814 research outputs found
Two brains in action: joint-action coding in the primate frontal cortex
Daily life often requires the coordination of our actions with those of another partner. After sixty years (1968-2018) of behavioral neurophysiology of motor control, the neural mechanisms which allow such coordination in primates are unknown. We studied this issue by recording cell activity simultaneously from dorsal premotor cortex (PMd) of two male interacting monkeys trained to coordinate their hand forces to achieve a common goal. We found a population of 'joint-action cells' that discharged preferentially when monkeys cooperated in the task. This modulation was predictive in nature, since in most cells neural activity led in time the changes of the "own" and of the "other" behavior. These neurons encoded the joint-performance more accurately than 'canonical action-related cells', activated by the action per se, regardless of the individual vs. interactive context. A decoding of joint-action was obtained by combining the two brains activities, using cells with directional properties distinguished from those associated to the 'solo' behaviors. Action observation-related activity studied when one monkey observed the consequences of the partner's behavior, i.e. the cursor's motion on the screen, did not sharpen the accuracy of 'joint-action cells' representation, suggesting that it plays no major role in encoding joint-action. When monkeys performed with a non-interactive partner, such as a computer, 'joint-action cells' representation of the "other" (non-cooperative) behavior was significantly degraded. These findings provide evidence of how premotor neurons integrate the time-varying representation of the self-action with that of a co-actor, thus offering a neural substrate for successful visuo-motor coordination between individuals.SIGNIFICANT STATEMENTThe neural bases of inter-subject motor coordination were studied by recording cell activity simultaneously from the frontal cortex of two interacting monkeys, trained to coordinate their hand forces to achieve a common goal. We found a new class of cells, preferentially active when the monkeys cooperated, rather than when the same action was performed individually. These 'joint-action neurons' offered a neural representation of joint-behaviors by far more accurate than that provided by the canonical action-related cells, modulated by the action per se regardless of the individual/interactive context. A neural representation of joint-performance was obtained by combining the activity recorded from the two brains. Our findings offer the first evidence concerning neural mechanisms subtending interactive visuo-motor coordination between co-acting agents
The complex hodological architecture of the macaque dorsal intraparietal areas as emerging from neural tracers and dw-mri tractography
In macaque monkeys, dorsal intraparietal areas are involved in several daily visuomotor actions. However, their border and sources of cortical afferents remain loosely defined. Combining retrograde histologic tracing and MRI diffusion-based tractography, we found a complex hodology of the dorsal bank of the intraparietal sulcus (db-IPS), which can be subdivided into a rostral intraparietal area PEip, projecting to the spinal cord, and a caudal medial intraparietal area MIP lacking such projections. Both include an anterior and a posterior sector, emerging from their ipsilateral, gradient-like connectivity profiles. As tractography estimations, we used the cross-sectional area of the white matter bundles connecting each area with other parietal and frontal regions, after selecting regions of interest (ROIs) corresponding to the injection sites of neural tracers. For most connections, we found a significant correlation between the proportions of cells projecting to all sectors of PEip and MIP along the continuum of the db-IPS and tractography. The latter also revealed âfalse positiveâ but plausible connections awaiting histologic validation
The complex hodological architecture of the macaque dorsal intraparietal areas as emerging from neural tracers and DW-MRI tractography
In macaque monkeys, dorsal intraparietal areas are involved in several daily visuo-motor actions. However, their border and sources of cortical afferents remain loosely defined. Combining retrograde histological tracing and MRI diffusion-based tractography we found a complex hodology of the dorsal bank of the IPS, which can be subdivided into a rostral area PEip, projecting to the spinal cord, and a caudal area MIP lacking such projections. Both include a rostral and a caudal sector, emerging from their ipsilateral, gradient-like connectivity profiles. As tractography estimations, we used the cross-sectional volume of the white matter bundles connecting each area with other parietal and frontal regions, after selecting ROIs corresponding to the injection sites of neural tracers. For most connections, we found a significant correlation between the proportions of cells projecting to all sectors of PEip and MIP along the continuum of the dorsal bank of the IPS and tractography. The latter also revealed âfalse positiveâ but plausible streamlines awaiting histological validation
The ACS LCID Project: On the origin of dwarf galaxy types: a manifestation of the halo assembly bias?
