Electronic Thesis and Dissertation Repository

Thesis Format

Monograph

Degree

Master of Science

Program

Neuroscience

Supervisor

Dr. Siobhan Schabrun

Affiliation

The Gray Centre of Mobility and Activity

Abstract

Homeostatic plasticity (HMP) regulates neural plasticity to prevent excessive neuronal excitability or silencing. Transcranial magnetic thetaburst stimulation (TBS) has been shown to induce HMP in the human cortex, however, the reliability is unknown. Reliability of TBS induced HMP was examined in the primary motor cortex (M1) of 20 healthy individuals using an excitatory and an inhibitory priming-test paradigm and assessed using motor evoked potentials (MEPs) from the first dorsal interosseous (FDI) muscle. MEPs were reliable for both protocols (ICCs between 0.61-0.99). HMP was not induced as expected (F(8) = 1.04, p = 0.408), although when accounting for order effects, HMP was evident (F(9)=3.85, p < 0.001), but contingent on first implemented protocol (t = -5.098, p < 0.001). Our findings provide important insights for designing HMP induction protocols, and this discovery of order effects suggests HMP may operate on a longer timescale than previously thought, warranting further investigation into the reliability of these methods.

Summary for Lay Audience

Your brain is capable of rapidly adapting and reorganizing itself in response to changes in the environment. This ability of the brain to ‘change’ is referred to as neuroplasticity. This is an important feature for learning, memory, and recovery of function after an injury or illness. Neuroplasticity allows the brain to change its wiring by strengthening or weaking its connections. However, this relies on a delicate balance of up- and down-regulation of these changes, and if left unchecked, this balance can become unstable and cause uncontrolled changes within the brain’s connections, making them either ‘too strong’ or ‘too weak’. To prevent this instability, a subtype of neuroplasticity termed homeostatic plasticity (HMP), naturally works as a mechanism to keep connections stable. Previous studies have used technologies that can safely manipulate brain activity to induce and study HMP in humans, however it is important to research whether these tools can produce consistent and reliable results. Our study tested the reliability of inducing HMP using one of these technologies, known as thetaburst stimulation (TBS). When TBS is applied over the area of the brain responsible for controlling the hand, it activates a pathway that causes muscle movement. This allows us to assess any changes in brain activity by placing electrode stickers on the hand that will record muscle activity in the form of something called motor evoked potentials (MEPs). For our study we used two paradigms, one excitatory and one inhibitory, in which we applied two consecutive blocks of TBS to try and induce HMP. This was repeated across three sessions for each to assess their reliability. We found that MEPs were reliable for both paradigms across sessions, meaning that we found consistent results that relate in some way. HMP was not induced as expected, however depending on which paradigm was applied first (excitatory or inhibitory) we could see HMP effects during the second paradigm. This means that HMP may be operating on a longer timescale than previously thought. These results provide insights into the way HMP studies are constructed and carried out.

Creative Commons License

Creative Commons Attribution 4.0 License
This work is licensed under a Creative Commons Attribution 4.0 License.

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