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Papers by Ben Williams

Research paper thumbnail of A prickly problem: developing a volunteer-friendly tool for monitoring populations of a terrestrial urban mammal, the West European hedgehog (Erinaceus europaeus)

Urban Ecosystems

Across Europe, hedgehogs (Erinaceus europaeus) appear to be in decline in both urban and rural la... more Across Europe, hedgehogs (Erinaceus europaeus) appear to be in decline in both urban and rural landscapes. Current methods used to monitor urban populations are, however, associated with several potential limitations. In this study, we conducted hedgehog footprint-tunnel surveys in 219 residential gardens across Reading, UK between May-September in 2013 and/or 2014; gardens were surveyed for five continuous days. Single-species occupancy models were used to investigate factors influencing hedgehog occupancy and two-species occupancy models were used to estimate a species interaction factor (SIF) between hedgehogs and (a) badgers (Meles meles), (b) foxes (Vulpes vulpes) and (c) dogs (Canis familiaris). The five-day survey protocol was associated with a falseabsence error rate of 0.1-0.4%, indicating that it was a reliable method for determining hedgehog presence; conversely, 34.7% of householders were not able to correctly predict hedgehog presence or absence. Hedgehogs were widely distributed across Reading, but detected in only 32-40% of gardens. None of the within-garden or outside-garden factors investigated significantly affected hedgehog occupancy in the single-species models, but the two-species models indicated that badgers (SIF = 0.471 ± 0.188), but not foxes (SIF = 0.954 ± 0.048) or dogs (SIF = 0.780 ± 0.228), negatively affected the presence of hedgehogs in gardens, although not significantly. Overall, footprint-tunnels represent a viable field method for monitoring urban hedgehog populations, however, other approaches are required to identify factors that make gardens Bhedgehog friendly^.

Research paper thumbnail of {"__content__"=>"Ancient duons may underpin spatial patterning of gene expression in Cleaves.", "sub"=>{"__content__"=>"4"}}

Proceedings of the National Academy of Sciences of the United States of America, Feb 20, 2018

If the highly efficient Cphotosynthesis pathway could be transferred to crops with the Cpathway t... more If the highly efficient Cphotosynthesis pathway could be transferred to crops with the Cpathway there could be yield gains of up to 50%. It has been proposed that the multiple metabolic and developmental modifications associated with Cphotosynthesis are underpinned by relatively few master regulators that have allowed the evolution of Cphotosynthesis more than 60 times in flowering plants. Here we identify a component of one such regulator that consists of a pair of-elements located in coding sequence of multiple genes that are preferentially expressed in bundle sheath cells of Cleaves. These motifs represent duons as they play a dual role in coding for amino acids as well as controlling the spatial patterning of gene expression associated with the Cleaf. They act to repress transcription of Cphotosynthesis genes in mesophyll cells. These duons are also present in the Cmodel, and, in fact, are conserved in all land plants and even some algae that use Cphotosynthesis. Cphotosynthesis...

Research paper thumbnail of A Trojan horse? Governmentality, private enterprise and school physical activity

Research paper thumbnail of Topology and purity for torsors

We study the homotopy theory of the classifying space of the complex projective linear groups to ... more We study the homotopy theory of the classifying space of the complex projective linear groups to prove that purity fails for PGL p-torsors on regular noetherian schemes when p is a prime. Extending our previous work when p = 2, we obtain a negative answer to a question of Colliot-Thélène and Sansuc, for all PGL p. We also give a new example of the failure of purity for the cohomological filtration on the Witt group, which is the first example of this kind of a variety over an algebraically closed field. CONTENTS 1. Introduction 1 2. Topology 3 2.1. The p-local cohomology of some Eilenberg-MacLane spaces 3 2.2. The p-local homotopy type of BSL p (C) 5 2.3. The p-local homotopy type of BPGL p (C) 8 3. Purity 9 3.1. Definitions 10 3.2. Purity for torsors 10 3.3. Local purity 12 3.4. Canonical factorization 12 3.5. The Witt group 13 References 14

Research paper thumbnail of Gradient copolymers of thiophene and pyrrole for photovoltaics

Computational Materials Science, 2015

The electronic properties of copolymers can be tuned by controlling their monomer ratio, and ther... more The electronic properties of copolymers can be tuned by controlling their monomer ratio, and therefore can potentially be used to improve charge separation in organic photovoltaic devices. Here we show evidence based on density functional theory calculations that it is possible to control the electronic structure of π-conjugated copolymers of thiophene and pyrrole to obtain a gradient in the band gap and both conduction and valence crystal orbital band levels by controlling their composition. Our calculations predict and optimal thiophene monomer fraction range between zero and 40% is needed in order to obtain the largest electronic structure gradients.

