Direct current (DC) potential is an objective measure of the functional state of the human organism. It is a sensitive and accurate indicator of short- and long-term adaptations to stress, adaptive capacities, and it is an important marker of athlete readiness. Sleep is posited to be the most efficacious strategy for improving recovery to enhance sport performance, and adequate sleep is considered vital to normal psycho-physiological function. Thus, optimal sleep may enhance the functional state, in turn enhancing an athlete’s adaptability to training stress. However, little is known about the relationship between sleep and DC potential. Therefore, the purpose of this study was to examine the effect of acute (one-night) and extended (two-night) sleep quantity and quality on DC potentials in collegiate American football players. Twenty-four Division 1 American football players (Age: 20.6 ± 1.30 yr; Height: 183.4 ± 6.40 cm; Body mass: 114.40 ± 24.60 kg) wore a wrist-worn actigraphy band seven days per week over the course of 136 days, which spanned the pre-season training camp and competitive season, to measure sleep quantity and quality. DC potential was assessed six days per week using the Omegawave Ltd (Espoo, Finland) athlete monitoring system either 30 minutes upon waking or 75-120 minutes prior to the onset of the football training session. Sleep quantity was stratified into duration categories and sleep quality was stratified within sleep latency, number of awakenings, and sleep efficiency variables. Sleep quantity and quality were evaluated using acute (one night) and extended (rolling average of two consecutive nights) sleep outcomes. Within subject comparisons of DC potential were made across sleep quantity and quality categories using repeated-measures analysis of variance to examine the influence of acute and extended sleep quantity and quality on DC potential outcomes. The level of significance was set at p ≤ 0.025. Statistically significant main effects were identified for acute sleep (F3,16 = 4.68, p < .02, η2p = 0.47) and extended sleep durations (F2,17 = 7.71, p < 0.005, η2p = 0.48). Specifically, for acute sleep durations, there was a 17.1% increase in DC potentials (3.59, p < 0.01, Cohen’s d = 0.52, SE 1.18) for sleep durations ≥ 7 hours to < 9 hours, compared to sleeping < 6. For extended sleep, there was a 20% increase in DC potentials (4.53, p < 0.002, Cohen’s d = 0.68, SE = 1.13) when recording a two-day sleep average of ≥ 7.5 hours and < 9 hours, compared to an extended sleep duration of < 6 hours. A statistically significant main effect was also identified for extended wake episodes (F2,19 = 4.5, p = 0.025, η2p = 0.32). For extended sleep periods with > 4 wake episodes there was a 12% increase in DC potentials (2.57 ± 2.24mV, p < 0.25, Cohen’s d = 0.34) compared to extended sleep periods with 2-3 wake episodes. There was not a significant effect of acute (p ≥ 0.20) sleep quality or extended latency (p > 0.18) and efficiency (p > 0.08) on DC potentials. These findings suggest that sleep quantity affects DC bio-potentials and thus the functional state of the athlete. Specifically, sleep durations between 7.00/7.50 to 9 hours correspond with higher measures of DC potentials compared to lesser durations. Given the effect of sleep quantity on biological markers for training adaptability, practitioners should prioritize sleep in the training process and educate athletes on proper sleep hygiene and sleep quantity to enhance their readiness to train.

It is well established that cathodal transcranial direct current stimulation (tDCS) can decrease the excitability of the primary motor cortex (M1) in humans. Despite the cortical inhibition caused by cathodal tDCS, it remains unknown how this intervention alters unrestrained dynamic reaching movements qualitatively. Accordingly, we designed this study to examine how cathodal tDCS impacts unrestrained dynamic reaching as measured by qualitative kinematic features and electromyography (EMG). Ten young, healthy adult subjects were recruited to participate in a two day protocol involving repetitively reaching to two different targets (large and small) both before and following cathodal tDCS applied over the contralateral M1 during one session and before and following sham tDCS over the same brain region during another session. We discovered that cathodal tDCS was not able to alter the kinematic features of reaching in these subjects but did degrade the EMG performance, specifically by increasing the amount of co-contraction between muscle pairs. Because co-contraction is an indicator of relatively unskilled performance, these results seem to indicate that cathodal tDCS of M1 preferentially disrupts the learning or execution of highly coordinated muscle firing patterns during dynamic reaching. This work adds to the growing body of knowledge about how tDCS applied over M1 affects our movements. Moreover, it leads us to believe that tDCS can be utilized to assist in rehabilitation of patient populations who suffer from neurological dysfunctions but EMG assessments may need to be included in order to more effectively assess the patient performance.

