Brain-machine-interfaceI requires a set of neurons to change their activity in order to control a cursor on a screen or a robotic arm. Neural activity from the primary motor cortex or the parietal reach region (PRR) is mapped to cursor or robotic arm movements through a decoder. Monkeys can learn to modulate their neural activity to become efficient at that task. In this paper (Hwang et al. 2013), Hwang and colleagues demonstrate that the mod.ulation of neural activity during learning is shaped by the existing neural structure.
Some kids are very efficient crawler. One can therefore wonder why these kids ever start walking. This question is central to the motor control field: what motivates people to change or adapt their behavior if they are already successful?
Usually, motor memories are created in the lab by perturbing the trajectory of the hand (e.g. force field perturbation Shadmerh and Mussa-Ivaldi, 1994) or the trajectory of the cursor (e.g. visuomotor rotation Krakauer et al. 2005). Initially, these perturbations lead to large errors that are reduced over the course of trials through learning. This learning depends on an error-dependent process (as discussed in Smith et al. 2006) that takes into account the error on one trial in order to update the motor commands for the next movements. Unfortunately, the resulting motor memories quickly fade away once the perturbation disappears. This forgetting of motor memories has been a major obstacle for the translation of motor learning paradigms to rehabilitation therapies because the beneficial effects of motor training do not last long. In a new paper, Shmuelof and colleagues present a novel technique that makes motor memories resistant to forgetting and opens up new avenues for the translation of motor learning paradigms to rehabilitation.
Writing is a very common skill but becomes difficult for disabled persons. Even when the limbs are not able to move anymore, the eyes still can. Therefore a method that could enable voluntary smooth pursuit eye movements and transform them into words would reestablish communication in severely disabled persons.
Until now, no one had succeeded in eliciting reliable smooth-pursuit eye movements in humans without a moving target on the screen. Smooth-pursuit eye movements in the absence of a target was only possible for short periods of time in anticipation of target motion onset (Barnes 2008) or during transient blanking of a moving target (Orban de Xivry et al.2008) but these movements cannot be voluntarily controlled. An article by Lorenceau (2012) precisely describes such an experiment.
Reorganization of motor networks
Anodal transcranial direct current stimulation (tDCS) of the dominant hemisphere improves learning and retention of a new skill performed with the dominant hand (e.g. Reis et al. 2009). These effects have been demonstrated several times in young healthy subjects for different learning tasks. In stroke patients, both anodal tDCS of the ipsilesional hemisphere (Hummel et al. 2005) and cathodal stimulation of the contralesional hemisphere improve motor functions of the paretic hand (Fregni et al. 2005). These tDCS protocols are balancing the activities of motor areas of both hemispheres by increasing activity in the ipsilesional hemisphere and decreasing activity in the contralesional hemisphere (Stagg et al.2012) and affect how the brain is reorganized after stroke (picture, Grefkes et al. 2011)
The question here is, if cathodal tDCS of the contralesional (unaffected) hemisphere is able to improve motor function of the paretic hand, can this protocol also improve motor learning in these patients?
Immobilization of an arm to favor the use of the other arm is a major component of Constrained-Induced Therapy (CIT). In stroke patients, this therapy improves the motor function of the affected hand. However, the neurological effect of this therapy has not been studied in non-patients populations.
In a recent paper published in the journal Neurology, Swiss scientists took advantage of arm immobilization after arm injury as a proxy for constrained-induced therapy in non-stroke patients. They investigated the effect of limb immobilization on brain structure, especially on gray and white matter plasticity.
or Memory interference at the single neuron level
Could Roger Federer be a world champion at tennis and table tennis at the same time? A new study suggests that it depends on the motor cortex neurons encoding those skills. If the same neurons are involved in tennis and table tennis, then the two tasks will interfere one with the other. If different sets of neurons are used, then Roger Federer could become a tennis table champion while maintaining his tennis ranking.
This text was written by Frédéric Crevecoeur and myself and was submitted to Journal of Neuroscience as a Journal Club article. Unfortunately, it was rejected. So, here it is...
