The Cognitive Effects of Light Color Temperature

The goal of the current studies is to explore the conditions by which light color temperature impacts cognitive abilities and the development of this relationship. Experiments 1 and 1A explored whether exposure to light fluctuating around a central color temperature leads to increases in attention in adult participants. Results showed that, under the dynamic lighting condition, participants’ showed a significant decrease in reaction time on a measure of sustained attention, beyond those of a static light source at a cooler color temperature. Experiment 2 tested whether preschool-aged and 7-year-old participants would show increases in attention and cognitive flexibility after exposure to light set to a cooler color temperature, as has been previously seen in older children and adults. While 7-year-olds showed no effect of the lighting condition, preschool-aged children exposed to cooler color temperature light showed significantly greater improvements in cognitive flexibility than controls, demonstrating that the relationship between light and cognition is present from an early age. Taken together, these studies add to the growing literature demonstrating that the spectral composition of light can lead to improvements in cognitive abilities

The Role of Representational Flexibility in Toddlers\u27 Manual Search

In the model room task, children watch as a miniature toy is hidden somewhere in a scale model of a room and are asked to find the larger version of the toy in the corresponding place in the actual room. Previous work has shown that children under age three often perform very poorly on this task. One prominent theory for their failure is that they lack the ability to understand the model as both a physical object and as a symbolic representation of the larger room. An alternative hypothesis is that they need to overcome weak, competing representations of where the object was on a previous trial, and where it is in the present trial, in order to succeed in their search. Children aged 33-39 months were tested on measures of inhibitory control, cognitive flexibility, recognition memory, and receptive vocabulary, as well as the model room task. Results showed that performance on the model room task was not predicted by measures of inhibitory control, cognitive flexibility or vocabulary, but was predicted by performance on the Delayed Recognition Span Test (DRST), a measure of recognition memory. These findings lend support to the theory of competing representations. Given the predictive nature of the recognition memory task and the task’s sensitivity to lesions in the hippocampus, implications for the development of the hippocampus and its role in success on the model room task are discussed

Fermi Surfaces and Where to Find Them: Quantum Oscillations in Kondo Insulators and High-Temperature Superconductors

The Fermi surface is a geometric concept that codifies the momenta of all electrons at the Fermi level. It is these electrons that underpin most physical properties of metals, and have made quantum oscillations, the experimental manifestation of the Fermi surface, a hallmark signature of metals. In this thesis we have studied systems that are distinctly non-Fermi liquid, and discovered that, contrary to this canon, quantum oscillations occur even in the absence of a Fermi liquid. We present the high magnetic field studies of two classes of correlated electron systems. We survey the striking quantum oscillations in the magnetisation of SmB₆ and YbB₁₂, two strongly-correlated Kondo insulators. We also present magnetic and transport measurements of underdoped YBa₂Cu₃O₆₊ₓ, a high-temperature superconductor, that prompt us to reinterpret the quantum oscillations previously associated with the non-superconducting normal state. The surprising observation of quantum oscillations in the magnetisation of Kondo insulating SmB₆, but unaccompanied by oscillations in the electrical resistance, has attracted much attention. Here, we detail magnetic torque measurements that establish the intrinsic, bulk nature of the quantum oscillations, and reveal a moderate angular dependence of the oscillation frequencies, characteristic of a bulk, three-dimensional Fermi surface. We identify a finite linear specific heat coefficient down to the lowest temperatures, a distinguishing feature between metals and insulators. We demonstrate that the measured finite linear specific heat coefficient is in good agreement with the density of states at the Fermi level estimated from quantum oscillations. The unconventional nature of the ground state of SmB₆ is further evidenced by a non-zero thermal conductivity that is enhanced in a magnetic field. Through an extensive suite of characterisation techniques we confirm the high purity of our single crystals, with material properties consistent with an impurity concentration of less than 0.05%, and therefore further establishing the intrinsic character of the observed quantum oscillations. In the search for other non-Fermi liquids that are host to a Fermi surface, we identify YbB₁₂ as the second Kondo insulator that exhibits intrinsic, bulk quantum oscillations. We present a detailed study of the de Haas-van Alphen oscillations, corresponding to a heavy semimetal Fermi surface. Our results show many similarities with the ground state of SmB₆, including the large absolute size of the quantum oscillations and a finite linear specific heat coefficient, but also some key differences, namely the heavy effective masses and the proximity to a magnetic-field-induced or applied-pressure-induced insulator-metal transition. The observation of quantum oscillations in underdoped YBa₂Cu₃O₆₊ₓ refocused efforts to understand the pseudogap ground state of cuprate superconductors. Distinct from the large hole orbits of the Fermi liquid-like overdoped regime, the pseudogap regime was found to be characterised by a small electron pocket and the absence of antinodal states. A proposal associated the quantum oscillations with a conventional metallic state that emerges at a magnetic field of ∼20 T, however magnetic and thermal measurements have been at odds with the destruction of the superconducting order parameter at such modest magnetic fields. We employ high-magnetic fields to explore the region characterised by quantum oscillations, in search for the origin of the missing antinodal states in underdoped YBa₂Cu₃O₆₊ₓ, and the true extent of superconductivity. We find that the measured quantum oscillations display a signature sawtooth waveform, that rule out vestigial residual density of states, and instead point towards a complete gapping of the antinodal regions. We present current-dependent transport measurements performed in DC magnetic fields, down to millikelvin temperatures, that reveal the high-field superconducting state to be characterised by non-ohmic signatures associated with a quantum vortex matter state. In contrast to previous proposals, the quantum oscillations are found to occur well within this gapped vortex phase, as established by their co-existence with zero resistivity and hysteretic torque magnetisation, that are found to persist to magnetic fields beyond 45 T.EPSRC studentshi

