Showing papers by "Louisiana State University published in 1989"
01 Jan 1989
TL;DR: In an autopsy study of the evolution of atherosclerotic lesions in young people, the coronary arteries and aortas of 1160 male and female subjects who died between full-term birth and age 29 years are obtained.
Abstract: In an autopsy study of the evolution of atherosclerotic lesions in young people, we obtained the coronary arteries and aortas of 1160 male and female subjects who died between full-term birth and age 29 years. In this article, we report the light and electron microscopic observations of the coronary arteries of 565 of these subjects in which we fixed the coronary arteries by perfusion with glutaraldehyde under pressure. From birth, the intima was always thicker in the half of the coronary artery circumference opposite the flow-divider wall of a bifurcation (eccentric thickening). In cases where we found lipid in the intima, there was always more in eccentric thickening. Isolated macrophage foam cells in the intima of infants were the earliest sign of lipid retention. These cells occurred in 45% of infants in the first 8 months of life but decreased subsequently. At puberty, more substantial accumulations of macrophage foam cells reappeared in more children. Foam cells were now accompanied by lipid droplets in existing smooth muscle cells and by thinly scattered extracellular lipid. Sixty-five percent of children between ages 12 and 14 years had such lesions. An additional 8% of children had progressed beyond this early stage and had developed advanced preatheroma or atheroma stages. Such advanced lesions, located only in areas of eccentric thickening, were characterized by the addition of massive extracellular lipid that displaced normal cells and matrix and, thus, damaged and weakened the arterial wall.(ABSTRACT TRUNCATED AT 250 WORDS)
795 citations
TL;DR: In this article, the authors test the proposition that the effectiveness of a particular strategic orientation (reactor, defender, analyzer, and prospector) is contingent upon the dynamics of the market.
Abstract: The authors test the proposition that the effectiveness of a particular strategic orientation—reactor, defender, analyzer, and prospector—is contingent upon the dynamics of the market. In mildly vo...
619 citations
TL;DR: In this paper, the authors focus on the treatment integrity of treatment, i.e., the degree to which a treatment i.i.d. a patient's treatment integrity.
Abstract: The consultation and prereferral intervention literature has ignored a fundamental aspect of behavior change: treatment integrity. Integrity of treatment refers to the degree to which a treatment i...
607 citations
TL;DR: The degree of membrane lipid peroxidative damage in erythrocytes was significantly correlated with the level of glycosylated hemoglobin, an index of mean glucose level for the preceding 3–4mo, which suggests that peroxidation of membrane lipids and accumulation of MDA occurs in ERY Throcytes of diabetic patients.
Abstract: Erythrocytes of diabetic patients have abnormal membrane properties. We examined in vivo membrane lipid peroxidation in erythrocytes of diabetic subjects and its possible relationship with hyperglycemia. Lipid peroxidation was assessed in fresh, untreated erythrocytes by quantitating thiobarbituric acid reactivity and an adduct of phospholipids and malonyldialdehyde (MDA), an end product of lipid peroxidation, with thin-layer chromatography of lipid extract of diabetic erythrocytes. There was a significantly increased membrane lipid peroxidation in diabetic erythrocytes compared with nondiabetic erythrocytes. The degree of membrane lipid peroxidative damage in erythrocytes was significantly correlated with the level of glycosylated hemoglobin, an index of mean glucose level for the preceding 3-4 mo. This suggests that peroxidation of membrane lipids and accumulation of MDA occurs in erythrocytes of diabetic patients.
607 citations
TL;DR: In this paper, an approach only recently investigated by aquatic toxicologists will be described and reviewed in detail, based on theories of oxyradical generation and subsequent oxidative stress in biological systems.
Abstract: The study of biochemical responses in aquatic animals comprises a vigorous area of inquiry within ecotoxicology for a number of reasons, including the perceived need for basic research in the field, the desire for highly sensitive biomarkers useful for biomonitoring and the particular concern for elevated rates of neoplasia observed in some aquatic systems. In this paper, an approach only recently investigated by aquatic toxicologists will be described and reviewed in detail. This approach is based on theories of oxyradical generation and subsequent oxidative stress in biological systems. Of particular concern to environmental toxicologists with respect to these phenomena are the abilities of a number of common and diverse compounds to undergo enzymatically facilitated redox cycling in cells and thereby generate oxyradicals under aerobic conditions. Mechanisms of oxyradical generation, toxicological consequences of these processes and endogenous antioxidant defense systems are described. In addition, methodologies for studying these phenomena are discussed and recent studies demonstrating their applicability to aquatic toxicology are reviewed.
