Daily modulation of the speed-accuracy trade-off.

Goal-oriented arm movements are characterized by a balance between speed and accuracy. The relation between speed and accuracy has been formalized by Fitts' law and predicts a linear increase in movement duration with task constraints. Up to now this relation has been investigated on a short-time scale only, that is during a single experimental session, although chronobiological studies report that the motor system is shaped by circadian rhythms. Here, we examine whether the speed-accuracy trade-off could vary during the day. Healthy adults carried out arm-pointing movements as accurately and fast as possible toward targets of different sizes at various hours of the day, and variations in Fitts' law parameters were scrutinized. To investigate whether the potential modulation of the speed-accuracy trade-off has peripheral and/or central origins, a motor imagery paradigm was used as well. Results indicated a daily (circadian-like) variation for the durations of both executed and mentally simulated movements, in strictly controlled accuracy conditions. While Fitts' law was held for the whole sessions of the day, the slope of the relation between movement duration and task difficulty expressed a clear modulation, with the lowest values in the afternoon. This variation of the speed-accuracy trade-off in executed and mental movements suggests that, beyond execution parameters, motor planning mechanisms are modulated during the day. Daily update of forward models is discussed as a potential mechanism.

Motor cortical plasticity induced by motor learning through mental practice.

Several investigations suggest that actual and mental actions trigger similar neural substrates. Motor learning via physical practice results in long-term potentiation (LTP)-like plasticity processes, namely potentiation of M1 and a temporary occlusion of additional LTP-like plasticity. However, whether this neuroplasticity process contributes to improve motor performance through mental practice remains to be determined. Here, we tested skill learning-dependent changes in primary motor cortex (M1) excitability and plasticity by means of transcranial magnetic stimulation (TMS) in subjects trained to physically execute or mentally perform a sequence of finger opposition movements. Before and after physical practice and motor-imagery practice, M1 excitability was evaluated by measuring the input-output (IO) curve of motor evoked potentials. M1 LTP and long-term depression (LTD)-like plasticity was assessed with paired-associative stimulation (PAS) of the median nerve and motor cortex using an interstimulus interval of 25 ms (PAS25) or 10 ms (PAS10), respectively. We found that even if after both practice sessions subjects significantly improved their movement speed, M1 excitability and plasticity were differentially influenced by the two practice sessions. First, we observed an increase in the slope of IO curve after physical but not after MI practice. Second, there was a reversal of the PAS25 effect from LTP-like plasticity to LTD-like plasticity following physical and MI practice. Third, LTD-like plasticity (PAS10 protocol) increased after physical practice, whilst it was occluded after MI practice. In conclusion, we demonstrated that MI practice lead to the development of neuroplasticity, as it affected the PAS25- and PAS10- induced plasticity in M1. These results, expanding the current knowledge on how MI training shapes M1 plasticity, might have a potential impact in rehabilitation.

Dominant vs. nondominant arm advantage in mentally simulated actions in right handers.

Although plentiful data are available regarding mental states involving the dominant-right arm, the evidence for the nondominant-left arm is sparse. Here, we investigated whether right-handers can generate accurate predictions with either the right or the left arm. Fifteen adults carried out actual and mental arm movements in two directions with varying inertial resistance (inertial anisotropy phenomenon). We recorded actual and mental movement times and used the degree of their similarity as an indicator for the accuracy of motor imagery/prediction process. We found timing correspondences (isochrony) between actual and mental right arm movements in both rightward (low inertia resistance) and leftward (high inertia resistance) directions. Timing similarities between actual and mental left arm movements existed for the leftward direction (low inertia resistance) but not for the rightward direction (high inertia resistance). We found similar results when participants reaching towards the midline of the workspace, a result that excludes a hemispace effect. Electromyographic analysis during mental movements showed that arm muscles remained inactivate, thus eliminating a muscle activation strategy that could explain intermanual differences. Furthermore, motor-evoked potentials enhancement in both right and left biceps brachii during mental actions indicated that subjects were actively engaged in mental movement simulation and that the disadvantage of the left arm cannot be attributed to the nonactivation of the right motor cortex. Our findings suggest that predictive mechanisms are more robust for the right than the left arm in right-handers. We discussed these findings from the perspective of the internal models theory and the dynamic-dominance hypothesis of laterality.

