Top-down modulation underlies our ability to selectively attend to relevant stimuli and to ignore irrelevant stimuli. Theories addressing neural mechanisms of top-down modulation are driven by ...studies that reveal increased magnitude of neural activity in response to directed attention, but are limited by a lack of data reporting modulation of neural processing speed, as well as comparisons with a perceptual (passive view) baseline necessary to evaluate the presence of enhancement and suppression. Utilizing functional MRI (fMRI) and event-related potential recordings (ERPs), we provide converging evidence that both the
of neural activity and the
of neural processing are modulated by top-down influences. Furthermore, both
and
occur relative to a perceptual baseline depending on task instruction. These findings reveal the fine degree of influence that goal-directed attention exerts upon activity within the visual association cortex. We further document capacity limitations in top-down enhancement corresponding with working memory performance deficits.
Celotno besedilo
Dostopno za:
DOBA, IZUM, KILJ, NUK, PILJ, PNG, SAZU, SIK, UILJ, UKNU, UL, UM, UPUK
Cognitive control permits us to make decisions about abstract actions, such as whether to e-mail versus call a friend, and to select the concrete motor programs required to produce those actions, ...based on our goals and knowledge. The frontal lobes are necessary for cognitive control at all levels of abstraction. Recent neuroimaging data have motivated the hypothesis that the frontal lobes are organized hierarchically, such that control is supported in progressively caudal regions as decisions are made at more concrete levels of action. We found that frontal damage impaired action decisions at a level of abstraction that was dependent on lesion location (rostral lesions affected more abstract tasks, whereas caudal lesions affected more concrete tasks), in addition to impairing tasks requiring more, but not less, abstract action control. Moreover, two adjacent regions were distinguished on the basis of the level of control, consistent with previous functional magnetic resonance imaging results. These results provide direct evidence for a rostro-caudal hierarchical organization of the frontal lobes.
Celotno besedilo
Dostopno za:
DOBA, IZUM, KILJ, NUK, PILJ, PNG, SAZU, UILJ, UKNU, UL, UM, UPUK
In this study, we assess the impact of normal aging on top-down modulation, a cognitive control mechanism that supports both attention and memory by the suppression and enhancement of sensory ...processing in accordance with task goals. Using fMRI (functional magnetic resonance imaging), we show that healthy older adults demonstrated a prominent deficit in the suppression of cortical activity associated with task-irrelevant representations, whereas enhancement of task-relevant activity was preserved. Moreover, this suppression-specific attention deficit correlated with impaired working memory performance.
Celotno besedilo
Dostopno za:
DOBA, IZUM, KILJ, NUK, PILJ, PNG, SAZU, UILJ, UKNU, UL, UM, UPUK
•Data on rotations of 46 binary and triple asteroid systems were collected.•An anti-correlation of secondary synchroneity with orbital eccentricity observed.•Libration angles of synchronous ...secondaries are less than 20° on most epochs.•A paucity of chaotic rotations among asynchronous secondaries is apparent.•An upper limit on the secondary equatorial axis ratios of 1.5 was found.
We collected data on rotations and elongations of 46 secondaries of binary and triple systems among near-Earth, Mars-crossing and small main belt asteroids. 24 were found or are strongly suspected to be synchronous (in 1:1 spin–orbit resonance), and the other 22, generally on more distant and/or eccentric orbits, were found or are suggested to have asynchronous rotations. For 18 of the synchronous secondaries, we constrained their librational angles, finding that their long axes pointed to within 20° of the primary on most epochs. The observed anti-correlation of secondary synchroneity with orbital eccentricity and the limited librational angles agree with the theories by Ćuk and Nesvorný (Ćuk, M., Nesvorný, D. 2010. Icarus 207, 732–743) and Naidu and Margot (Naidu, S.P., Margot, J.-L. 2015. Astron. J. 149, 80). A reason for the asynchronous secondaries being on wider orbits than synchronous ones may be longer tidal circularization time scales at larger semi-major axes. The asynchronous secondaries show relatively fast spins; their rotation periods are typically <10 h. An intriguing observation is a paucity of chaotic secondary rotations; with an exception of (35107) 1991 VH, the secondary rotations are single-periodic with no signs of chaotic rotation and their periods are constant on timescales from weeks to years. The secondary equatorial elongations show an upper limit of a2/b2∼1.5. The lack of synchronous secondaries with greater elongations appears consistent, considering uncertainties of the axis ratio estimates, with the theory by Ćuk and Nesvorný that predicts large regions of chaotic rotation in the phase space for a2/b2≳2. Alternatively, secondaries may not form or stay very elongated in gravitational (tidal) field of the primary. It could be due to the secondary fission mechanism suggested by Jacobson and Scheeres (Jacobson, S.A., Scheeres, D.J. 2011. Icarus 214, 161–178), as its efficiency is correlated with the secondary elongation. Sharma (Sharma, I. 2014. Icarus 229, 278–294) found that rubble-pile satellites with a2/b2≲1.5 are more stable to finite structural perturbations than more elongated ones. It appears that more elongated secondaries, if they originally formed in spin fission of parent asteroid, are less likely to survive intact and they more frequently fail or fission.
