Showing papers by "Boston University published in 1987"

A massively parallel architecture for a self-organizing neural pattern recognition machine

Abstract: A neural network architecture for the learning of recognition categories is derived. Real-time network dynamics are completely characterized through mathematical analysis and computer simulations. The architecture self-organizes and self-stabilizes its recognition codes in response to arbitrary orderings of arbitrarily many and arbitrarily complex binary input patterns. Top-down attentional and matching mechanisms are critical in self-stabilizing the code learning process. The architecture embodies a parallel search scheme which updates itself adaptively as the learning process unfolds. After learning self-stabilizes, the search process is automatically disengaged. Thereafter input patterns directly access their recognition codes without any search. Thus recognition time does not grow as a function of code complexity. A novel input pattern can directly access a category if it shares invariant properties with the set of familiar exemplars of that category. These invariant properties emerge in the form of learned critical feature patterns, or prototypes. The architecture possesses a context-sensitive self-scaling property which enables its emergent critical feature patterns to form. They detect and remember statistically predictive configurations of featural elements which are derived from the set of all input patterns that are ever experienced. Four types of attentional process—priming, gain control, vigilance, and intermodal competition—are mechanistically characterized. Top—down priming and gain control are needed for code matching and self-stabilization. Attentional vigilance determines how fine the learned categories will be. If vigilance increases due to an environmental disconfirmation, then the system automatically searches for and learns finer recognition categories. A new nonlinear matching law (the ⅔ Rule) and new nonlinear associative laws (the Weber Law Rule, the Associative Decay Rule, and the Template Learning Rule) are needed to achieve these properties. All the rules describe emergent properties of parallel network interactions. The architecture circumvents the noise, saturation, capacity, orthogonality, and linear predictability constraints that limit the codes which can be stably learned by alternative recognition models.

System for self-organization of stable category recognition codes for analog input patterns

Abstract: A neural network includes a feature representation field which receives input patterns. Signals from the feature representation field select a category from a category representation field through a first adaptive filter. Based on the selected category, a template pattern is applied to the feature representation field, and a match between the template and the input is determined. If the angle between the template vector and a vector within the representation field is too great, the selected category is reset. Otherwise the category selection and template pattern are adapted to the input pattern as well as the previously stored template. A complex representation field includes signals normalized relative to signals across the field and feedback for pattern contrast enhancement.

The prevalence of knee osteoarthritis in the elderly. The Framingham Osteoarthritis Study.

Abstract: To investigate the prevalence of osteoarthritis (OA) of the knee in elderly subjects, we studied the Framingham Heart Study cohort, a population-based group. During the eighteenth biennial examination, we evaluated the cohort members for OA of the knee by use of medical history, physical examination, and anteroposterior (standing) radiograph of the knees. Radiographs were obtained on 1,424 of the 1,805 subjects (79%). Their ages ranged from 63-94 years (mean 73). Radiographs were read by a radiologist who specializes in bone and joint radiology, and were graded 0-4 according to the scale described by Kellgren and Lawrence. OA was defined as grade 2 changes (definite osteophytes), or higher, in either knee. Radiographic evidence of OA increased with age, from 27% in subjects younger than age 70, to 44% in subjects age 80 or older. There was a slightly higher prevalence of radiographic changes of OA in women than in men (34% versus 31%); however, there was a significantly higher proportion of women with symptomatic disease (11% of all women versus 7% of all men; P = 0.003). The age-associated increase in OA was almost entirely the result of the marked age-associated increase in the incidence of OA in the women studied. This study extends current knowledge about OA of the knee to include elderly subjects, and shows that the prevalence of knee OA increases with age throughout the elderly years.

