Human motor cortical plasticity and upper limb performance

Abstract

The capacity of the adult human nervous system to alter the strength of connections between neurons and between networks of neurons is an exciting area of research providing novel insights into the mechanisms involved in learning, memory and recovery following brain damage. In recent years, it has become clear that both afferent input into the motor cortex and the learning of a new motor task can drive cortical reorganisation. This thesis is concerned with the functional significance of this plasticity, in both normal subjects and stroke patients, and with the question of whether stimulation - induced plasticity can lead to improved fine motor performance. My initial experiments were conducted to determine the optimal method of analysing responses to transcranial magnetic stimulation ( TMS ), and to investigate aspects of motor performance as the hand performs a precision task to grasp and lift an object. Studies on normal subjects showed that there is little difference between the dominant and non - dominant hands performing this task, but the type of grip used influences grip - force control. An investigation of stroke patients performing this task demonstrated that certain parameters were sensitive to differences between the affected and unaffected hands and these parameters were highly correlated with stroke - specific functional outcome measures. The induction of plastic change in the human motor cortex can be induced by repetition of movements, performing a complex motor task or stimulation of the peripheral afferents and / or the motor cortex itself. I observed that the application of so - called " associative stimulation " to two hand muscles in normal subjects increased the excitability of the corticospinal projection to those muscles, and improved performance times on a subsequent motor task to a greater extent than subjects receiving a control intervention. I then applied associative stimulation to the affected hand of stroke patients in conjunction with rehabilitation, which improved their ability to perform the dextrous grip - lift task. This is the first study to show that this method of inducing motor cortical plasticity can also lead to functional improvements in stroke patients. These studies confirm that using afferent stimulation to drive cortical reorganisation is associated with improved function and fine motor performance in both normal subjects and stroke patients.