Human Performance on Hard Non-Euclidean Graph Problems: Vertex Cover

Recent studies on a computationally hard visual optimization problem, the Traveling Salesperson Problem (TSP), indicate that humans are capable of finding close to optimal solutions in near-linear time. The current study is a preliminary step in investigating human performance on another hard problem, the Minimum Vertex Cover Problem, in which solvers attempt to find a smallest set of vertices that ensures that every edge in an undirected graph is incident with at least one of the selected vertices. We identify appropriate measures of performance, examine features of problem instances that impact performance, and describe strategies typically employed by participants to solve instances of the Vertex Cover problem

The Role of the Goal in Solving Hard Computational Problems: Do People Really Optimize?

The role that the mental, or internal, representation plays when people are solving hard computational problems has largely been overlooked to date, despite the reality that this internal representation drives problem solving. In this work we investigate how performance on versions of two hard computational problems differs based on what internal representations can be generated. Our findings suggest that problem solving performance depends not only on the objective difficulty of the problem, and of course the particular problem instance at hand, but also on how feasible it is to encode the goal of the given problem. A further implication of these findings is that previous human performance studies using NP-hard problems may have, surprisingly, underestimated human performance on instances of problems of this class. We suggest ways to meaningfully frame human performance results on instances of computationally hard problems in terms of these problems’ computational complexity, and present a novel framework for interpreting results on problems of this type. The framework takes into account the limitations of the human cognitive system, in particular as it applies to the generation of internal representations of problems of this class

Dynamic Evocation of Hand Action Representations During Sentence Comprehension

This document is copyrighted by the American Psychological Association or one of its allied publishers. This article is intended solely for the personal use of the individual user and is not to be disseminated broadly. When listening to a sentence describing an interaction with a manipulable object, understanding the actor’s intentions is shown to have a striking influence on action representations evoked during comprehension. Subjects performed a cued reach and grasp response while listening to a context sentence. Responses were primed when they were consistent with the proximal intention of an actor (“John lifted the cell phone...”), but this effect was evanescent and appeared only when sentences mentioned the proximal intention first. When the sentence structure was changed to mention the distal intention first (“To clear the shelf...”), priming effects were no longer context specific and actions pertaining to the function of an object were clearly favored. These results are not compatible with a straightforward mental-simulation account of sentence comprehension but instead reflect a hierarchy of intentions distinguishing how and why actions are performed

Different frequencies and triggers of canyon filling and flushing events in Nazaré Canyon, offshore Portugal

Submarine canyons are one of the most important pathways for sediment transport into ocean basins. For this reason, understanding canyon architecture and sedimentary processes has importance for sediment budgets, carbon cycling, and geohazard assessment. Despite increasing knowledge of turbidity current triggers, the down-canyon variability in turbidity current frequency within most canyon systems is not well constrained. New AMS radiocarbon chronologies from canyon sediment cores illustrate significant variability in turbidity current frequency within Nazaré Canyon through time. Generalised linear models and Cox proportional hazards models indicate a strong influence of global sea level on the frequency of turbidity currents that fill the canyon. Radiocarbon ages from basin sediment cores indicate that larger, canyon-flushing turbidity currents reaching the Iberian Abyssal Plain have a significantly longer average recurrence interval than turbidity currents that fill the canyon. The recurrence intervals of these canyon-flushing turbidity currents also appear to be unaffected by long-term changes in global sea level. Furthermore, canyon-flushing and canyon-filling have very different statistical distributions of recurrence intervals. This indicates that the factors triggering, and thus controlling the frequency of canyon-flushing and canyon-filling events are very different. Canyon-filling appears to be predominantly triggered by sediment instability during sea level lowstand, and by storm and nepheloid transport during the present day highstand. Canyon-flushing exhibits time-independent behaviour. This indicates that a temporally random process, signal shredding, or summation of non-random processes that cannot be discerned from a random signal, are triggering canyon flushing events

Radio source calibration for the VSA and other CMB instruments at around 30 GHz

Accurate calibration of data is essential for the current generation of CMB experiments. Using data from the Very Small Array (VSA), we describe procedures which will lead to an accuracy of 1 percent or better for experiments such as the VSA and CBI. Particular attention is paid to the stability of the receiver systems, the quality of the site and frequent observations of reference sources. At 30 GHz the careful correction for atmospheric emission and absorption is shown to be essential for achieving 1 percent precision. The sources for which a 1 percent relative flux density calibration was achieved included Cas A, Cyg A, Tau A and NGC7027 and the planets Venus, Jupiter and Saturn. A flux density, or brightness temperature in the case of the planets, was derived at 33 GHz relative to Jupiter which was adopted as the fundamental calibrator. A spectral index at ~30 GHz is given for each. Cas A,Tau A, NGC7027 and Venus were examined for variability. Cas A was found to be decreasing at $0.394 \pm 0.019$ percent per year over the period March 2001 to August 2004. In the same period Tau A was decreasing at $0.22\pm 0.07$ percent per year. A survey of the published data showed that the planetary nebula NGC7027 decreased at $0.16\pm 0.04$ percent per year over the period 1967 to 2003. Venus showed an insignificant ($1.5 \pm 1.3$ percent) variation with Venusian illumination. The integrated polarization of Tau A at 33 GHz was found to be $7.8\pm 0.6$ percent at pa $= 148^\circ \pm 3^\circ$.}Comment: 13 pages, 15 figures, submitted to MNRA