An important aspect of cognitive neuroscience is to localise specific brain regions involved in cognitive tasks, and to determine the mediating brain processes. There are several investigative approaches towards this, but amongst them, only transcranial magnetic stimulation (TMS) is able to interfere with the brain in such a way as to show the critical involvement of a brain region in a particular behaviour. TMS can be applied in normal subjects during the performance of a cognitive task and the resulting disruption of activity in the targeted brain region leads to an alteration in, or suspension of, behaviour consequent upon that brain activity. More recently, another brain stimulation technique has emerged that may also be able to contribute to the investigation of human cognition. Transcranial direct current stimulation (tDCS) applies a weak direct current to a targeted brain region, modulating cortical excitability and thereby altering the behavioural output. tDCS may be able to provide information that complements TMS and other investigative techniques by modulating behaviour in a way that depends on the role the brain region is carrying out in the task. This thesis describes a series of experiments in which TMS and tDCS were applied to two well-studied cognitive behaviours, working memory (WM) and mental rotation (MR). WM is the temporary retention of information that can be manipulated in order to guide behaviour. The most popular psychological model of WM proposes a multi-modal central executive (CE) that acts upon information stored in dedicated buffers (Baddeley, 1986). The dorsolateral prefrontal cortex (DLPFC) is a strong candidate as a key CE node (D'Esposito & Postle, 2000; Petrides, 2000b; Smith & Jonides, 1997; Stuss & Knight, 2002). MR is a visuo-cognitive process by which an image can be mentally modified into an orientation other than the one in which it is displayed (Corballis & McLaren, 1984). The area centred around the intraparietal sulcus is a brain key region for MR (Alivisatos & Petrides, 1996; Harris et al., 2000; Jordan et al., 2001). The work presented in this thesis examines the roles of the DLPFC and posterior parietal cortex (PPC) in WM and MR, respectively, and also highlights some of the methodological issues that are necessary to consider in order to produce reliable virtual lesions. The studies were carried out in young healthy volunteers, and were approved by the institutional ethics committee. In one study, repetitive TMS (rTMS) was shown to disrupt the manipulation of verbal information held in WM when administered over the right DLPFC, a result which supports a process-based segregation of the human prefrontal cortex for WM. Low- and high-frequency rTMS did not disrupt performance on another popular test of executive processing, n-back, a result which suggests that specific stimulation and task conditions must be met in order to produce virtual lesions, but also questions the critical importance of recruitment of the DLPFC for a running span task. rTMS applied to the right PPC replicated results from a previous TMS investigation, supporting the critical role this region in the rotation of images (Harris & Miniussi, 2003). When the left PPC was stimulated, impairment was produced only for the rotation of inverted stimuli. A role for the left PPC in the rotation of objects-as-a-whole is proposed based on these findings. The use of tDCS in the investigation of WM and MR is amongst the first to be described. Stimulation of the left DLPFC led to decreased performance accuracy on a verbal WM task in a polarity-specific manner. The pattern of results produced supports the role of the DLPFC as a node of a CE. tDCS over the left DLPFC did not modulate n-back task performance, a result which supports the TMS results that the involvement of the left DLPFC is not critical to the successful performance of the n-back task, although methodological issues remain of concern in relation to this conclusion. MR was not affected by tDCS applied to the right PPC and this result is most likely a direct demonstration of the importance of electrode montage. In conclusion, these studies show that rTMS and tDCS can be usefully applied to create virtual cortical lesions or modulate cortical excitability during the performance of cognitive tasks in humans, and can play an important role in investigating cognitive neuropsychological models. More widespread use of these techniques to complement lesion studies and functional neuroimaging is recommended.

