Title

Stimulus parameters influencing cortical responses by transcranial magnetic stimulation targeting I-wave dynamics : duration, intensity and interval between interventions

Date of Award

2011

Degree Type

Thesis

Degree Name

Doctor of Philosophy

School

School of Exercise and Health Sciences

Faculty

Computing, Health and Science

First Advisor

Professor Kazunori (Ken) Nosaka

Second Advisor

Professor Gary Thickbroom

Abstract

Transcranial magnetic stimulation (TMS) modulates corticomotor excitability and plasticity; however, the magnitude of the effects appears to depend on a range of stimulus parameters. Three possible critical factors of repetitive TMS (rTMS) interventions are the duration of application, the intensity of the stimulation and the interval between interventions. However, no previous studies have systematically investigated the effect of these factors. The purpose of this research was to investigate the effects of duration (Study 1), intensity (Study 2) and interval between interventions (Study 3) comprising of equi-intensity paired pulses (1.5 ms between pulses) corresponding to indirect (I)-wave periodicity (iTMS) on cortical excitability changes. To assess cortical excitability changes, single-pulse motor evoked potential (MEP) amplitude before and following the intervention, and paired-pulse motor evoked potential (iMEP) amplitude during the intervention from the first dorsal interosseous (FDI) muscle were recorded in the studies. In Study 1, the effect of intervention duration was investigated by comparing 15 min and 30 min to test the hypothesis that doubling the duration of the intervention would further increase the excitability. Ten (7 women and 3 men) healthy individuals (21 - 27 years) performed the 15- and 30-min interventions in a counterbalanced, crossover fashion, and changes in iMEP amplitudes during the intervention and MEP amplitudes before and for 30 min following the intervention were compared between the interventions by a two-way repeated measures ANOVA. Paired-pulse iMEP amplitude increased (p<0.001) by 98% for 15 min and 121% for 30 min from the first to the last minute, without a significant difference between the interventions for the changes. The increases occurred in the first half of the 30-min intervention, and the changes in the first 15-min were similar between interventions. Following the intervention, changes in MEP amplitude were not significantly differentbetween interventions, although the MEP amplitude increased (p<0.05) for 30 min post-intervention. It was concluded that the longer intervention had no additional effect, perhaps as a result of homeostatic regulation. In Study 2, the effects of stimulation intensity on iMEP and MEP amplitude changes were examined by comparing three different suprathreshold intensities, evoking iMEP amplitudes of 0.3 mV, 0.6 mV or 1.2 mV, during a 15-min iTMS intervention and for 2 hours following the intervention. Fifteen individuals (8 women and 7 men, 18 - 39 years) received two intensities in a randomised, counterbalanced order, and tested on two occasions separated by a week (10 participants for each intensity). Changes in iMEP and MEP amplitudes were compared across the three intensities by two-way repeated measures ANOVA. Significant (p<0.01) increases in iMEP amplitude from the baseline were evident for the intermediate intensity (0.6 mV) by 125%, but no significant changes in iMEP amplitude were found for the low (0.3 mV) and high (1.2 mV) intensity interventions, although iMEP amplitude was always greater for the high than low intensity. Significant increases in MEP amplitude were evident only for the intermediate and high intensities, which returned to baseline by 100 min. In conclusion, the intermediate and high intensity interventions increase corticomotor excitability to a similar extent post-intervention, whilst the lower intensity was unable to modulate MEP amplitude. The final study (Study 3) consisted of two experiments to investigate whether a previous iTMS intervention would affect the subsequent intervention(s), and multiple interventions would produce cumulative effects. Ten participants (5 women and 5 men, 20 - 42 years) were recruited for the first experiment in which two bouts of 15-min iTMS interventions separated by 7 days were compared. In the second experiment, another 10 participants (5 women and 5 men, 18 - 43 years) were tested for four bouts of 15-min iTMS interventions separated by 2 days for the first three bouts and by 7 days between the 3rd and 4th bouts. Changes in iMEP and MEP amplitude during and for 45 min following the intervention were compared between two bouts or amongst four bouts by a two-way repeated measures ANOVA. Significant (p<0.005) increases in iMEP amplitude from baseline were evident during the first and second interventions without a significant difference between the bouts. A significant increase (p<0.02) in MEP amplitude was also similar between bouts. In the second experiment, the increases in iMEP and MEP amplitude from baseline were similar to those seen in the first experiment, and no significant differences were evident for the changes amongst four bouts. These results suggest that the effects of iTMS intervention do not remain for a long time, and the iTMS intervention does not appear to produce any carry over effects between bouts spaced more than 2 days apart. The research has clarified the effects of intensity, duration and interval between interventions on changes in cortical excitability. Further studies are necessary to investigate the combination of the factors, and other stimulus parameters, for example, the number of sessions, the intervals between sessions incorporating rest or recovery periods, the site of stimulation and orientation of the coil, to establish effective rTMS intervention protocols.

http://library.ecu.edu.au/record=b2239749


Link to Full Text

Share

 
COinS