New methods for spiroketal synthesis

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Neuroimaging of binocular vision in human amblyopia

Amblyopia is a visual developmental condition that usually occurs when one eye receives abnormal input. For many years amblyopia was thought to be untreatable beyond 8 years old, after which the visual system would become functionally monocular. Recent research has shown that binocular mechanisms do remain intact in amblyopia and therefore investigating the nature of the deficit is crucial for understanding where neural problems arise and how they can be treated. Chapter 3 used population receptive field (pRF) modelling to further understand the cortical problems caused by amblyopia. Findings suggest that neurons responding to the amblyopic eye have reduced spatial resolution within striate and extrastriate areas. Chapters 4 and 5 aimed to test the predictions of different computational models of amblyopia using functional magnetic resonance imaging (fMRI) and electroencephalography (EEG), within the same group of participants. This is the first study to use a model driven approach to directly compare both neuroimaging methods within the same participants. The pattern of fMRI responses from the amblyopic eye showed evidence of a response gain effect and unbalanced interocular suppression, whereas EEG responses showed evidence of a contrast gain shift. Finally, Chapter 6 used EEG to objectively measure visual improvements, following treatment for amblyopia in children and adults. Measurable steady-state EEG responses were found for both groups; however, there was no convincing evidence of improvements in amblyopic eye responses throughout treatment. The studies undertaken in this thesis contribute to the wider understanding of the neural basis of amblyopia. Two different neuroimaging methods are compared, which has enabled insight into how current computational models of amblyopia could be improved. It is hoped that this research will further the development of treatments for amblyopia, by providing more insight into how binocular visual processes break down between the eyes

What are we going to do about a problem like polymer chemistry? Develop new methods of delivery to improve understanding of a demanding interdisciplinary topic

Following collaboration between two chemistry lecturers and an academic developer an attempt was made to enhance the learning of students within a chemistry module through the adaptation of the delivery of content material. This paper reports a piece of practitioner led research which considered how effective the approach developed was upon the level of student understanding and the process through which this occurred. The module delivery was altered from an emphasis on the transmission of knowledge through a traditional lecture format, to rotating small group problem based sessions and the use of concept maps. Student feedback and higher grades achieved appear to demonstrate it was effective

Promoting Soviet culture in Britain: the history of the Society for Cultural Relations Between the Peoples of the British Commonwealth and the USSR, 1924–45

ArticleThis is the author accepted manuscript. The final version is available via http://dx.doi.org/10.5699/modelangrevi.108.2.0571This article examines the history of the Society for Cultural Relations between the Peoples of the British Commonwealth and the USSR (SCR) from its inception to the end of the Second World War. It argues that although the Society toed the Party line, it was controlled by British left-wing intellectuals rather than by Soviet agents or agencies. The SCR is shown to have been a broad church in the period, whose membership included intellectuals from a wide range of fields with varied interests in Soviet culture. Its history, it is argued, reveals the significance of Soviet culture for British intellectuals and one route of its dissemination in Britain

Neural markers of suppression in impaired binocular vision

Even after conventional patching treatment, individuals with a history of amblyopia typically lack good stereo vision. This is often attributed to atypical suppression between the eyes, yet the specific mechanism is still unclear. Guided by computational models of binocular vision, we tested explicit predictions about how neural responses to contrast might differ in individuals with impaired binocular vision. Participants with a history of amblyopia (N = 25), and control participants with typical visual development (N = 19) took part in the study. Neural responses to different combinations of contrast in the left and right eyes, were measured using both electroencephalography (EEG) and functional magnetic resonance imaging (fMRI). Stimuli were sinusoidal gratings with a spatial frequency of 3c/deg, flickering at 4 Hz. In the fMRI experiment, we also ran population receptive field and retinotopic mapping sequences, and a phase-encoded localiser stimulus, to identify voxels in primary visual cortex (V1) sensitive to the main stimulus. Neural responses in both modalities increased monotonically with stimulus contrast. When measured with EEG, responses were attenuated in the weaker eye, consistent with a fixed tonic suppression of that eye. When measured with fMRI, a low contrast stimulus in the weaker eye substantially reduced the response to a high contrast stimulus in the stronger eye. This effect was stronger than when the stimulus-eye pairings were reversed, consistent with unbalanced dynamic suppression between the eyes. Measuring neural responses using different methods leads to different conclusions about visual differences in individuals with impaired binocular vision. Both of the atypical suppression effects may relate to binocular perceptual deficits, e.g. in stereopsis, and we anticipate that these measures could be informative for monitoring the progress of treatments aimed at recovering binocular vision