584 citations
TL;DR: Oxygen transport through the air spaces of the stem and roots of aquatic macrophytes into the root zone supports nitrification of NH,+, with the NOJ- formed diffusing into the adjacent anaerobic zone where it undergoes #denitrification.
Abstract: Oxygen transport through the air spaces (aerenchyma tissue) of the stem and roots of aquatic macrophytes into the root zone supports nitrification of NH,+, with the NOJ- formed diffusing into the adjacent anaerobic zone where it undergoes #denitrification. To test this hypothesis, we conducted a growth chamber study to determine the transformation of applied 15NH,+-N to r5N, in the root zone of three aquatic macrophytes: rice (Oryza sativa L.), pickerel weed (Pontecferia cordata L.), and soft rush (Juncus e&us L.). Detection of gaseous 15N, in the air above the floodwater of the soil column with aquatic plants provided direct evidence of nitrification-dcnitrification in the root zone, while such losses were not measurable for soil columns without plants. Air spaces in aquatic plants can also function as conduits for denitrified gases from anaerobic sediments to the atmosphere. Maximal 15N, flux due to this process was 102, 113, and 122 mg N m-2 d-l for soft rush, rice, and pickerel weed. This N loss mechanism has important agronomic and ecological consequences.
543 citations
TL;DR: It is suggested that 22:6 synthesized from 18:3 by the liver is secreted into the bloodstream in lipoproteins, taken up by brain and retina, and incorporated into cell membranes.
Abstract: Docosahexaenoic acid [22:6 omega 3; 22:6(4, 7, 10, 13, 16, 19)] is concentrated in phospholipids of cellular membranes from brain and retina. Although linolenic acid [18:3 omega 3; 18:3(9, 12, 15)] is the major omega 3 fatty acid of mouse dams' milk, 22:6 is the prevalent omega 3 fatty acid in serum and tissues. Intraperitoneal injection of [1-14C]18:3 into 3-day-old mouse pups resulted in liver and serum lipid labeling that was initially high, followed by a rapid decline. In contrast, labeling of brain and retinal lipids were initially low and increased with time. Labeled 22:6 first appeared in liver 2 hr after injection and later in brain and retina. We suggest that 22:6 synthesized from 18:3 by the liver is secreted into the bloodstream in lipoproteins, taken up by brain and retina, and incorporated into cell membranes. We hypothesize that the 22:6 requirements of membranes (e.g., during synaptogenesis, photoreceptor membrane biogenesis, or repair after ischemic injury or neurodegenerative disorders) are met by a signal that is sent by the appropriate tissues to the liver to evoke the secretion of 22:6-containing lipoproteins.
438 citations
TL;DR: In this article, implicit learning is conceptualized as an automatic, memory-based mechanism for detecting patterns of family resemblance among exemplars, which is similar to conscious problem-solvers' implicit learning mechanisms for discovering and controlling task variables.
Abstract: Four experiments are reported in which subjects gained extensive experience with artificial grammars in explicit and implicit processing tasks. Results indicated that (a) implicit processing was sufficient for learning a finite state grammar but was inadequate for learning another type of grammar based on logical rules, (b) Subjects were able to communicate some of their implicit knowledge of the grammars to another person, (c) Consistent with rule induction but not memory array models of learning, verbal protocols indicated there was no tendency to converge on the same set of cues used to identify valid strings, (d) A synergistic learning effect occurred when both implicit and explicit processing tasks were used in the grammar based on logical rules but not in the finite state grammar. A theoretical framework is proposed in which implicit learning is conceptualized as an automatic, memory-based mechanism for detecting patterns of family resemblance among exemplars. Explicit learning mechanisms for discovering and controlling task variables are similar to conscious problem solving. These processes include attempts to form a mental representation of the task, searching memory for knowledge of analogous systems, and attempts to build and test mental models of task performance (Gentner & Stevens, 1983; JohnsonLaird, 1983). Implicit learning is thought to be an alternate mode of learning that is automatic, nonconscious, and more powerful than explicit thinking for discovering nonsalient covariance between task variables (Lewicki, 1986; Reber, 1969, 1976; Reber & Allen, 1978). Demonstrations of implicit learning of artificial grammars typically involve comparisons between groups of subjects who experience exemplars of a grammar under (a) instructions to figure out the rules of the grammar (rule discovery instructions) or (b) instructions that require attention to the exemplars without attempting to determine the rules of the grammar (e.g., groups of subjects asked to memorize the exemplars for a subsequent memory test). Typically, groups of subjects who implicitly learned the grammar do as well or better on subsequent attempts to discriminate between new valid versus invalid strings as subjects who
427 citations
TL;DR: The applicability of this approach for multidrug residue extraction from a single sample is demonstrated for compounds representing the organophosphate, benzimidazole anthelmintic and beta-lactam antibiotic drug classes.
