The influence of posterior parietal cortex on extrastriate visual activity: A concurrent TMS and fast optical imaging study

The posterior parietal cortex (PPC) is a critical node in attentional and saccadic eye movement networks of the cerebral cortex, exerting top-down control over activity in visual cortex. Here, we sought to further elucidate the properties of PPC feedback by providing a time-resolved map of functional connectivity between parietal and occipital cortex using single-pulse TMS to stimulate the left PPC while concurrently recording fast optical imaging data from bilateral occipital cortex. Magnetic stimulation of the PPC induced transient ipsilateral occipital activations (BA 18) 24 to 48ms post-TMS. Concurrent TMS and fast optical imaging results demonstrate a clear influence of PPC stimulation on activity within human extrastriate visual cortex and further extend this time- and space-resolved method for examining functional connectivity

An Investigation of Short-Term Plasticity in Human Motor Cortex

Transcranial magnetic stimulation (TMS) produces a transient magnetic field that activates underlying cortical tissue by eliciting an electrical discharge of the neurons in the targeted area. Repetitive TMS (rTMS) uses patterns of repetitive TMS pulses and has been reliably shown to produce changes in the state of cortical excitability outlasting the time of stimulation. One such protocol that has demonstrated states of increased excitability is intermittent theta burst stimulation (iTBS). This method applies high-frequency bursts (50Hz) of pulses every 200 ms in trains of ten bursts. The effects of and differences between rTMS protocols have been investigated since gaining popularity in the 1990’s, however, there are still many unknowns regarding the neurophysiological changes that accompany this plasticity. Much research on these effects takes place in motor cortex due to reliable and quantifiable measures of cortical excitability observed there. Here, I sought to further investigate the effects of iTBS on inter-hemispheric changes in in motor cortex using EEG simultaneously recorded with TMS pulses. That is, I examined if iTBS conducted over right motor cortex would lead to measureable changes in excitability indices of left motor cortex. I quantified changes in right and left motor cortex excitability with measurements of TMS-evoked potentials (TEPs) and motor-evoked potentials (MEPs) elicited through blocks of single pulse TMS immediately following iTBS and 30 minutes post-iTBS. I compared the effects of the iTBS condition to those found in a control condition where a sham version of iTBS was administered. Results indicate differential modulations of cortical TEPs between hemispheres, including an initial enhancement of the P30 in right hemisphere, coinciding with sustained suppression in left hemisphere, and an enhancement of the P190 during left hemisphere stimulation

Studying the cortical state with transcranial magnetic stimulation

Cortical excitability and connectivity describe the state of the cerebral cortex. They reflect the ability of neurons to respond to input and the way information flows in the neuronal networks. These properties can be assessed with transcranial magnetic stimulation (TMS), which enables direct and noninvasive modulation of cortical activity. Electrophysiological or hemodynamic recordings of TMS-evoked activity or behavioral measures of the stimulation effect characterize the state of the cortex during and as a result of the stimulation. In the research reported in this Thesis, the ability of TMS to inform us about the cortical state is studied from different points of view. First, we examine the relationships between different measures of cortical excitability to better understand the physiology behind them; we show how cortical background activity is related to motor cortical excitability and how the evoked responses reflect the excitability. Second, this study addresses the questions whether the TMS-evoked responses include stimulation-related artifacts, how these artifacts are generated, and how they can be avoided or removed. Specifically, we present a method to remove the artifacts from TMS-evoked electroencephalographic (EEG) signals arising as a result of cranial muscle stimulation. The use of TMS-EEG has been limited to relatively medial sites because of these artifacts, but the new method enables studying the cortical state even when stimulating areas near the cranial muscles, especially lateral sites. Finally, this work provides new information about brain function. The mechanisms how the brain processes visually guided timed motor actions are elucidated. Moreover, we show that cortical excitability as measured with TMS-evoked EEG increases during the course of wakefulness and decreases during sleep, which contributes to our understanding of what happens in the brain during wakefulness that makes us feel tired and why the brain needs sleep. The study also shows the sensitivity of the TMS-EEG measurement to changes in the state of the cortex. Accordingly, we demonstrate the power of TMS in studying the cortical state

Depicting Transcranial Magnetic Stimulation from a Neuronal Perspective

Transcranial magnetic stimulation (TMS) is a widely used non-invasive tool to study and modulate human brain functions. However, our knowledge of TMS neurophysiology is largely limited due to various difficulties in studying neural activities in vivo in real-time under the strong electromagnetic interference generated by TMS. In this work, the author first reviews the state-of-the-art knowledge in the neurophysiology of TMS and highlights the need to conduct further studies at the level of individual neurons and their neural networks. The second chapter is dedicated to the technical development of a research platform that allows faithful concurrent TMS and EEP recording in laboratory rodents. Building on this methodology, the third chapter focuses on characterizing the dynamics of neuronal activities in M1 evoked by a monophasic single-pulse TMS (mspTMS) that triggers unilateral MEPs in the forearm. Bringing the investigation further, the fourth chapter explores the mechanism of TMS-evoked intermediate excitation, a period of synaptic-mediated excitatory neuronal activities in M1 triggered by TMS, in the cortico-basal ganglia-thalamo-cortical (CBGTC) and the cortico-cerebello-thalamo-cortical (CCTC) loop. Finally, the fifth chapter summarizes the results from this work, discusses their limitations and implications, and proposes future research

The Cognitive Neuroscience Toolkit for the Neuroeconomist: A Functional Overview

This article provides the beginning neuroeconomist with an introductory overview to the different methods used in human neuroscience. It describes basic strengths and weaknesses of each technique, points to examples of how each technique has been used in neuroeconomic studies, and provides key tutorial references that contain more detailed information. In addition to this overview, the article presents a framework that organizes human neuroscience methods functionally, according to whether they provide tests of the association between brain activity and cognition or behavior, or whether they test the necessity or the sufficiency of brain activity for cognition and behavior. This framework demonstrates the utility of a multimethod research approach, because converging evidence from tests of association, necessity, and sufficiency provides the strongest inference regarding brain–behavior relationships. Set against this goal of converging evidence, human neuroscience studies in neuroeconomics currently rely far too heavily on methods that test association, most notably functional magnetic resonance imaging (MRI)