1,180 research outputs found
Neural Coding of Sound Envelope in Reverberant Environments
Speech reception depends critically on temporal modulations in the amplitude envelope of the speech signal. Reverberation encountered in everyday environments can substantially attenuate these modulations. To assess the effect of reverberation on the neural coding of amplitude envelope, we recorded from single units in the inferior colliculus (IC) of unanesthetized rabbit using sinusoidally amplitude modulated (AM) broadband noise stimuli presented in simulated anechoic and reverberant environments. Although reverberation degraded both rate and temporal coding of AM in IC neurons, in most neurons, the degradation in temporal coding was smaller than the AM attenuation in the stimulus. This compensation could largely be accounted for by the compressive shape of the modulation input–output function (MIOF), which describes the nonlinear transformation of modulation depth from acoustic stimuli into neural responses. Additionally, in a subset of neurons, the temporal coding of AM was better for reverberant stimuli than for anechoic stimuli having the same modulation depth at the ear. Using hybrid anechoic stimuli that selectively possess certain properties of reverberant sounds, we show that this reverberant advantage is not caused by envelope distortion, static interaural decorrelation, or spectral coloration. Overall, our results suggest that the auditory system may possess dual mechanisms that make the coding of amplitude envelope relatively robust in reverberation: one general mechanism operating for all stimuli with small modulation depths, and another mechanism dependent on very specific properties of reverberant stimuli, possibly the periodic fluctuations in interaural correlation at the modulation frequency.National Institutes of Health (U.S.) (Grant R01DC002258)National Institutes of Health (U.S.) (Grant P30DC0005209)Paul and Daisy Soros Fellowships for New American
Direct Measurement of intermediate-range Casimir-Polder potentials
We present the first direct measurements of Casimir-Polder forces between
solid surfaces and atomic gases in the transition regime between the
electrostatic short-distance and the retarded long-distance limit. The
experimental method is based on ultracold ground-state Rb atoms that are
reflected from evanescent wave barriers at the surface of a dielectric glass
prism. Our novel approach does not require assumptions about the potential
shape. The experimental data confirm the theoretical prediction in the
transition regime.Comment: 4 pages, 3 figure
Phase-sensitive detection of Bragg scattering at 1D optical lattices
We report on the observation of Bragg scattering at 1D atomic lattices. Cold
atoms are confined by optical dipole forces at the antinodes of a standing wave
generated by the two counter-propagating modes of a laser-driven high-finesse
ring cavity. By heterodyning the Bragg-scattered light with a reference beam,
we obtain detailed information on phase shifts imparted by the Bragg scattering
process. Being deep in the Lamb-Dicke regime, the scattered light is not
broadened by the motion of individual atoms. In contrast, we have detected
signatures of global translatory motion of the atomic grating.Comment: 4 pages, 4 figure
The role of Mie scattering in the seeding of matter-wave superradiance
Matter-wave superradiance is based on the interplay between ultracold atoms
coherently organized in momentum space and a backscattered wave. Here, we show
that this mechanism may be triggered by Mie scattering from the atomic cloud.
We show how the laser light populates the modes of the cloud, and thus imprints
a phase gradient on the excited atomic dipoles. The interference with the atoms
in the ground state results in a grating, that in turn generates coherent
emission, contributing to the backward light wave onset. The atomic recoil
'halos' created by the scattered light exhibit a strong anisotropy, in contrast
to single-atom scattering
Multiple Reflections and Diffuse Scattering in Bragg Scattering at Optical Lattices
We study Bragg scattering at 1D atomic lattices. Cold atoms are confined by
optical dipole forces at the antinodes of a standing wave generated inside a
laser-driven cavity. The atoms arrange themselves into an array of lens-shaped
layers located at the antinodes of the standing wave. Light incident on this
array at a well-defined angle is partially Bragg-reflected. We measure
reflectivities as high as 30%. In contrast to a previous experiment devoted to
the thin grating limit [S. Slama, et al., Phys. Rev. Lett. 94, 193901 (2005)]
we now investigate the thick grating limit characterized by multiple
reflections of the light beam between the atomic layers. In principle multiple
reflections give rise to a photonic stop band, which manifests itself in the
Bragg diffraction spectra as asymmetries and minima due to destructive
interference between different reflection paths. We show that close to
resonance however disorder favors diffuse scattering, hinders coherent multiple
scattering and impedes the characteristic suppression of spontaneous emission
inside a photonic band gap
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