Research paper thumbnail of Use of Frequency Response Metrics to Assess the Planning and Operating Requirements for Reliable Integration of Variable Renewable Generation

An interconnected electric power system is a complex system that must be operated within a safe f... more An interconnected electric power system is a complex system that must be operated within a safe frequency range in order to reliably maintain the instantaneous balance between generation and load. This is accomplished by ensuring that adequate resources are available to respond to expected and unexpected imbalances and restoring frequency to its scheduled value in order to ensure uninterrupted electric service to customers. Electrical systems must be flexible enough to reliably operate under a variety of "change" scenarios. System planners and operators must understand how other parts of the system change in response to the initial change, and need tools to manage such changes to ensure reliable operation within the scheduled frequency range. This report presents a systematic approach to identifying metrics that are useful for operating and planning a reliable system with increased amounts of variable renewable generation which builds on existing industry practices for frequency control after unexpected loss of a large amount of generation. The report introduces a set of metrics or tools for measuring the adequacy of frequency response within an interconnection. Based on the concept of the frequency nadir, these metrics take advantage of new information gathering and processing capabilities that system operators are developing for wide-area situational awareness. Primary frequency response is the leading metric that will be used by this report to assess the adequacy of primary frequency control reserves necessary to ensure reliable operation. It measures what is needed to arrest frequency decline (i.e., to establish frequency nadir) at a frequency higher than the highest set point for underfrequency load shedding within an interconnection. These metrics can be used to guide the reliable operation of an interconnection under changing circumstances.

Research paper thumbnail of Extracting Motion Primitives from Natural Handwriting Data

Lecture Notes in Computer Science, 2006

For the past 10 years it has become clear that biological movement is made up of sub-routine type... more For the past 10 years it has become clear that biological movement is made up of sub-routine type blocks, or motor primitives, with a central controller timing the activation of these blocks, creating synergies of muscle activation. This paper shows that it is possible to use a factorial hidden Markov model to infer primitives in handwriting data. These primitives are not predefined in terms of location of occurrence within the handwriting, and they are not limited or defined by a particular character set. Also, the variation in the data can to a large extent be explained by timing variation in the triggering of the primitives. Once an appropriate set of primitives has been inferred, the characters can be represented as a set of timings of primitive activations, along with variances, giving a very compact representation of the character. Separating the motor system into a motor primitive part, and a timing control gives us a possible insight into how we might create scribbles on paper.

Research paper thumbnail of Evidence that blocking is due to associative deficit: Blocking history affects the degree of subsequent associative competition

Psychonomic Bulletin & Review, 1996

Rats were trained on an appetitive discriminated operant procedure according to a blocking design... more Rats were trained on an appetitive discriminated operant procedure according to a blocking design, in which training with a compound stimulus (A + B) was given with or without prior training with A alone, Stimulus B was then presented in compound with a new stimulus (B + C compound). The ability of Stimulus B to block acquisition of control by Stimulus C was decreased by B having itself been blocked by A during the prior training. The results imply that the mechanism of blocking during the initial blocking procedure was a reduced association between Stimulus B and the reinforcer, and not a performance deficit such as that postulated by comparator models of learning.

Research paper thumbnail of Taxometric analysis supports a dimensional latent structure for schizotypy

Personality and Individual Differences, 2008

The existence of a discrete class of people vulnerable to schizophrenia spectrum disorders is the... more The existence of a discrete class of people vulnerable to schizophrenia spectrum disorders is the most replicated finding of taxometric research. Evidence for such a “taxon” has been obtained with diverse measures of schizotypy in clinical, high-risk, and normal samples. However, ...

Research paper thumbnail of Intraoperative Hyperglycemia and Perioperative Outcomes in Cardiac Surgery Patients

Mayo Clinic Proceedings, 2005

To estimate the magnitude of association between intraoperative hyperglycemia and perioperative o... more To estimate the magnitude of association between intraoperative hyperglycemia and perioperative outcomes in patients who underwent cardiac surgery. We conducted a retrospective observational study of consecutive adult patients who underwent cardiac surgery between June 10, 2002, and August 30, 2002, at the Mayo Clinic, a tertiary care center in Rochester, Minn. The primary independent variable was the mean intraoperative glucose concentration. The primary end point was a composite of death and infectious (sternal wound, urinary tract, sepsis), neurologic (stroke, coma, delirium), renal (acute renal failure), cardiac (new-onset atrial fibrillation, heart block, cardiac arrest), and pulmonary (prolonged pulmonary ventilation, pneumonia) complications developing within 30 days after cardiac surgery. Among 409 patients who underwent cardiac surgery, those experiencing a primary end point were more likely to be male and older, have diabetes mellitus, undergo coronary artery bypass grafting, and receive insulin during surgery (P< or =.05 for all comparisons). Atrial fibrillation (n=105), prolonged pulmonary ventilation (n=53), delirium (n=22), and urinary tract infection (n=16) were the most common complications. The initial, mean, and maximal intraoperative glucose concentrations were significantly higher in patients experiencing the primary end point (P<.01 for all comparisons). In multivariable analyses, mean and maximal glucose levels remained significantly associated with outcomes after adjusting for potentially confounding variables, including postoperative glucose concentration. Logistic regression analyses indicated that a 20-mg/dL increase in the mean intraoperative glucose level was associated with an increase of more than 30% in outcomes (adjusted odds ratio, 1.34; 95% confidence Interval, 1.10-1.62). Intraoperative hyperglycemia is an independent risk factor for complications, including death, after cardiac surgery.

