8,954 research outputs found
Hierarchical control over effortful behavior by rodent medial frontal cortex : a computational model
The anterior cingulate cortex (ACC) has been the focus of intense research interest in recent years. Although separate theories relate ACC function variously to conflict monitoring, reward processing, action selection, decision making, and more, damage to the ACC mostly spares performance on tasks that exercise these functions, indicating that they are not in fact unique to the ACC. Further, most theories do not address the most salient consequence of ACC damage: impoverished action generation in the presence of normal motor ability. In this study we develop a computational model of the rodent medial prefrontal cortex that accounts for the behavioral sequelae of ACC damage, unifies many of the cognitive functions attributed to it, and provides a solution to an outstanding question in cognitive control research-how the control system determines and motivates what tasks to perform. The theory derives from recent developments in the formal study of hierarchical control and learning that highlight computational efficiencies afforded when collections of actions are represented based on their conjoint goals. According to this position, the ACC utilizes reward information to select tasks that are then accomplished through top-down control over action selection by the striatum. Computational simulations capture animal lesion data that implicate the medial prefrontal cortex in regulating physical and cognitive effort. Overall, this theory provides a unifying theoretical framework for understanding the ACC in terms of the pivotal role it plays in the hierarchical organization of effortful behavior
Milky Way Kinematics: Measurements at the Subcentral Point of the Fourth Quadrant
We use atomic hydrogen (HI) data from the Southern Galactic Plane Survey to
study the kinematics of the fourth quadrant of the Milky Way. By measuring the
terminal velocity as a function of longitude throughout the fourth Galactic
quadrant we have derived the most densely sampled rotation curve available for
the Milky Way between 3 < R < 8 kpc. We determine a new joint rotation curve
fit for the first and fourth quadrants, which can be used for kinematic
distances interior to the Solar circle. From our data we place new limits on
the peak to peak variation of streaming motions in the fourth quadrant to be
~10 km/s. We show that the shape of the average HI profile beyond the terminal
velocity is consistent with gas of three velocity dispersions, a cold component
with km/s, a warmer component with km/s and a
fast component with km/s. Examining the widths with Galactic
radius we find that the narrowest two components show little variation with
radius and their small scale fluctuations track each other very well,
suggesting that they share the same cloud-to-cloud motions. The width of the
widest component is constant until R<4 kpc, where it increases sharply.Comment: 36 pages, 10 figures, accepted to ApJ. Full electronic version of
table 1 available at
ftp://ftp.atnf.csiro.au/pub/people/nmcclure/papers/velocity_tab1.te
Preparation of monotectic alloys having a controlled microstructure by directional solidification under dopant-induced interface breakdown
Monotectic alloys having aligned spherical particles of rods of the minor component dispersed in a matrix of the major component are prepared by forming a melt containing predetermined amounts of the major and minor components of a chosen monotectic system, providing in the melt a dopant capable of breaking down the liquid solid interface for the chosen alloy, and directionally solidfying the melt at a selected temperature gradient and a selected rate of movement of the liquid-solid interface (growth rate). Shaping of the minor component into spheres or rods and the spacing between them are controlled by the amount of dopant and the temperature gradient and growth rate values. Specific alloy systems include Al Bi, Al Pb and Zn Bi, using a transition element such as iron
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