title: Using neural fields to model motor evoked potentials due to transcranial magnetic stimulation

authors: Wilson, Marcus T.

abstract: Neural field models, in which populations of neurons are described through ensemble-averaged properties such as firing rate and axonal flux, have been well-used to describe electroencephalogram (EEG) effects in natural sleep, anaesthesia and epilepsy. Changes in EEG due to stimulation by electromagnetic fields have also been studied. However, experimental data concerning transcranial magnetic stimulation (TMS) is dominated by measurements of motor-evoked potentials (MEPs). To use neural fields for TMS, a model of the MEP is required. In this work, an existing model has been augmented by addition of populations of cells describing stages of the MEP pathway previously neglected by neural field models. The model includes coupled populations of excitatory and inhibitory layer 2/3 cortical neurons, stimulated by an external field. These populations feed an additional population of layer 5 neurons, which also couples weakly to the external field. The layer 5 neurons feed motoneurons, from which a measure of the motor-evoked potential is constructed. After some tuning of parameters, the extra populations have resulted in a better model of the MEP than previously present in neural field models. The model reproduces the non-linear response to an increase in stimulation amplitude. With a paired-pulse protocol it reproduces the experimental effects of intracortical facilitation and long-interval intracortical inhibition, though it shows only weak short-interval intracortical inhibition. The experimental direct and indirect waves of activation from the layer 5 neurons are not adequately reproduced. Small changes in synaptic weight can give much greater changes in MEP response. In conclusion, the model is promising but it raises questions about whether neural field models are really appropriate for the study of many TMS effects.

text: Using neural fields to model motor evoked potentials due to transcranial magnetic stimulation Neural field models, in which populations of neurons are described through ensemble-averaged properties such as firing rate and axonal flux, have been well-used to describe electroencephalogram (EEG) effects in natural sleep, anaesthesia and epilepsy. Changes in EEG due to stimulation by electromagnetic fields have also been studied. However, experimental data concerning transcranial magnetic stimulation (TMS) is dominated by measurements of motor-evoked potentials (MEPs). To use neural fields for TMS, a model of the MEP is required. In this work, an existing model has been augmented by addition of populations of cells describing stages of the MEP pathway previously neglected by neural field models. The model includes coupled populations of excitatory and inhibitory layer 2/3 cortical neurons, stimulated by an external field. These populations feed an additional population of layer 5 neurons, which also couples weakly to the external field. The layer 5 neurons feed motoneurons, from which a measure of the motor-evoked potential is constructed. After some tuning of parameters, the extra populations have resulted in a better model of the MEP than previously present in neural field models. The model reproduces the non-linear response to an increase in stimulation amplitude. With a paired-pulse protocol it reproduces the experimental effects of intracortical facilitation and long-interval intracortical inhibition, though it shows only weak short-interval intracortical inhibition. The experimental direct and indirect waves of activation from the layer 5 neurons are not adequately reproduced. Small changes in synaptic weight can give much greater changes in MEP response. In conclusion, the model is promising but it raises questions about whether neural field models are really appropriate for the study of many TMS effects.

year: 2018

title: Modelling single cell electroporation with bipolar pulse parameters and dynamic pore radii

authors: Talele, Sadhana

subjects: electroporation

abstract: We develop a model of single spherical cell electroporation and simulate spatial and temporal aspects of the transmembrane potential and pore radii as an effect of any form of applied electric field. The extent of electroporation in response to sinusoidal electric pulses of two different frequencies in a range of extracellular conductivity for two different cell radii are compared. Results show that pore radii tend to be more normalized for AC fields. The relative difference in fractional pore area is reduced by the use of a 1 MHz sinusoidal applied electric field over a 100 kHz field.

text: Modelling single cell electroporation with bipolar pulse parameters and dynamic pore radii We develop a model of single spherical cell electroporation and simulate spatial and temporal aspects of the transmembrane potential and pore radii as an effect of any form of applied electric field. The extent of electroporation in response to sinusoidal electric pulses of two different frequencies in a range of extracellular conductivity for two different cell radii are compared. Results show that pore radii tend to be more normalized for AC fields. The relative difference in fractional pore area is reduced by the use of a 1 MHz sinusoidal applied electric field over a 100 kHz field.

year: 2010

title: Maori thinking with a dead white male: Philosophizing in the realm of Novalis

authors: Mika, Carl Te Hira

subjects: Novalis

abstract: In this paper I shall discuss my experiences of referring to Novalis in the context of Maori postcolonialism and metaphysical philosophy. As with other methods of research, from a Maori perspective one always alights on and then carries the effects of the philosopher that stands behind the method, whether the philosopher is silent or explicit. This important onto-epistemological interaction, in a general sense, acknowledges for the indigenous person that one is always ‘within’ the world and not detached from it. The maligned dead white male hence unavoidably becomes the highly constructive, living impulse behind what is to become fresh and innovative indigenous thinking.

text: Maori thinking with a dead white male: Philosophizing in the realm of Novalis In this paper I shall discuss my experiences of referring to Novalis in the context of Maori postcolonialism and metaphysical philosophy. As with other methods of research, from a Maori perspective one always alights on and then carries the effects of the philosopher that stands behind the method, whether the philosopher is silent or explicit. This important onto-epistemological interaction, in a general sense, acknowledges for the indigenous person that one is always ‘within’ the world and not detached from it. The maligned dead white male hence unavoidably becomes the highly constructive, living impulse behind what is to become fresh and innovative indigenous thinking.

year: 2014