421 citations
TL;DR: Results demonstrate that a whole virus vaccine is highly effective in inducing immune responses that can protect against lentivirus infection and AIDS-like disease.
Abstract: A vaccine against human immunodeficiency virus (HIV) would be highly effective in stopping the acquired immunodeficiency syndrome (AIDS) epidemic. A comprehensive evaluation of potential vaccine methodologies can be made by means of the simian model for AIDS, which takes advantage of the similarities in viral composition and disease potential between simian immunodeficiency virus (SIV) infection of rhesus macaques and HIV infection in humans. Immunization with a formalin-inactivated whole SIV vaccine potentiated with either alum and the Syntex adjuvant threonyl muramyl dipeptide (MDP) or MDP alone resulted in the protection of eight of nine rhesus monkeys challenged with ten animal-infectious doses of pathogenic virus. These results demonstrate that a whole virus vaccine is highly effective in inducing immune responses that can protect against lentivirus infection and AIDS-like disease.
411 citations
TL;DR: In this article, a study of wetland values in coastal Louisiana was conducted, which employed both willingness-to-pay and energy analysis-based methodologies and were able to bracket a range of values within which they feel fairly confident the true value lies.
TL;DR: Observations that both allopurinol and superoxide dismutase attenuate reperfusion-induced leukocytes adherence and extravasation are consistent with the hypothesis that xanthine oxidase-derived oxidants initiate the leukocyte infiltration induced by reperfusions of ischemic intestine.
Abstract: Xanthine oxidase-derived oxidants and leukocytes have been implicated in the microvascular injury associated with reperfusion of ischemic intestine. The objective of this study was to determine whether xanthine oxidase-derived oxidants play a role in the leukocyte-microvascular interactions initiated by ischemia-reperfusion. Adherence and extravasation of leukocytes were monitored in cat mesenteric venules subjected to 1 h of ischemia (blood flow reduced to 20% of control) and reperfusion. Leukocyte rolling velocity, vessel diameter, and red cell velocity were also measured in control (untreated) animals and in animals pretreated with either allopurinol or superoxide dismutase. The responses of venular blood flow, wall shear rate, and leukocyte rolling velocity to ischemia and reperfusion did not differ between the three experimental groups. In control animals, 1 h of ischemia was associated with significant adherence and extravasation of leukocytes with reperfusion greatly enhancing these responses. Allopurinol treatment did not alter the responses to ischemia per se, yet it largely prevented the further increment in adherence and extravasation associated with reperfusion. Superoxide dismutase treatment attenuated the leukocyte responses elicited by both ischemia and reperfusion. Our observations that both allopurinol and superoxide dismutase attenuate reperfusion-induced leukocyte adherence and extravasation are consistent with the hypothesis that xanthine oxidase-derived oxidants initiate the leukocyte infiltration induced by reperfusion of ischemic intestine.
TL;DR: Elevated glucose levels can cause the peroxidation of membrane lipids in human RBC, and a significant positive correlation was observed between the extent of glucose-induced membrane lipid per oxidation and the osmotic fragility of treated RBC.
TL;DR: In this article, a 9-item Treatment Evaluation Inventory (TEI-Short Form) with a 5-point scale, consistent anchors on the scale, and simplified text and instructions is presented.
Abstract: Kazdin's (1980a) Treatment Evaluation Inventory (TEI) is the major instrument used to assess parents' acceptance of procedures for behavior problem children. The length of the TEI, however, as well as problems with its scaling and wording limits its value as a clinical research instrument. In the present study, three experiments were conducted to develop a modified TEI. In Experiment 1, 153 parents completed the TEI to evaluate a behavioral treatment for noncompliant and oppositional children. A factor analysis of the data was used to obtain a reliable factor structure for the TEI and to construct a 9-item TEI-Short Form (TEI-SF) with a 5-point scale, consistent anchors on the scale, and simplified text and instructions. Experiment 2 evaluated the psychometric characteristics of the TEI and the TEI-SF. These data indicated the TEI-SF is a sound alternative to the original TEI. Experiment 3 compared the readability and completion time of the two instruments.