Stochastic Trends and Economic Fluctuations

Abstract: Are business cycles mainly the result of permanent shocks to productivity? This paper uses a long-run restriction implied by a large class of real-business-cycle models -identifying permanent productivity shocks as shocks to the common stochastic trend in output, consumption, and investment -to provide new evidence on this question. Econometric tests indicate that this common-stochastic-trend / cointegration implication is consistent with postwar U.S. data. However, in systems with nominal variables, the estimates of this common stochastic trend indicate that permanent productivity shocks typically explain less than half of the business-cycle variability in output, consumption, and investment. (JEL E32, C32) A central, surprising, and controversial result of some current research on real business cycles is the claim that a common stochastic trend-the cumulative effect of permanent shocks to productivity-underlies the bulk of economic fluctuations. If confirmed, this finding would imply that many other forces have been relatively unimportant over historical business cycles, including the monetary and fiscal policy shocks stressed in traditional macroeconomic analysis. This paper shows that the hypothesis of a common stochastic productivity trend has a set of econometric implications that allows us to test for its presence, measure its importance, and extract estimates of its realized value. Applying these procedures to consumption, investment, and output for the postwar United States, we find results that both support and contradict this claim in the real-businesscycle literature. The U.S. data are consistent with the presence of a common

The genetic defect causing familial Alzheimer's disease maps on chromosome 21

Abstract: Alzheimer's disease is a leading cause of morbidity and mortality among the elderly. Several families have been described in which Alzheimer's disease is caused by an autosomal dominant gene defect. The chromosomal location of this defective gene has been discovered by using genetic linkage to DNA markers on chromosome 21. The localization on chromosome 21 provides an explanation for the occurrence of Alzheimer's disease-like pathology in Down syndrome. Isolation and characterization of the gene at this locus may yield new insights into the nature of the defect causing familial Alzheimer's disease and possibly, into the etiology of all forms of Alzheimer's disease.

Echocardiographic criteria for left ventricular hypertrophy: the Framingham heart study

Abstract: Of 6,148 original cohort and offspring subjects of the Framingham Heart Study who underwent routine evaluation, a healthy group of 347 men (aged 42 ± 12 years) and 517 women (aged 43 ± 12 years) was identified to develop echocardiographic criteria for left ventricular (LV) hypertrophy. Healthy subjects were defined as normotensive, receiving no cardiac or antihypertensive medications, nonobese and free of cardiopulmonary disease. Echocardiographic criteria (in accordance with the American Society of Echocardiography convention) for LV hypertrophy, based on mean plus 2 standard deviations for LV mass, LV mass corrected for body surface area and LV mass corrected for height in this healthy sample are, respectively: 294 g, 150 g/m2 and 163 g/m In men and 198 g, 120 g/m2 and 121 g/m in women. Criteria based on LV mass/height result in higher prevalence rates of LV hypertrophy than LV mass/body surface area while still correctIng for body size. The prevalence of LV hypertrophy in the entire study population (using LV mass/height criteria) is 16% in men and 19% in women. Until outcome guided criteria for LV hypertrophy are developed, application of sex-specific criteria based on a healthy population distribution of LV mass offer the best approach to echocardiographic diagnosis of LV hypertrophy.

Cingulate cortex of the rhesus monkey: II. Cortical afferents.

Abstract: Cortical projections to subdivisions of the cingulate cortex in the rhesus monkey were analyzed with horseradish peroxidase and tritiated amino acid tracers. These projections were evaluated in terms of an expanded cytoarchitectural scheme in which areas 24 and 23 were divided into three ventrodorsal parts, i.e., areas 24a-c and 23a-c. Most cortical input to area 25 originated in the frontal lobe in lateral areas 46 and 9 and orbitofrontal areas 11 and 14. Area 25 also received afferents from cingulate areas 24b, 24c, and 23b, from rostral auditory association areas TS2 and TS3, from the subiculum and CA1 sector of the hippocampus, and from the lateral and accessory basal nuclei of the amygdala (LB and AB, respectively). Areas 24a and 24b received afferents from areas 25 and 23b of cingulate cortex, but most were from frontal and temporal cortices. These included the following areas: frontal areas 9, 11, 12, 13, and 46; temporal polar area TG as well as LB and AB; superior temporal sulcus area TPO; agranular insular cortex; posterior parahippocampal cortex including areas TF, TL, and TH and the subiculum. Autoradiographic cases indicated that area 24c received input from the insula, parietal areas PG and PGm, area TG of the temporal pole, and frontal areas 12 and 46. Additionally, caudal area 24 was the recipient of area PG input but not amygdalar afferents. It was also the primary site of areas TF, TL, and TH projections. The following projections were observed both to and within posterior cingulate cortex. Area 29a-c received inputs from area 46 of the frontal lobe and the subiculum and in turn it projected to area 30. Area 30 had afferents from the posterior parietal cortex (area Opt) and temporal area TF. Areas 23a and 23b received inputs mainly from frontal areas 46, 9, 11, and 14, parietal areas Opt and PGm, area TPO of superior temporal cortex, and areas TH, TL, and TF. Anterior cingulate areas 24a and 24b and posterior areas 29d and 30 projected to area 23. Finally, a rostromedial part of visual association area 19 also projected to area 23. The origin and termination of these connections were expressed in a number of different laminar patterns. Most corticocortical connections arose in layer III and to a lesser extent layer V, while others, e.g., those from the cortex of the superior temporal sulcus, had an equal density of cells in both layers III and V.(ABSTRACT TRUNCATED AT 400 WORDS)

