Thesis (M.S.)--University of Michigan, 1985.

Time-varying fields are widely used to extend the accessible range of trapping potentials for ultracold atoms. This work explores two very different examples of such fields, in the radiofrequency and optical regimes, whose interactions with trapped atoms can both be described in terms of the dressed atom picture. Forming the basis of this work are radiofrequency dressed adiabatic potentials based on macroscopic trapping coils. Atoms are confined at the south pole of the resultant oblate spheroidal trapping surfaces. This work describes the extension of these potentials by two different methods: the application of multiple dressing radiofrequencies, and addition of a rapidly-scanned optical dipole trap. This is the first experimental demonstration of a multiple-radiofrequency dressed adiabatic potential, explored using ultracold ⁸⁷Rb atoms confined in a highly configurable double well. Due to the independent generation of each constituent dressing frequency, the depth of each trapping well and the height of the barrier are easily manipulated, enabling precise and reliable transfer of atoms between the available trapping geometries. Experimental work includes an exploration of the potential-shaping capabilities of the three-radiofrequency system, and characterisation of the potential landscape using radiofrequency spectroscopy with good agreement to the eigenvalues numerically calculated using Floquet theory. This initial exploration of multiple-radiofrequency techniques lays the groundwork for applications in studying double well physics in a two-dimensional system, and independent state or species selective manipulation of trapped atoms. The potential shaping capabilities of this method can also be extended by applying additional trapping frequencies. In a supplementary line of experimental work, an optical dipole trapping system has been constructed, and the trapping beam aligned to the lower surface of the radiofrequency dressed trapping shell in order to sculpt the radial confinement. Beam shaping is achieved using an acousto-optic deflector, which can be used to produce either a composite array of static deflected beams, a rapidly-scanned painted potential, or some combination of the two approaches. The development and extension of the experimental apparatus required to implement these enhanced dressed state potentials is explored, and the challenges of their experimental implementation considered.

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published_or_final_version / Psychology / Doctoral / Doctor of Philosophy

published_or_final_version / Orthopaedic Surgery / Doctoral / Doctor of Philosophy

No description available.

This dissertation consists of a review of the N2 component of the ERP and five experiments investigating the role of complex visual object categorization in modulating the N2 and two other ERP components: the P300, and a late prefrontal positivity. In the review, we focus on paradigms that elicit N2 components with an anterior scalp distribution, namely cognitive control, novelty, and sequential matching, arguing that the anterior N2 should be divided into separate control- and mismatch-related subcomponents. The experiments manipulated categorical typicality and the presence of conflicting information as participants categorized multi-featured artificial animals. In Experiments 1 and 2, rule-irrelevant features were correlated with particular categories during training. During transfer, participants applied a one- dimensional rule to stimuli with category-congruent, category-incongruent, or novel rule-irrelevant features. Category-incongruent and novel features delayed RT and P300 latency, but had no effect on the N2. Experiment 3 used a two-dimensional rule to create conflict between rule-relevant features. Conflict resulted in prolonged RTs, P300 latency, and larger amplitudes of a prefrontal positive component, but had no impact on the N2. Novel features did enhance the N2 relative to frequent features. In Experiments 4 and 5, participants categorized stimuli using a more complex three dimensional rule. Conflicting stimuli shared two features with one prototype and one feature with a second prototype while prototypes contained no conflicting information. A third category contained stimuli with either common or novel features. Again, perceptual novelty, but not conflict, increased the amplitude of the N2. Compared to prototypes, stimuli with conflicting information slowed reaction times but had no effect on P300 latency, instead enhancing a late prefrontal positive component. These results suggest limitations on the generality of the N2's sensitivity to conflicting information, while confirming its sensitivity to attended visual novelty. We suggest that, while P300 latency tracks stimulus evaluation time, application of a complex categorization rule requires a later stage of evaluation involving prefrontal cortex. In very complex rules, computations indexed by the P3 may be terminated early in favor of computations in PFC.

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No description available.

This masters thesis examines the hemispheric activation pattern of the cognitive processes involved in a complex mental rotations test (MRT) (Vandenberg and Kuse, 1978) using Steady-State Probe Topography (SSPT) (Silberstein et al, 1990) as a method to index brain activity. The Steady State Visually Evoked Potential (SSVEP) was recorded from 64 electrode sites using a multichannel electrode helmet, and elicited by a 13 Hz sinusoidal visual flicker, whilst the subjects were performing a visual vigilance Baseline task and the MRT. Forty-one right handed subjects (twenty male and twenty-one female) were used. In the MRT the subjects were required to choose the two figures which correctly matched the criterion figure in the centre. The figures were three-dimensional objects represented in two-dimensions on a computer screen. A significant finding of this study was that when all the subjects were considered as one group, no noticeable lateralization in cerebral activation associated with mental rotation was evident. When analyzing the results for the subjects, partitioned into two groups according to gender, evidence was found suggesting that the cortical processing associated with mental rotation may be more localized bilaterally in the males than the females. However, no noticeable lateralization effects for mental rotation were found in the males or females, and hence no gender differences in hemispheric lateralization was evident. An important finding was the emergence of gender differences in hemispheric lateralization in subsets of subjects performing with higher spatial ability. A left hemisphere lateralization for mental rotation was associated with the Best Performance Male group. The Best Performance Female group showed the opposite effect, where a right hemisphere lateralization was associated with better performance on the task. The lateralization effect appeared to be stronger in the Best Performance Males than the Best Performance Females. An important conclusion from this study is that when examining for hemispheric lateralization effects in mental rotation, and possibly other visual-spatial tasks, not only gender effects need to be considered, but the level of spatial ability in the comparison groups needs also to be taken into account.