Base vectors of sequential finger movements: modulation of muscle activation and motor cortex excitability - an explorative study

Fine hand motor control influences daily activities as grasping, writing, and
typing. The control of the hand is complex as a large number of mechanical
degrees of freedom have to be controlled simultaneously. Mechanical and neural
coupling between the different muscles can simplify the control. However, these
couplings also constraint certain movements, especially those where fingers are
controlled individually. At a cortical level, fine hand motor control is
accomplished by adjusting cortical excitability. The excitability of instructed
fingers is increased, while that of surrounding fingers is decreased. In order
to analyse how the brain controls complex finger movements, a sequential finger
task is introduced. The muscle activity and force production of this task is
compared to that of other tasks, among others to the muscle activity and force
production of involuntary movement due to transcranial magnetic stimulation
(TMS). This allows us to analyse how the brain builds a representation of
complex movements, en what muscle combinations are used as a base to build new,
dynamic movements.
The results of this study might also teach us something about the neurological
disorder dystonia. Patients with focal hand dystonia show a lack of neural
focus during the execution of complex dynamic tasks. Understanding how these
tasks are controlled in healthy, young subjects might allow us to see if there
is a difference in the representations of these movements in patients with
focal hand dystonia.

Our objective is to investigate how complex finger movements like fast
sequences are controlled by the brain. Muscle activation patterns, studied by
multi-channel EMG on both intrinsic and extrinsic hand muscles, and force
traces are compared between different types of movements. The influence of M1
excitability is studied by inducing involuntary muscle and force activation by
TMS. By relating these patterns, we will determine which movements the brain
sees as *base vectors*.

The study population will be contacted through the SONA system of the Radboud
University Nijmegen.
This explorative study exists of 4 sessions over 4 days.
After signing the consent form, the participants get 2 questionnaires: 1 to
determine the dominant hand, and 1 to check if the participant can undergo TMS.
In the first session, the resting motor threshold is determined for TMS. When
this is higher than 60% of the stimulator output, the experiment is terminated
here. The first session ends with learning 16 different 3-finger press
sequences. This takes approximately 1 hour. In session 2 and 3, the same
sequences are further taught, each time for about 1 hour. The fourth session
consists of TMS, single finger presses, and sequential presses. In the TMS
part, 300 single pulses are given over a grid of 30 points over M1. During the
single finger presses, one finger at a time is pressed for 3 seconds. This is
repeated 3 times for each finger. The sequence task consists of executing the
learned sequences as fast and correct as possible, with rest in between the
different sequences. Each sequence is repeated 2 times per block, and the
series exists of 8 blocks.

The risk assessment teaches that the risks associated with this study are
negligible. The longest sessions takes maximum 3 hours, where a break is
incorporated everey 5 - 10 minutes. The participants can determine the length
of the breaks themselves. The most common side effect of TMS is a light
transient headache (2 - 4% occurrence). A severe transient headache is uncommon
(0.3 - 0.5% occurrence).