Nosocomial pneumonia in intubated patients given sucralfate as compared with antacids or histamine type 2 blockers. The role of gastric colonization.

Abstract: Gram-negative nosocomial pneumonia may result from retrograde colonization of the pharynx from the stomach, and this may be more likely when the gastric pH is relatively high. We studied the rate of nosocomial pneumonia among 130 patients given mechanical ventilation in an intensive care unit who were receiving as prophylaxis for stress ulcer either sucralfate (n = 61), which does not raise gastric pH, or conventional treatment with antacids, histamine type 2 (H2) blockers, or both (n = 69). At the time of randomization to treatment, the two groups were similar in age, underlying diseases, and severity of acute illness. Patients in the sucralfate group had a higher proportion of gastric aspirates with a pH less than or equal to 4 (P less than 0.001) and significantly lower concentrations of gram-negative bacilli (P less than 0.05) in gastric aspirates, pharyngeal swabs, and tracheal aspirates than did patients in the antacid-H2-blocker group. The rate of pneumonia was twice as high in the antacid-H2 group as in the sucralfate group (95 percent confidence interval, 0.89 to 4.58; P = 0.11). Gram-negative bacilli were isolated more frequently from the tracheal aspirates of patients with pneumonia who were receiving antacids or H2 blockers. Mortality rates were 1.6 times higher in the antacid-H2 group than in the sucralfate group (95 percent confidence interval, 0.99 to 2.50; P = 0.07). Although our results fell just short of statistical significance when they were analyzed according to intention to treat, they suggest that agents that elevate gastric pH increase the risk of nosocomial pneumonia in patients receiving ventilation by favoring gastric colonization with gram-negative bacilli. We conclude that in patients receiving mechanical ventilation, the use of a prophylactic agent against stress-ulcer bleeding that preserves the natural gastric acid barrier against bacterial overgrowth may be preferable to antacids and H2 blockers.

Architecture and frontal cortical connections of the premotor cortex (area 6) in the rhesus monkey

Abstract: The premotor cortex (area 6) has several architectonic sectors that can be delineated on the basis of cytoarchitectonic and myeloarchitectonic features. Area 6 may be broadly subdivided into a dorsal and a ventral sector at the spur of the arcuate sulcus. Dorsal 6 lacks a granular layer IV, but ventral 6 has an emergent layer IV that separates laminae III and V. Dorsal 6 has a higher myelin content than ventral 6. Dorsal area 6 is further subdivided into a caudal and a rostral sector on the basis of the presence of large pyramidal cells in the caudal but not in the rostral sector. The rostral sector of area 6 can be subdivided into a medial region distinguished from a more laterally situated area by the presence of more compact and darkly stained cells in layers III and V. Ventral area 6 can be subdivided into an upper and lower division. The upper part has more prominent pyramidal cells in layers III and V, and a better developed outer Baillarger band and vertical plexus than the lower division. The efferent and afferent connections of area 6 were studied with anterograde and retrograde tracers. The frontal connections of dorsal area 6 are restricted to neighboring dorsal frontal regions. Only the caudal sector of dorsal area 6 is connected with the motor cortex. In contrast, ventral area 6 is not only connected with the prefrontal cortex, but also directly with the motor cortex, the parainsular gustatory area, and with somatosensory areas in the frontal operculum. The widespread connections of ventral area 6 may be related to the specialization of the head, neck, and face structures that are represented ventrally within the premotor cortex.

