Afferent Inhibition is the reduction in motor output when Transcranial Magnetic Stimulation (TMS) of the motor cortex is preceded by peripheral nerve stimulation. Afferent inhibition can be subdivided into two circuits of Short- (SAI) and Long-Afferent Inhibition (LAI). Reliability reflects the repeatability of a measure and can be measured in terms of both absolute and relative reliability. Relative reliability refers to the ability of a measure to identify individuals on repeated testing, measured through the Intraclass Correlation Coefficient (ICC); absolute reliability is the repeatability of scores through repeated testing, measured through Standard Error of Measurement (SEM) and Smallest Detectable Change (SDC). Current literature has highlighted only the intersession reliability of SAI and LAI, but measures of the intrasession reliability are also needed. This study aims to quantify the intrasession reliability of SAI and LAI, alongside identifying the minimum number of trials needed to obtain a reliable measure. 30 healthy individuals (21.17 ± 2.84 years) took part in one session, with SAI and LAI obtained three times at 30-minute intervals. To identify the minimum number of trials required to reliably elicit SAI and LAI, relative reliability was assessed at running intervals of every 5 trials. Results indicate that SAI had moderate–high, and LAI had high-excellent relative reliability. Both SAI and LAI had high amounts of measurement error. LAI was seen to have high relative reliability when only 5 frames of data were included, whereas for SAI, ~20-30 frames of data resulted in high relative reliability. For LAI, a minimal sample size of 19 is needed to have an SDCGroup < 10, whereas for SAI, a sample size of 22 is needed to achieve the same. These results can be used to inform future work regarding the clinical utility of these measures, particularly in terms of their diagnostic ability. / Thesis / Master of Science (MSc)

Evidence indicates attention can alter afferent inhibition, a Transcranial Magnetic Stimulation (TMS) evoked measure of cortical inhibition following somatosensory input. This measure is emerging as a valuable tool for clinical assessment of sensorimotor function. However, the reliability of the measure remains relatively low. Further, attention is capable of modifying the magnitude of afferent inhibition. Therefore, for afferent inhibition to become an assessment with translation within and beyond the research lab, the reliability of the measure must be improved. Controlling the focus of attention may be one method to improve the reliability of afferent inhibition. In the present study, two experiments were conducted. One to assess the biological effects of attention on SAI and LAI, and the other to address whether the reliability of SAI and LAI are altered in the presence of varying attentional demands. The magnitude of short- and long-latency afferent inhibition (SAI and LAI, respectively) was assessed under four conditions with varying attentional demands focused on the somatosensory input that mediates SAI and LAI circuits. Further, the reliability of SAI and LAI was assessed with and without directed attention to the relevant somatosensory input to explore whether attention to the tactile stimulation can improve intrasession and intersession reliability of these measures. Thirty individuals participated in four conditions; three conditions were identical in their physical parameters and varied only in the focus of directed attention (visual attend, tactile attend, non- directed attend) and one condition consisted of no external physical parameters (no stimulation). Reliability was measured by repeating conditions at three time points to assess intrasession and intersession reliability. Results indicate the magnitude of SAI and LAI were not modulated by varied attention. Reliability assessments demonstrated that the attention manipulations increased intrasession and intersession reliability of SAI and LAI compared to the no stimulation condition. This research exposes the influence of attention, and its impact on the reliability of afferent inhibition. By quantifying these influences, this research has identified new information to inform the design of TMS research in sensorimotor integration. / Thesis / Master of Science in Kinesiology / Attention can alter transcranial magnetic stimulation (TMS) evoked afferent inhibition. Measures of afferent inhibition are emerging as valuable tools for clinical assessments of sensorimotor function. However, the reliability of afferent inhibition remains relatively low, limiting its value in the clinic. Afferent inhibition is increased when the one’s attention is focused on the peripheral nerve stimulation used to elicit afferent inhibition. However, it is unknown whether afferent inhibition, with attention directed to somatosensory input, will improve the reliability of these measures. This is important as it suggests that changes to the methodology used to acquire afferent inhibition can improve the reliability of this measure, thereby increasing the opportunity for translation to the clinic. The goal of this study was to assess the influence of attention on afferent inhibition circuits, short afferent inhibition (SAI) and long afferent inhibition (LAI) and determine whether attention modulation would increase the reliability of afferent inhibition.

This thesis describes a series of experiments to investigate the effect of thermal stimulation on corticospinal excitability using transcranial magnetic stimulation (TMS). Experiment I showed that innocuous cooling or warming of a single digit, produced short-lasting and mixed patterns of modulation only during actual thermal stimulation, with the inhibition being the most common pattern observed. In line with this finding, cooling stimulation applied to a larger area (i.e. multi-digits) produced variable but more sustained modulation in motor evoked potential (MEP) amplitude in the post-cooling phase (Exp II). Notably, the responses to cooling in terms of either suppressed or enhanced corticospinal excitability tended to be fairly consistent in a given individual with repeated applications. When examining possible sources of the observed variable MEP modulation, we found that individual characteristics such as age, sex and changes in skin temperature had no major influences. We hypothesized that the variability of responses might be related to individual differences in the excitability of intra-cortical circuits involved in sensorimotor integration. To test this hypothesis, we performed Experiment III using conditioning TMS paradigms. This experiment revealed that TMS markers of sensorimotor integration (SAI and SAF levels) were good predictors of individual variations in cooling-induced modulation in corticospinal excitability. This provided evidence supporting the role of SAI and SAF as markers to predict individual’s response to focal thermal stimulation. The identification of such predictors could enhance the therapeutic applicability of this form of stimulation in neurorehabilitation. Collectively, these findings advance our understanding of the neurophysiological basis of thermal stimulation and shed light on the development of a more rational application of neurofacilitation techniques based on afferent stimulation in clinical populations, such as stroke survivors.

Primary somatosensory cortex (SI) is an important cortical structure involved in receiving and relaying sensory inputs to condition primary motor cortex (M1). The functional interaction between SI and M1 is important for motor control by providing surround inhibition, which is the inhibition of muscles not involved in the movement and in learning new motor skills. This interconnection is known as short-latency afferent inhibition (SAI) and may be probed using Transcranial magnetic stimulation and peripheral nerve stimulation. SAI is dependent on the afferent volley as increasing the nerve stimulation intensity increases the depth of SAI. Individuals with spinal cord injury show reductions in SAI evoked in lower limb and this may be a contributing factor to the impairments in motor control seen within this population. SAI has yet to be investigated in the upper limb in individuals with chronic cervical SCI and this thesis examines these alterations. Two experiments were performed examining M1 excitability (motor evoked potentials), SI excitability (somatosensory evoked potentials) and the interconnection between SI and M1 (SAI). The first Experiment investigated alterations in these measures in individuals with SCI while the second Experiment investigated these measures as a function of the afferent volley. The collective results from Experiment 1 indicate that motor evoked potentials and SAI are reduced but somatosensory evoked potentials are similar to controls. Further data from Experiment 2 indicate that SAI and SEPs increase as a function of the afferent volley and indicate that alterations seen in individuals with SCI may be due to cortical plasticity in the synapses from SI to M1 or within M1. The novel findings of this thesis have indicated aberrant cortical circuits in individuals with SCI and have indicated potential synapses that may be targets for TMS plasticity protocols to alter and restore function to these circuits. / Thesis / Master of Science (MSc)