Modulation of motor overflow and surround inhibition during sequential finger movements - an explorative study

Fine motor control of the hand influences daily activities as grasping and
writing. While most human hand movement involves grasping, some require single
finger movement. Two neural mechanisms are associated with the execution of
single finger movements: motor overflow, and surround inhibition (SI). Motor
overflow describes the involuntary muscle activation of nearby muscles, while
SI focuses the movement on a single muscle by inhibiting surrounding muscles.
To understand how fine hand movements are controlled, we introduce a sequential
task, to simulate skilled performances as typing. During this task, motor
overflow is monitored. The effect of the task on SI is tested by comparing SI
before and after the task.
The results of this study might teach us, among others, about the mechanisms of
dystonia . This neurological condition typically features task-and
context-specific abnormal posturing due to sustained muscle contractions
interfering with the performance of motor tasks. Focal hand dystonia (FHD) can
be characterised by a pattern of cocontraction of the agonist and antagonist
muscles in the forearm and hand. People with FHD have been shown to lack SI,
and are known to generate motor overflow. Currently, it is not clear if there
is a relation, let alone a causal link between these characteristics.

Our objective is to investigate how individual finger movements are controlled
within a sequential task. To study the presence and changes over time of motor
overflow, multi-channel EMG on both intrinsic and extrinsic hand muscles are
used. Changes in muscle excitability due to the training are determined with
TMS. We will study if these two modulations - motor overflow and excitability
- are related to each other.

Subjects will be approached in person, via telephone, or social media to inform
them in short about the experiment and to hear if they might be interested to
participate. After signing the informed consent, participants are given two
questionnaires: one in order to determine the handedness, and one to check if
the participant can undergo TMS. Surround inhibition is determined by TMS
before the sequential task as a normative value. After this, the participant is
asked to performed a predefined finger sequence for 30 minutes. During this,
muscle activity is monitored with EMG. After performing the task, surround
inhibition is determined again at 4 time points: 0 - 10 - 20 - 30 minutes after
the end of the task.

The risk assessment teaches us that the risk associated with this study are
negligible. The experiment takes 3 hours, with the longest task (only performed
once) taking 30 minutes. Most comon side effect of TMS is a light transient
headache (2 - 4% occurrence). A severe transient headache is uncommon (0.3 -
0.5% occurrence).