TL;DR: In this article, the authors investigated the relationship between specific personality variables and job performance in a sample (N= 43) of accountants and found that three personality scales (orientation towards work, degree of ascendancy, and degree and quality of interpersonal orientation) are significantly related to important aspects of job performance.
Abstract: It is suggested that personality variables are significant predictors of job performance when carefully matched with the appropriate occupation and organization. The present study investigates the relationship between specific personality variables and job performance in a sample (N= 43) of accountants. The results indicate that even with the effects of cognitive ability taken into account, three personality scales (orientation towards work; degree of ascendancy; and degree and quality of interpersonal orientation) are significantly related to important aspects of job performance. It is suggested that the overall validity of selection strategies might be improved with the addition of measures of relevant personality dimensions when appropriately matched to an occupation and organization.
TL;DR: In this article, four studies were conducted to assess the accuracy with which consumers perceive objective price-quality relationships and found that, overall, consumers perceive object quality relationships as more important than product quality relationships.
Abstract: Four studies were conducted to assess the accuracy with which consumers perceive objective price-quality relationships. Results across four studies indicate that, overall, consumers perceive object...
TL;DR: In this article, the incidence relation of a graph G=(V. E) was viewed as an order relation on its vertices and edges, i.e. if and only of a is a vertex and b is an edge incident on a.
Abstract: We view the incidence relation of a graph G=(V. E) as an order relation on its vertices and edges, i.e. a
TL;DR: Findings are taken as evidence that, although the different meanings of ambiguous stimuli may be processed in parallel by all subjects, an interpretive bias operates such that anxiety-prone individuals tend to become preferentially aware of the more threatening meaning of such events.
Abstract: In previous studies, we have established that anxiety states are characterized by an attentional bias that favors the processing of threatening stimuli. In the present study we extend this finding to ambiguous stimuli, specifically, homophones with spellings that correspond to either a threatening or a neutral meaning. As predicted, clinically anxious subjects used the threatening spellings relatively more than did controls, whereas recovered subjects were intermediate in this respect. Threatening words were associated with greater skin conductance responses than were neutral words, but the groups did not differ in their electrodermal reactions to homophones. We take these findings as evidence that, although the different meanings of ambiguous stimuli may be processed in parallel by all subjects, an interpretive bias operates such that anxiety-prone individuals tend to become preferentially aware of the more threatening meaning of such events.
TL;DR: In this paper, the authors examined the influence of feedback sign and feedback credibility on goal setting and task performance, and found that feedback sign had a main effect on subjects' performance.
TL;DR: The present study contrasts implicit (word completion) with explicit (cued recall) memory and shows that indices of bias for emotional material derived from the two types of memory are independent of one another.
Abstract: Previous investigations of recall and recognition for threatening information in clinically anxious subjects have yielded equivocal results. The present study contrasts implicit (word completion) with explicit (cued recall) memory and shows that indices of bias for emotional material derived from the two types of memory are independent of one another. The explicit measure was correlated with trait anxiety scores, but did not clearly distinguish between subjects with clinical anxiety states and normal control subjects. On the implicit memory measure, clinically anxious subjects produced more threat word completions, but only from a set to which they had recently been exposed. These results are taken as evidence that internal representations of threat words are more readily or more persistently activated in anxiety states, although they are not necessarily better elaborated.
TL;DR: It appeared that simple intestinal obstruction of the colon or small bowel in the absence of necrotic bowel was associated with bacterial translocation, and the most common bacteria cultured was Escherichia coli.