The electrochemical treatment (EChT) of tumours entails thattumour tissue is treated with a continuous direct currentthrough two or more electrodes placed in or near the tumour.Promising results have been reported from clinical trials inChina, where more than ten thousand patients have been treatedwith EChT during the past ten years. Before clinical trials canbe conducted outside of China, the underlying destructionmechanism behind EChT must be clarified and a reliabledose-planning strategy has to be developed. One approach inachieving this is through mathematical modelling. Mathematical models, describing the physicochemical reactionand transport processes of species dissolved in tissuesurrounding platinum anodes and cathodes, during EChT, aredeveloped and visualised in this thesis. The consideredelectrochemical reactions are oxygen and chlorine evolution, atthe anode, and hydrogen evolution at the cathode. Concentrationprofiles of substances dissolved in tissue, and the potentialprofile within the tissue itself, are simulated as functions oftime. In addition to the modelling work, the thesis includes anexperimental EChT study on healthy mammary tissue in rats. Theresults from the experimental study enable an investigation ofthe validity of the mathematical models, as well as of theirapplicability for dose planning. The studies presented in this thesis have given a strongindication of the destruction mechanism involved in EChT. It isshown by the modelling work, in combination with theexperiments, that the most probable cause of tissue destructionis acidification at the anode and alkalisation at the cathode.The pH profiles obtained from the theoretical models have showngood correlation with the experimentally measured destructionzones, assuming that a pH above and below certain values causetissue destruction. This implies that the models presented inthis thesis could be of use in predicting the tumourdestruction produced through EChT, and thereby provide a basisfor a systematic dose planning of clinical treatments.Moreover, the models can serve as valuable tools in optimisingthe operating conditions of EChT. Modelling work of theanode processes has explained the roleof chlorine in the underlying destruction mechanism behindEChT. It is found that the reactions of chlorine with tissueplay important roles as generators of hydrogen ions. Thecontribution of these reactions to the acidification of tissue,surrounding the anode, is strongly dependent on the appliedcurrent density and increases with decreasing currentdensity. <b>Keywords:</b>cancer, direct current, dose planning,electrochemical treatment (EChT), electrotherapy, mathematicalmodelling, tumour.

The performance of a novel micro-thermodielectric power generation device (MTDPG) was investigated in order to determine if thermodielectric power generation can compete with current portable power generation technologies. Thermodielectric power generation is a direct energy conversion technology that converts heat directly into high voltage direct current. It requires dielectric (i.e., capacitive) materials whose charge storing capabilities are a function of temperature. This property is exploited by heating these materials after they are charged; as their temperature increases, their charge storage capability decreases, forcing them to eject a portion of their surface charge to an appropriate electronic storage device. Previously, predicting the performance of a thermodielectric power generator was hindered by a poor understanding of the materials thermodynamic properties and the affect unsteady heat transfer losses have on system performance. In order to improve predictive capabilities in this study, a thermodielectric equation of state was developed that describes the relationship between the applied electric field, the surface charge stored by the thermodielectric material, and its temperature. This state equation was then used to derive expressions for the material's thermodynamic states (internal energy, entropy), which were subsequently used to determine the optimum material properties for power generation. Next, a numerical simulation code was developed to determine the heat transfer capabilities of a micro-scale parallel plate heat recuperator (MPPHR), a device designed specifically to a) provide the unsteady heating and cooling necessary for thermodielectric power generation and b) minimize the unsteady heat transfer losses of the system. The previously derived thermodynamic equations were then incorporated into the numerical simulation code, creating a tool capable of determining the thermodynamic performance of an MTDPG, in terms of the thermal efficiency, percent Carnot efficiency, and energy/power density, when the material properties and the operating regime of the MPPHR were varied. The performance of the MTDPG was optimized for an operating temperature range of 300 500 K. The optimization predicted that the MTDPG could provide a thermal efficiency of 29.7 percent. This corresponds to 74.2 percent of the Carnot efficiency. The power density of this MTDPG depends on the operating frequency and can exceed 1,000,000 W/m3.

