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Human acute neurophysiological responses to magnetically-induced alternating current densities of up to 100 mA.m-2
Abstracts / Brain Stimulation 10 (2017) 346e540
electroencephalography (TMSeEEG). We first characterized the neural signature of SICI and ICF in M1 in terms of TMS evoked potentials (TEPs) and spectral power modulation. Subsequently these paradigms were applied in DLPFC to determine whether similar neural signatures were evident. With TMS at M1, SICI and ICF led to bidirectional modulation (inhibition and facilitation respectively) of P30 and P60 TEP amplitude which correlated with MEP amplitude changes. With DLPFC stimulation, P60 was bidirectionally modulated by SICI and ICF in the same manner as for M1 stimulation, while P30 was absent. The sole modulation of early TEP components is in contradistinction to other measures such as long interval intracortical inhibition and may reflect modulation of short latency excitatory and inhibitory postsynaptic potentials (EPSPs and IPSPs). Overall the data suggest that SICI and ICF can be recorded using TMS-EEG in DLPFC providing non-invasive measures of glutamatergic and GABAA receptor mediated neurotransmission. This may facilitate future research attempting to ascertain the role of these neurotransmitters in the pathophysiology and treatment of neurological and psychiatric disorders. Keywords: TMS-EEG, SICI, ICF, DLPFC [0648] CONTINUOUS THETA-BURST STIMULATION OF MEDIAL PREFRONTAL CORTEX ENHANCES SCHEMA-LINKED ENCODING A.N. Vorobiova*1, A.A. Shpektor 1, 2, M. Feurra 1. 1 National Research University Higher School of Economics, Russia; 2 University of Oxford, UK Introduction: It has long been established that the existence of prior knowledge (or schema representation) to which new information can be related, facilitates memory encoding, consolidation and retrieval. Schema is interpreted as a network of strongly interrelated neocortical representations. Recent studies show the role of the medial prefrontal cortex (mPFC) for establishing a link between representations of objects based on their congruency to either prior knowledge or to each other, thereby facilitating memory functions. Methods: We used MRI-navigated continuous theta-burst stimulation (cTBS) delivered over the mPFC to modulate encoding of object-scene paired associations at different levels of congruency (i.e. differently fitting to subject’s knowledge about real world). Stimulation was applied by using a novel parabolic-shaped focal coil that allows deeper brain stimulation than conventional one. Sham and conventional stimulation were included as controls. The next day subjects were asked to perform a stimulation-free retrieval memory task. Results: We found that mPFC-cTBS, delivered before encoding, selectively increased memory retrieval performance (item recognition and associative memory) only for congruent items (compared to incongruent and intermediate) with respect to stimulation controls. Discussion: Our findings represent the first causal evidence on the role of mPFC in associative memory. The effect is condition-specific and well controlled. The facilitatory effect of cTBS is likely to be due to the intrinsic inhibitory mechanisms of mPFC towards the medial temporal lobe (MTL) which also play a role in consolidation processes of congruent associations. Keywords: associative memory, schema, medial prefrontal cortex, continuous theta-burst stimulation [0650] HUMAN ACUTE NEUROPHYSIOLOGICAL RESPONSES TO MAGNETICALLY-INDUCED ALTERNATING CURRENT DENSITIES OF UP TO 100 MA.M-2 A. Legros*1, 2, J. Modolo 3,1, M. Corbacio 1, S. Davarpanah Jazi 1, S. Villard 1, D. Goulet 4, M. Plante 4, M. Souques 5, F. Deschamps 6, G. Ostiguy 4. 1 LHRI and e de Montpellier, France; Western University, Canada; 2 EuroMov, Universit 3 INSERM, France; 4 Hydro-Qu ebec, Canada; 5 EDF, France; 6 RTE, France Introduction: Living in our modern environment implies that we are constantly subjected to various types of electromagnetic exposures coming from different artificial sources, such as mobile phones, power-lines or electrical appliances. As a consequence, the question of possible biological effects from this type of exposure arises, and international organizations such as ICNIRP (International Commission on Non-Ionizing Radiation Protection) and IEEE-ICES (Institute of Electrical and Electronics Engineers