Binocular summation revisited: beyond √2

Our ability to detect faint images is better with two eyes than with one, but how great is this improvement? A meta-analysis of 65 studies published across more than five decades shows definitively that psychophysical binocular summation (the ratio of binocular to monocular contrast sensitivity) is significantly greater than the canonical value of √2. Several methodological factors were also found to affect summation estimates. Binocular summation was significantly affected by both the spatial and temporal frequency of the stimulus, and stimulus speed (the ratio of temporal to spatial frequency) systematically predicts summation levels, with slow speeds (high spatial and low temporal frequencies) producing the strongest summation. We furthermore show that empirical summation estimates are affected by the ratio of monocular sensitivities, which varies across individuals, and is abnormal in visual disorders such as amblyopia. A simple modeling framework is presented to interpret the results of summation experiments. In combination with the empirical results, this model suggests that there is no single value for binocular summation, but instead that summation ratios depend on methodological factors that influence the strength of a nonlinearity occurring early in the visual pathway, before binocular combination of signals. Best practice methodological guidelines are proposed for obtaining accurate estimates of neural summation in future studies, including those involving patient groups with impaired binocular vision

Induced superconductivity in the two-dimensional topological insulator phase of cadmium arsenide

Hybrid structures between conventional, s-wave superconductors and two-dimensional topological insulators (2D TIs) are a promising route to topological superconductivity. Here, we investigate planar Josephson junctions fabricated from hybrid structures that use thin films of cadmium arsenide (Cd3As2) as the 2D TI material. Measurements of superconducting interference patterns in a perpendicular magnetic field are used to extract information about the spatial distribution of the supercurrent. We show that the interference patterns are distinctly different in junctions with and without mesa-isolation, respectively. In mesa-defined junctions, the bulk of the 2D TI appears to be almost completely shunted by supercurrent flowing along the edges, while the supercurrent is much more uniform across the junction when the Cd3As2 film extends beyond the device. We discuss the possible origins of the observed behaviors.Comment: Accepted for publication in APL Material

Zeeman field-induced two-dimensional Weyl semimetal phase in cadmium arsenide

We report a topological phase transition in quantum-confined cadmium arsenide (Cd3As2) thin films under an in-plane Zeeman field when the Fermi level is tuned into the topological gap via an electric field. Symmetry considerations in this case predict the appearance of a two-dimensional Weyl semimetal (2D WSM), with a pair of Weyl nodes of opposite chirality at charge neutrality that are protected by space-time inversion (C2T) symmetry. We show that the 2D WSM phase displays unique transport signatures, including saturated resistivities on the order of h/e^2 that persist over a range of in-plane magnetic fields. Moreover, applying a small out-of-plane magnetic field, while keeping the in-plane field within the stability range of the 2D WSM phase, gives rise to a well-developed odd integer quantum Hall effect, characteristic of degenerate, massive Weyl fermions. A minimal four-band k.p model of Cd3As2, which incorporates first-principles effective g factors, qualitatively explains our findings.Comment: Accepted for publication in Physical Review Letter

Two-dimensional topological insulator state in cadmium arsenide thin films

Two-dimensional topological insulators (2D TIs) are a highly desired quantum phase but few materials have demonstrated clear signatures of a 2D TI state. It has been predicted that 2D TIs can be created from thin films of three-dimensional TIs by reducing the film thickness until the surface states hybridize. Here, we employ this technique to report the first observation of a 2D TI state in epitaxial thin films of cadmium arsenide, a prototype Dirac semimetal in bulk form and a 3D TI in thin films. Using magnetotransport measurements with electrostatic gating, we observe a Landau level spectrum and quantum Hall effect that are in excellent agreement with those of an ideal 2D TI. Specifically, we observe a crossing of the zeroth Landau levels at a critical magnetic field. We show that the film thickness can be used to tune the critical magnetic field. Moreover, a larger change in film thickness causes a transition from a 2D TI to a 2D trivial insulator, just as predicted by theory. The high degree of tunability available in epitaxial cadmium arsenide heterostructures can thus be used to fine-tune the 2D TI, which is essential for future topological devices.Comment: Accepted for publication in Physical Review Letters. Fixed issues with the display of Fig. 3 in the pdf in this versio