Jorge Cervantes

Limitations of Existing Mutation Rate Heuristics and How a Rank GA Overcomes Them

Using a set of different search metrics and a set of model landscapes we theoretically and empirically study how ldquooptimalrdquo mutation rates for the simple genetic algorithm (SGA) depend not only on the fitness landscape, but also on population size and population state. We discuss the limitations of current mutation rate heuristics, showing that any fixed mutation rate can be expected to be suboptimal in terms of balancing exploration and exploitation. We then develop a mutation rate heuristic that offers a better balance by assigning different mutation rates to different subpopulations. When the mutation rate is assigned through a ranking of the population, according to fitness for example, we call the resulting algorithm a Rank GA. We show how this Rank GA overcomes the limitations of other heuristics on a set of model problems showing under what circumstances it might be expected to outperform a SGA with any choice of mutation rate.

Laparoscopic Common Bile Duct Exploration

Abstract. Since the introduction of laparoscopic cholecystectomy, the management of common bile duct (CBD) stones has undergone significant change. Preoperative endoscopic retrograde cholangiopancreatography (ERCP) with sphincterotomy is now routinely done in cases where the diagnosis of choledocholithiasis is suspected preoperatively, with clearance of the bile ducts before laparoscopic cholecystectomy. Intraoperative discovery of CBD stones by cholangiography represents a challenge to the surgeon, who must make a decision about when to perform laparoscopic CBD exploration, convert to open surgery, or send the patient for ERCP during the postoperative period. Because ERCP has a definite failure rate, laparoscopic CBD exploration can be a treatment option. Among 2500 laparoscopic cholecystectomies done by our group from January 1991 to June 1997, 50 patients (2%) underwent laparoscopic CBD exploration, 13 by the transcystic technique and 37 by choledocotomy, with a conversion rate of 8% and a hospital stay of 4.3 days. One patient died from complicated pancreatitis following ERCP and unsuccessful extraction of a CBD stone. We obtained our goal of a CBD free of stones in 92% of the cases. We conclude that laparoscopic CBD exploration is an effective method for treating choledocolithiasis that allows management of this pathology in one stage, although it requires advanced laparoscopic skills and adequate equipment.

Optimal mutation rates for genetic search

Using a set of model landscapes we examine how different mutation rates affect different search metrics. We show that very universal heuristics, such as 1/N and the error threshold, can generally be improved upon if one has some qualitative information about the landscape. In particular, we show in the case of multiple optima (signals) how mutation affects which signal dominates and how passing between the dominance of one to another depends on the relative height and size of the peaks and their relative positions in the configuration space.

Changes in gallbladder surgery: comparative study 4 years before and 4 years after laparoscopic cholecystectomy.

Operative procedures on the gallbladder and biliary ducts have undergone a profound transformation since the introduction of laparoscopic techniques in general surgery. As the benefits of minimally invasive procedures become universally known, patients are seeking surgery at an earlier stage, resulting in an increased number of cases for elective surgery and a considerable reduction in emergency operations, morbidity, need for intraoperative cholangiography (IOC), fewer common bile duct (CBD) explorations, shortened hospital stay, and reduced overall costs. The early criteria for IOC and the need for CBC explorations must be reevaluated in view of the observed changes and appropriate modifications made. Looking at the present trends, it seems that the routine use of IOC is not justified in the average patient who presents for laparoscopic cholecystectomy with no history of jaundice or pancreatitis, normal liver function tests, and a normal-size CBD on ultrasonography. Under those conditions, the chance of leaving an unsuspected stone in the CBD is less than 1%; and if it happens the stone can be easily retrieved by endoscopic sphincterotomy as an outpatient procedure.RésuméLes techniques chirurgicales utilisées pour la chirurgie biliaire ont connu des changements radicaux depuis l’introduction de la chirurgie laparoscopique. Au fur et à mesure que les avantages des procédés peu invasifs deviennent connus universellement, les patients euxmêmes cherchent à se faire opérer à un stade plus précoee. Ceci augmente alors le nombre de patients opérés à froid, diminuant le nombre d’interventions nécessaires en urgence, la morbidité, la nécessité de pratiquer une cholangiographie per-opératoire (CPO), le besoin d’explorer la voie biliaire principale (VBP), la durée d’hospitalisation et les couts. Les indications du temps jadis de la CPO et de l’exploration de la VBP doivent être revues et actualisées. En tenant compte des tendances actuelles, il paraît que la CPO de routine n’est plus justifiée chez le patient d’aujourd’hui qui se présente pour une cholécystectomie laparoscopique (CL) sans antécédent d’ictère ou de pancréatite, avec des tests hépatiques normaux et une VBP de taille normale à l’échographie. Dans ces conditions, les chances de laisser en place un calcul insoupçonné dans la VBP est inférieure à 1% et si cet incident se produit, le calcul peut généralement etre retiré par la sphinctérotomie endoscopique pratiquée en ambulatoire.ResumenLos procedimientos operatorios sobre la vesícula biliar y los canales biliares han sufrido una transformatión profunda desde la introductión de las técnicas laparoscópicas a la cirugía general. En la medida que los beneficios de los procedimientos mínimamente invasores son conocidos universalmente, los pacientes solicitan la cirugía en fases tempranas de su enfermedad, lo cual resulta en un número creciente de casos de cirugía electiva y en una considerable reductión de las operaciones de emergencia, así como de la morbilidad, de la necesidad de realizar colangiogramas intraoperatorios (CIO), de la incidencia de exploraciones de la vía biliar (EVB), de la estancia hospitalaria y de los costos globales. Los viejos criterios para realizar CIO y para determinar la necesidad de EVB deben ser reevaluados a la luz de los cambios y modificaciones ocurridas. Al contemplar las tendencias actuales parece que el uso rutinario de CIO no se justifica en el paciente común que se presenta a colecistectomía laparoscópica sin historia de ictericia o de pancreatitis, con pruebas de función hepáticas normales y con un colédoco normal en el ultrasonido. Bajo tales condiciones, las posibilidades de dejar un cálculo no sospechado en el colédoco son menores a 1% y si esto ocurre, el cálculo puede ser fácilmente removido mediante esfmterotomía endoscópica practicada como procedimiento de consulta externa.

