3,807 research outputs found
Disproportionate Frequency Representation in the Inferior Colliculus of Doppler-Compensating Greater Horseshoe Bats. Evidence for an Acoustic Fovea
1. The inferior colliculus of 8 Greater Horseshoe bats (Rhinolophus ferrumequinun) was systematically sampled with electrode penetrations covering the entire volume of the nucleus. The best frequencies and intensity thresholds for pure tones (Fig. 2) were determined for 591 neurons. The locations of the electrode penetrations within the inferior colliculus were histologically verified.
2. About 50% of all neurons encountered had best frequencies (BF) in the frequency range between 78 and 88 kHz (Table 1, Fig. 1A). Within this frequency range the BFs between 83.0 and 84.5 kHz were overrepresented with 16.3% of the total population of neurons (Fig. 1B). The frequencies of the constant frequency components of the echoes fall into this frequency range.
3. The representation of BFs expressed as number of neurons per octave shows a striking correspondence to the nonuniform innervation density in the afferent innervation of the basilar membrane (Bruns and Schmieszek, in press). The high innervation density of the basilar membrane in the frequency band between 83 and 84.5 kHz coincides with the maximum of the distribution of number of neurons per octave across frequency in the inferior colliculus (Fig. 1 C).
4. The disproportionate representation of frequencies in the auditory system of the greater horseshoe bat is described as an acoustical fovea functioning in analogy to the fovea in the visual system. The functional importance of the Doppler-shift compensation for such a foveal mechanism in the auditory system of horseshoe bats is related to that of tracking eye movements in the visual system
The glass transition and the Coulomb gap in electron glasses
We establish the connection between the presence of a glass phase and the
appearance of a Coulomb gap in disordered materials with strongly interacting
electrons. Treating multiparticle correlations in a systematic way, we show
that in the case of strong disorder a continuous glass transition takes place
whose Landau expansion is identical to that of the Sherrington-Kirkpatrick spin
glass. We show that the marginal stability of the glass phase controls the
physics of these systems: it results in slow dynamics and leads to the
formation of a Coulomb gap
Electroreflectance spectroscopy in self-assembled quantum dots: lens symmetry
Modulated electroreflectance spectroscopy of semiconductor
self-assembled quantum dots is investigated. The structure is modeled as dots
with lens shape geometry and circular cross section. A microscopic description
of the electroreflectance spectrum and optical response in terms of an external
electric field () and lens geometry have been considered. The field
and lens symmetry dependence of all experimental parameters involved in the
spectrum have been considered. Using the effective mass formalism
the energies and the electronic states as a function of and dot
parameters are calculated. Also, in the framework of the strongly confined
regime general expressions for the excitonic binding energies are reported.
Optical selection rules are derived in the cases of the light wave vector
perpendicular and parallel to . Detailed calculation of the Seraphin
coefficients and electroreflectance spectrum are performed for the InAs and
CdSe nanostructures. Calculations show good agreement with measurements
recently performed on CdSe/ZnSe when statistical distribution on size is
considered, explaining the main observed characteristic in the
electroreflectance spectra
Neural delays shape selectivity to interaural intensity differences in the lateral superior olive
Neurons in the lateral superior olive (LSO) respond selectively to interaural intensity differences (IIDs), one of the chief cues used to localize sounds in space. LSO cells are innervated in a characteristic pattern: they receive an excitatory input from the ipsilateral ear and an inhibitory input from the contralateral ear. Consistent with this pattern, LSO cells generally are excited by sounds that are more intense at the ipsilateral ear and inhibited by sounds that are more intense at the contralateral ear. Despite their relatively homogeneous pattern of innervation, IID selectivity varies substantially from cell to cell, such that selectivities are distributed over the range of IIDs that would be encountered in nature. For some time, researchers have speculated that the relative timing of the excitatory and inhibitory inputs to an LSO cell might shape IID selectivity. To test this hypothesis, we recorded from 50 LSO cells in the free-tailed bat while presenting stimuli that varied in interaural intensity and in interaural time of arrival. The results suggest that, for more than half of the cells, the latency of inhibition was several hundred microseconds longer than the latency of excitation. Increasing the intensity to the inhibitory ear shortened the latency of inhibition and brought the timing of the inputs from the two ears into register. Thus, a neural delay of the inhibition helped to define the IID selectivity of these cells, accounting for a significant part of the variation in selectivity among LSO cells
