1,721,668 research outputs found
Neural correlates of cognitive control of reaching movements in the dorsal premotor cortex of rhesus monkeys
Full text linkMirabella G, Pani P, Ferraina S. Neural correlates of cognitive control of reaching movements in the dorsal premotor cortex of rhesus monkeys. J Neurophysiol 106: 1454-1466, 2011. First published June 22, 2011; doi: 10.1152/jn.00995.2010.-Canceling a pending movement is a hallmark of voluntary behavioral control because it allows us to quickly adapt to unattended changes either in the external environment or in our thoughts. The countermanding paradigm allows the study of inhibitory processes of motor acts by requiring the subject to withhold planned movements in response to an infrequent stop-signal. At present the neural processes underlying the inhibitory control of arm movements are mostly unknown. We recorded the activity of single units in the rostral and caudal portion of the dorsal premotor cortex (PMd) of monkeys trained in a countermanding reaching task. We found that among neurons with a movement-preparatory activity, about one-third exhibit a modulation before the behavioral estimate of the time it takes to cancel a planned movement. Hence these neurons exhibit a pattern of activity suggesting that PMd plays a critical role in the brain networks involved in the control of arm movement initiation and suppression.Mirabella G, Pani P, Ferraina S. Neural correlates of cognitive control of reaching movements in the dorsal premotor cortex of rhesus monkeys. J Neurophysiol 106: 1454-1466, 2011. First published June 22, 2011; doi: 10.1152/jn.00995.2010.-Canceling a pending movement is a hallmark of voluntary behavioral control because it allows us to quickly adapt to unattended changes either in the external environment or in our thoughts. The countermanding paradigm allows the study of inhibitory processes of motor acts by requiring the subject to withhold planned movements in response to an infrequent stop-signal. At present the neural processes underlying the inhibitory control of arm movements are mostly unknown. We recorded the activity of single units in the rostral and caudal portion of the dorsal premotor cortex (PMd) of monkeys trained in a countermanding reaching task. We found that among neurons with a movement-preparatory activity, about one-third exhibit a modulation before the behavioral estimate of the time it takes to cancel a planned movement. Hence these neurons exhibit a pattern of activity suggesting that PMd plays a critical role in the brain networks involved in the control of arm movement initiation and suppression
Axisymmetric deformations of neutron stars and gravitational-wave astronomy
No full textEinstein's theory of general relativity predicts that the only stationary configuration of an isolated black hole is the Kerr spacetime, which has a unique multipolar structure and a spherical shape when nonspinning. This is in striking contrast to the case of other self-gravitating objects, which instead can in principle have arbitrary deformations even in the static case. Here, we develop a general perturbative framework to construct stationary stars with small axisymmetric deformations and study explicitly compact stars with an intrinsic quadrupole moment. The latter can be sustained, for instance, by crust stresses or strong magnetic fields. While our framework is general, we focus on quadrupolar deformations of neutron stars induced by an anisotropic crust, which continuously connect to spherical neutron stars in the isotropic limit. Deformed neutron stars might provide a more accurate description for isolated and binary compact objects and can be used to improve constraints on the neutron-star equation of state through gravitational-wave detections and through the observation of low-mass X-ray binaries. We argue that, for values of the dimensionless intrinsic quadrupole moment of few percent or higher (which can be sustained by an elastic crust with ordinary parameters), the effect of the deformation is stronger than that of tidal interactions in coalescing neutron-star binaries, and might also significantly affect the electromagnetic signal from accreting neutron stars. Current observational bounds on the post-Newtonian coefficients in the gravitational waveform signal from GW170817 do not exclude that the neutron stars in the binary had some significant intrinsic deformation
Proposta didattica per lo studio dell'interazione tra due sfere conduttrici elettricamente cariche
No full textTidal deformability of dressed black holes and tests of ultralight bosons in extended mass ranges