Research paper thumbnail of Conditioned reinforcement versus time to reinforcement in chain schedules

Journal of the Experimental Analysis of Behavior, 1990

Pigeons were trained on three-component chain schedules in which the initial component was either... more Pigeons were trained on three-component chain schedules in which the initial component was either a fixed-interval or variable-interval schedule. The middle and terminal components were varied among fixed-interval fixed-interval, variable-interval variable-interval, and an interdependent variable-interval variable-interval schedule in which the sum of the durations of the two variable-interval components was always equal to the sum of the fixed-interval fixed-interval components. At issue was whether the response rate in the initial component was controlled by its time to primary reinforcement or by the temporal parameters of the stimulus correlated with the middle terminal link. The fixed-interval initial-link schedule maintained much lower response rates than the variable-interval initial-link schedule regardless of the schedules in the middle and terminal links. Nevertheless, the intervening schedules played some role: With fixed-interval schedules in the initial links, response rates were consistently highest with independent variable-interval schedules in the middle and terminal links and intermediate with the interdependent variable-interval schedules; these initial-link differences were predicted by the response rates in the middle link of the chain. With variable-interval schedules in the initial links, response rates were lowest with the fixed-interval fixed-interval schedules following the initial link and were not systematically different for the two types of variable-interval variableinterval schedules. The results suggest that time to reinforcement itself accounts for little if any variance in initial-link responding. Key words: chain schedules, conditioned reinforcement, time to reinforcement, fixed-interval schedules, variable-interval schedules, key peck, pigeons This research was supported by NIMH Research Grant RO 1 MH4297. Requests for reprints should be addressed to the first author,

Research paper thumbnail of Elicited responding to signals for reinforcement: the effects of overall versus local changes in reinforcement probability1

Journal of the Experimental Analysis of Behavior, 1976

Pigeons were studied on a three-component multiple schedule where all reinforcement was independe... more Pigeons were studied on a three-component multiple schedule where all reinforcement was independent of responding. Two components were cued by different keylights and were associated with different rates of reinforcement. The third was always a no-key period associated with extinction. After a few sessions, pecking was elicited by the keylights signalling the reinforcement and continued to be maintained indefinitely. The duration and sequence of the three components were varied to determine if the primary controlling variable was differences in the overall probability of reinforcement, or if it was the immediate change in reinforcement signalled by the onset and/or offset of the stimulus. Both variables were found to control behavior. When 30-sec components were used, the primary controlling variable was the overall probability of reinforcement, but when 3-min components were used, overall probability had little effect. Control by local changes in reinforcement also occurred, although the type of local control varied both across subjects and experimental conditions. Some behaviors were controlled more by the change in reinforcement signalled by the onset of the stimulus, while others were controlled more by the change signalled by the offset of the stimulus.

Research paper thumbnail of Molar versus local reinforcement probability as determinants of stimulus value

Journal of the Experimental Analysis of Behavior, 1993

During one component of a multiple schedule, pigeons were trained on a discrete-trial concurrent ... more During one component of a multiple schedule, pigeons were trained on a discrete-trial concurrent variable-interval variable-interval schedule in which one alternative had a high scheduled rate of reinforcement and the other a low scheduled rate of reinforcement. When the choice proportion between the alternatives matched their respective relative reinforcement frequencies, the obtained probabilities of reinforcement (reinforcer per peck) were approximately equal. In alternate components of the multiple schedule, a single response alternative was presented with an intermediate scheduled rate of reinforcement. During probe trials, each alternative of the concurrent schedule was paired with the constant alternative. The stimulus correlated with the high reinforcement rate was preferred over that with the intermediate rate, whereas the stimulus correlated with the intermediate rate of reinforcement was preferred over that correlated with the low rate of reinforcement. Preference on probe tests was thus determined by the scheduled rate of reinforcement. Other subjects were presented all three alternatives individually, but with a distribution of trial frequency and reinforcement probability similar to that produced by the choice patterns of the original subjects. Here, preferences on probe tests were determined by the obtained probabilities of reinforcement. Comparison of the two sets of results indicates that the availability of a choice alternative, even when not responded to, affects the preference for that alternative. The results imply that models of choice that invoke only obtained probability of reinforcement as the controlling variable (e.g., melioration) are inadequate.

Research paper thumbnail of Conditioned reinforcement dynamics in three-link chained schedules

Journal of the Experimental Analysis of Behavior, 1997

In two experiments rats were trained on three-link concurrent-chains schedules of reinforcement. ... more In two experiments rats were trained on three-link concurrent-chains schedules of reinforcement. In Experiment 1, additional entries to one terminal link were added during one of the middle links to a baseline schedule that was otherwise equal for the two chains, and, depending on the condition, these additional terminal-link presentations ended either in food or in no food. When food occurred, preference was always in favor of the chain with the additional terminal-link presentations (which also entailed a higher rate of reinforcement). When no food occurred at the end of the additional terminal links, the outcome depended on the nature of the stimuli associated with these additional terminal links. When stimuli different from the reinforced baseline terminal links were used for the no-food terminal links, preference was against the choice alternative that led to the extra periods of extinction. When the same stimulus was used for the two kinds of terminal links, preference was near indifference, that is, significantly greater than when different stimuli were used. In Experiment 2, rats learned repeated reversals of a simultaneous discrimination under a three-link concurrentchains schedule, in which the food or no-food choice outcomes were delayed until the end of the chain. Different conditions were defined by the point in the chain at which differential stimuli occurred. When the middle and terminal links provided no differential stimuli, discrimination was acquired more slowly than when differential stimuli occurred in both links. When differential stimuli occurred in the middle but not the terminal links, acquisition rates were intermediate. Both experiments together show that the effects of stimuli in a chain schedule are due partly to the time to food correlated with the stimuli and partly to the time to the next conditioned reinforcer in the sequence.