Spin rate distribution of small asteroids Pravec, P.; Harris, A.W.; Vokrouhlický, D. ...
Icarus (New York, N.Y. 1962),
10/2008, Letnik:
197, Številka:
2
Journal Article
Recenzirano
Odprti dostop
The spin rate distribution of main belt/Mars crossing (MB/MC) asteroids with diameters 3–15 km is uniform in the range from
f
=
1
to 9.5 d
−1, and there is an excess of slow rotators with
f
<
1
d
−1. ...The observed distribution appears to be controlled by the Yarkovsky–O'Keefe–Radzievskii–Paddack (YORP) effect. The magnitude of the excess of slow rotators is related to the residence time of slowed down asteroids in the excess and the rate of spin rate change outside the excess. We estimated a median YORP spin rate change of
≈
0.022
d
−1
/
Myr
for asteroids in our sample (i.e., a median time in which the spin rate changes by 1 d
−1 is
≈
45
Myr
), thus the residence time of slowed down asteroids in the excess is
≈
110
Myr
. The spin rate distribution of near-Earth asteroids (NEAs) with sizes in the range 0.2–3 km (∼5 times smaller in median diameter than the MB/MC asteroids sample) shows a similar excess of slow rotators, but there is also a concentration of NEAs at fast spin rates with
f
=
9
–
10
d
−1
. The concentration at fast spin rates is correlated with a narrower distribution of spin rates of primaries of binary systems among NEAs; the difference may be due to the apparently more evolved population of binaries among MB/MC asteroids.
Ranging from the 1840s through the early twenty-first century, this study of shared political, economic, and cultural histories fills significant gaps in our understanding of Paraguayan-U.S. ...relations. Frank O. Mora and Jerry W. Cooney tell how an initially rocky beginning between the two countries, marked by diplomatic posturing, shows of military force, and failed business schemes, gave way to a calmer period during which the United States backed Paraguay's territorial claims against its neighbors, prospects grew brighter for American entrepreneurs, and Paraguay embraced Pan-Americanism. It was not until the 1930s that the two countries engaged in earnest as the United States attempted to mediate the Chaco War between Paraguay and Bolivia. Then, as the authors write, "hemispheric solidarity in World War II, the cold war in Latin America, the 'balance of power' among states in the Río de la Plata, and the question of U.S. support for, or aid to, Latin American dictators" became matters of mutual interest. The dictatorship of Alfredo Stroessner (1954-89) spanned much of this era, and a shared attitude of realpolitik typified U.S.-Paraguayan relations during his rule. Post-Stroessner, the United States has stood by Paraguay during its transition to democracy, despite lingering concerns about such issues as drug trafficking and intellectual piracy. The countries should grow closer with time, the authors conclude, if Paraguay resists the continent's leftward political shift and remains a solid partner in U.S. antiterror initiatives in South America.
Tumbling asteroids Pravec, P.; Harris, A.W.; Scheirich, P. ...
Icarus (New York, N.Y. 1962),
2005, 2005-1-00, 20050101, Letnik:
173, Številka:
1
Journal Article
Recenzirano
We present both a review of earlier data and new results on non-principal axis rotators (tumblers) among asteroids. Among new tumblers found, the best data we have are for 2002 TD
60, 2000 WL
107, ...and (54789) 2001 MZ
7—each of them shows a lightcurve with two frequencies (full terms with linear combinations of the two frequencies are present in the lightcurve). For 2002 TD
60, we have constructed a physical model of the NPA rotation. Other recent objects which have been found to be likely tumblers based on their lightcurves that do not fit with a single periodicity are 2002 NY
40, (16067) 1999 RH
27, and (5645) 1990 SP. We have done a statistical analysis of the present sample of the population of NPA rotators. It appears that most asteroids larger than
∼
0.4
km
with estimated damping timescales (Harris, 1994, Icarus 107, 209) of 4.5 byr and longer are NPA rotators. The statistic of two short-period tumblers (
D
=
0.04
and 0.4 km) with non-zero tensile strength suggests that for them the quantity
μ
Q
/
T
, where
μ is the mechanical rigidity,
Q is the elastic dissipation factor, and
T is a spin excitation age (i.e., a time elapsed since the last significant spin excitation event), is greater by two to four orders of magnitude than the larger, likely rubble-pile tumblers. Among observational conditions and selection effects affecting detections of NPA rotations, there is a bias against detection of low-amplitude (small elongation) tumblers.
