Combined recruitment of two fixation chains during cyclic movements of one arm

Voluntary adduction-abduction movements of one arm in the horizontal plane discharge a reaction torque which would rotate the trunk in the direction opposite to arm acceleration. Rotation is impeded by muscular fixation chains that exert forces counterbalancing the reaction torque. We examined how two different fixation chains cooperate in stabilising the trunk during the above movements. Standing subjects (n = 6), with shoulders ante-flexed, performed cyclic adductions-abductions of the right arm (1.5 Hz) while grasping a fixed handle with the left hand. In this set-up, reaction torque is contrasted by: (1) a leg fixation chain, exerting on the ground a torque around the vertical axis (Tz), recorded by a force platform; and (2) a left arm fixation chain, exerting on the handle a force in the medial-lateral direction (Fh), recorded by a load cell. Subjects performed 20 trials (15 cycles each). It was found that Tz and Fh underwent sinusoidal changes at the same frequency as arm movements and contributed in counteracting the reaction torque. The intensity of the handle grip, monitored by EMG activity in the left Flexor Digitorum Superficialis, was changed from trial to trial and kept constant during each trial. As grip strength increased, Fh amplitude increased linearly while amplitude of Tz linearly decreased. In conclusion, voluntarily strengthening the handle grip progressively deviates the postural actions from the legs to the left arm.     

Viewing objects and planning actions: on the potentiation of grasping behaviours by visual objects

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.     

Motor timing and the preparation for sequential actions

Motor timing is essential for performing self-initiated movement sequences. Here, we investigated how sequence rhythm, or the timing for co-ordinating movements within a sequence, contributes to action preparation, compared with other processes occurring during sequence planning. First, we recorded the readiness potential (RP) in a condition of complex sequence rhythm and in condition of high demand on the timing for sequence initiation. We found that sequence rhythm and sequence initiation are independent processes, with sequence initiation contributing to early RP. Second, we compared the RP recorded in a condition of complex sequence rhythm and in a condition of complex sequence order, in which a complex combination of finger sub-movements had to be correctly ordered within a sequence. We found that sequence rhythm and sequence order share common processes occurring late RP. We suggest that the preparation for movement involves independent processes devoted to different aspects of motor timing and sequencing.     

The intrinsic link between motor behavior and temporal cognition

The debate about the cognitive mechanisms behind human temporal processing has raged for decades without a clear resolution. The theory presented here describes a different perspective to the traditional accounts on the issue, namely, that motor behaviors or sequences of motor behaviors provide a means of reproducing time intervals. Evidence behind this perspective includes tapping strategies (exemplified by musicians), counting strategies, and neuropsychological results showing activation of motor areas during temporal cognitive tasks. I propose that motor behaviors aid human timing by offering a set of processes that consistently take a set amount of time to accomplish. Motor behaviors also allow segmentation of larger intervals into smaller intervals that are easier to estimate. I conclude with a discussion of implications of this perspective on temporal cognition.     

Postural configuration affects the perception of earth-based space during pitch tilt

This study investigates the relative contribution of body parts in the elaboration of a whole-body egocentric attraction phenomenon previously observed during earth-based judgments. This was addressed through a particular earth-based task requiring estimating the possibility of passing under a projected line, imagining a forward horizontal displacement. Different postural configurations were tested, involving whole-body tilt, trunk tilt alone or head tilt alone. Two legs positions relative to the trunk were manipulated. Results showed systematic deviations of the subjective “passability” toward the tilt, linearly related to the tilt magnitude. For each postural configuration, the egocentric influence appeared to be highly dependent on the position of trunk and head axes, whereas the legs position appeared not relevant. When compared to the whole-body tilt condition, tilting the trunk alone consistently reduced the amount of the deviation toward the tilt, whereas tilting the head alone consistently increased it. Our results suggest that several specific effects from multiple body parts can account for the global deviation of the estimates observed during whole-body tilt. Most importantly, we support that the relative contribution of the body segments could mainly depend on a reweighting process, probably based on the reliability of sensory information available for a particular postural set.     

Modulating Fitts's Law: perceiving targets at the last placeholder

Fitts's Law predicts increasing movement times (MTs) with increasing movement amplitudes; however, when targets are placed in a structured perceptual array containing placeholders, MTs to targets in the last position are shorter than predicted. We conducted three experiments to determine if this modulation has a perceptual cause. Experiment 1, which used extremely diminished (three pixel) placeholders, showed that the modulation is not due to perceptual interference from neighboring placeholders. Experiment 2, which measured reaction times using a target detection task, showed that the modulation does not result from speeded perceptual processing at the last position of the array. Experiment 3, which measured accuracy using a masked letter-discrimination task, showed that the modulation does not result from the increased quality of perceptual representation at the last position of the array. Overall, these findings suggest that the changes in effectiveness of visual processing (less interference, speeded processing, and increased quality) at the last position in the perceptual array do not drive the modulation. Thus, while the locus of the Fitts's Law modulation appears to be in the movement planning stage, it is likely not due to perceptual mechanisms.     