Research paper thumbnail of A re-examination of local contrast in multiple schedules1

Journal of the Experimental Analysis of Behavior, 1975

Pigeons were presented with multiple schedules of alternating 90-sec components. When components ... more Pigeons were presented with multiple schedules of alternating 90-sec components. When components in which grain was never presented alternated with components in which grain was presented on a variable-interval schedule, the average rate of responding in the variable-interval components increased, showing overall positive behavioral contrast. Unlike previous reports, this study found that the response rates for all birds increased toward the end of the variable-interval components as training proceeded. This increase in local response rate disappeared when the multiple schedule was composed solely of variable-interval components and reappeared when the variable-interval components were again alternated with extinction. This finding cannot be predicted or explained by recent theories of behavioral contrast based on autoshaping, and thus questions their sufficiency. We suggest that this local response-rate increase results from the predictable change from high to low density of reinforcement at the end of the fixed-duration component. Thus, the present effect apparently illustrates a different type of interaction between components of a multiple schedule than that described by previous theories of contrast. In a given procedure, either or both types of interaction may occur; neither provides a complete account of behavioral contrast.

Research paper thumbnail of Conditioned Reinforcement in Chain Schedules When Time to Reinforcement is Held Constant

Journal of the Experimental Analysis of Behavior, 2013

Two alternative approaches describe determinants of responding to a stimulus temporally distant f... more Two alternative approaches describe determinants of responding to a stimulus temporally distant from primary reinforcement. One emphasizes the temporal relation of each stimulus to the primary reinforcer, with relative proximity of the stimulus determining response rate. A contrasting view emphasizes immediate consequences of responding to the stimulus, the key factor being the conditioned reinforcement value of those immediate consequences. To contrast these approaches, 4 pigeons were exposed to a two-component multiple schedule with three-link chain schedules in each component. Only middle-link stimuli differed between chains. Baseline reinforcement probabilities were 0.50 for both chains; during discrimination phases it was 1.0 for one chain and 0.0 for the other. During discrimination phases pigeons responded more to the reinforcement-correlated middle link than to the extinction-correlated middle link, demonstrating that responding was affected by the probability change. Terminal link responding was also higher in the reinforced chain, even though the terminal link stimulus was identical in both chains. Of greatest interest is initial link responding, which was temporally most distant from reinforcement. Initial link responding, necessarily equal in the two chains, was significantly higher during the 1.0/0.0 discrimination phases, even though overall reinforcement probability remained constant. For 3 of 4 birds, in fact, initial-link response rates were higher than terminal-link response rates, an outcome that can be ascribed only to the potent conditioned reinforcement properties of the middle-link stimulus during the discrimination phases. Results are incompatible with any account of chain behavior based solely on relative time to reinforcement.

Research paper thumbnail of School Desegregation: The Non-Traditional Sociological Perspective

The Journal of Negro Education, 1978

Research paper thumbnail of A test of the melioration theory of matching

Journal of Experimental Psychology: Animal Behavior Processes, 1989

Melioration theory entails that matching in concurrent schedules occurs because the subjects equa... more Melioration theory entails that matching in concurrent schedules occurs because the subjects equalize the local reinforcement rates (reinforcers received for each alternative divided by the time allocated to each alternative). The role of local reinforcement rates was tested by using multiple schedules in which one component involved an alternative with a high absolute rate of reinforcement and a low local reinforcement rate while the second component involved an alternative with a low absolute rate and a high local rate. These alternatives were then presented simultaneously in probe trials to determine preference between them. Contrary to melioration, the absolute rate of reinforcement, not the local rate, was the controlling variable.

Research paper thumbnail of Unramified division algebras do not always contain Azumaya maximal orders

Inventiones mathematicae, 2013

We show that, in general, over a regular integral noetherian affine scheme X of dimension at leas... more We show that, in general, over a regular integral noetherian affine scheme X of dimension at least 6, there exist Brauer classes on X for which the associated division algebras over the generic point have no Azumaya maximal orders over X. Despite the algebraic nature of the result, our proof relies on the topology of classifying spaces of algebraic groups.

Research paper thumbnail of Does neonatal ocular misalignment predict later abnormality?

Eye, 2001

Purpose A longitudinal prospective study was carried out to ascertain the significance of neonata... more Purpose A longitudinal prospective study was carried out to ascertain the significance of neonatal ocular misalignments. Methods Preschool vision screening and hospital records were examined to determine the visual outcome of 1150 infants classified into 'often' (> 15% of waking hours), 'occasionally' « 15%) or 'never' having an ocular misalignment (neonatal squint) in the first 8 weeks of life. X2 and Fisher's exact tests and ANOV A were used to analyse the data. Results When compared with infants who had squinted occasionally or never, frequent squinting in the neonatal period (which occurred in 7.7% of the subjects) was significantly associated with having been prescribed spectacles (p = 0.04), both for hypermetropia (p = 0.04) and for myopia (p = 0.05). Frequent squinters also had a higher incidence of significant esodeviation (p = 0.04) and were more likely to be > 21 days premature (p = 0.05). Small numbers of abnormalities made statistical analysis limited, but there were weak trends towards more myopic and oblique astigmatism in the 'never' group. The esotropias in the 'often' group were more frequently intermittent than those found in the other groups. Conclusions Occasional squinting in the first 8 weeks of life appears to be normal neonatal behaviour. Frequent squinting trebles the chances of developing a significant esodeviation or refractive error severe enough to require spectacles before 5 years of age but incidence of abnormality still does not exceed 9%.

