Wumpus Protocol Analysis

This report describes research done at the Artificial Intelligence Laboratory of the Massachusetts institute of Technology. Support for the Laboratory's artificial intelligence research is provided in part by the Advanced Research Projects Agency of the Department of Defense under Office of Naval Research contract N00014-75-C-0643.The goal of this research was to assist in the creation of a new, improved Wumpus advisor by taking protocols of ten people learning to play Wumpus with a human coach. It was hoped that by observing these subjects learn Wumpus from a human coach, that insights would be gained into how the computer coach could be modified or extended. In particular, attention was paid to the representations subjects used, the goals they pursued, and the problems they had as well as to the teaching methods used by the human versus the computer coach.MIT Artificial Intelligence Laboratory Department of Defense Advanced Research Projects Agenc

The Role of Intermediate Abstractions in Understanding Science and Mathematics

Acquiring powerful abstractions -- i.e., representations that enable one to reason about key aspects of a domain in an economical and generic form - should be a primary goal of learning. The most effective means for achieving this goal is not. we argue, the "topdown" approach of traditional curricula where students are first presented with an abstraction, such as F=ma. and then with examples of how it applies in a variety of contexts. Nor do we advocate the "bottom-up" approach proposed by situated cognition theorists. Instead, we argue for a "middle-out approach where students are introduced to new domains via intermediate abstractions in the form of mechanistic causal models. These models serve as "conceptual eyeglasses" that unpack causal mechanisms implicit in abstractions such as F=ma. Tney are readily mappable to a vanety of real-worid contexts since their objects and operators are generic and causal. Intermediate abstractions thus give meaning to higher-order abstractions as well as to real-world situations, provide a link between the two, and a route to understanding

Explorations in Understanding How Physical Systems Work

This paper presents a theory of how to enable people to understand how physical systems work Two key hypotheses have emerged from our research. The first is that in order to understand a physical system, students need to acquire causal mental models for how the system works. Further, it is not enough to have just a single mental model. Students need alternative mental models that represent the systems behavior from different, but coordinated, perspectives, such as at the macroscopic and microscopic levels.The second hypothesis is that in order to make causal understanding feasible in the initial stages of learning, students have to be introduced to simplified models. These models then get gradually refined into more sophisticated mental models. W e will present a theory outlining (1) the properties of an easily learnable, coherent set of initial models, and (2) the types of evolutions needed for students to acquire a more powerful set of models with broad utility

Reductionistic Conceptual Models and the Acquisition of Electrical Expertise

Our objective has been to determine whether woii^ing with reductionistic models reduces students' mis- conceptions, and increases the coherence and flexibility of their expertise as they solve problems and generate expla- nations. W e conducted experimental trials of an interac- tive learning environment that provides models of circuit behavior. In these trials, w e examined students' perfor- mance on a variety of circuit problems before and after they worked with either (a) a "transport" model alone, or (b) the transport model augmented with explanations of its processes in terms of a "particle" model. T h e posttest re- sults reveal that, while both groups performed well o n a wide range of tasks, students w h o received the particle- model explanations achieved higher levels of performance on tasks that require an understanding of voltage and its distribution. W e conjecture that this is due to the particle model providing students with a mechanistic model for charge distribution that is consistent with the behavior of the transport model and that inhibits the construction and use of certain common misconceptions

Geometrical and Physical Properties of Circumbinary Discs in Eccentric Stellar Binaries