Purpose: This study contributes to a growing literature focused on institutional complexity, or the phenomenon of competing institutional logics, in PK-12 public education. Responding to calls for ...more nuanced characterizations of district-level administration, our purpose was to identify the logics of curriculum leadership in one large school district in the Western U.S. and examine how district leaders made sense of their multiple demands. Research Methods: Employing case study methods, we collected curricular documents and conducted 21 interviews of district-level curriculum leaders, principals, and teachers. We used inductive coding to identify patterns in the district's culture of curriculum leadership and then drew upon institutional theory to associate these patterns with particular institutional logics. Findings: We found three coexisting logics of curriculum leadership in the school district: a logic of uniformity, which called for teachers’ fidelity to a prescriptive curriculum; a logic of participation, which valued teacher voice and shared leadership; and a logic of performance, which placed a premium on measurable student achievement. These logics were often incompatible or contradictory, producing mixed messages and an institutional complexity that remained largely unacknowledged among leaders. Implications: Tensions among logics of curriculum leadership may not always result in open contestation, yet if leaders do not resolve them at the district level, teachers may be unsupported in their efforts to reconcile competing demands in their classroom practices. Identifying specific logics of curriculum leadership and considering their interactions may help leaders make sense of conflicting values and better support teachers in their classroom decision making.
Neuropsychiatric Issues in Parkinson’s Disease Cooney, Jeffrey W.; Stacy, Mark
Current Neurology and Neuroscience Reports,
05/2016, Letnik:
16, Številka:
5
Journal Article, Book Review
Recenzirano
Cognitive and neuropsychiatric symptoms are common in Parkinson’s Disease and may surpass motor symptoms as the major factors impacting patient quality of life. The symptoms may be broadly separated ...into those associated with the disease process and those that represent adverse effects of treatment. Symptoms attributed to the disease arise from pathologic changes within multiple brain regions and are not restricted to dysfunction in the dopaminergic system. Mood symptoms such as depression, anxiety, and apathy are common and may precede the development of motor symptoms by years, while other neuropsychiatric symptoms such as cognitive impairment, dementia, and psychosis are more common in later stages of the disease. Neuropsychiatric symptoms attributed to treatment include impulse control disorders, pathologic use of dopaminergic medications, and psychosis. This manuscript will review the current understanding of neuropsychiatric symptoms in Parkinson’s Disease.
Photometric data on 17 binary near-Earth asteroids (15 of them are certain detections, two are probables) were analysed and characteristic properties of the near-Earth asteroid (NEA) binary ...population were inferred. We have found that binary systems with a secondary-to-primary mean diameter ratio
D
s
/
D
p
⩾
0.18
concentrate among NEAs smaller than 2 km in diameter; the abundance of such binaries decreases significantly among larger NEAs. Secondaries show an upper size limit of
D
s
=
0.5
–
1
km
. Systems with
D
s
/
D
p
⩽
0.5
are abundant but larger satellites are significantly less common. Primaries have spheroidal shapes and they rotate rapidly, with periods concentrating between 2.2 to 2.8 h and with a tail of the distribution up to ∼4 h. The fast rotators are close to the critical spin for rubble piles with bulk densities about 2 g/cm
3. Orbital periods show an apparent cut-off at
P
orb
∼
11
h
; closer systems with shorter orbital periods have not been discovered, which is consistent with the Roche limit for strengthless bodies. Secondaries are more elongated on average than primaries. Most, but not all, of their rotations appear to be synchronized with the orbital motion; nonsynchronous secondary rotations may occur especially among wider systems with
P
orb
>
20
h. The specific total angular momentum of most of the binary systems is similar to within ±20% and close to the angular momentum of a sphere with the same total mass and density, rotating at the disruption limit; this suggests that the binaries were created by mechanism(s) related to rotation near the critical limit and that they neither gained nor lost significant amounts of angular momentum during or since formation. A comparison with six small asynchronous binaries detected in the main belt of asteroids suggests that the population extends beyond the region of terrestrial planets, but with characteristics shifted to larger sizes and longer periods. The estimated mean proportion of binaries with
D
s
/
D
p
⩾
0.18
among NEAs larger than 0.3 km is
15
±
4
%
. Among fastest rotating NEAs larger than 0.3 km with periods between 2.2 and 2.8 h, the mean proportion of such binaries is (66
+10
−12)%.
PDF
