1,038 research outputs found
The Effect of Access Regulation on Broadbnd Deployment
This paper quetions whether or not mandatory unbundling delayed DSL deployment by BellSouth. By exploiting a law change in Kentucky and variation in access prices across markets, I find that deregulation in Kentucky triggered deployment, but no evidence that access prices affected deployment pace. An upper bound on the welfare loss due to late deployment in rural markets is $21 million. The findings are consistent with a positive effect of deregulation on the profitability of upgrades, but can also result from a strategic delay which is part of the bargaining between the firm and the regulator.Access regulation; Broadband; Hazard model; Technology adoption
Cutoff on Graphs and the Sarnak-Xue Density of Eigenvalues
It was recently shown by Lubetzky and Peres (2016) and by Sardari (2018) that
Ramanujan graphs, i.e., graphs with the optimal spectrum, exhibit cutoff of the
simple random walk in optimal time and have optimal almost-diameter. We prove
that this spectral condition can be replaced by a weaker condition, the
Sarnak-Xue density of eigenvalues property, to deduce similar results.
We show that a family of Schreier graphs of the
-action on the projective line satisfies the
Sarnak-Xue density condition, and hence exhibit the desired properties. To the
best of our knowledge, this is the first known example of optimal cutoff and
almost-diameter on an explicit family of graphs that are neither random nor
Ramanujan
Development of Auditory Selective Attention: Why Children Struggle to Hear in Noisy Environments
Childrenās hearing deteriorates markedly in the presence of unpredictable noise. To explore why, 187 school-age children (4ā11 years) and 15 adults performed a tone-in-noise detection task, in which the masking noise varied randomly between every presentation. Selective attention was evaluated by measuring the degree to which listeners were influenced by (i.e., gave weight to) each spectral region of the stimulus. Psychometric fits were also used to estimate levels of internal noise and bias. Levels of masking were found to decrease with age, becoming adult-like by 9ā11 years. This change was explained by improvements in selective attention alone, with older listeners better able to ignore noise similar in frequency to the target. Consistent with this, age-related differences in masking were abolished when the noise was made more distant in frequency to the target. This work offers novel evidence that improvements in selective attention are critical for the normal development of auditory judgments
Frequency-Domain Coherent Control of Femtosecond Two-Photon Absorption: Intermediate-Field vs. Weak-Field Regime
Coherent control of femtosecond two-photon absorption in the
intermediate-field regime is analyzed in detail in the powerful frequency
domain using an extended 4th-order perturbative description. The corresponding
absorption is coherently induced by the weak-field non-resonant two-photon
transitions as well as by four-photon transitions involving three absorbed
photons and one emitted photons. The interferences between these two groups of
transitions lead to a difference between the intermediate-field and weak-field
absorption dynamics. The corresponding interference nature (constructive or
destructive) strongly depends on the detuning direction of the pulse spectrum
from half the two-photon transition frequency. The model system of the study is
atomic sodium, for which both experimental and theoretical results are
obtained. The detailed understanding obtained here serves as a basis for
coherent control with rationally-shaped femtosecond pulses in a regime of
sizable absorption yields.Comment: 25 pages, 5 figure
Multi-Channel Selective Femtosecond Coherent Control Based on Symmetry Properties
We present and implement a new scheme for extended multi-channel selective
femtosecond coherent control based on symmetry properties of the excitation
channels. Here, an atomic non-resonant two-photon absorption channel is
coherently incorporated in a resonance-mediated (2+1) three-photon absorption
channel. By proper pulse shaping, utilizing the invariance of the two-photon
absorption to specific phase transformations of the pulse, the three-photon
absorption is tuned independently over order-of-magnitude yield range for any
possible two-photon absorption yield. Noticeable is a set of two-photon dark
pulses inducing widely-tunable three-photon absorption
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Reduction of internal noise in auditory perceptual learning
This paper examines what mechanisms underlie auditory perceptual learning. Fifteen normal hearing adults performed two-alternative, forced choice, pure tone frequency discrimination for four sessions. External variability was introduced by adding a zero-mean Gaussian random variable to the frequency of each tone. Measures of internal noise, encoding efficiency, bias, and inattentiveness were derived using four methods (model fit, classification boundary, psychometric function, and double-pass consistency). The four methods gave convergent estimates of internal noise, which was found to decrease from 4.52 to 2.93āHz with practice. No group-mean changes in encoding efficiency, bias, or inattentiveness were observed. It is concluded that learned improvements in frequency discrimination primarily reflect a reduction in internal noise. Data from highly experienced listeners and neural networks performing the same task are also reported. These results also indicated that auditory learning represents internal noise reduction, potentially through the re-weighting of frequency-specific channels
The Role of Response Bias in Perceptual Learning
Sensory judgments improve with practice. Such perceptual learning is often thought to reflect an increase in perceptual sensitivity. However, it may also represent a decrease in response bias, with unpracticed observers acting in part on a priori hunches rather than sensory evidence. To examine whether this is the case, 55 observers practiced making a basic auditory judgment (yes/no amplitude-modulation detection or forced-choice frequency/amplitude discrimination) over multiple days. With all tasks, bias was present initially, but decreased with practice. Notably, this was the case even on supposedly ābias-free,ā 2-alternative forced-choice, tasks. In those tasks, observers did not favor the same response throughout (stationary bias), but did favor whichever response had been correct on previous trials (nonstationary bias). Means of correcting for bias are described. When applied, these showed that at least 13% of perceptual learning on a forced-choice task was due to reduction in bias. In other situations, changes in bias were shown to obscure the true extent of learning, with changes in estimated sensitivity increasing once bias was corrected for. The possible causes of bias and the implications for our understanding of perceptual learning are discussed
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