Research paper thumbnail of A prickly problem: developing a volunteer-friendly tool for monitoring populations of a terrestrial urban mammal, the West European hedgehog (Erinaceus europaeus)

Urban Ecosystems

Across Europe, hedgehogs (Erinaceus europaeus) appear to be in decline in both urban and rural la... more Across Europe, hedgehogs (Erinaceus europaeus) appear to be in decline in both urban and rural landscapes. Current methods used to monitor urban populations are, however, associated with several potential limitations. In this study, we conducted hedgehog footprint-tunnel surveys in 219 residential gardens across Reading, UK between May-September in 2013 and/or 2014; gardens were surveyed for five continuous days. Single-species occupancy models were used to investigate factors influencing hedgehog occupancy and two-species occupancy models were used to estimate a species interaction factor (SIF) between hedgehogs and (a) badgers (Meles meles), (b) foxes (Vulpes vulpes) and (c) dogs (Canis familiaris). The five-day survey protocol was associated with a falseabsence error rate of 0.1-0.4%, indicating that it was a reliable method for determining hedgehog presence; conversely, 34.7% of householders were not able to correctly predict hedgehog presence or absence. Hedgehogs were widely distributed across Reading, but detected in only 32-40% of gardens. None of the within-garden or outside-garden factors investigated significantly affected hedgehog occupancy in the single-species models, but the two-species models indicated that badgers (SIF = 0.471 ± 0.188), but not foxes (SIF = 0.954 ± 0.048) or dogs (SIF = 0.780 ± 0.228), negatively affected the presence of hedgehogs in gardens, although not significantly. Overall, footprint-tunnels represent a viable field method for monitoring urban hedgehog populations, however, other approaches are required to identify factors that make gardens Bhedgehog friendly^.

Research paper thumbnail of {"__content__"=>"Ancient duons may underpin spatial patterning of gene expression in Cleaves.", "sub"=>{"__content__"=>"4"}}

Proceedings of the National Academy of Sciences of the United States of America, Feb 20, 2018

If the highly efficient Cphotosynthesis pathway could be transferred to crops with the Cpathway t... more If the highly efficient Cphotosynthesis pathway could be transferred to crops with the Cpathway there could be yield gains of up to 50%. It has been proposed that the multiple metabolic and developmental modifications associated with Cphotosynthesis are underpinned by relatively few master regulators that have allowed the evolution of Cphotosynthesis more than 60 times in flowering plants. Here we identify a component of one such regulator that consists of a pair of-elements located in coding sequence of multiple genes that are preferentially expressed in bundle sheath cells of Cleaves. These motifs represent duons as they play a dual role in coding for amino acids as well as controlling the spatial patterning of gene expression associated with the Cleaf. They act to repress transcription of Cphotosynthesis genes in mesophyll cells. These duons are also present in the Cmodel, and, in fact, are conserved in all land plants and even some algae that use Cphotosynthesis. Cphotosynthesis...

Research paper thumbnail of A Trojan horse? Governmentality, private enterprise and school physical activity

Research paper thumbnail of Topology and purity for torsors

We study the homotopy theory of the classifying space of the complex projective linear groups to ... more We study the homotopy theory of the classifying space of the complex projective linear groups to prove that purity fails for PGL p-torsors on regular noetherian schemes when p is a prime. Extending our previous work when p = 2, we obtain a negative answer to a question of Colliot-Thélène and Sansuc, for all PGL p. We also give a new example of the failure of purity for the cohomological filtration on the Witt group, which is the first example of this kind of a variety over an algebraically closed field. CONTENTS 1. Introduction 1 2. Topology 3 2.1. The p-local cohomology of some Eilenberg-MacLane spaces 3 2.2. The p-local homotopy type of BSL p (C) 5 2.3. The p-local homotopy type of BPGL p (C) 8 3. Purity 9 3.1. Definitions 10 3.2. Purity for torsors 10 3.3. Local purity 12 3.4. Canonical factorization 12 3.5. The Witt group 13 References 14

Research paper thumbnail of Gradient copolymers of thiophene and pyrrole for photovoltaics

Computational Materials Science, 2015

The electronic properties of copolymers can be tuned by controlling their monomer ratio, and ther... more The electronic properties of copolymers can be tuned by controlling their monomer ratio, and therefore can potentially be used to improve charge separation in organic photovoltaic devices. Here we show evidence based on density functional theory calculations that it is possible to control the electronic structure of π-conjugated copolymers of thiophene and pyrrole to obtain a gradient in the band gap and both conduction and valence crystal orbital band levels by controlling their composition. Our calculations predict and optimal thiophene monomer fraction range between zero and 40% is needed in order to obtain the largest electronic structure gradients.

