Since the discovery of stress-generated potentials (SGPs) in bone by Fukada and Yasuda in 1957, researchers have tried to understand their origin and function in the maintenance of bone. There have been a variety of methods attempting to quantify these SGPs in both wet and dry bone. In this study, I prepared both dry and wet beams of cortical bovine bone and subjected them to mechanical deformation in cantilever bending. Mechanical testing was performed to explore how the magnitude of the SGPs was affected by hydration levels, strain, and pressure gradients associated with various load magnitudes and deformation rates. Signals that were collected from the dry bone samples were attributed to motion artifact resulting from the movement of the materials testing machine and load cell. The SGPs from wet bone, on the other hand, consistently produced exponentially decaying signals following deformation that were maintained throughout held deformation and produced an SGP of opposite magnitude upon release of deformation. The exponentially decaying SGP signal produced after application of a step load to wet bone samples was determined to fit a two-term exponential equation (V(t) = Aet/τ1 + Cet/τ2). The first term, made up of the A-coefficient and τ1, was found to be dependent on deformation rate whereas the second term, containing the C-coefficient and τ2, was dependent on load magnitude. The sum of the two coefficients determine the maximum voltage the SGP can reach. Additionally, samples were left to air dry for one hour and tested intermittently throughout that time period. SGP signals diminished significantly over the hour, therefore, it has been concluded that the majority of the SGP signal is due to streaming potentials caused by ionic fluid movement within the bone upon deformation. / Thesis / Master of Applied Science (MASc) / Mechanical deformation of bone produces electrical signals known as stress-generated potentials (SGPs). In this study, I mechanically tested wet beams of bone to assess how the SGPs were affected by hydration levels, load magnitudes, and deformation rates. Dry bone samples did not produce any acceptable SGP signals. The SGPs from wet bone, however, produced repeatable signals that decayed following deformation. With a step load input, the decaying SGP signal fit a two-term exponential equation (V(t) = Aet/τ1 + Cet/τ2). The first term, made up of the A-coefficient and τ1, was found to be dependent on deformation rate whereas the second term, containing the C-coefficient and τ2, was dependent on load magnitude. The two coefficients, the A and C-coefficient, together determine the maximum voltage the SGP can reach. The result of this work showed that SGPs in bone are dependent on tissue hydration and vary with load magnitude and deformation rate.

Mouse IHCs, the main sensory cells of the mammalian cochlea, were studied using whole-cell patch clamp techniques. Recordings were made from 453 IHCs (ranging from EI4.5-P20) positioned at the two extremes of the cochlea. Changes in the properties of K+ currents, spiking activity, and Ca2+ currents along with synaptic vesicle exocytosis, were investigated. IRCs begin to express voltage-gated K+ currents from as early as EI4.5. During the next few days the appearance of the Ca2 + current allows IHCs to fire broad spontaneous action potentials. The growth of these currents from just after birth speeds up each action potential and increases the spike frequency with basal cells spiking more frequently than apical cells. The disappearance of spiking activity, that occurs just before the onset of hearing (P12), changes IHCs into mature sound transducers. It is likely that spiking activity is important for the reorganization of neuronal connections during early development as previously suggested in other systems (Moody, 1998). In order for spiking activity to function as a developmental signal to the afferent fibres, neurotransmitter must be released from IHCs. Capacitance measurements (indicative of synaptic vesicle exocytosis i.e. neurotransmitter release) indicated that IHCs responded to an action potential with exocytosis from late embryonic stages. Immature cells contain at least two kinetically distinct vesicle pools that seem to become depleted following stimuli greater than 1.5 s. By contrast, exocytosis in mature cells is more graded, with no apparent depletion for the same stimulus duration, and shows a higher Ca2+ -efficiency. These changes may enable IHCs to relay accurate auditory information onto afferent fibres. Overall, the results suggest that IHC maturation goes through different stages in order to fulfill their role in cochlear development incorporating changes in their basolateral currents and synaptic machinery. The aim of this thesis was to examine developmental changes in the properties of spiking activity among apical and basal IHCs and investigate any differences that exist between the two regions. The ability of IHCs to respond to an action potential with exocytosis was studied at different developmental stages and the maturation of vesicle release properties was compared between the two extremes of the cochlea.