Industry Effects and Appropriability Measures in the Stock Markets Valuation of R&D and Patents

Abstract: This paper examines the stock market's valuation of a firm's innovative activity. We estimate the market's relative valuation of firms' tangible and intangible assets, focusing on knowledge capital in the form of accumulated "stocks" of R&D and patents. We tried to improve upon our estimates of the stock market's valuation of knowledge capital embodied in such "stocks" by bringing in measures of the appropriability environment facing a firm from the Yale Survey on Industrial Research and Development. The responses to Survey questions about the effectiveness of patents as a mechanism for protecting the returns from innovation turn out to be of some use: there is evidence of an interaction between industry level measures of the effectiveness of patents and the market's valuation of a firm's past R&D and patenting performance, as well as its current R&D moves. We find no evidence, however, that other appropriability mechanisms differ enough across industries to leave measurable traces in our data. The structure of the Yale Survey makes it possible to estimate the sampling error in the appropriability measures derived from it. This information was used by us in an errors-in-variables context, but with little success. In the absence of R&D variables, our estimates imply that a two standard deviation increase in our index of patent effectiveness would raise the value of a patent held by our average firm from $0.4 million to $1.0 million. When R&D variables are introduced into the equations, the patents variables become insignificant - R&D expenditures are a better measure of input to the innovative function of firms than patents are of its output - but we estimate that the same experiment would induce changes in q of between 10 and 27 percent for the average firm, approximately doubling the market's valuation of this kind of capital.

Cingulate cortex of the rhesus monkey: I. Cytoarchitecture and thalamic afferents

Abstract: The cytoarchitecture and thalamic afferents of cingulate cortex were evaluated in the rhesus monkey (Macaca mulatto). Area 24 has three divisions of which area 24a is adjacent to the callosal sulcus and has the least laminar differentiation. Area 24b has more clearly defined layers II, III, and Va, and area 24c, which forms the lower bank of the anterior cingulate sulcus, has a particularly dense layer III. Area 23 also has three divisions, each of which has a distinct layer IV. Area 23a is adjacent to the callosal sulcus and has the thinnest layers II–IV, which have the same cell density as layers V and VI. Area 23b has the largest pyramids in layers IIIc and Va, and area 23c, in the depths of the posterior cingulate sulcus, has the broadest external and thinnest internal pyramidal layers. Finally, areas 29 and 30 are located in the posterior depths of the callosal sulcus. Two divisions of area 29 are apparent: one with a granular layer directly adjacent to layer I (area 29a–c) and another with differentiation of layers III and IV (area 29d). Area 30 has a dysgranular layer IV. Injections of the retrograde tracer horseradish peroxidase (HRP) were made into subdivisions of cingulate cortex in the monkey. Area 25 received thalamic input mainly from the midline parataenial (Pt), central densocellular (Cdc), and reuniens nuclei as well as from the dorsal parvicellular division of the mediodorsal nucleus (MDpc). A less dense projection also originated in the intralaminar parafascicular (Pf), central superior, and limitans (Li) nuclei as well as the medial division of the anterior nuclei (AM). Areas 24a and 24b received most thalamic afferents from fusiform and multipolar cells in the Cdc and Pf nuclei with fewer from the ventral anterior (VA) and MDpc and MD densocellular (MDdc) nuclei and only minor input from AM. Most input to premotor cingulate area 24c appeared to originate in VA, MDdc, and Li. Area 29 received the most dense input from nuclei traditionally associated with limbic cortex including the anteroventral (AV), anterodorsal (AD), and laterodorsal (LD) nuclei. Areas 23a and 23b, in contrast, did not receive AV, AD, or LD input, but the greatest proportion of their thalamic afferents arose in AM. Less-pronounced input also came from the lateroposterior (LP), medial pulvinar, and MDdc nuclei. This latter nucleus projected more to area 23b than to areas 30 or 23a. Anterior medial nucleus efferents to cingulate cortex were of particular note for two reasons. First, AM projected primarily to posterior cingulate areas with area 23 receiving its principal thalamic input from AM. Second, projections to areas 30, 23a, and 23b were topographically organized with ventral areas 30 and 23a receiving from the central core of AM. While the more dorsally located area 23b received input from peripheral and medial. In light of the extensive projections of Cds, Csl, and Pf to anterior cingulate cortex, it is proposed that the midline and intralaminar thalamic nuclie be classified as part of limbic thalamus along with the anterior, LD, and MD nuclei. Furthermore, although AM projects mainly to posterior cingulate cortex, it also has light projections to area 25 and minor input to area 24. As suhc, AM is the only limbic thalamic nucleus that has such widespread projections to cingulate cortex. Finally, visually evoked activity in area 23 may be the result of projections from the LP and medial pulvinar.

