- Email: [email protected]
Contributions to decision neuroscience
Journal of Economic Psychology 31 (2010) 764–766
Contents lists available at ScienceDirect
Journal of Economic Psychology journal homepage: www.elsevier.com/locate/joep
Editorial
Contributions to decision neuroscience Scanning methodologies such as functional Magnetic Resonance Imaging (fMRI), electroencelography (EEG) and others have allowed neuroscientists to focus on brain activations during complex real life behaviours. Researchers have now moved beyond the clinical setting to study a wide range of basic processes in healthy subjects. This emerging discipline is broadly referred to as cognitive neuroscience. Within this field several groups have focussed on the study of decision-making, giving rise to the field of decision neuroscience. Decision neuroscience encompasses the behaviours of the individual in society in a wide range of decision-making contexts from economic decisions to health issues, communication and purchasing of goods; whether to purchase a product or not is a basic unit of economic and consumption behaviour. Decision neuroscience has thus initiated a multidisciplinary research agenda for economics, psychology and consumer behaviour. Within the nascent field of decision neuroscience several special interest groups have formed that focus for example on economic and public policy questions (referred to as neuroeconomics) or on marketing questions (referred to as consumer neuroscience). All the papers in this special issue of the Journal of Economic Psychology: Decision Neuroscience originate from the Conference on Neuroeconomics (‘‘ConNecs”) held at the Copenhagen Business School on May 15–16, 2008, which was attended by a large group of researchers involved in various aspects of decision neuroscience. From the many valuable contributions submitted, a number have been selected for this special issue to demonstrate the variety of the research carried out. From important theoretical contributions like that of Bechara and Reimann to more applied examples such as the paper by Ohme et al., the papers vary in the methodology used, ranging from fMRI over EEG to psycho-physiological procedures, and in their area of application, from general decision theory in economics to marketing and communication. The editors hope that this selection will provide readers with a thorough and varied introduction to the fast growing field of decision neuroscience. In ‘‘The somatic marker framework as a neurological theory of decision making”, Bechara and Reimann review the findings on what is probably the most important contemporary theory of emotional impact on decision-making. The theory originated with Damasio’s and Bechara’s joint work in the early 90s and has found confirmation in numerous studies since then. The Iowa gambling task, in particular, has been a major instrument in this research work. Essentially, the theory posits that decision makers store the consequences of alternative choices effectively. It is based upon observations with patients with lesions on the frontal lobes of the brain and in studies with healthy individuals. fMRI scanning has been a critical instrument in this development and the following areas in the brain have been identified as critical for emotional influences in accordance with the somatic marker theory (see Fig. 1): the medial prefrontal cortex with a particular focus on its ventromedial and orbitofrontal parts, the dorsal lateral prefrontal cortex and the amygdala. While Bechara and Reimann examine theoretical formulations of decision neuroscience, Ramsøy and Skov expand on this by reviewing genetically-driven changes in neurotransmitter levels as a factor influencing human decision-making. In ‘‘How genes make up your mind: Individual biological differences and value-based decisions” they depart from classical neurological work on the role of dopamine and serotonin in Parkinson’s and other mental diseases. They point out that variations in biological factors may account for individual differences in the processes underlying value-based decision-making. They argue that emotions are associated with neural processes whereby neural transmitters such as dopamine and opioid are important; these play a particular role for computing expected rewards. On the other hand, negative emotions are particularly related to brain areas where the neurotransmitter serotonin is important. Following a general discussion, the authors point to the role of ordinary-level variations in dopamine and serotonin in human decision-making. Specifically, they demonstrate that dopamine levels influence risk-taking in gambling tasks and that serotonin levels influence moods that may in turn influence decision-making. They argue that increased levels of serotonin influence anxiety and may generate an increased fear of losing, of risk or even ambiguity. Conversely, low levels of serotonin are related to depression, in which responses to reward and losses are lower and thus decisions will be less influenced by emotional factors. They then review a range of evidence suggesting that drug-induced alterations in dopamine and serotonin levels may influence decisionmaking. In a final section they argue that genetic differences may cause individual differences in dopamine and serotonin and other neurotransmitter levels. As a consequence they underline the need to test models of decision-making where 0167-4870/$ - see front matter Ó 2010 Published by Elsevier Ltd. doi:10.1016/j.joep.2010.03.001
Editorial / Journal of Economic Psychology 31 (2010) 764–766
765