Surface spin waves in superconducting and insulating ferromagnets

Surface magnetization waves are studied on a semi-infinite magnetic medium in the perpendicular geometry. Both superconducting and insulating ferromagnets are considered. Exchange and dipole energies are taken into account, as well as retardation effects. At large wave vectors, the spectrum for a superconductor and insulator is the same, though for the former the branch is terminated much earlier than for the latter due to excitation of plasmons. At small wave vectors, the surface wave is more robust in the superconductor since it is separated from the bulk continuum by a finite gap.Comment: 4 pages, 2 figure

Using linear initialisation to improve speed of convergence and fully-trained error in Autoencoders

Good weight initialisation is an important step in successful training of Artificial Neural Networks. Over time a number of improvements have been proposed to this process. In this paper we introduce a novel weight initialisation technique called the Straddled Matrix Initialiser. This initialisation technique is motivated by our assumption that major, global-scale relationships in data are linear with only smaller effects requiring complex non-linearities. Combination of Straddled Matrix and ReLU activation function initialises a Neural Network as a de facto linear model, which we postulate should be a better starting point for optimisation given our assumptions. We test this by training autoencoders on three datasets using Straddled Matrix and seven other state-of-the-art weight initialisation techniques. In all our experiments the Straddeled Matrix Initialiser clearly outperforms all other methods

The predictive value of small versus diminutive adenomas for subsequent advanced neoplasia

Background and Aims Patients with previous colorectal adenomas are at increased risk of colorectal cancer. Current guidelines for postpolypectomy surveillance intervals treat all tubular adenomas 1 to 9 mm in size with low-grade dysplasia as carrying the same level of risk. We evaluated whether 6 to 9 mm adenomas detected at colonoscopy are associated with greater risk of advanced neoplasia at follow-up compared with baseline 1 to 5 mm adenomas. Methods We retrospectively evaluated a colonoscopy database at a single U.S. academic center. Patients with baseline examinations demonstrating tubular adenomas 1 to 9 mm in size with low-grade dysplasia and no advanced adenomas were included. Follow-up colonoscopies were performed at least 200 days later and were assessed for incident advanced neoplasia (cancer, high-grade dysplasia, adenoma ≥10 mm in size, or villous elements). Results There were 2477 qualifying baseline colonoscopies. The absolute risk of metachronous advanced neoplasia increased from 3.6% in patients with 1 to 5 mm adenomas to 6.9% in patients with at least 1 adenoma of 6 to 9 mm (P = .001). Patients with 5 or more adenomas 1 of which was at least 6 to 9 mm had the highest risk of advanced neoplasia at follow-up (10.4%, P = .006). When only screening colonoscopies were considered, all baseline groups (1-2 adenomas, 3-4 adenomas, ≥5 adenomas) with adenomas 6 to 9 mm in size had an increased risk for metachronous advanced neoplasia (odds ratio [OR], 4.07; 95% confidence interval [CI], 1.50-11.04; OR, 4.91; 95% CI, 1.44-16.75; OR, 4.71; 95% CI, 1.30-17.05, respectively). Conclusions Patients with baseline small (6-9 mm) adenomas have an increased risk of advanced lesions on follow-up compared with patients with only diminutive (1-5 mm) adenomas. Postpolypectomy guidelines should consider risk stratification based on small versus diminutive adenomas