Time-of-day effects on the internal simulation of motor actions: psychophysical evidence from pointing movements with the dominant and non-dominant arm.

It is well known that circadian rhythms modulate human physiology and behavior at various levels. However, chronobiological data concerning mental and sensorimotor states of motor actions are still lacking in the literature. In the present study, we examined the effects of time-of-day on two important aspects of the human motor behavior: prediction and laterality. Motor prediction was experimentally investigated by means of imagined movements and laterality by comparing the difference in temporal performance between right and left arm movements. Ten healthy participants had to actually perform or to imagine performing arm-pointing movements between two targets at different hours of the day (i.e., 08:00, 11:00, 14:00, 17:00, 20:00, and 23:00 h). Executed and imagined movements were accomplished with both the right and left arm. We found that both imagined and executed arm pointing movements significantly fluctuated through the day. Furthermore, the accuracy of motor prediction, investigated by the temporal discrepancy between executed and imagined movements, was significantly better in the afternoon (i.e., 14:00, 17:00, and 20:00 h) than morning (08:00 and 11:00 h) and evening (23:00 h). Our results also revealed that laterality was not stable throughout the day. Indeed, the smallest temporal differences between the two arms appeared at 08:00 and 23:00 h, whereas the largest ones occurred at the end of the morning (11:00 h). The daily variation of motor imagery may suggest that internal predictive models are flexible entities that are continuously updated throughout the day. Likewise, the variations in temporal performance between the right and the left arm during the day may indicate a relative independence of the two body sides in terms of circadian rhythms. In general, our findings suggest that cognitive (i.e., mental imagery) and motor (i.e., laterality) states of human behavior are modulated by circadian rhythms.

Circadian modulation of mentally simulated motor actions: implications for the potential use of motor imagery in rehabilitation.

BACKGROUND: . Mental practice through motor imagery improves subsequent motor performance and thus mental training is considered to be a potential tool in neuromotor rehabilitation. OBJECTIVE: . The authors investigated whether a circadian fluctuation of the motor imagery process occurs, which could be relevant in scheduling mental training in rehabilitation programs. METHODS: . The executed and imagined durations of walking and writing movements were recorded every 3 hours from 8 AM to 11 PM in healthy participants. The authors made a cosinor analysis on the temporal features of these movements to detect circadian rhythms. Temporal differences between executed and imagined movements as well as their variability during the day were also quantified. RESULTS: . Circadian rhythms were detected for both the executed and the imagined movements. Furthermore, these rhythms covaried between them and with body temperature. The participants' ability to internally simulate their movements also fluctuated significantly during the day. The isochrony between the executed and the imagined movements was exclusively observed between 2 PM and 8 PM. In the morning (8 AM and 11 AM) and the evening (11 PM), the durations of the imagined movements were significantly longer than the durations of executed movements. CONCLUSIONS: . Predictive internal models fluctuate in a circadian basis, as do many other physiological parameters. It could be important to take into consideration the time of day in the planning of rehabilitation programs using physical or mental training.

The influence of eye movements on the temporal features of executed and imagined arm movements.

The very close coordination between eye and hand indicates that eye movements are parts of the neural processes underlying the planning and control of arm movements. Eye movements are fundamental during observed actions and play a functional role in visual mental imagery. However, the role of eye movements during imagined actions is still unknown. Here, we report the timing features of eye and arm pointing movements for nine healthy participants in four conditions: Executed movements with orientation saccades (Eyes Free) or with no saccades (Eyes Motionless), and Imagined movements with Eyes Free or with Eyes Motionless. The first result was a facilitation effect of saccades upon both executed and imagined arm movements as revealed by the shorter arm movement durations in Eyes Free than in Eyes Motionless. Another interesting finding was that executed and imagined movements preserved their temporal similarities in both Eyes Free and Eyes Motionless, suggesting that the accuracy of motor representations is not dependent on the presence or lack of eye movements. This result and the close similarities between the EOG patterns accompanying both executed and imagined arm pointing movements in Eyes Free, argue in favour of a similar neurocognitive network in executed and imagined actions. We propose that internal forward models provide fine estimations of the temporal features of imagined arm movements, whatever they accompanied, or not, by orientation saccades.