Transcranial magnetic stimulation (TMS) has been a powerful tool to understand the function of the motor cortex. It has been used to investigate its role in decision-making (Michelet et al., 2010), movement preparation (Soto et al., 2010) and, in a recent study by Gritsenko and colleagues in the Journal of Neuroscience, knowledge of limb dynamics (Gritsenko et al., 2011).
Knowledge of limb dynamics is essential for the control of reaching movements. Indeed, shoulder motion produces interaction torques at the elbow and movements of the elbow generate torques at the shoulder joint. This complex inter-segmental dynamics must clearly be taken into account during movement planning and execution in order to produce the typical straight paths that characterizes natural reaching movements (Hollerbach and Flash, 1982). The source of interaction torque compensation has been a matter of debate for years. Some argue that compensation takes place at the level of the spinal cord (Bizzi et al., 2000) whereas others argue for a more direct control of muscle activation mediated by the primary motor cortex (M1) (Todorov, 2000; Scott, 2004). Whether the descending motor commands accounts for inter-segmental dynamics remains a significant open question at the centre of spinal versus cortical control hypotheses.
Sketches of spinal and cortical control hypothese in the extreme case. qS,E represent the shoulder and elbow angles and tS,E represent the shoulder and elbow torques. According to the spinal control hypothesis, the descending motor command (u1, right panel) encodes high-level features of the intended movement such as the location of the reach target. The control command that compensates for interaction torques (u2) could be mediated by sub-cortical pathways including the spinal cord. Alternatively, the descending motor command (u, left panel) could account for the location of the reach target as well as shoulder and elbow torques that result from interaction dynamics.
Skill leaning elicits changes in the brain (Yarrow et al. 2009). In humans and rats, it leads to a massive increase in the connections between neurons in the primary motor cortex, the area that is responsible for the control of movements. Over time and training, the more useful connections are reinforced while the weaker ones are discarded. Such a profound reorganization of the area underlying the control of movements, should impact a large part of the movement repertoire or at least the part that encompasses the use of the same limbs or effectors as those required for the skill. Gentner et al. (2010) investigated how musical practice reorganizes the primary motor cortex and impacts daily life tasks such as grasping.
Professional musicians practice their skills several dozens of hours per week. This extensive practice is known to produce lasting changes in the primary motor cortex (Pascual-Leone et al. 1995; Elbert et al. 1995), the area that controls our movements. Does this practice influence how they use their fingers to grasp a glass of wine or throw a ball?
In their study, Gentner and colleagues demonstrate that brain reorganization due to intensive musical practice leads to different patterns of hand movements. More impressively, the more the patterns of hand movements were biased towards musical practice, the less similar they were to the patterns of hand movements used for daily life activities.
Have you ever tried to tickle yourself?
It is actually not possible.
This impossibility relates to sensory attenuation, namely the reduction in sensitivity of the brain to the sensory consequences of the actions that it has produced. The sensory consequences represent all the changes that the brain can measure via its sensors (vision, touch, sound, muscle elongation ...) and that result from its own action. For instance, someone's own voice sounds very different to him when he hears himself talking than when he hears his own voice from a recording device. Namely, the brain decreases its sensitivity to the voice that it controls in order to preserve its sensitivity to external sounds. This diminished sensitivity is the hallmark of sensory attenuation or sensory cancellation.
Predicted and actual songs cancel each other
Sensory attenuation relies on forward models. The role of a forward model is to predict the future sensory consequences of an action from the motor commands sent to the muscles. These forward models exist in many species, even in insects (Webb 2004). The male cricket rubs one wing against the other in order to produce a song. While the male cricket is singing, some auditory neurons have a reduced responsiveness to its own song. This reduction results from the cancellation between the predicted and actual song produced. In other word, those particular neurons fire very differently in response to a song produced by the cricket and in response to the exact same song produced by another animal.
written by Jean-Jacques Orban de Xivry
Scientist in the motor control field.