Research paper thumbnail of Use of Frequency Response Metrics to Assess the Planning and Operating Requirements for Reliable Integration of Variable Renewable Generation

An interconnected electric power system is a complex system that must be operated within a safe f... more An interconnected electric power system is a complex system that must be operated within a safe frequency range in order to reliably maintain the instantaneous balance between generation and load. This is accomplished by ensuring that adequate resources are available to respond to expected and unexpected imbalances and restoring frequency to its scheduled value in order to ensure uninterrupted electric service to customers. Electrical systems must be flexible enough to reliably operate under a variety of "change" scenarios. System planners and operators must understand how other parts of the system change in response to the initial change, and need tools to manage such changes to ensure reliable operation within the scheduled frequency range. This report presents a systematic approach to identifying metrics that are useful for operating and planning a reliable system with increased amounts of variable renewable generation which builds on existing industry practices for frequency control after unexpected loss of a large amount of generation. The report introduces a set of metrics or tools for measuring the adequacy of frequency response within an interconnection. Based on the concept of the frequency nadir, these metrics take advantage of new information gathering and processing capabilities that system operators are developing for wide-area situational awareness. Primary frequency response is the leading metric that will be used by this report to assess the adequacy of primary frequency control reserves necessary to ensure reliable operation. It measures what is needed to arrest frequency decline (i.e., to establish frequency nadir) at a frequency higher than the highest set point for underfrequency load shedding within an interconnection. These metrics can be used to guide the reliable operation of an interconnection under changing circumstances.

Research paper thumbnail of Extracting Motion Primitives from Natural Handwriting Data

Lecture Notes in Computer Science, 2006

For the past 10 years it has become clear that biological movement is made up of sub-routine type... more For the past 10 years it has become clear that biological movement is made up of sub-routine type blocks, or motor primitives, with a central controller timing the activation of these blocks, creating synergies of muscle activation. This paper shows that it is possible to use a factorial hidden Markov model to infer primitives in handwriting data. These primitives are not predefined in terms of location of occurrence within the handwriting, and they are not limited or defined by a particular character set. Also, the variation in the data can to a large extent be explained by timing variation in the triggering of the primitives. Once an appropriate set of primitives has been inferred, the characters can be represented as a set of timings of primitive activations, along with variances, giving a very compact representation of the character. Separating the motor system into a motor primitive part, and a timing control gives us a possible insight into how we might create scribbles on paper.

Research paper thumbnail of Evidence that blocking is due to associative deficit: Blocking history affects the degree of subsequent associative competition

Psychonomic Bulletin & Review, 1996

Rats were trained on an appetitive discriminated operant procedure according to a blocking design... more Rats were trained on an appetitive discriminated operant procedure according to a blocking design, in which training with a compound stimulus (A + B) was given with or without prior training with A alone, Stimulus B was then presented in compound with a new stimulus (B + C compound). The ability of Stimulus B to block acquisition of control by Stimulus C was decreased by B having itself been blocked by A during the prior training. The results imply that the mechanism of blocking during the initial blocking procedure was a reduced association between Stimulus B and the reinforcer, and not a performance deficit such as that postulated by comparator models of learning.

Research paper thumbnail of Taxometric analysis supports a dimensional latent structure for schizotypy

Personality and Individual Differences, 2008

The existence of a discrete class of people vulnerable to schizophrenia spectrum disorders is the... more The existence of a discrete class of people vulnerable to schizophrenia spectrum disorders is the most replicated finding of taxometric research. Evidence for such a “taxon” has been obtained with diverse measures of schizotypy in clinical, high-risk, and normal samples. However, ...

Research paper thumbnail of Intraoperative Hyperglycemia and Perioperative Outcomes in Cardiac Surgery Patients

Mayo Clinic Proceedings, 2005

To estimate the magnitude of association between intraoperative hyperglycemia and perioperative o... more To estimate the magnitude of association between intraoperative hyperglycemia and perioperative outcomes in patients who underwent cardiac surgery. We conducted a retrospective observational study of consecutive adult patients who underwent cardiac surgery between June 10, 2002, and August 30, 2002, at the Mayo Clinic, a tertiary care center in Rochester, Minn. The primary independent variable was the mean intraoperative glucose concentration. The primary end point was a composite of death and infectious (sternal wound, urinary tract, sepsis), neurologic (stroke, coma, delirium), renal (acute renal failure), cardiac (new-onset atrial fibrillation, heart block, cardiac arrest), and pulmonary (prolonged pulmonary ventilation, pneumonia) complications developing within 30 days after cardiac surgery. Among 409 patients who underwent cardiac surgery, those experiencing a primary end point were more likely to be male and older, have diabetes mellitus, undergo coronary artery bypass grafting, and receive insulin during surgery (P< or =.05 for all comparisons). Atrial fibrillation (n=105), prolonged pulmonary ventilation (n=53), delirium (n=22), and urinary tract infection (n=16) were the most common complications. The initial, mean, and maximal intraoperative glucose concentrations were significantly higher in patients experiencing the primary end point (P<.01 for all comparisons). In multivariable analyses, mean and maximal glucose levels remained significantly associated with outcomes after adjusting for potentially confounding variables, including postoperative glucose concentration. Logistic regression analyses indicated that a 20-mg/dL increase in the mean intraoperative glucose level was associated with an increase of more than 30% in outcomes (adjusted odds ratio, 1.34; 95% confidence Interval, 1.10-1.62). Intraoperative hyperglycemia is an independent risk factor for complications, including death, after cardiac surgery.