Receptor-mediated transcytosis of transferrin across the blood-brain barrier.

Abstract: The perfusion of rat brain with 125I-transferrin resulted in a receptor-mediated uptake of transferrin into the endothelium of the blood-brain barrier followed by its detection in the brain. During a pulse-chase experiment, 125I-transferrin accumulated in the endothelial cells during the pulse, with a decrease of this intraendothelial radioactivity during the chase associated with a concomitant increase in the nonvascular elements of the brain. The receptor-mediated movement of transferrin across the blood-brain barrier suggests that the brain may derive its iron through the transcytosis of iron-loaded transferrin across the brain microvasculature. We discuss the likelihood that aluminum and other potentially toxic heavy metals, which also bind tightly to transferrin, may enter the brain by this pathway. We also discuss the possibility that other large molecules including neuroactive peptides and neurotrophic viruses may enter the brain through a similar receptor-mediated, vesicular transcytotic route.

Routh-Hurwitz criterion in the examination of eigenvalues of a system of nonlinear ordinary differential equations.

Abstract: In stability analysis of nonlinear systems, the character of the eigenvalues of the Jacobian matrix (i.e., whether the real part is positive, negative, or zero) is needed, while the actual value of the eigenvalue is not required. We present a simple algebraic procedure, based on the Routh-Hurwitz criterion, for determining the character of the eigenvalues without the need for evaluating the eigenvalues explicitly. This procedure is illustrated for a system of nonlinear ordinary differential equations we have studied previously. This procedure is simple enough to be used in computer code, and, more importantly, makes the analysis possible even for those cases where the secular equation cannot be solved.

Sleep problems in early childhood: continuities, predictive factors, and behavioral correlates.

Abstract: A longitudinal study, based on interviews with 308 middle-class, preponderantly white mothers, provided an opportunity to evaluate the continuity, predictive factors, and behavioral correlates of sleep problems in young children. When their children were 8 months old, 10% of the mothers reported that their babies woke three or more times per night, 8% reported that the babies took an hour or more to settle after waking, 5% complained that their own sleep was severely disrupted by the child, and 18% reported at least one of these problems. At 3 years of age, 29% of the children had difficulty getting to bed and/or falling asleep or staying asleep. Of children with a sleep problem at 8 months of age, 41% still had a problem at 3 years of age, whereas only 26% of children without a problem at 8 months of age had a problem at 3 years of age (P less than .001). Among children with sleep problems at 8 months of age, mothers' depressed feelings were the only measured demographic or psychosocial factor associated with persistent sleep problems (P = .02). A separate analysis indicated that these depressed feelings did not appear to be a consequence of the child's sleep problem. Future studies should evaluate how maternal depression interacts with other factors to result in persistent sleep problems. Children with persistent sleep problems were more likely to have behavior problems, especially tantrums (P less than .02) and behavior management problems (P less than .01), than were children without persistent sleep problems (P less than .02).(ABSTRACT TRUNCATED AT 250 WORDS)

