Measuring cosmic microwave background radiation anisotropy on medium angular scales

Thesis (Ph. D.)--Massachusetts Institute of Technology, Dept. of Physics, 1995.Includes bibliographical references (leaves 105-107).by Jason L. Puchalla.Ph.D

Observations of the anisotropy in the cosmic microwave background by the FIRS, SK93, and MSAM-I experiments

The observations and results from the FIRS, SK93, and MSAM-1, experiments are discussed. These experiments search for anisotropy in the cosmic microwave background over a range in angular scale from 180 deg to 0.5 deg and a range in frequency from 26 to 680 GHz. Emphasis is placed on the observing strategy and potential systematic errors. Contamination of the data by galactic sources is addressed. Future directions are indicated. The results for all three experiments, as found by us and others, are given in the context of the standard CDM model, Q(sub CDM), and the model-independent band-power estimates

Selectable light-sheet uniformity using tuned axial scanning

Light-sheet fluorescence microscopy (LSFM) is an optical sectioning technique capable of rapid three-dimensional (3D) imaging of a wide range of specimens with reduced phototoxicity and superior background rejection. However, traditional light-sheet generation approaches based on elliptical or circular Gaussian beams suffer an inherent trade-off between light-sheet thickness and area over which this thickness is preserved. Recently, an increase in light-sheet uniformity was demonstrated using rapid biaxial Gaussian beam scanning along the lateral and beam propagation directions. Here we apply a similar scanning concept to an elliptical beam generated by a cylindrical lens. In this case, only z-scanning of the elliptical beam is required and hence experimental implementation of the setup can be simplified. We introduce a simple dimensionless uniformity statistic to better characterize scanned light-sheets and experimentally demonstrate custom tailored uniformities up to a factor of 5 higher than those of un-scanned elliptical beams. This technique offers a straightforward way to generate and characterize a custom illumination profile that provides enhanced utilization of the detector dynamic range and field of view, opening the door to faster and more efficient 2D and 3D imaging

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Light-sheet fluorescence microscopy (LSFM) is an optical sectioning technique capable of rapid three-dimensional (3D) imaging of a wide range of specimens with reduced phototoxicity and superior background rejection. However, traditional light-sheet generation approaches based on elliptical or circular Gaussian beams suffer an inherent trade-off between light-sheet thickness and area over which this thickness is preserved. Recently, an increase in light-sheet uniformity was demonstrated using rapid biaxial Gaussian beam scanning along the lateral and beam propagation directions. Here we apply a similar scanning concept to an elliptical beam generated by a cylindrical lens. In this case, only z-scanning of the elliptical beam is required and hence experimental implementation of the setup can be simplified. We introduce a simple dimensionless uniformity statistic to better characterize scanned light-sheets and experimentally demonstrate custom tailored uniformities up to a factor of 5 higher than those of un-scanned elliptical beams. This technique offers a straightforward way to generate and characterize a custom illumination profile that provides enhanced utilization of the detector dynamic range and field of view, opening the door to faster and more efficient 2D and 3D imaging

Recording spikes from a large fraction of the ganglion cells in a retinal patch. Nat Neurosci.

Simultaneous recording from most neurons in a neural circuit has not been accomplished anywhere in the vertebrate brain. The retina is promising for such a systematic study, because its modular organization implies that recording from a small patch of ganglion cells should sample its full functional diversity 1 . Although multi-electrode array technology was developed almost 10 years ago 2 , so far we can record only from small fractions of ganglion cells over the array: about 15% of cells in salamander 2,3 and in rabbit 4 . The limitation is not in recording signals from many ganglion cells, but rather in sorting the signals into spike trains from individual neurons. Despite considerable interest in algorithms designed to improve spike sorting, no general solution has emerged Here we report the development of a new method of multielectrode recording and spike sorting that uses a dense array and combines signals from up to 30 electrodes to sort spikes; this method can be thought of as a generalization of tetrode recording 8 . We first find the average voltage pattern on the array when a ganglion cell fires a spike and then use an iterative algorithm to match multiple spike patterns to the raw data. Because every ganglion cell occupies a unique position in space, and because extracellular signals decay rapidly with distance, each ganglion cell produces a unique pattern of activity on the dense array. This unique pattern can be used to identify the source of overlapping spikes, which might appear ambiguous if we were to use only a single electrode. By retrograde labeling of the ganglion cells, we can compare the number of cells over the array to the number of isolated, single-unit spike trains. This comparison shows that all or nearly all of the retinal ganglion cells in a patch of the retina are recorded. This technical advance promises to yield insights into how populations of ganglion cells encode visual stimuli and how the retinal circuitry processes its visual inputs. RESULTS Determination of array spacing Electrical activity was recorded by placing the ganglion cell layer against a planar multi-electrode array of either hexagonal The cell density, as determined both by retrograde labeling and by electron microscopy of the optic nerve (Methods), was moderate (∼1,400 cells/mm 2 ) and roughly uniform across the retina. At this density, the average spacing between ganglion cells was about 27 µm, which meant that the array should have an electrode near to every ganglion cell. We measured the amplitude of the voltage deflection when ganglion cells fired an action potential and found that this amplitude could be fit by an exponential function of the distance from the primary electrode Identification of templates The first step in spike sorting is to find the typical voltage waveform observed on the array when each ganglion cell fires a spike, a pattern that is called the 'spike template' . We found the times when each ganglion cell fired its spike in isolation from the spikes and slow potentials fired from other neurons. The template was an average over all these 'clean' spike waveforms. Although it was easy to identify isolated spikes, they usually comprised a small subset of all of the spikes produced by a ganglion cell, and thus their identification was not by itself sufficient for spike sorting. Specifically, we started by finding all the times in the raw data that contained a possible spike, which we defined as a voltag