individual differences are taken into consideration. As areas for future research they point to the interplay between genetic and experience-based factors in decision-making, and the role of genes/neurotransmitters in determining choices. Risk plays a crucial role in (economic) decision-making and has thus received considerable attention in the decision neuroscience literature. The same holds for the phenomenon of intertemporal choice. The paper by Jamison and Wegener that has won the best paper award during the ConNecs focuses on these phenomena. They point out that the terms ‘‘intertemporal choice”, ‘‘delay discounting”, and ‘‘time preference” refer to the same construct, namely individuals trading off outcomes to be received at different points in time. This is seen as fundamentally different from the idea of ‘‘prospection”, where the respondent imagines himself at some future point in time. The paper claims that the way in which people make evaluations when faced with a choice of outcomes to appear at different times is different from the form that prospection normally assumes, when ‘‘an agent is simply asked to imagine him or herself at some future point in time”. In the authors’ view, when people compare alternatives that occur at different times they apply evaluation procedures similar to those that would occur if they were asked to evaluate the future mental state of others; the processes encompassed by the term ‘‘theory of mind” are relevant here. The brain systems involved when individuals attempt to empathise with others are basically the same as when they empathise with themselves in the future. The areas of the brain that they involve are the temporal poles in the medial prefrontal cortex and areas both within and in proximity to the tempo-parietal junction. In reviewing the available literature, the authors find evidence for their proposition and report findings from a study where these areas are activated when people evaluate immediate monetary rewards compared with delayed monetary rewards varying from one week to one month and one year. In discussing future research, they stress the need to explain the extent to which individuals, who are more or less proficient in predicting the behaviour of others, are good at predicting their own future actions. The authors discuss the social implications of temporal choice. Many conflicting situations – healthwise and otherwise – in which the individual is placed involve the comparison of immediate rewards with future rewards. If the way in which future alternatives are evaluated is better understood and parameters are identified that can improve this evaluation procedure and make it more efficient, it could have considerable social implications. DaSilva, Moreira and Matsushita focus on risk-seeking behaviour among pre-school children. Based on research with monkeys, which were found to prefer risky choices in a gambling task, they propose that humans initially prefer risky choices but eventually learn by experience to avoid them. To explain this they conducted a study with 100 4–6-year-olds, finding a similar tendency to prefer risky choices but also an ability to learn to avoid risky choices in subsequent tasks. In a more general sense the findings suggest that individual differences in the preference for risky choices may depend upon previous opportunities (and ability) to learn about the respective consequences of risky vs. non-risky choices. Thus, differences in this respect may relate to pathological gambling and similar behaviours. In the subsequent paper, ‘‘Celebrities and shoes on the female brain: The neural correlates of product evaluation in the context of fame”, Stallen, Smidts et al. focus on the neural correlates of product evaluations in situations where celebrities are associated with the products to be evaluated. The authors depart from general consumer research findings that celebrity endorsements influence the effects of advertising, resulting in preferred brands and advertisements. They point out that whereas earlier work demonstrated that the pairing of a positively valence stimuli – a celebrity – with a product resulted in a conditioning process where effect is transferred from the neutral stimuli to the product, little is known about why this result occurs. The authors offer two possible explanations. First, they suggest that the perception of the celebrity may bring to mind facts about them, or suggest a personal relationship with them. Alternatively, the celebrity may trigger retrieval of implicit memories which are not dependent on conscious recall. In both cases, emotions may be influenced in terms of how positive or negative they are and/or in terms of how far they are aroused. To explore these processes, the authors report findings from an experiment where young female subjects were presented with photographs of equally attractive famous and non-famous faces together with photographs of products. In a preliminary behavioural test the authors found that shoes presented with famous faces were better recalled. Conversely, the purchase intention was not significantly different between shoes presented with or without famous faces. In the fMRI part of the experiment it was found that the perception of celebrity faces generated activity in the right medial orbitofrontal cortex when the combination of famous face/shoes was compared with non-famous face/shoes. The authors interpret this as being due to the generation of associations between famous faces and the product. The influence on the evaluation of shoes, however, was less certain. Moreover, the number of subjects in this condition was too small for a significant testing. The authors