Bioenergetics of lactic acid bacteria: cytoplasmic pH and osmotolerance

Abstract: Lactic acid bacteria maintain a cytoplasm that is more alkaline than the medium, but whose pH decreases as the medium is acidified during growth and fermentation. Streptococci generally acidify the cytoplasm from approximately pH 7.6 to 5.7 (external pH 4.5) before growth and then fermentation cease. The internal enzyme machinery of these anaerobic fermenters thus tolerates a fairly wide range in internal proton concentration. Lactobacilli tolerate a significantly more acidic cytoplasmic pH of 4.4 (external pH 3.5). However, when the cytoplasmic pH decreases below a threshold pH, which depends on the organism cellular functions are inhibited. Fermentation end-products, such as organic acids or alcohols, exert their deleterious effects by bringing about acidification of the cytoplasm below the permissible pH. Organic acids, which act as protonophores, or solvents, which perturb membrane phospholipids, at high concentrations increase the inward leak of H+ so that H+ efflux is not rapid enough to alkalinize the cytoplasm. The membrane pH gradient is thus dissipated. A specific strain of Lactobacillus acidophilus has been found to be unusually osmotolerant. The osmoresistance is due to the cells' capacity to accumulate glycine betaine by a transport carrier that is activated, but not induced, by high medium osmotic pressure.

The Hippocampal Formation of the Primate Brain

Abstract: The long-standing interest of neuroscientists in the hippocampal formation has occurred from the perspective of two rather different points of view. On the one hand, the comparatively simple cytoarchitecture of the hippocampal formation and the laminar segregation of its extrinsic and intrinsic afferents make it an interesting and relatively simple model system for experimental manipulation and investigation. Chapter 8 in this volume, by Schwartzkroin and Mueller, is a thoughtful review of how effective the hippocampus has been as a model system for neurophysiological studies using the in vitro slice preparation. Other investigations have utilized the hippocampal formation to study the effects of genetic defects in neuronal development (e.g., Nowakowski and Davis, 1985), collateral sprouting induced by deafferentation (e.g., Lynch et al. 1972; Cotman and Nadler, 1978), or intracerebral transplantation (e.g., Frotscher and Zimmer, 1986). While most of the studies that use the hippocampal formation as a model system have focused on the rodent, the other major interest in the hippocampal formation has grown out of a variety of clinical and experimental data derived from humans and nonhuman primates. These latter investigations have focused on the disruption of certain aspects of the memory process produced by hippocampal lesions as first described in human neurosurgical patients (Scoville and Milner, 1957; Penfield and Milner, 1958) and subsequently investigated in nonhuman primates (e.g., Moss et al. 1981).

Risk of ventricular arrhythmias in left ventricular hypertrophy: The Framingham Heart Study

Abstract: The association of ventricular arrhythmias with left ventricular (LV) hypertrophy was examined in 6,218 participants in the Framingham Heart Study. Electrocardiographic (ECG) LV hypertrophy was present in 171 subjects and echocardiographic hypertrophy was detected in 869. Echocardiographic LV hypertrophy was associated with increased risk for each of 6 ventricular arrhythmia grades in men (relative risk up to 8.9, p less than 0.01), and 4 of 6 grades in women (p less than 0.05). Similarly, men with ECG LV hypertrophy were at increased risk for 4 of 6 arrhythmia grades (p less than 0.05). However, owing to low prevalence ECG LV hypertrophy was not associated with arrhythmia in women. After adjustment for age, sex, systolic blood pressure, valvular heart disease, angina pectoris and acute myocardial infarction, the association of echocardiographic but not ECG LV hypertrophy with ventricular arrhythmia remained significant (p less than 0.001). Thus, echocardiographic LV hypertrophy is more prevalent and more sensitive for ventricular arrhythmias than ECG LV hypertrophy.

Non-Market-Based Transfers of Wealth and Power: A Research Framework for Family Businesses:

Abstract: Family businesses are basically owner-managed enterprises with the family Involved within the business. When, to family ties within the business, is added the biological inevitability of an eventua...

Strategic groups, asymmetrical mobility barriers and contestability

Abstract: This paper examines the literature on strategic groups and extends it: first, by clarifying the distinction between two alternate approaches to forming groups, emphasizing that multivariate group analysis can be used to conserve information not only summarize it as bivariate grouping does; second, by focusing attention on the importance of asymmetrical mobility barriers and competitive advantage in the process of industry consolidation and concentration; and third, by linking groups and the concept of contestability in an effort to make some progress towards explaining the evolution of industry structure.

