Observing the loss and revival of long-range phase coherence through disorder quenches

Relaxation of quantum systems is a central problem in nonequilibrium physics. In contrast to classical systems, the underlying quantum dynamics results not only from atomic interactions but also from the long-range coherence of the many-body wave function. Experimentally, nonequilibrium states of quantum fluids are usually created using moving objects or laser potentials, directly perturbing and detecting the system’s density. However, the fate of long-range phase coherence for hydrodynamic motion of disordered quantum systems is less explored, especially in three dimensions. Here, we unravel how the density and phase coherence of a Bose–Einstein condensate of 6Li2 molecules respond upon quenching on or off an optical speckle potential. We find that, as the disorder is switched on, long-range phase coherence breaks down one order of magnitude faster than the density of the quantum gas responds. After removing it, the system needs two orders of magnitude longer times to reestablish quantum coherence, compared to the density response. We compare our results with numerical simulations of the Gross–Pitaevskii equation on large three-dimensional grids, finding an overall good agreement. Our results shed light on the importance of long-range coherence and possibly long-lived phase excitations for the relaxation of nonequilibrium quantum many-body systems.

Comparative investigation of coherence factor weighting methods for an annular array photoacoustic microscope

To achieve fine visualization of the peripheral microvascular networks, we have developed a photoacoustic (PA) microscope equipped with a four-channel annular array transducer. The quality of PA images processed with Delay-and-Sum (DAS) method is degraded by off-axis signals. Thus, to achieve higher image quality for the PA microscope, this study evaluated the efficacy of the five coherence factor weighting methods: coherence factor, sign coherence factor, phase coherence factor, circular coherence factor, and vector coherence factor. Using PA signals acquired from a 100 µm microtube and the skin microvessels, we generated PA images with DAS and one of the weighting methods, and quantitatively evaluated the image quality by calculating the sharpness, contrast ratio, and contrast-to-noise ratio. The results showed the phase coherence factor and the vector coherence factor methods were more effective to clearly visualize the microvascular structure, in terms of vessel sharpening and noise suppression performances, than the other methods.

Measurement and Characterisation of a Diffuse Acoustic Field Using a Phased Array

A diffuse acoustic field has been increasingly used to infer temporal changes in the structures, such as early dislocations and microcracking. This study explores three different techniques to characterise acoustic field by using a single ultrasonic phased array. The first two techniques are proposed to measure spatial uniformity of wave field by examining differences in the integral of energy and the maximum energy respectively at multiple inspection locations. The third one is developed to evaluate the degree of phase coherence between propagating waves transmitted sequentially by two neighbouring array elements. The efficacy of these techniques are investigated by examining their metrics on simulations and well-known samples. The results suggest that two selected metrics can be used to quantitatively estimate the diffuse field start time as well as the field size by comparing their value with the idealised diffuse state (15% for the energy integral metric, ηarea and 1 for the phase coherence metric, ηphase) and identifying the convergence start point.

Modulation of gamma spectral amplitude and connectivity during reaching predicts peak velocity and movement duration

Voluntary movements are accompanied by increased oscillatory activity or synchronization in the gamma range (> 25.5 Hz) within the sensorimotor system. Despite the extensive literature about movement-related gamma synchronization, the specific role of gamma oscillations for movement control is still debated. In this study, we characterized movement-related gamma oscillatory dynamics and its relationship with movement characteristics based on 256-channels EEG recordings in 64 healthy subjects while performing fast and uncorrected reaching movements to targets located at three distances. We found that movement-related gamma synchronization occurred during both movement planning and execution, albeit with different gamma peak frequencies and topographies. Also, the amplitude of gamma synchronization in both planning and execution increased with target distance. Additional analysis of phase coherence revealed a gamma-coordinated long-range network involving occipital, frontal and central regions during movement execution. Gamma synchronization amplitude and phase coherence pattern reliably predicted peak velocity amplitude and timing, thus suggesting that cortical gamma oscillations play a significant role in the selection of appropriate kinematic parameters during planning and in their implementation during movement execution.

Neural language processing across time, space, frequency and age: MEG-MVPA classification of intertrial phase coherence

Language is a key part of human cognition. Whereas many neurocognitive abilities decline with age, for language the picture is much less clear and how exactly language processing changes with aging is still unknown. To investigate this, we employed magnetoencephalography (MEG) and recorded neuromagnetic brain responses to auditory linguistic stimuli in healthy participants of younger and older age using a passive task-free paradigm and a range of different linguistic stimulus contrasts, which enabled us to assess neural language processes at multiple levels (lexical, semantic, morphosyntactic). By using machine learning-based classification algorithms to scrutinise intertrial phase coherence of MEG responses in source space, we found significant differences between younger and older participants across several frequency bands and for all tested processing types, which shows multiple changes in the brain’s neurolinguistic circuits which may be due to both healthy aging in general and compensatory processes in particular.

O024 Weight loss surgery in obese patients with obstructive sleep apnoea is associated with increased inter-hemispheric electroencephalographic coherence

Introduction Electroencephalogram (EEG) analysis of obstructive sleep apnoea (OSA) patients has shown reductions in both delta band frequency power and interhemispheric coherence. Weight loss surgery is increasing in popularity and is often effective in reducing the symptoms and severity of OSA. No study has examined the effects of weight loss surgery on the sleep EEG of OSA patients. Methods Twenty patients underwent diagnostic polysomnography (PSG) prior to- and twelve months after weight loss surgery. Quantitative EEG analysis was conducted to calculate spectral power (fast fourier transform with four second overlapping windows) as well as amplitude and phase coherence between the two hemispheres (C3/C4 electrodes). Results Weight loss surgery was successful in reducing weight (137±17 kg vs 102±17kg P<0.001 for pre- and post-surgery, respectively), and OSA severity (Apnoea-Hypopnoea Index: 26±15 events/hour vs 10±12 events/hour P<0.001 for pre- and post-surgery, respectively). Stage N1 proportions were slightly decreased (12±7% vs 9±8%; P=0.039 for pre- and post-surgery, respectively) but all other stages were unchanged. Increases in interhemispheric phase coherence was observed following weight loss surgery, with significant changes observed in theta (-0.07±0.09 degrees vs -0.03±0.03 degrees P=0.035), alpha (-0.12±0.13 degrees vs -0.03±0.06 degrees P=0.008), sigma (-0.19±0.18 degrees vs -0.07±0.07 degrees P=0.011) and beta (-0.29±0.25 degrees vs -0.11±0.09 degrees P=0.004) band frequencies. There were no differences in EEG spectral power or interhemispheric amplitude coherence. Discussion Sleep EEG coherence, a putative marker of neurocognitive susceptibility in OSA, improves following weight loss surgery. Further studies are needed to determine the functional consequences of these EEG changes.