Columbia University

About: Columbia University is a education organization based out in New York, New York, United States. It is known for research contribution in the topics: Population & Poison control. The organization has 95695 authors who have published 224027 publications receiving 12838453 citations. The organization is also known as: Columbia University in the City of New York & King's College of New York.

Is Silence Golden? An Empirical Analysis of Firms that Stop Giving Quarterly Earnings Guidance in the post Regulation-FD period

Abstract: We investigate a sample of 75 firms that publicly renounced quarterly EPS guidance in the post-FD period (10/2000 to 10/2004). We find that stoppers have poor trailing earnings and stock return performance. We document an average -3.8% three-day return around the announcement to stop guidance and such reaction is associated with poor future performance and an increase in systematic risk. After the elimination of guidance, stock prices lead earnings less but there is no change in overall stock return volatility or analyst following. However, analyst forecast dispersion increases and forecast accuracy decreases following firms' decision to stop guiding.

Using Channel Hopping to Increase 802.11 Resilience to Jamming Attacks

Abstract: 802.11a, b, and g standards were designed for deployment in cooperative environments, and hence do not include mechanisms to protect from jamming attacks. In this paper, we explore how to protect 802.11 networks from jamming attacks by having the legitimate transmission hop among channels to hide the transmission from the jammer. Using a combination of mathematical analysis and prototype experimentation in an 802.11a environment, we explore how much throughput can be maintained in comparison to the maintainable throughput in a cooperative, jam-free environment. Our experimental and analytical results show that in today's conventional 802.11a networks, we can achieve up to 60% of the original throughput. Our mathematical analysis allows us to extrapolate the throughput that can be maintained when the constraint on the number of orthogonal channels used for both legitimate communication and for jamming is relaxed.

The formation of massive, compact galaxies at z = 2 in the Illustris simulation

Abstract: Massive, quiescent galaxies at high redshift have been found to be considerably more compact than galaxies of similar mass in the local universe. How these compact galaxies formed has yet to be determined, though several progenitor populations have been proposed. Here we investigate the formation processes and quantify the assembly histories of such galaxies in Illustris, a suite of hydrodynamical cosmological simulations encompassing a sufficiently large volume to include rare objects, while simultaneously resolving the internal structure of galaxies. We select massive ( 10 11 M ) and compact (stellar half-mass radius < 2 kpc) galaxies from the simulation at z = 2. Within the Illustris suite, we find that these quantities are not perfectly converged, but are reasonably reliable for our purposes. The resulting population is composed primarily of quiescent galaxies, but we also find several star-forming compact galaxies. The simulated compact galaxies are similar to observed galaxies in star formation activity and appearance. We follow their evolution at high redshift in the simulation and find that there are multiple pathways to form these compact galaxies, dominated by two mechanisms: (i) intense, centrally concentrated starbursts generally triggered by gas-rich major mergers between z 2 4, reducing the galaxies’ half-mass radii by a factor of a few to below 2 kpc, and (ii) assembly at very early times when the universe was much denser; the galaxies formed compact and remained so until z 2.

Functional Connectome of the Striatal Medium Spiny Neuron

Abstract: Dopamine system disorders ranging from movement disorders to addiction and schizophrenia involve striatal medium spiny neurons (MSNs), yet their functional connectivity has been difficult to determine comprehensively. We generated a mouse with conditional channelrhodopsin-2 expression restricted to medium spiny neurons and assessed the specificity and strength of their intrinsic connections in the striatum and their projections to the globus pallidus and the substantia nigra. In the striatum, medium spiny neurons connected with other MSNs and tonically active cholinergic interneurons, but not with fast-spiking GABA interneurons. In the globus pallidus, medium spiny neurons connected strongly with one class of electrophysiologically identified neurons, but weakly with the other. In the substantia nigra, medium spiny neurons connected strongly with GABA, but not with dopamine neurons. Projections to the globus pallidus showed solely D2-mediated presynaptic inhibition, whereas projections to the substantia nigra showed solely D1-mediated presynaptic facilitation. This optogenetic approach defines the functional connectome of the striatal medium spiny neuron.