Research paper thumbnail of Conditioned reinforcement versus time to reinforcement in chain schedules

Journal of the Experimental Analysis of Behavior, 1990

Pigeons were trained on three-component chain schedules in which the initial component was either... more Pigeons were trained on three-component chain schedules in which the initial component was either a fixed-interval or variable-interval schedule. The middle and terminal components were varied among fixed-interval fixed-interval, variable-interval variable-interval, and an interdependent variable-interval variable-interval schedule in which the sum of the durations of the two variable-interval components was always equal to the sum of the fixed-interval fixed-interval components. At issue was whether the response rate in the initial component was controlled by its time to primary reinforcement or by the temporal parameters of the stimulus correlated with the middle terminal link. The fixed-interval initial-link schedule maintained much lower response rates than the variable-interval initial-link schedule regardless of the schedules in the middle and terminal links. Nevertheless, the intervening schedules played some role: With fixed-interval schedules in the initial links, response rates were consistently highest with independent variable-interval schedules in the middle and terminal links and intermediate with the interdependent variable-interval schedules; these initial-link differences were predicted by the response rates in the middle link of the chain. With variable-interval schedules in the initial links, response rates were lowest with the fixed-interval fixed-interval schedules following the initial link and were not systematically different for the two types of variable-interval variableinterval schedules. The results suggest that time to reinforcement itself accounts for little if any variance in initial-link responding. Key words: chain schedules, conditioned reinforcement, time to reinforcement, fixed-interval schedules, variable-interval schedules, key peck, pigeons This research was supported by NIMH Research Grant RO 1 MH4297. Requests for reprints should be addressed to the first author,

Research paper thumbnail of Elicited responding to signals for reinforcement: the effects of overall versus local changes in reinforcement probability1

Journal of the Experimental Analysis of Behavior, 1976

Pigeons were studied on a three-component multiple schedule where all reinforcement was independe... more Pigeons were studied on a three-component multiple schedule where all reinforcement was independent of responding. Two components were cued by different keylights and were associated with different rates of reinforcement. The third was always a no-key period associated with extinction. After a few sessions, pecking was elicited by the keylights signalling the reinforcement and continued to be maintained indefinitely. The duration and sequence of the three components were varied to determine if the primary controlling variable was differences in the overall probability of reinforcement, or if it was the immediate change in reinforcement signalled by the onset and/or offset of the stimulus. Both variables were found to control behavior. When 30-sec components were used, the primary controlling variable was the overall probability of reinforcement, but when 3-min components were used, overall probability had little effect. Control by local changes in reinforcement also occurred, although the type of local control varied both across subjects and experimental conditions. Some behaviors were controlled more by the change in reinforcement signalled by the onset of the stimulus, while others were controlled more by the change signalled by the offset of the stimulus.

Research paper thumbnail of Molar versus local reinforcement probability as determinants of stimulus value

Journal of the Experimental Analysis of Behavior, 1993

During one component of a multiple schedule, pigeons were trained on a discrete-trial concurrent ... more During one component of a multiple schedule, pigeons were trained on a discrete-trial concurrent variable-interval variable-interval schedule in which one alternative had a high scheduled rate of reinforcement and the other a low scheduled rate of reinforcement. When the choice proportion between the alternatives matched their respective relative reinforcement frequencies, the obtained probabilities of reinforcement (reinforcer per peck) were approximately equal. In alternate components of the multiple schedule, a single response alternative was presented with an intermediate scheduled rate of reinforcement. During probe trials, each alternative of the concurrent schedule was paired with the constant alternative. The stimulus correlated with the high reinforcement rate was preferred over that with the intermediate rate, whereas the stimulus correlated with the intermediate rate of reinforcement was preferred over that correlated with the low rate of reinforcement. Preference on probe tests was thus determined by the scheduled rate of reinforcement. Other subjects were presented all three alternatives individually, but with a distribution of trial frequency and reinforcement probability similar to that produced by the choice patterns of the original subjects. Here, preferences on probe tests were determined by the obtained probabilities of reinforcement. Comparison of the two sets of results indicates that the availability of a choice alternative, even when not responded to, affects the preference for that alternative. The results imply that models of choice that invoke only obtained probability of reinforcement as the controlling variable (e.g., melioration) are inadequate.

Research paper thumbnail of Conditioned reinforcement dynamics in three-link chained schedules

Journal of the Experimental Analysis of Behavior, 1997

In two experiments rats were trained on three-link concurrent-chains schedules of reinforcement. ... more In two experiments rats were trained on three-link concurrent-chains schedules of reinforcement. In Experiment 1, additional entries to one terminal link were added during one of the middle links to a baseline schedule that was otherwise equal for the two chains, and, depending on the condition, these additional terminal-link presentations ended either in food or in no food. When food occurred, preference was always in favor of the chain with the additional terminal-link presentations (which also entailed a higher rate of reinforcement). When no food occurred at the end of the additional terminal links, the outcome depended on the nature of the stimuli associated with these additional terminal links. When stimuli different from the reinforced baseline terminal links were used for the no-food terminal links, preference was against the choice alternative that led to the extra periods of extinction. When the same stimulus was used for the two kinds of terminal links, preference was near indifference, that is, significantly greater than when different stimuli were used. In Experiment 2, rats learned repeated reversals of a simultaneous discrimination under a three-link concurrentchains schedule, in which the food or no-food choice outcomes were delayed until the end of the chain. Different conditions were defined by the point in the chain at which differential stimuli occurred. When the middle and terminal links provided no differential stimuli, discrimination was acquired more slowly than when differential stimuli occurred in both links. When differential stimuli occurred in the middle but not the terminal links, acquisition rates were intermediate. Both experiments together show that the effects of stimuli in a chain schedule are due partly to the time to food correlated with the stimuli and partly to the time to the next conditioned reinforcer in the sequence.