Abstract: • Indirect clinical evidence has accumulated indicating that the gut may be a reservoir for microorganisms causing systemic infection in man. Our experimental results, in a variety of animal models, demonstrate that bacteria can translocate across the mucosal barrier and cause systemic infections. To determine directly whether bacterial translocation occurs in man, we cultured mesenteric lymph nodes (MLNs) obtained at laparotomy from 42 patients, none of whom were clinically infected. Ten (59%) of 17 patients with intestinal obstruction (none of whom had necrotic bowel) had bacteria in their MLNs, in contrast to one (4%) of 25 patients operated on for other reasons. The most common bacteria cultured from the MLNs wasEscherichia coli. Thus, it appeared that simple intestinal obstruction of the colon or small bowel in the absence of necrotic bowel was associated with bacterial translocation. (Arch Surg1989;124:699-701)
TL;DR: Growing evidence now suggests that the PGE group of prostaglandins can serve as a negative feedback modulator of immune response, and in vitro effects of other cyclooxygenase‐derived products, such as PGI2 and thromboxane A2 (TXA2), have not been well established, perhaps because of their instability in aqueous media.
Abstract: The implication that essential fatty acids (EFA) can affect immune response was based on the observation that EFA deficiency can accentuate or improve symptoms of certain autoimmune diseases in animals, and that supplementation of linoleic acid to animals reversed such effects. Furthermore, treatment of animals with cyclooxygenase inhibitors abrogated the effect of linoleic acid. Administration of cyclooxygenase inhibitors to animals enhanced both cell-mediated and humoral immune responses. In vitro studies have shown that prostaglandin E (PGE) group inhibits both T and B lymphocyte functions; it is suggested that effects of EFA on immune response are, in part, mediated through eicosanoids. Growing evidence now suggests that the PGE group of prostaglandins can serve as a negative feedback modulator of immune response. However, in vitro effects of other cyclooxygenase-derived products, such as PGI2 and thromboxane A2 (TXA2) have not been well established, perhaps because of their instability in aqueous media. Unlike the PGE group, some of lipoxygenase-derived products of arachidonic acid have shown immunostimulatory effects, as assessed by lymphokine production in vitro. Whether such effects can be seen in vivo remains to be determined. Some lipoxygenase-derived products with strong chemotactic action may indirectly influence immune response by modulating the population of antigen-presenting macrophages in tissues. Thus, the net effect of eicosanoids synthesized in macrophages on modulating immune response may depend on relative amounts of cyclooxygenase-derived products as compared with lipoxygenase-derived products. Macrophages are the major source of eicosanoids among immunocompetent cells. The profile of eicosanoids, produced in vitro by macrophages, varies with type of stimuli and anatomical sites. It can also be affected by the fatty acid composition of tissue lipids, which in turn can be modified by the composition of dietary EFA. Whether manipulating dietary EFA can modulate immune response in normal humans and animals needs to be determined.
TL;DR: In this article, the authors measured sediment accretion and accumulation rates with feldspar marker horizons in the vicinity of natural waterways and man-made canals with spoil banks in the rapidly subsiding environment of coastal Louisiana.
Abstract: Recent (6–12 month) marsh sediment accretion and accumulation rates were measured with feldspar marker horizons in the vicinity of natural waterways and man-made canals with spoil banks in the rapidly subsiding environment of coastal Louisiana. Annual accretion rates in aSpartina alterniflora salt marsh in the Mississippi deltaic plain averaged 6 mm in marsh adjacent to canals compared to 10 mm in marsh adjacent to natural waterways. The rates, however, were not statistically significantly different. The average rate of sediment accretion in the same salt marsh region for a transect perpendicular to a canal (13 mm yr−1) was significantly greater than the rate measured for a transect perpendicular to a natural waterway (7 mm yr−1). Measurements of soil bulk density and organic matter content from the two transects were also different. This spatial variability in accretion rates is probably related to (1) spoil bank influences on local hydrology; and (2) a locally high rate of sediment input from lateral erosion associated with pond enlargement. In a brackishSpartina patens marsh on Louisiana’s Chenier plain, vertical accretion rates were the same along natural and canal waterways (3–4 mm yr−1) in a hydrologically restricted marsh region. However, the accretion rates for both waterways were significantly lower than the rates along a nonhydrologically restricted natural waterway nearby (11 mm yr−1). The vertical accretion of matter displayed semi-annual differences in the brackish marsh environment.
TL;DR: Results indicate that superoxide mediates reperfusion-induced leukocyte adherence and that endothelial cells are required for this superoxide-mediated adherence.