<p>The electrochemical treatment (EChT) of tumours entails thattumour tissue is treated with a continuous direct currentthrough two or more electrodes placed in or near the tumour.Promising results have been reported from clinical trials inChina, where more than ten thousand patients have been treatedwith EChT during the past ten years. Before clinical trials canbe conducted outside of China, the underlying destructionmechanism behind EChT must be clarified and a reliabledose-planning strategy has to be developed. One approach inachieving this is through mathematical modelling.</p><p>Mathematical models, describing the physicochemical reactionand transport processes of species dissolved in tissuesurrounding platinum anodes and cathodes, during EChT, aredeveloped and visualised in this thesis. The consideredelectrochemical reactions are oxygen and chlorine evolution, atthe anode, and hydrogen evolution at the cathode. Concentrationprofiles of substances dissolved in tissue, and the potentialprofile within the tissue itself, are simulated as functions oftime. In addition to the modelling work, the thesis includes anexperimental EChT study on healthy mammary tissue in rats. Theresults from the experimental study enable an investigation ofthe validity of the mathematical models, as well as of theirapplicability for dose planning.</p><p>The studies presented in this thesis have given a strongindication of the destruction mechanism involved in EChT. It isshown by the modelling work, in combination with theexperiments, that the most probable cause of tissue destructionis acidification at the anode and alkalisation at the cathode.The pH profiles obtained from the theoretical models have showngood correlation with the experimentally measured destructionzones, assuming that a pH above and below certain values causetissue destruction. This implies that the models presented inthis thesis could be of use in predicting the tumourdestruction produced through EChT, and thereby provide a basisfor a systematic dose planning of clinical treatments.Moreover, the models can serve as valuable tools in optimisingthe operating conditions of EChT.</p><p>Modelling work of theanode processes has explained the roleof chlorine in the underlying destruction mechanism behindEChT. It is found that the reactions of chlorine with tissueplay important roles as generators of hydrogen ions. Thecontribution of these reactions to the acidification of tissue,surrounding the anode, is strongly dependent on the appliedcurrent density and increases with decreasing currentdensity.</p><p><b>Keywords:</b>cancer, direct current, dose planning,electrochemical treatment (EChT), electrotherapy, mathematicalmodelling, tumour.</p>

Parts of the auxiliary power systems at Fortum's hydro power stations are usingdirect current, which is fed from the generators at the plant and converted byrectifiers. As photovoltaic solar cells produce direct current there are severalhypothetical advantages to use solar power for the auxiliary power supply, e.g.enabling more of the power from the generator to be sold to the grid. It eliminatesthe need of an inverter, conversion losses are avoided and less load is put on therectifiers. However the exclusion of an inverter also prevents the solar cells to have adirect connection to the grid, which in turn makes them ineligible for the Swedishgovernmental solar power investment support program. The lesser load on therectifiers will not affect their lifetime according to manufacturers and thus achieves noeconomic gain. Avoiding conversion losses will increase the gain from the producedelectricity by enabling even more power to be sold to the grid. The economic gainfrom avoiding conversion losses is however too small to gain any feasibility in a smallsolar power installation at a hydro power plant, as the small size will make itexpensive in terms of investment per Wp.

Anodal transcranial direct current stimulation (tDCS) is an intervention which is thought to enhance motor learning in healthy and stroke-injured states, when applied adjunctively during skill learning. We set out to investigate whether anodal tDCS might enhance functional rehabilitation from incomplete tetraplegic SCI. To address current limitations in the measurement of task-dependent skill, a novel integrated skill training and measurement task, the Motor Skill Rehabilitation Task (MSRT) was designed and developed. Measures of performance from this task delivered the functional measure of spatial motor skill learning, Task Productivity Rate (TPR). TPR was analysed and validated as a univariate dependent outcome, which is of potential importance to the future development of clinical measures measuring goal-directed motor skills. The MSRT was included alongside conventional behavioural measures in a repeated-measures RCT pilot study, the first to investigate the effect of anodal tDCS on rehabilitation of motor skill from chronic spinal cord injury. Adjunctive application of anodal tDCS had a statistically significant benefit upon retention of skill in the incomplete spinal cord injured population, but only when the independent factor of sensory acuity was included in the analysis. Differences between the development of task-dependent skill and generic dexterity over time suggested that spatial skill development was subject to an interaction of short-term and lasting effects. A larger study in healthy persons further investigated these phenomena, also applying Transcranial Magnetic Stimulation (TMS)–evoked measurements to investigate intervention-dependent effects upon the excitability of projections between the primary motor cortex and muscles involved in the prehension task. The findings revealed that active tDCS did not enhance skill learning at 7 days beyond the training period, but did significantly alter the development of motor skill following a period of learning and subsequent skill consolidation which was associated with underlying perturbation of motor control strategy. Significant and divergent patterns of cortical plasticity were evoked in projections to muscles necessary for reaching and grasping. The main findings of this thesis do not support anodal tDCS as an effective adjunctive means of enhancing spatial motor skill in rehabilitation from incomplete tetraplegic SCI. If applied in patient populations, the clinical benefits of anodal tDCS may be contingent both on the nature of the sensorimotor deficit affecting upper limb function and the spatial demands of the behavioural task. The findings of this project serve to inform further research in relation to the effect of anodal tDCS on the brain and behavioural outcomes, the potential for efficacy in target patient groups and the sensitivity of outcome measures to spatial and temporal dimensions of practical motor skills.

