Assessment of learning tomography using Mie theory

In Optical diffraction tomography, the multiply scattered field is a nonlinear function of the refractive index of the object. The Rytov method is a linear approximation of the forward model, and is commonly used to reconstruct images. Recently, we introduced a reconstruction method based on the Beam Propagation Method (BPM) that takes the nonlinearity into account. We refer to this method as Learning Tomography (LT). In this paper, we carry out simulations in order to assess the performance of LT over the linear iterative method. Each algorithm has been rigorously assessed for spherical objects, with synthetic data generated using the Mie theory. By varying the RI contrast and the size of the objects, we show that the LT reconstruction is more accurate and robust than the reconstruction based on the linear model. In addition, we show that LT is able to correct distortion that is evident in Rytov approximation due to limitations in phase unwrapping. More importantly, the capacity of LT in handling multiple scattering problem are demonstrated by simulations of multiple cylinders using the Mie theory and confirmed by experimental results of two spheres

A Learning Approach to Optical Tomography

We describe a method for imaging 3D objects in a tomographic configuration implemented by training an artificial neural network to reproduce the complex amplitude of the experimentally measured scattered light. The network is designed such that the voxel values of the refractive index of the 3D object are the variables that are adapted during the training process. We demonstrate the method experimentally by forming images of the 3D refractive index distribution of cells

Heterodyne detection of multiply scattered monochromatic light with a multipixel detector

International audienceA new technique is presented for measuring the spectral broadening of light that has been multiply scattered from scatterers in motion. In our method the scattered light is detected by a heterodyne receiver that uses a CCD as a multipixel detector. We obtain the frequency spectrum of the scattered light by sweeping the heterodyne local oscillator frequency. Our detection scheme combines a high optical etendue (product of the surface by the detection solid angle) with an optimal detection of the scattered photons (shot noise). Using this technique, we measure, in vivo, the frequency spectrum of the light scattered through the breast of a female volunteer

Single spontaneous photon as a coherent beamsplitter for an atomic matterwave

In spontaneous emission an atom in an excited state undergoes a transition to the ground state and emits a single photon. Associated with the emission is a change of the atomic momentum due to photon recoil. Photon emission can be modified close to surfaces and in cavities. For an ion, localized in front of a mirror, coherence of the emitted resonance fluorescence has been reported. In free space experiments demonstrated that spontaneous emission destroys motional coherence. Here we report on motional coherence created by a single spontaneous emission event close to a mirror surface. The coherence in the free atomic motion is verified by atom interferometry. The photon can be regarded as a beamsplitter for an atomic matterwave and consequently our experiment extends the original recoiling slit Gedanken experiment by Einstein to the case where the slit is in a robust coherent superposition of the two recoils associated with the two paths of the quanta.Comment: main text: 5 pages, 4 figure; supplementary information: 8 pages, 1 figur

Nitric oxide and peptide neurohormones activate cGMP synthesis in the crab stomatogastric nervous system

In the neural circuits that comprise the crustacean stomatogastric nervous system (STNS), synaptically delivered neurotransmitters and circulating neurohormones elicit a wide range of rhythmic motor outputs. However, functional roles for second messengers in this system are poorly understood. Here we demonstrate two different signaling pathways that control the synthesis of 3',5'-cGMP in the crab STNS. One pathway is activated by nitric oxide (NO) and is mediated by a cytoplasmic guanylate cyclase. A second pathway is stimulated by peptide-containing extracts from a crab neurohemal organ that activate a membrane-associated guanylate cyclase. Using whole-mount immunocytochemistry to localize individual cGMP-containing cells, we find that NO elevates intracellular cGMP in a small subset of STNS neurons. Immunopositive cells are found predominantly in the stomatogastric ganglion, with a few additional cells located in the oesophageal and commissural ganglia. Crab tissues differ in their sensitivities to NO and to the peptide-containing extract. The NO-mediated pathway is apparently restricted to the nervous system, whereas the peptidemediated pathway is present in every tissue tested. The results of these experiments demonstrate that multiple signaling pathways involving cGMP are present in the STNS and suggest that this second messenger may help control the metabolic and physiological status of these motor circuits

Nitric Oxide Inhibits the Rate and Strength of Cardiac Contractions in the Lobster Homarus americanus by Acting on the Cardiac Ganglion

The lobster heart is synaptically driven by the cardiac ganglion, a spontaneously bursting neural network residing within the cardiac lumen. Here, we present evidence that nitric oxide (NO) plays an inhibitory role in lobster cardiac physiology. (1) NO decreases heartbeat frequency and amplitude. Decreased frequency is a direct consequence of a decreased ganglionic burst rate. Decreased amplitude is an indirect consequence of decreased burst frequency, attributable to the highly facilitating nature of the synapses between cardiac ganglion neurons and muscle fibers (although, during prolonged exposure to NO, amplitude recovers to the original level by a frequency-independent adaptation mechanism). NO does not alter burst duration, spikes per burst, heart muscle contractility, or amplitudes of synaptic potentials evoked by stimulating postganglionic motor nerves. Thus, NO acts on the ganglion, but not on heart muscle. (2) Two observations suggest that NO is produced within the lobster heart. First, immunoblot analysis shows that nitric oxide synthase (NOS) is strongly expressed in heart muscle relative to other muscles. Second

Questioning the rise of gelatinous zooplankton in the World's oceans

During the past several decades, high numbers of gelatinous zooplankton species have been reported in many estuarine and coastal ecosystems. Coupled with media-driven public perception, a paradigm has evolved in which the global ocean ecosystems are thought to be heading toward being dominated by “nuisance” jellyfish. We question this current paradigm by presenting a broad overview of gelatinous zooplankton in a historicalcontext to develop the hypothesis that population changes reflect the human-mediated alteration of global ocean ecosystems. To this end, we synthesize information related to the evolutionary context of contemporary gelatinous zooplankton blooms, the human frame of reference forchanges in gelatinous zooplankton populations, and whether sufficient data are available to have established the paradigm. We conclude that the current paradigm in which it is believed that there has been a global increase in gelatinous zooplankton is unsubstantiated, and we develop a strategy for addressing the critical questions about long-term, human-related changes in the sea as they relate to gelatinous zooplankton blooms