the stair with the greatest riser height subjects thought they could climb without outside support or use of hands. Dimensional and dynamic data were collected and analyzed to reveal the invariant relationships that sustain action preparation and execution. All subjects chose the same proportion between stair height and distance covered before mounting the stair, as expressed by the invariant angle (alpha). While the geometric invariant relationship was picked up as a visual guide prior to action, there was a dynamic invariance in the forces applied during actual execution. To establish whether the invariance still held in extreme cases, two perturbed conditions were introduced in which stair distances were changed, forcing subjects to execute a foot-strike, either very far from or near to the stair, before climbing it, so as to reveal any significant adaptations the climber would undertake to avoid slips or falls. Older and younger subjects applied appropriate visual and motor guidance by scaling their motor capabilities to the environmental dimensions.

(Dis-)Harmony in movement: effects of musical dissonance on movement timing and form

Abstract While the origins of consonance and dissonance in terms of acoustics, psychoacoustics and physiology have been debated for centuries, their plausible effects on movement synchronization have largely been ignored. The present study aimed to address this by investigating whether, and if so how, consonant/dissonant pitch intervals affect the spatiotemporal properties of regular reciprocal aiming movements. We compared movements synchronized either to consonant or to dissonant sounds and showed that they were differentially influenced by the degree of consonance of the sound presented. Interestingly, the difference was present after the sound stimulus was removed. In this case, the performance measured after consonant sound exposure was found to be more stable and accurate, with a higher percentage of information/movement coupling (tau coupling) and a higher degree of movement circularity when compared to performance measured after the exposure to dissonant sounds. We infer that the neural resonance representing consonant tones leads to finer perception/action coupling which in turn may help explain the prevailing preference for these types of tones.