503
- International Committee on Electromagnetic Safety) are responsible for issuing limits and recommendations to ensure public health and worker safety. In the context of the so-called Extremely Low Frequencies (ELF, <300 Hz), the time-varying magnetic field (MF) from the exposure induces electric fields and currents within biological structures, possibly leading to biological effects including on neuronal functions. In this frequency range, limits and recommendations are based on the exposure threshold at which acute effects are observed on synaptic communication in humans, hence resulting in modulated functional outcomes. Methods: Our team is working since 2005 towards experimentally establishing these thresholds in human through the study of magnetically induced cognitive (e.g. short term memory) or motor (e.g. tremor, postural stability) modulations, changes in functional brain activity, and acute visual perceptions called magnetophosphenes. Results: This presentation will give an overview of recent human studies investigating the impact of ELF exposures on neurophysiological outcomes, and will report new results establishing the threshold for an acute neurophysiological response in humans: magnetophosphene perception. Conclusion: Knowing and understanding the implications of this threshold for an acute neurophysiological response to a time-varying MF is critical from an international guidelines standpoint. Also, since the induced electric fields and current densities resulting from such stimuli are comparable to those produced by tDCS and tACS, possible translational applications will be discussed. Keywords: Magnetophosphenes, Human study, Extremely Low Frequency Magnetic Field, Threshold effect [0655] TRANSCRANIAL ALTERNATING CURRENT STIMULATION FOR BOOSTING CORTICOSPINAL EXCITABILITY AND RESTITUTION OF INTRACORTICAL INHIBITION IN ELDERLY M. Christova*1, 2, S. Fresnoza 3, E. Gallasch 1, A. Ischebeck 3. 1 Medical University of Graz, Austria; 2 University of Applied Sciences Graz, Austria; 3 University of Graz, Austria The oscillatory alpha-band activity (8-12 Hz) reflects the GABAergic inhibition that blocks task irrelevant activity, thereby increasing the signal-tonoise ratio within the brain networks. In aging, there is a parallel decrease in alpha activity and GABA- mediated inhibitory processes with a profound effect on cognitive abilities. Application of transcranial alternating current stimulation (tACS) was shown to modulate brain oscillation, cortical excitability and behaviour. EEG studies suggest a state dependent effect of tACS: upregulation of alpha oscillations only in conditions of low prestimulation alpha power. Therefore, tACS might be especially useful in elderly with reduced alpha power. To explore the state/age-dependent effect of tACS on cortical excitability, tACS at individual alpha frequency (IAF) with 1.5 mA current was applied to the left motor cortex of 12 young and 12 old volunteers for 10min. Single and paired-pulse transcranial magnetic stimulation (TMS) was applied to assess the corticospinal and intracortical excitability changes immediately, 1h and 2h post tACS. The results showed age/state-dependent effect of tACS applied at IAF on motor cortical excitability. Stimulation among elderly individuals with slower oscillation and less alpha power caused a significant increase in corticospinal excitability at lower TMS intensities and GABA-mediated intracortical inhibition after tACS. Younger participants with higher oscillation frequency and alpha power showed significant increase in corticospinal excitability at higher TMS intensities only, while intracortical inhibition and facilitation were not affected. The study compares for the first time the after effect of tACS at IAF on the cortical excitability of younger and elderly brain. Data further add on the growing evidence that the tACS effect is dependent on the alpha activity state before stimulation. tACS can be considered as a promising tool that can safely modulate oscillatory activity for studying brain functions and can be given as an individualized treatment regimen for patients. [0659] VAGUS NERVE STIMULATION THERAPY FOR CHRONIC MEDICAL DISORDERS L.L. Carpenter*1, 2. 1 Butler Hospital, USA; 2 Brown University, USA