Coronary risk prediction in adults (the Framingham Heart Study)

Abstract: The Framingham Heart Study, an ongoing prospective study of adult men and women, has shown that certain risk factors can be used to predict the development of coronary artery disease. These factors include age, gender, total cholesterol level, high density lipoprotein cholesterol level, systolic blood pressure, cigarette smoking, glucose intolerance and cardiac enlargement (left ventricular hypertrophy on electrocardiogram or enlarged heart on chest x-ray). Calculators and computers can be easily programmed using a multivariate logistic function that allows calculation of the conditional probability of cardiovascular events. These determinations, based on experience with 5,209 men and women participating in the Framingham study, estimate coronary artery disease risk over variable periods of follow-up. Modeled incidence rates range from 80% over an arbitrarily selected 6-year interval; however, they are typically

Intergenerational Transfers and Savings

Abstract: In recent years the role of intergenerational transfers in the process of wealth accumulation has been the subject of substantial empirical and theoretical analysis. The key question stimulating this research is what is the main explanation for savings? Is it primarily accumulation for retirement as claimed by Albert Ando, Richard Brumberg, and Franco Modigliani in their celebrated Life Cycle Model of Savings? Is it primarily intentional accumulation for intergenerational transfers? Or is it primarily precautionary savings, much of which may be bequeathed because of imperfections in annuity markets? This paper examines a range of findings on the importance of intergenerational transfers. The strong conclusion that emerges from this evidence is that intergenerational transfers play a very important, if not a key, role in aggregate wealth accumulation. While intergenerational transfers figure very large in savings, the precise motivation for such transfers is unclear. Intergenerational altruism might appear the most likely candidate, but at least sane stylized facts, such as the equal allocation of bequests among children, are strongly at adds with the altruism model. Other explanations involving imperfect insurance arrangements or payments for child services do not appear capable of explaining the substantial amounts of transfers actually observed. Sorting cut the relative contributions of different models to intergenerational transfers and the precise role of intergenerational transfers in the process of wealth accumulation remains an intriguing and exciting enterprise.

Voluntary control of motor units in human antagonist muscles: coactivation and reciprocal activation

Abstract: 1. Myoelectric (ME) activity of several motor units was detected simultaneously from the human flexor pollicis longus and extensor pollicis longus muscles, the only two muscles that control the interphalangeal joint of the thumb. The ME signals were detected while the subjects produced isometric force outputs to track three different paradigms: triangular trajectories, random-force trajectories requiring both flexion and extension contractions, and net zero force resulting from stiffening the joint by voluntarily coactivating both muscles. 2. The ME signals were decomposed into their constituent motor-unit action potential trains. The firing rate behavior of the concurrently active motor units was studied using cross-correlation techniques. 3. During isometric contractions, the firing rates of motor units within a muscle were greatly cross-correlated with essentially zero time shift with respect to each other. This observation confirms our previous report of this behavior, which has been called common drive. Common drive was also found among the motor units of the agonist and antagonist muscles during voluntary coactivation to stiffen the interphalangeal joint. This observation suggests two interesting points: 1) that the common drive mechanism has a component of central origin, and 2) that the central nervous system may control the motoneuron pools of an agonist-antagonist muscle pair as if they were one pool when both are performing the same task. 4. During force reversals, the firing rates of motor units reverse in an orderly manner: earlier recruited motor units decrease their firing rate before later recruited motor units. This orderly reversal of firing rates is consistent with the concept of orderly recruitment and derecruitment. 5. A control scheme is suggested to explain the behavior of the motor units in both muscles during force reversal. It consists of centrally mediated reciprocally organized flexion and extension commands along with a common coactivation command to both muscles. This control scheme allows for coactivation and reciprocal activation of an agonist-antagonist set. 6. The agonist-antagonist pair was observed to generate a net force in two control modalities: proportional activation and reciprocal activation. In proportional activation, the agonist-antagonist set is coactivated during either of two states: when uncertainty exists in the required task or when a compensatory force contraction is perceived to be required.(ABSTRACT TRUNCATED AT 400 WORDS)