Research paper thumbnail of A re-examination of local contrast in multiple schedules1

Journal of the Experimental Analysis of Behavior, 1975

Pigeons were presented with multiple schedules of alternating 90-sec components. When components ... more Pigeons were presented with multiple schedules of alternating 90-sec components. When components in which grain was never presented alternated with components in which grain was presented on a variable-interval schedule, the average rate of responding in the variable-interval components increased, showing overall positive behavioral contrast. Unlike previous reports, this study found that the response rates for all birds increased toward the end of the variable-interval components as training proceeded. This increase in local response rate disappeared when the multiple schedule was composed solely of variable-interval components and reappeared when the variable-interval components were again alternated with extinction. This finding cannot be predicted or explained by recent theories of behavioral contrast based on autoshaping, and thus questions their sufficiency. We suggest that this local response-rate increase results from the predictable change from high to low density of reinforcement at the end of the fixed-duration component. Thus, the present effect apparently illustrates a different type of interaction between components of a multiple schedule than that described by previous theories of contrast. In a given procedure, either or both types of interaction may occur; neither provides a complete account of behavioral contrast.

Research paper thumbnail of Conditioned Reinforcement in Chain Schedules When Time to Reinforcement is Held Constant

Journal of the Experimental Analysis of Behavior, 2013

Two alternative approaches describe determinants of responding to a stimulus temporally distant f... more Two alternative approaches describe determinants of responding to a stimulus temporally distant from primary reinforcement. One emphasizes the temporal relation of each stimulus to the primary reinforcer, with relative proximity of the stimulus determining response rate. A contrasting view emphasizes immediate consequences of responding to the stimulus, the key factor being the conditioned reinforcement value of those immediate consequences. To contrast these approaches, 4 pigeons were exposed to a two-component multiple schedule with three-link chain schedules in each component. Only middle-link stimuli differed between chains. Baseline reinforcement probabilities were 0.50 for both chains; during discrimination phases it was 1.0 for one chain and 0.0 for the other. During discrimination phases pigeons responded more to the reinforcement-correlated middle link than to the extinction-correlated middle link, demonstrating that responding was affected by the probability change. Terminal link responding was also higher in the reinforced chain, even though the terminal link stimulus was identical in both chains. Of greatest interest is initial link responding, which was temporally most distant from reinforcement. Initial link responding, necessarily equal in the two chains, was significantly higher during the 1.0/0.0 discrimination phases, even though overall reinforcement probability remained constant. For 3 of 4 birds, in fact, initial-link response rates were higher than terminal-link response rates, an outcome that can be ascribed only to the potent conditioned reinforcement properties of the middle-link stimulus during the discrimination phases. Results are incompatible with any account of chain behavior based solely on relative time to reinforcement.

Research paper thumbnail of School Desegregation: The Non-Traditional Sociological Perspective

The Journal of Negro Education, 1978

Research paper thumbnail of A test of the melioration theory of matching

Journal of Experimental Psychology: Animal Behavior Processes, 1989

Melioration theory entails that matching in concurrent schedules occurs because the subjects equa... more Melioration theory entails that matching in concurrent schedules occurs because the subjects equalize the local reinforcement rates (reinforcers received for each alternative divided by the time allocated to each alternative). The role of local reinforcement rates was tested by using multiple schedules in which one component involved an alternative with a high absolute rate of reinforcement and a low local reinforcement rate while the second component involved an alternative with a low absolute rate and a high local rate. These alternatives were then presented simultaneously in probe trials to determine preference between them. Contrary to melioration, the absolute rate of reinforcement, not the local rate, was the controlling variable.

Research paper thumbnail of Unramified division algebras do not always contain Azumaya maximal orders

Inventiones mathematicae, 2013

We show that, in general, over a regular integral noetherian affine scheme X of dimension at leas... more We show that, in general, over a regular integral noetherian affine scheme X of dimension at least 6, there exist Brauer classes on X for which the associated division algebras over the generic point have no Azumaya maximal orders over X. Despite the algebraic nature of the result, our proof relies on the topology of classifying spaces of algebraic groups.

Research paper thumbnail of Does neonatal ocular misalignment predict later abnormality?

Eye, 2001

Purpose A longitudinal prospective study was carried out to ascertain the significance of neonata... more Purpose A longitudinal prospective study was carried out to ascertain the significance of neonatal ocular misalignments. Methods Preschool vision screening and hospital records were examined to determine the visual outcome of 1150 infants classified into 'often' (> 15% of waking hours), 'occasionally' « 15%) or 'never' having an ocular misalignment (neonatal squint) in the first 8 weeks of life. X2 and Fisher's exact tests and ANOV A were used to analyse the data. Results When compared with infants who had squinted occasionally or never, frequent squinting in the neonatal period (which occurred in 7.7% of the subjects) was significantly associated with having been prescribed spectacles (p = 0.04), both for hypermetropia (p = 0.04) and for myopia (p = 0.05). Frequent squinters also had a higher incidence of significant esodeviation (p = 0.04) and were more likely to be > 21 days premature (p = 0.05). Small numbers of abnormalities made statistical analysis limited, but there were weak trends towards more myopic and oblique astigmatism in the 'never' group. The esotropias in the 'often' group were more frequently intermittent than those found in the other groups. Conclusions Occasional squinting in the first 8 weeks of life appears to be normal neonatal behaviour. Frequent squinting trebles the chances of developing a significant esodeviation or refractive error severe enough to require spectacles before 5 years of age but incidence of abnormality still does not exceed 9%.