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Object recognition memory (ORM) refers to both recognition of an object and the memory of having seen it before. In humans, ORM has been investigated using functional neuroimaging and electrophysiological techniques with tests of episodic memory retrieval involving recollection of previously studied items. Processes involved in the maintenance of a mental state adopted for the performance of a retrieval task (retrieval mode) appear to involve right frontal neural regions. More transient processes occurring at the time of item recollection (retrieval success) have shown scalp activity over parietal and right frontal regions. This activity is thought to originate in the medial temporal lobes and the underlying right frontal cortex respectively. The aforementioned findings have been derived mainly from studies using verbal stimuli. It is uncertain whether the same neural regions are involved in object recollection. It is also uncertain whether sustained modal and transient item-related activity involve the same or different right frontal regions. In this study, steady-state probe topography (SSPT) was used to investigate both sustained and transient processes involved in the retrieval of abstract pictorial objects from memory. The ability to vary the evaluation period of the steady-state visually evoked potential (SSVEP) allows investigation of cognitive processes occurring over different time scales. Neural regions involved in sustained modal processes were identified by examining the SSVEP values averaged over the duration of a memory retrieval task. Sustained SSVEP effects were observed over right fronto-temporal regions. Neural regions involved in transient retrieval success processes were identified by comparing the transient SSVEP responses to tasks with different memory loads. Comparison of a higher with a lower memory load condition showed SSVEP effects over parieto-temporal and right inferior frontal regions. Larger differences between memory loads gave effects that were larger and more right lateralized. Retrieval mode and retrieval success processes showed SSVEP effects over different right frontal regions. It was also found that, in contrast to the left lateralized parietal ERP response to recollected verbal stimuli, the SSVEP effects produced with abstract pictorial shapes showed a more bilateral pattern. This was considered to reflect the relatively non-verbalizable pictorial nature of the stimuli.

Novices and experts show distinct differences in the performance of many tasks. Experts may perform a task quickly and accurately with seemingly little attention or effort, whilst novices will perform the same task more slowly and with great effort. The transition from novice to expert performance occurs only after extended practice and has been conceptualized as a transition from controlled to automatic processing, and has been modeled as a reduction in attention or cognitive resources. Alternatively, based on findings relating to learning in the domain of number arithmetic, it has also been modeled as a transition from an algorithmic, or computationally-based process, to the use of memory retrieval. However, relatively few studies have investigated the changes in brain activity associated with such a transition. In this study, the Steady-State Probe Topography technique was used to investigate differences in the topography of the Steady-State Visual Evoked Potential (SSVEP) between an unpracticed and a well-practiced analogue of number arithmetic, Alphabet arithmetic. Subjects showed decreases in response time with practice that followed a power law and were suggestive of automatization. During initial, unpracticed performance of the task, processing of the Alphabet Arithmetic equations was characterised by increased SSVEP amplitude and decreased latency in frontal regions, whilst after extended practice, performance was characterised by reduced SSVEP amplitude and increased latency. It is suggested that the frontal activity during the initial, unpracticed stage of the task implicates a role for working memory, whilst the amplitude decrease and latency increase observed in the well-practiced task may reflect a reduction in excitation, consistent with ideas of an improvement in brain efficiency, and possibly an increase in inhibitory processes.

Thesis (Ph. D.)--University of California, San Diego and San Diego State University, 2006. / Title from first page of PDF file (viewed December 13, 2006). Available via ProQuest Digital Dissertations. Vita. Includes bibliographical references.

Thesis (M.A.)--York University, 2002. Graduate Programme in Kinesiology and Health Science. / Typescript. Includes bibliographical references (leaves 63-66). Also available on the Internet. MODE OF ACCESS via web browser by entering the following URL: http://wwwlib.umi.com/cr/yorku/fullcit?pMQ71617.

published_or_final_version / Medical Sciences / Master / Master of Medical Sciences