Near-Field Microwave Microscopy on nanometer length scales
The Near-Field Microwave Microscope (NSMM) can be used to measure ohmic
losses of metallic thin films. We report on the presence of a new length scale
in the probe-to- sample interaction for the NSMM. We observe that this length
scale plays an important role when the tip to sample separation is less than
about 10nm. Its origin can be modeled as a tiny protrusion at the end of the
tip. The protrusion causes deviation from a logarithmic increase of capacitance
versus decreasing height of the probe above the sample. We model this
protrusion as a cone at the end of a sphere above an infinite plane. By fitting
the frequency shift of the resonator versus height data (which is directly
related to capacitance versus height) for our experimental setup, we find the
protrusion size to be 3nm to 5nm. For one particular tip, the frequency shift
of the NSMM relative to 2 micrometers away saturates at a value of about -1150
kHz at a height of 1nm above the sample, where the nominal range of sheet
resistance values of the sample are 15 ohms to 150 ohms. Without the
protrusion, the frequency shift would have followed the logarithmic dependence
and reached a value of about -1500 kHz.Comment: 6 pages, 7 figures (included in 6 pages
Persistent Homology in Sparse Regression and its Application to Brain Morphometry
Sparse systems are usually parameterized by a tuning parameter that
determines the sparsity of the system. How to choose the right tuning parameter
is a fundamental and difficult problem in learning the sparse system. In this
paper, by treating the the tuning parameter as an additional dimension,
persistent homological structures over the parameter space is introduced and
explored. The structures are then further exploited in speeding up the
computation using the proposed soft-thresholding technique. The topological
structures are further used as multivariate features in the tensor-based
morphometry (TBM) in characterizing white matter alterations in children who
have experienced severe early life stress and maltreatment. These analyses
reveal that stress-exposed children exhibit more diffuse anatomical
organization across the whole white matter region.Comment: submitted to IEEE Transactions on Medical Imagin
Electron-spectroscopic investigation of metal-insulator transition in Sr2Ru1-xTixO4 (x=0.0-0.6)
We investigate the nature and origin of the metal-insulator transition in
Sr2Ru1-xTixO4 as a function of increasing Ti content (x). Employing detailed
core, valence, and conduction band studies with x-ray and ultraviolet
photoelectron spectroscopies along with Bremsstrahlung isochromat spectroscopy,
it is shown that a hard gap opens up for Ti content greater than equal to 0.2,
while compositions with x<0.2 exhibit finite intensity at the Fermi energy.
This establishes that the metal-insulator transition in this homovalent
substituted series of compounds is driven by Coulomb interaction leading to the
formation of a Mott gap, in contrast to transitions driven by disorder effects
or band flling.Comment: Accepted for publication in Phys. Rev.
Unequal Bequests
Using data from the Health and Retirement Study (HRS), we make two contributions to the literature on end-of-life transfers. First, we show that unequal bequests are much more common than generally recognised, with one-third of parents with wills planning to divide their estates unequally among their children. These plans for unequal division are particularly concentrated in complex families, that is, families with stepchildren and families with genetic children with whom the parent has had no contact (e.g., children from previous marriages). We find that in complex families past and current contact between parents and children reduces or eliminates unequal bequests. Second, although the literature focuses on the bequest intentions of parents who have made wills, we find that many elderly Americans have not made wills. Although the probability of having a will increases with age, 30 percent of HRS respondents aged 70 and over have no wills. Of HRS respondents who died between 1995 and 2010, 38 percent died intestate (i.e., without wills). Thus, focusing exlusively on the bequest intentions of parents who have made wills provides an incomplete and misleading picture of end-of-life transfers
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