In a previous work (Pichardo et al. 2005), we studied stable configurations for circumstellar discs in eccentric binary systems. We searched for "invariant loops": closed curves (analogous to stable periodic orbits in time-independent potentials) that change shape with the binary orbital phase, as test particles in them move under the influence of the binary potential. This approach allows us to identify stable configurations when pressure forces are unimportant, and dissipation acts only to prevent gas clouds from colliding with one another. We now extend this work to study the main geometrical properties of circumbinary discs. We have studied more than 100 cases with a range in eccentricity 0 .le. e .le. 0.9, and mass ratio 0.1 .le. q .le. 0.9. Although gas dynamics may impose further restrictions, our study sets lower stable bounds for the size of the central hole in a simple and computationally cheap way, with a relation that depends on the eccentricity and mass ratio of the central binary. We extend our previous studies and focus on an important component of these systems: circumbinary discs. The radii for stable orbits that can host gas in circumbinary discs are sharply constrained as a function of the binary's eccentricity. The circumbinary disc configurations are almost circular, with eccentricity e_d < 0.15, but if the mass ratio is unequal the disk is offset from the center of mass of the system. We compare our results with other models, and with observations of specific systems like GG Tauri A, UY Aurigae, HD 98800 B, and Fomalhaut, restricting the plausible parameters for the binary.Comment: 12 pages, 10 figures and 6 tables. MNRAS, accepte

Increasing dominance of large lianas in Amazonian forests

Ecological orthodoxy suggests that old-growth forests should be close to dynamic equilibrium, but this view has been challenged by recent findings that neotropical forests are accumulating carbon and biomass, possibly in response to the increasing atmospheric concentrations of carbon dioxide. However, it is unclear whether the recent increase in tree biomass has been accompanied by a shift in community composition. Such changes could reduce or enhance the carbon storage potential of old-growth forests in the long term. Here we show that non-fragmented Amazon forests are experiencing a concerted increase in the density, basal area and mean size of woody climbing plants (lianas). Over the last two decades of the twentieth century the dominance of large lianas relative to trees has increased by 1.7–4.6% a year. Lianas enhance tree mortality and suppress tree growth, so their rapid increase implies that the tropical terrestrial carbon sink may shut down sooner than current models suggest. Predictions of future tropical carbon fluxes will need to account for the changing composition and dynamics of supposedly undisturbed forests

Bioelectric Effects of Intense Nanosecond Pulses

Electrical models for biological cells predict that reducing the duration of applied electrical pulses to values below the charging time of the outer cell membrane (which is on the order of 100 ns for mammalian cells) causes a strong increase in the probability of electric field interactions with intracellular structures due to displacement currents. For electric field amplitudes exceeding MV/m, such pulses are also expected to allow access to the cell interior through conduction currents flowing through the permeabilized plasma membrane. In both cases, limiting the duration of the electrical pulses to nanoseconds ensures only nonthermal interactions of the electric field with subcellular structures. This intracellular access allows the manipulation of cell functions. Experimental studies, in which human cells were exposed to pulsed electric fields of up to 300 kY/cm amplitude with durations as short as 3 ns, have confirmed this hypothesis and have shown that it is possible to selectively alter the behavior and/or survival of cells. Observed nanosecond pulsed effects at moderate electric fields include intracellular release of calcium and enhanced gene expression, which could have long term implications on cell behavior and function. At increased electric fields, the application of nanosecond pulses induces a type of programmed cell death, apoptosis, in biological cells. Cell survival studies with 10 ns pulses have shown that the viability of the cells scales inversely with the electrical energy density, which is similar to the ‘dose’ effect caused by ionizing radiation. On the other hand, there is experimental evidence that, for pulses of varying durations, the onset of a range of observed biological effects is determined by the electrical charge that is transferred to the cell membrane during pulsing. This leads to an empirical similarity law for nanosecond pulse effects, with the product of electric field intensity, pulse duration, and the square root of the number of pulses as the similarity parameter. The similarity law allows one not only to predict cell viability based on pulse parameters, but has also been shown to be applicable for inducing platelet aggregation, an effect which is triggered by internal calcium release. Applications for nanosecond pulse effects cover a wide range: from a rather simple use as preventing biofouling in cooling water systems, to advanced medical applications, such as gene therapy and tumor treatment. Results of this continuing research are leading to the development of wound healing and skin cancer treatments, which are discussed in some detail