Differential dynamics of spatial and non-spatial stimulus-response compatibility effects: a dual task LRP study

Choice reaction times are shorter when stimulus and response features are compatible rather than incompatible. Recent studies revealed that spatial compatibility effects in Simon tasks are strongly attenuated when there is temporal overlap with a different high-priority task. In contrast, non-spatial variants of the Simon task appear to be unaffected by task overlap. The present study used the lateralized readiness potential (LRP) within a dual task design to elucidate the dynamics underlying these differential effects for a color and a spatial variant of the Simon task. In the color version there was no sign of early response priming by irrelevant stimulus features in the LRP. The color compatibility effect was independent of task overlap and reflected in the LRP onset latency. In contrast, in the spatial version, priming by irrelevant stimulus location showed up and was mirrored by early LRP activation. Response priming and the corresponding Simon effect, however, were present only in case of little temporal overlap with the primary task. The absence of spatial compatibility effects at strong temporal overlap suggests that response conflicts due to stimulus-related priming depend on the availability of processing resources.     

Do equilibrium constraints modulate postural reaction when viewing imbalance?

Action observation and action execution are tightly coupled on a neurophysiological and a behavioral level, such that visually perceiving an action can contaminate simultaneous and subsequent action execution. More specifically, observing a model in postural disequilibrium was shown to induce an increase in observers' body sway. Here we reciprocally questioned the role of observers' motor system in the contagion process by comparing participants' body sway when watching displays of antero-posterior vs. lateral imbalance. Indeed, during upright standing, biomechanical constraints differ along the antero-posterior (A-P) and medio-lateral (M-L) axes; hence an impact of observers' postural constraints on the contagion response would result in different reactions to both types of stimuli. In response to the displays, we recorded greater area of center of pressure (CoP) displacement when watching forward/backward compared to left/right imbalance. In addition, after normalizing A-P and M-L CoP displacements by a control condition (fixation cross), A-P CoP path length when viewing forward imbalance tended to be higher than M-L CoP path length when viewing imbalance to the left or right. These results indicate that postural contagion is promoted when the display is compatible with observers' motor stabilization strategy which is mainly oriented along the A-P axis. In terms of clinical application, this study brings new indications for adaptation of observational training devices in rehabilitation programs.     

Valence,form, and content of self-talk predict sport type and level of performance

In this paper, we aimed to test whether we could predict sport type (badminton or running) and marathon proficiency from the valence, form, and content of the athletes’ self-reported inner speech. Additionally, we wanted to assess the difference between self-talk during high intensity and low intensity exercise. The present study corroborated existing research – we were able to predict both sport type in Study 1 and intensity level as well as marathon proficiency in Study 2 from questionnaire data using machine learning models. In Study 1, we found that badminton players engage more in worry and anxiety-control while runners are more prone to task disengagement. Interestingly, it seemed in Study 2 that the more participants engaged in condensed, positive, and repetitive self-talk when not pushing themselves, the slower their fastest marathons and half marathons were. We discuss potential explanations for these findings and make suggestions for future research.     

Effects of Developmental Task Constraints on Kinematic Synergies during Catching in Children with Developmental Delays

Abstract

The aim of this study was to examine effects of a task intervention on kinematic synergies in catching. Participants were young children (5.58?±?0.52?years) with the lowest scores on two-hand catching, according to assessments with the Test of Gross Motor Development-2 (TGMD-2) and were allocated into two groups. The constraints group took part in an 8-week intervention, whereas the control group experienced a typical physical education. Both groups were assessed with motor development and kinematic coordination measures with a catching task with a ball thrown from 2?m distance. Kinematic variables were recorded using a wireless motion capture system. A principal component analysis (PCA) was used to measure the kinematic synergies formed among active body parts. Two synergies that emerged in catching were mainly utilised for “reaching” and “catching” the ball. The control group tended to re-organise the majority of active body parts into two functional units in all phases, whereas the constraints group adapted their active parts into functional units according to the requirement of the novel movement in the transfer task. The findings of this study suggested that task constraints could facilitate object control by re-organisation of active body parts into functional synergies to achieve successful performance.