We discuss how knowledge of the whole evolutionary history of dwarf galaxies,
including details on the early star formation events, can provide insight on
the origin of the different dwarf galaxy types. We suggest that these types may
be imprinted by the early conditions of formation rather than being only the
result of a recent morphological transformation driven by environmental
effects. We present precise star formation histories of a sample of Local Group
dwarf galaxies, derived from colour-magnitude diagrams reaching the oldest
main-sequence turnoffs. We argue that these galaxies can be assigned to two
basic types: fast dwarfs that started their evolution with a dominant and short
star formation event, and slow dwarfs that formed a small fraction of their
stars early and have continued forming stars until the present time (or
almost). These two different evolutionary paths do not map directly onto the
present-day morphology (dwarf spheroidal vs dwarf irregular). Slow and fast
dwarfs also differ in their inferred past location relative to the Milky Way
and/or M31, which hints that slow dwarfs were generally assembled in lower
density environments than fast dwarfs. We propose that the distinction between
a fast and slow dwarf galaxy reflects primarily the characteristic density of
the environment where they form. At a later stage, interaction with a large
host galaxy may play a role in the final gas removal and ultimate termination
of star formation.Comment: 7 pages, 3 figures, ApJ Letters, submitted. Comments welcom
The orientation and kinematics of inner tidal tails around dwarf galaxies orbiting the Milky Way
Using high-resolution collisionless N-body simulations we study the
properties of tidal tails formed in the immediate vicinity of a two-component
dwarf galaxy evolving in a static potential of the Milky Way (MW). The stellar
component of the dwarf is initially in the form of a disk and the galaxy is
placed on an eccentric orbit motivated by CDM-based cosmological simulations.
We measure the orientation, density and velocity distribution of the stars in
the tails. Due to the geometry of the orbit, in the vicinity of the dwarf,
where the tails are densest and therefore most likely to be detectable, they
are typically oriented towards the MW and not along the orbit. We report on an
interesting phenomenon of `tidal tail flipping': on the way from the pericentre
to the apocentre the old tails following the orbit are dissolved and new ones
pointing towards the MW are formed over a short timescale. We also find a tight
linear relation between the velocity of stars in the tidal tails and their
distance from the dwarf. Using mock data sets we demonstrate that if dwarf
spheroidal (dSph) galaxies in the vicinity of the MW are tidally affected their
kinematic samples are very likely contaminated by tidally stripped stars which
tend to artificially inflate the measured velocity dispersion. The effect is
stronger for dwarfs on their way from the peri- to the apocentre due to the
formation of new tidal tails after pericentre. Realistic mass estimates of dSph
galaxies thus require removal of these stars from kinematic samples.Comment: 8 pages, 7 figures, accepted for publication in MNRA
The kinematic status and mass content of the Sculptor dwarf spheroidal galaxy
We present VLT/FLAMES spectroscopic observations (R~6500) in the CaII triplet
region for 470 probable kinematic members of the Sculptor (Scl) dwarf
spheroidal galaxy. The accurate velocities (+- 2 km/s) and large area coverage
of Scl allow us to measure a velocity gradient of 7.6_{-2.2}^{+3.0} km/s
deg^{-1} along the Scl projected major axis, likely a signature of intrinsic
rotation. We also use our kinematic data to measure the mass distribution
within this system. By considering independently the kinematics of the distinct
stellar components known to be present in Scl, we are able to relieve known
degeneracies, and find that the observed velocity dispersion profiles are best
fitted by a cored dark matter halo with core radius r_c= 0.5 kpc and mass
enclosed within the last measured point M(< 1.8 kpc)=3.4 +- 0.7 x 10^8 M_sun,
assuming an increasingly radially anisotropic velocity ellipsoid. This results
in a mass-to-light ratio of 158+-33 (M/L)_sun inside 1.8 kpc. An NFW profile
with concentration c=20 and mass M(< 1.8 kpc) = 2.2_{-0.7}^{+1.0} x 10^8 M_sun
is also statistically consistent with the observations, but it tends to yield
poorer fits for the metal rich stars.Comment: 4 pages, 3 figures. Accepted for publication in ApJL. Some revision
to the text after iteration with refere
Reaching activity in parietal area V6A of macaque: eye influence on arm activity or retinocentric coding of reaching movements?