No full textThe deformability of a compact object under the presence of a tidal perturbation is encoded in the tidal Love numbers (TLNs), which vanish for isolated black holes in vacuum. We show that the TLNs of black holes surrounded by matter fields do not vanish and can be used to probe the environment around binary black holes. In particular, we compute the TLNs for the case of a black hole surrounded by a scalar condensate under the presence of scalar and vector tidal perturbations, finding a strong power-law behavior of the TLN in terms of the mass of the scalar field. Using this result as a proxy for gravitational tidal perturbations, we show that future gravitational-wave detectors like the Einstein Telescope and LISA can impose stringent constraints on the mass of ultralight bosons that condensate around black holes due to accretion or superradiance. Interestingly, LISA could measure the tidal deformability of dressed black holes across the range from stellar-mass (≈ 102M ) to supermassive (≈ 107M ) objects, providing a measurement of the mass of ultralight bosons in the range (10-17 - 10-13) eV with less than 10% accuracy, thus filling the gap between other superradiance-driven constraints coming from terrestrial and space interferometers. Altogether, LISA and Einstein Telescope can probe tidal effects from dressed black holes in the combined mass range (10-17 - 10-11) eV
Psychopathological heterogeneity in opium drug addicts | [Eterogeneità psicopatologica nel tossicodipendente da oppiacei]
No full textBlack Hole Superradiant Instability from Ultralight Spin-2 Fields
Full text linkUltralight bosonic fields are compelling dark-matter candidates and arise in a variety of beyond standard model scenarios. These fields can tap energy and angular momentum from spinning black holes through superradiant instabilities, during which a macroscopic bosonic condensate develops around the black hole. Striking features of this phenomenon include gaps in the spin-mass distribution of astrophysical black holes and a continuous gravitational-wave (GW) signal emitted by the condensate. So far these processes have been studied in great detail for scalar fields and, more recently, for vector fields. Here we take an important step forward in the black hole superradiance program by computing, analytically, the instability timescale, direct GW emission, and stochastic background, in the case of massive tensor (i.e., spin-2) fields. Our analysis is valid for any black hole spin and for small boson masses. The instability of massive spin-2 fields shares some properties with the scalar and vector cases, but its phenomenology is much richer, for example, there exist multiple modes with comparable instability timescales, and the dominant GW signal is hexadecapolar rather than quadrupolar. Electromagnetic and GW observations of spinning black holes in the mass range M∈(1,1010) M can constrain the mass of a putative spin-2 field in the range 10-22 mb c2/eV 10-10. For 10-17 mb c2/eV 10-15, the space mission LISA could detect the continuous GW signal for sources at redshift z=20, or even larger
Impact of multiple modes on the black-hole superradiant instability
No full textUltralight bosonic fields in the mass range of approximately (10-20-10-11) eV can trigger a superradiant instability that extracts energy and angular momentum from an astrophysical black hole with mass M∼(5,1010)M, forming a nonspherical, rotating condensate around it. So far, most studies of the evolution and end state of the instability have been limited to initial data containing only the fastest growing superradiant mode. By studying the evolution of multimode data in a quasiadiabatic approximation, we show that the dynamics is much richer and depends strongly on the energy of the seed, on the relative amplitude between modes, and on the gravitational coupling. If the seed energy is a few percent of the black-hole mass, a black hole surrounded by a mixture of superradiant and nonsuperradiant modes with comparable amplitudes might not undergo a superradiant unstable phase, depending on the value of the boson mass. If the seed energy is smaller, as in the case of an instability triggered by quantum fluctuations, the effect of nonsuperradiant modes is negligible. We discuss the implications of these findings for current constraints on ultralight fields with electromagnetic and gravitational-wave observations
Extreme mass ratio inspirals with spinning secondary: A detailed study of equatorial circular motion
No full textExtreme-mass-ratio inspirals (EMRIs) detectable by the future Laser Interferometer Space Antenna provide a unique way to test general relativity and fundamental physics. Motivated by this possibility, here we study in detail the EMRI dynamics in the presence of a spinning secondary, collecting and extending various results that appeared in previous work and also providing useful intermediate steps and new relations for the first time. We present the results of a frequency-domain code that computes gravitational-wave fluxes and the adiabatic orbital evolution for the case of circular, equatorial orbits with (anti)aligned spins. The spin of the secondary starts to affect the gravitational-wave phase at the first post-adiabatic order (as does the first-order conservative self-force) and introduces a detectable dephasing, which can be used to measure it at the 5-25% level, depending on individual spins. In a companion paper we discuss the implication of this effect for tests of the Kerr bound
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