electroencephalography (TMSeEEG). We first characterized the neural signature of SICI and ICF in M1 in terms of TMS evoked potentials (TEPs) and spectral power modulation. Subsequently these paradigms were applied in DLPFC to determine whether similar neural signatures were evident. With TMS at M1, SICI and ICF led to bidirectional modulation (inhibition and facilitation respectively) of P30 and P60 TEP amplitude which correlated with MEP amplitude changes. With DLPFC stimulation, P60 was bidirectionally modulated by SICI and ICF in the same manner as for M1 stimulation, while P30 was absent. The sole modulation of early TEP components is in contradistinction to other measures such as long interval intracortical inhibition and may reflect modulation of short latency excitatory and inhibitory postsynaptic potentials (EPSPs and IPSPs). Overall the data suggest that SICI and ICF can be recorded using TMS-EEG in DLPFC providing non-invasive measures of glutamatergic and GABAA receptor mediated neurotransmission. This may facilitate future research attempting to ascertain the role of these neurotransmitters in the pathophysiology and treatment of neurological and psychiatric disorders. Keywords: TMS-EEG, SICI, ICF, DLPFC [0648] CONTINUOUS THETA-BURST STIMULATION OF MEDIAL PREFRONTAL CORTEX ENHANCES SCHEMA-LINKED ENCODING A.N. Vorobiova*1, A.A. Shpektor 1, 2, M. Feurra 1. 1 National Research University Higher School of Economics, Russia; 2 University of Oxford, UK Introduction: It has long been established that the existence of prior knowledge (or schema representation) to which new information can be related, facilitates memory encoding, consolidation and retrieval. Schema is interpreted as a network of strongly interrelated neocortical representations. Recent studies show the role of the medial prefrontal cortex (mPFC) for establishing a link between representations of objects based on their congruency to either prior knowledge or to each other, thereby facilitating memory functions. Methods: We used MRI-navigated continuous theta-burst stimulation (cTBS) delivered over the mPFC to modulate encoding of object-scene paired associations at different levels of congruency (i.e. differently fitting to subject’s knowledge about real world). Stimulation was applied by using a novel parabolic-shaped focal coil that allows deeper brain stimulation than conventional one. Sham and conventional stimulation were included as controls. The next day subjects were asked to perform a stimulation-free retrieval memory task. Results: We found that mPFC-cTBS, delivered before encoding, selectively increased memory retrieval performance (item recognition and associative memory) only for congruent items (compared to incongruent and intermediate) with respect to stimulation controls. Discussion: Our findings represent the first causal evidence on the role of mPFC in associative memory. The effect is condition-specific and well controlled. The facilitatory effect of cTBS is likely to be due to the intrinsic inhibitory mechanisms of mPFC towards the medial temporal lobe (MTL) which also play a role in consolidation processes of congruent associations. Keywords: associative memory, schema, medial prefrontal cortex, continuous theta-burst stimulation [0650] HUMAN ACUTE NEUROPHYSIOLOGICAL RESPONSES TO MAGNETICALLY-INDUCED ALTERNATING CURRENT DENSITIES OF UP TO 100 MA.M-2 A. Legros*1, 2, J. Modolo 3,1, M. Corbacio 1, S. Davarpanah Jazi 1, S. Villard 1, D. Goulet 4, M. Plante 4, M. Souques 5, F. Deschamps 6, G. Ostiguy 4. 1 LHRI and e de Montpellier, France; Western University, Canada; 2 EuroMov, Universit 3 INSERM, France; 4 Hydro-Qu ebec, Canada; 5 EDF, France; 6 RTE, France Introduction: Living in our modern environment implies that we are constantly subjected to various types of electromagnetic exposures coming from different artificial sources, such as mobile phones, power-lines or electrical appliances. As a consequence, the question of possible biological effects from this type of exposure arises, and international organizations such as ICNIRP (International Commission on Non-Ionizing Radiation Protection) and IEEE-ICES (Institute of Electrical and Electronics Engineers