Choledocholithiasis: New Approach to an Old Problem

Abstract. The old problem of diagnosis and treatment of choledocholithiasis is analyzed in light of recent developments in the field of radiology and surgery. The traditional indications for common bile duct (CBD) exploration resulted in a large number of unnecessary manipulations of the bile ducts. The same is now true of the widespread use of endoscopic retrograde cholangiopancreatography. With the advent of magnetic resonance imaging cholangiography, which can clearly demonstrate CBD calculi by means of a noninvasive, complication-free technology, we should abandon the old indications for CBD exploration and be more conservative regarding the diagnosis and management of this condition. In an era of minimally invasive surgery we are seeing a significant reduction in the number of patients with choledocholithiasis.

Reflections on the 50th anniversary of the first abdominal aortic aneurysm resection.

Fifty years ago, on March 29, 1951, Charles Dubost in France performed the first successful resection of an abdominal aortic aneurysm and replaced it with a cadaver homograft, thereby initiating the modern era of surgical correction of a common entity. This article offers a summary of the events leading to that milestone in vascular surgery.

Inguinal Hernia in the New Millennium

Abstract.Since 1884, when Edoardo Bassini started the modern era of surgical correction of inguinal hernia (today the most common procedure performed by the general surgeon), many techniques have been introduced, some short-lived, others with proven long-term results. At the start of the new millennium, the surgeon has three clear alternatives: tension repairs, tension-free repairs, and laparoscopic procedures. This paper analyzes these three options and offers an update regarding the pros and cons of the most commonly performed operations in surgical centers around the world.

Common bile duct stones revisited after the first operation 110 years ago.

In 1999 we commemorated the 110th anniversary of the first successful common bile duct (CBD) exploration. It was done by Knowsley Thornton in London on May 9, 1889, only 7 years after the ground-breaking operation by Carl Augustus Langenbuch, who had inaugurated the modern era of gallbladder surgery with a cholecystectomy performed on Wilhem Daniels on July 15, 1882 at the San Lazarous Hospital in Berlin.

Some Implications of System Dynamics Analysis of Discrete-Time Recurrent Neural Networks for Learning Algorithms Design

It is not clear so far what the implications of bifurcations in Discrete-Time Recurrent Neural Networks dynamics are with respect to learning algorithms. Previous studies discussed different phenomena in a general purpose framework, and here we are going to discuss in more detail. We perform an analysis of the dynamics of a neuron with feedback in order to find the different behaviors that it shows depending on the magnitude of the offset weight, the input weight and the feedback weight. We calculate the bifurcation manifolds that show the regions where the neuron behavior changes. We discuss the implications that these findings can have for the design of DTRNN learning algorithms.

Rank Based Evolution of Real Parameters on Noisy Fitness Functions: Evolving a Robot Neurocontroller

We present a Rank Based Evolutionary Algorithm for representations in the real numbers. We introduce a new Rank Based Selection operator and a new variation of a Rank Based Mutation that act in a representation using real numbers. The problem in which we tested the algorithm was to evolve a fixed topology feed forward artificial neural network that is used as a controller for a robot. In order to be successful, the robot must be able to use both proximity sensors and video input but there is some level of noise in them. The test results show how the proposed operators are suitable for this kind of problems where the fitness landscape is noisy and where little else is known about it.