Parietal area V6A contains neurons modulated by the direction of gaze as well as neurons able to code the direction of arm movement. The present study was aimed to disentangle the gaze effect from the effect of reaching activity upon single V6A neurons. To this purpose, we used a visuomotor task in which the direction of arm movement remained constant while the animal changed the direction of gaze. Gaze direction modulated reach-related activity in about two-thirds of tested neurons. In several cases, modulations were not due to the eye-position signal per se, the apparent eye-position modulation being just an epiphenomenon. The real modulating factor was the location of reaching target with respect to the point gazed by the animal, that is, the retinotopic coordinates towards which the action of reaching occurred. Comparison of neural discharge of the same cell during execution of foveated and non-foveated reaching movements, performed towards the same or different spatial locations, confirmed that in a part of V6A neurons reaching activity is coded retinocentrically. In other neurons, reaching activity is coded spatially, depending on the direction of reaching movement regardless of where the animal was looking at. The majority of V6A reaching neurons use a system that encompasses both of these reference frames. These results are in line with the view of a progressive visuomotor transformation in the dorsal visual stream, that changes the frame of reference from the retinocentric one, typically used by the visual system, to the arm-centred one, typically used by the motor system
The Dynamical and Chemical Evolution of Dwarf Spheroidal Galaxies
We present a large sample of fully self-consistent hydrodynamical
Nbody/Tree-SPH simulations of isolated dwarf spheroidal galaxies (dSphs). It
has enabled us to identify the key physical parameters and mechanisms at the
origin of the observed variety in the Local Group dSph properties. The initial
total mass (gas + dark matter) of these galaxies is the main driver of their
evolution. Star formation (SF) occurs in series of short bursts. In massive
systems, the very short intervals between the SF peaks mimic a continuous star
formation rate, while less massive systems exhibit well separated SF bursts, as
identified observationally. The delay between the SF events is controlled by
the gas cooling time dependence on galaxy mass. The observed global scaling
relations, luminosity-mass and luminosity-metallicity, are reproduced with low
scatter. We take advantage of the unprecedentedly large sample size and data
homogeneity of the ESO Large Programme DART, and add to it a few independent
studies, to constrain the star formation history of five Milky Way dSphs,
Sextans, LeoII, Carina, Sculptor and Fornax. For the first time, [Mg/Fe] vs
[Fe/H] diagrams derived from high-resolution spectroscopy of hundreds of
individual stars are confronted with model predictions. We find that the
diversity in dSph properties may well result from intrinsic evolution. We note,
however, that the presence of gas in the final state of our simulations, of the
order of what is observed in dwarf irregulars, calls for removal by external
processes.Comment: 21 Pages, 19 figures ; Accepted for publication in A&A. Higher
resolution version may be downloaded here :
http://obswww.unige.ch/~revaz/publications/aa2009_1173
Simulations of the formation and evolution of isolated dwarf galaxies - II. Angular momentum as a second parameter
We show results based on a large suite of N-Body/SPH simulations of isolated,
flat dwarf galaxies, both rotating and non-rotating. The main goal is to
investigate possible mechanisms to explain the observed dichotomy in radial
stellar metallicity profiles of dwarf galaxies: dwarf irregulars (dIrr) and
flat, rotating dwarf ellipticals (dE) generally possess flat metallicity
profiles, while rounder and non-rotating dEs show strong negative metallicity
gradients. These simulations show that flattening by rotation is key to
reproducing the observed characteristics of flat dwarf galaxies, proving
particularly efficient in erasing metallicity gradients. We propose a
"centrifugal barrier mechanism" as an alternative to the previously suggested
"fountain mechanism" for explaining the flat metallicity profiles of dIrrs and
flat, rotating dEs. While only flattening the dark-matter halo has little
influence, the addition of angular momentum slows down the infall of gas, so
that star formation (SF) and the ensuing feedback are less centrally
concentrated, occurring galaxy-wide. Additionally, this leads to more
continuous SFHs by preventing large-scale oscillations in the SFR
("breathing"), and creates low density holes in the ISM, in agreement with
observations of dIrrs. Our general conclusion is that rotation has a
significant influence on the evolution and appearance of dwarf galaxies, and we
suggest angular momentum as a second parameter (after galaxy mass as the
dominant parameter) in dwarf galaxy evolution. Angular momentum differentiates
between SF modes, making our fast rotating models qualitatively resemble dIrrs,
which does not seem possible without rotation.Comment: Accepted for publication in MNRAS | 19 pages, 20 figures | extra
online content available (animations) : on the publisher's website / on the
YouTube channel for the astronomy department of the University of Ghent :
http://www.youtube.com/user/AstroUGent / YouTube playlist specifically for
this article :
http://www.youtube.com/user/AstroUGent#grid/user/EFAA5AAE5C5E474
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