503
- International Committee on Electromagnetic Safety) are responsible for issuing limits and recommendations to ensure public health and worker safety. In the context of the so-called Extremely Low Frequencies (ELF, <300 Hz), the time-varying magnetic field (MF) from the exposure induces electric fields and currents within biological structures, possibly leading to biological effects including on neuronal functions. In this frequency range, limits and recommendations are based on the exposure threshold at which acute effects are observed on synaptic communication in humans, hence resulting in modulated functional outcomes. Methods: Our team is working since 2005 towards experimentally establishing these thresholds in human through the study of magnetically induced cognitive (e.g. short term memory) or motor (e.g. tremor, postural stability) modulations, changes in functional brain activity, and acute visual perceptions called magnetophosphenes. Results: This presentation will give an overview of recent human studies investigating the impact of ELF exposures on neurophysiological outcomes, and will report new results establishing the threshold for an acute neurophysiological response in humans: magnetophosphene perception. Conclusion: Knowing and understanding the implications of this threshold for an acute neurophysiological response to a time-varying MF is critical from an international guidelines standpoint. Also, since the induced electric fields and current densities resulting from such stimuli are comparable to those produced by tDCS and tACS, possible translational applications will be discussed. Keywords: Magnetophosphenes, Human study, Extremely Low Frequency Magnetic Field, Threshold effect [0655] TRANSCRANIAL ALTERNATING CURRENT STIMULATION FOR BOOSTING CORTICOSPINAL EXCITABILITY AND RESTITUTION OF INTRACORTICAL INHIBITION IN ELDERLY M. Christova*1, 2, S. Fresnoza 3, E. Gallasch 1, A. Ischebeck 3. 1 Medical University of Graz, Austria; 2 University of Applied Sciences Graz, Austria; 3 University of Graz, Austria The oscillatory alpha-band activity (8-12 Hz) reflects the GABAergic inhibition that blocks task irrelevant activity, thereby increasing the signal-tonoise ratio within the brain networks. In aging, there is a parallel decrease in alpha activity and GABA- mediated inhibitory processes with a profound effect on cognitive abilities. Application of transcranial alternating current stimulation (tACS) was shown to modulate brain oscillation, cortical excitability and behaviour. EEG studies suggest a state dependent effect of tACS: upregulation of alpha oscillations only in conditions of low prestimulation alpha power. Therefore, tACS might be especially useful in elderly with reduced alpha power. To explore the state/age-dependent effect of tACS on cortical excitability, tACS at individual alpha frequency (IAF) with 1.5 mA current was applied to the left motor cortex of 12 young and 12 old volunteers for 10min. Single and paired-pulse transcranial magnetic stimulation (TMS) was applied to assess the corticospinal and intracortical excitability changes immediately, 1h and 2h post tACS. The results showed age/state-dependent effect of tACS applied at IAF on motor cortical excitability. Stimulation among elderly individuals with slower oscillation and less alpha power caused a significant increase in corticospinal excitability at lower TMS intensities and GABA-mediated intracortical inhibition after tACS. Younger participants with higher oscillation frequency and alpha power showed significant increase in corticospinal excitability at higher TMS intensities only, while intracortical inhibition and facilitation were not affected. The study compares for the first time the after effect of tACS at IAF on the cortical excitability of younger and elderly brain. Data further add on the growing evidence that the tACS effect is dependent on the alpha activity state before stimulation. tACS can be considered as a promising tool that can safely modulate oscillatory activity for studying brain functions and can be given as an individualized treatment regimen for patients. [0659] VAGUS NERVE STIMULATION THERAPY FOR CHRONIC MEDICAL DISORDERS L.L. Carpenter*1, 2. 1 Butler Hospital, USA; 2 Brown University, USA