Abstract: The objective of this study was to determine whether superoxide mediates the leukocyte-endothelial cell interactions elicited by reperfusion (reoxygenation) of ischemic (hypoxic) tissues. Mesenteric and intestinal blood flows were reduced to 20% of control for 1 h, followed by 1 h of reperfusion. Sixty minutes after reperfusion, red blood cell velocity (Vr), leukocyte rolling velocity (Vw), and the number of adherent leukocytes were measured in mesenteric venules. Then, either human superoxide dismutase (hSOD), hydrogen peroxide-inactivated hSOD, or MoAb IB4 (a monoclonal antibody against the leukocyte adhesion molecule CD18) was injected intravenously. Ten minutes later, repeat measurements were obtained and compared with pretreatment values. hSOD attenuated reperfusion-induced neutrophil adherence and increased Vw/Vr, an index of the fracture stress between leukocytes and endothelium. Peroxide-inactivated hSOD did not alter any parameter. MoAb IB4 attenuated reperfusion-induced adherence but did not alter Vw/Vr. In a correlate study, cultured bovine microvascular endothelium was exposed to 30 min of anoxia, followed by 60 min of reoxygenation. Cat neutrophils were added during reoxygenation. Reoxygenation-induced leukocyte adherence was attenuated by either hSOD or MoAb IB4 but not by inactivated hSOD. Adherence of phorbol 12-myristate 13-acetate-activated cat neutrophils to plastic was unaffected by hSOD or inactive hSOD, yet MoAb IB4 virtually abolished the response. These results indicate that superoxide mediates reperfusion-induced leukocyte adherence and that endothelial cells are required for this superoxide-mediated adherence.
TL;DR: In this paper, a computational model of the acquisition of the gender, number, and case paradigm for the German definite article was developed, using a connectionist algorithm developed by Rumelhart, Hinton, and Williams.
TL;DR: Field and greenhouse experiments in which salinity and elevation were manipulated demonstrated that the response of freshwater marshes to saltwater intrusion may be variable and dependent upon a number of factors including species composition; level, duration, and abruptness of exposure to saline water; flooding depth; a source of propagules of more salt-tolerant species.
TL;DR: The development of a continuum of care that is coordinated across the mental health and non-mental-health systems that naturally occur in all children's lives has the potential to vastly improve mental health services to children, youth, and families.
Abstract: Throughout this century, people in the United States have been concerned about the serious deficiencies in the mental health care of our children. Despite eloquent needs assessment and recommendations for remediation, most of the unserved needs and deficiencies of our mental-health-care-delivery system remain the same. This article reviews the current status of mental health services to children, youth, and families to highlight the necessity of an integrated system of mental health care. The development of a continuum of care that is coordinated across the mental health and non-mental-health systems that naturally occur in all children's lives has the potential to vastly improve mental health services to children, youth, and families.
TL;DR: The present assessment of awareness was based on an objective definition of awareness, rather than a subjective definition as in Lewicki et al.'s study, which revealed that learning relied on perceptual aspects of the task.
Abstract: Lewicki, Czyzewska, and Hoffman (1987) demonstrated learning without awareness in a visual search task. Rules determined target location on every seventh trial on the basis of target locations in the preceding six trials. Learning was demonstrated by negative transfer effects when the rules were changed. When questioned afterwards, the subjects could not describe the rules and denied awareness of them. This experiment was designed to replicate that of Lewicki et al. and to test several hypotheses about this apparent learning without awareness. Transfer conditions were included to determine whether rule learning was primarily perceptual or motor. The present assessment of awareness was based on an objective definition of awareness, rather than a subjective definition as in Lewicki et al.'s study. Their effect was replicated, and the transfer conditions revealed that learning relied on perceptual aspects of the task. The objective measure of awareness provided further evidence that subjects were unaware of the rules. Language: en
TL;DR: In this article, a theoretical framework is proposed in which subjects draw on two separate but interacting knowledge structures to perform simulated process control tasks, one based on memory for past experiences (close analogies) and the other based on one's current mental model of the task.
Abstract: Four experiments in which subjects learned to control two versions of a complex simulated process control task show that verbalizable knowledge of procedures used to perform these tasks is very limited and is acquired late in learning. Individual learning curves associated with these tasks showed sudden improvements in performance, which were not accompanied by a similar increase in verbalizable knowledge. It was also found that verbal instructions consisting of exemplar memorization, strategies for rule induction, simple heuristics, and experts' instructions were all effective in enhancing novice subjects' performance. A theoretical framework is proposed in which subjects draw on two separate but interacting knowledge structures to perform these tasks. One knowledge structure is based on memory for past experiences (close analogies), and the other is based on one's current mental model of the task. Implicit sets of competing rules that control response selection are derived from both sources of knowledge...