The main objective of this research was to study DC conductor corona loss (CL) and audible noise (AN) in the context of local climatic conditions, through corona cage measurements, and do a comparative analysis of the measured data with results available in literature and EPRI TLW software simulation results. The ultimate aim was to assess the applicability of the software to our local conditions and hence determine, if necessary, appropriate correction factors for application in HYDC transmission line designs. For this study, short term measurements of corona AN and currents were carried out in an indoor meshed cylindrical corona cage, under DC and AC voltages. The cage was later converted into a short test line and some of the measurements repeated. The DC supply was obtained from a two stage ±500 kV Walton-Cockroft generator. The AC voltages were obtained from a 2x 100 kV, 50 Hz, AC test transformer set. The tests were performed using single solid and stranded aluminum conductors with three different diameters (1.6 cm, 2.8 cm, and 3.5 cm). All the measurements were carried out at low altitude. A CoroCAM I camera was used to determine the corona inception gradients and to observe the corona activity at different surface gradients and under different voltages and polarities. AN measurements at different conductor surface gradients were done using the Rohde&Schwarz and the Bruel&Kjaer sound level meters. To obtain the frequency spectra, a Bruel&Kjaer octave-band filter set attached to the sound level meter was used. The measured data was corrected for both height and length effects, and then compared with simulations from the EPRI-TLW software through curve fitting. A digital micro-ammeter connected to the centre of the cage through a 560 .Q measuring resistor was used to measure the corona current. Current pulses were viewed using a digital storage oscilloscope. To verify the corona current results obtained from the cage measurements, current measurements were also done for a point-plane spark gap. The corona current data was, later on, used to evaluate the total corona power loss for DC. The results obtained from test line measurements were used to compare the CL and AN for different configurations. The effect of the space charge under DC voltages was assessed through current measurements. The measurements were done with the cage covered with an aluminum foil to trap the charge and then repeated with the cage uncovered. On the test line, the space charge effect was investigated using a high power fan blowing along the conductor, to simulate the wind factor. The results of this study have shown the characteristics of corona discharges under different system voltages. The results also give an understanding of how factors such as conductor surface conditions and size, polarity and system voltage affect CL and AN. Both CL and AN were found to increase with conductor size for the same conductor surface gradient and to be higher for stranded conductors. Positive polarity DC and AC noise levels were higher than the negative polarity levels. CL under positive polarity DC was lower than the negative polarity loss. The effects of space of space charge were noted to some extend. The comparison of test line results and cage results showed that CL depends more on the gap size and the shorter the gap the higher the loss. Hence CL results were not compared with the software simulations. The comparison between the corrected AN results and the software simulations showed a very good agreement. The comparison was done for the 3.5 cm and the 2.8 cm diameter conductors under both positive and negative polarities. The trends compared through curve fitting were quite similar and the trend line equations were of the same order of magnitude. The magnitudes of the corrected noise levels were higher than the CRIEPI and BPA predictions but closer to the EPRl prediction. Generally there is a very good and encouraging agreement between the available literature, simulation results and the results obtained from the laboratory measurements. It is proposed, as part of further studies, to extend this work to high altitude regions and use bundled conductors as well. Consideration of different and larger test configurations will provide an understanding of the effects of geometry on corona discharges. Space charge analysis will also assist in determining the effect of space charge on different configurations. / Thesis (M.Sc.Eng.)-University of KwaZulu-Natal, Durban, 2007.

The aim of this study was to assess the electrical performance of the Cahora Bassa HVDC scheme. For practical reasons a database was developed to hold and analyse the performance data. Microsoft® Access 2002 relational database management system was chosen for this work. The principle of simplicity and flexibility were used in the design of the database. The Apollo year 2002 faults data was populated into the database. The database stored the information in a format that enables the user to extract results and the information required by Ciqre, The Ciqre Working Group 14 collects performance information from all the participating HVDC schemes around the world annually. The Apollo converter station's 2002 performance data was compared to other similar HVDC schemes that submitted to Cigre in the year 2002. In addition performance trends were drawn from similar schemes that submitted to Cigre over the past years. The Apollo performance data is quite comparable to other similar schemes in 2002 and over the past years. / Thesis (M.Sc.)-University of KwaZulu-Natal, 2005.