Learning, transfer and interference in bimanual arm movements

Composition and decomposition in bimanual dynamic learning  Our ability to skilfully manipulate an object often involves the motor system learning to compensate for the dynamics of the object. When the two arms learn to manipulate a single object they can act cooperatively, whereas when they manipulate separate objects they control each object independently. One project examined how learning transfers between these two bimanual contexts by applying force fields to the arms. The results suggest that the representations of dynamics for uncoupled and coupled contexts are partially independent.  Additional support for this hypothesis was found by showing significant learning of opposing curl fields when the context, coupled versus uncoupled, was alternated with the curl field direction. These results suggest that the motor system is able to use partially separate representations for dynamics of the two arms acting on a single object and two arms acting on separate objects. For further details see the publication here.

Context Dependent Partitioning of Motor Learning in Bimanual Movements Another another project  examined the effect of bimanual movement context on interference between opposing perturbations using pairs of contexts, in which the relative direction of movement between the two arms was different across the pair. When each perturbation direction was associated with a different bimanual context, such as movement of the arms in the same direction versus movement in the opposite direction, interference was dramatically reduced. In addition, we examined a bimanual context in which one arm was moved passively and show that the reduction in interference requires active movement. For further details see the publication here.

Separate representations of dynamics in rhythmic and discrete movements: Evidence from motor learning This project examined interference in a motor learning paradigm to test whether rhythmic and discrete movements employ at least partially separate neural representations. Subjects were required to make circular movements of their right hand while they were exposed to a velocity-dependent force field that perturbed the circularity of the movement path. The direction of the force field perturbation reversed at the end of each block of 20 revolutions and only when subjects alternated between blocks of rhythmic and discrete movements, such that each was  uniquely associated with one of the perturbation directions, interference was significantly reduced. These results provide further evidence that the two different movements actions, rhythmic and discrete, employ at least partially separate control mechanisms in the motor system. For further details see the publication here.

Robotic interfaces for the investigation of human arm movements
I worked on the design and construction of a modular, general purpose, two-dimensional planar manipulandum (vBOT) primarily optimized for dynamic learning paradigms. Such robotic manipulanda are extensively used in investigation of the motor control of human arm movements. The design minimizes the intrinsic dynamics of the manipulandum without active compensation.

A modular planar robotic manipulandum with end-point torque control
A novel variant of the design, the WristBOT, can apply torques at the handle using an add-on cable drive mechanism. In a second variant, the StiffBOT, a more rigid arm can be substituted and zero backlash belts can be used, making the StiffBOT more suitable for the study of stiffness. The three variants can be used with custom built display rigs, mounting, and air tables, which I also designed.

Daniel Wolpert's lab currently has 2 WristBOTs, 4 vBOTs and 1 StiffBOT. In addition, two vBOTs have been supplied to Chris Miall's lab, 1 vBOT to Steve Jackson's lab and 1 WristBOT to Randy Flanagan's lab. For further details on the vBOT manipulandum, see the publication here.

Control and signal conditioning electronics for the vBOTs

I worked on the design and construction of motor controller units used to drive the vBOTs. These units are based on Maxon Motor's switching amplifiers. Additional circuitry was built to implement safety features and a high current passive power supply was used to drive the amplifiers and auxiliary electronics. For further details see the publication here.

In addition I designed and build 6-channel filter boxes to filter the output of analogue force transducers. These provide buffering and low-pass filtering of the force transducer signals before they are read by A/D converters on a host PC.

Statistics of natural arm movements

Humans use their arms to engage in a wide variety of motor tasks during everyday life. However, little is known about the statistics of these natural arm movements. I developed a portable motion-tracking system that could be worn by subjects as they went about their daily routine outside of a laboratory setting.

The statistics of natural movements are reflected in motor errors We found that the well-documented symmetry bias is reflected in the relative incidence of movements made during everyday tasks. Specifically, symmetric and anti-symmetric movements are predominant at low frequencies, whereas only symmetric movements are predominant at high frequencies. Moreover, the statistics of natural movements, that is, their relative incidence, correlated with subjects’ performance on a laboratory-based phase-tracking task. For further details see the publication here.