emphasise the need for further research, and to include male respondents. Two other papers also investigate the neuropsychology of advertising effects. In the first, entitled ‘‘Using the P3a to gauge automatic attention to interactive television advertising”, Treleaven et al. demonstrate the use of a special version of eventrelated potential obtained with EEG labelled P3a, which focuses on the occurrence of particularly strong effects in the frontal central region of the brain. They argue for the use of this methodology rather than fMRI, since it is more cost efficient and can be applied relatively easily to a larger number of subjects. It is argued that the occurrence of the P3a reflects attention. The authors report on a project where they compared the ERP data with self reports on attention, preferences and other behavioural and cognitive measures. In the study, a 32-channel EEG cap was used while respondents were exposed to brand logos and advertisements. The exposure situations were manipulated so that some were presented as interactive television and others as regular advertising. Basically, they found that the first of two (the most demanding) forms of interactive advertising generated more attention, whereas neither of them had any impact on preferences for the advertised items. This brings
766
Editorial / Journal of Economic Psychology 31 (2010) 764–766
to mind a somewhat similar finding in the aforementioned paper by Stallen, Smidts et al., where the presence of celebrities together with selected shoes influenced attention but not preferences for the shoes. The second paper by Ohme et al. also applies frontal EEG to investigate neural responses to television advertising. Fundamental to their procedure is the assumption made by Davidson (2004) that asymmetries in frontal EEG reflect more or less emotional processing, with left-brain activities reflecting positive approach tendencies and the right brain processing negative avoidance-related activity. In the same context, Ohme et al. report findings elsewhere confirming his asymmetry model with measures testing facial muscle movements. In their study they compared responses to three 30-s commercials for the same brand, where the first part contained an emotional element in communicating the value of colours on a TV screen and the last part concentrated on product claims, brand name and product category information. Ohme et al. demonstrated that one of the three ads gave rise to significantly more left-brain dominance than the two others. They also showed a tendency for the processing of the ensuing product information to be biased in the same direction. However, it is noteworthy that the effects of the emotional part were significantly larger than those identified for the product-related part of the commercials. Their observation is in line with Stallen, Smidts et al. findings of limited purchase intent communicated by celebrities, and the less extreme purchase intent effect Treleaven et al. found in their P3a experiment. One of the topics of interest in the field of consumer neuroscience is to study the effectiveness of alternative communication, be it advertising or more complex messages. The paper by Ohme et al. falls into this category, and those by Treleaven et al. and by Stallen, Smidts et al. could also be seen in this light. They all represent intelligent attempts to test the situation that occurs when people are exposed to complex messages over an extended duration in time. While they suggest paths that future developments may take, neither provides the final answer to the ultimate question: What market communication has more impact than others? The papers discussed here have one thing in common: They all call for further research in the field of decision neuroscience. Summing up the challenges of this enterprise, the final submission by Hubert reviews the state of existing research and suggests directions for the future. Firstly, she looks at the current state of ‘neuroeconomics’ and discusses how the findings in this area may influence more general studies within cognitive neuroscience and individual decision-making in economics and other areas. She asserts that recent decision neuroscience is primarily explorative, striving to add a new theoretical perspective. Moreover, her thesis is that neuroeconomics research will move from mainly inductive studies to more deductive experimental approaches. She also points out a need for a revision of the concept of emotions in the light of physiological measures of the same, calling for research to be interdisciplinary. Finally she argues that future acceptance of neuroeconomics and consumer neuroscience within the scientific community will depend on the success of neurophysiological descriptions of classical theories, models and well-known phenomena. Taken together, these contributions demonstrate the wide spectrum of issues of interest in the nascent field of decision neuroscience. We hope that this wide range encourages the reader to gain more insights into this fascinating field of research. If it does so, this special issue will have fulfilled its goal. Copenhagen, Friedrichshafen, Fontainebleau, Spring 2010 Flemming Hansen Copenhagen Business School, Solbjerg Plads 3, CMC/Afsætningsøkonomi, 2000 Frederiksberg, Denmark E-mail address: [email protected] (F. Hansen) Peter Kenning Chair of Marketing, Zeppelin University, Am Seemooser Horn 20, 88045 Friedrichshafen, Germany E-mail address: [email protected] (P. Kenning) Hilke Plassmann Marketing Area, INSEAD, Boulevard de Constance, 77300 Fontainebleau, France E-mail address: [email protected] (H. Plassmann) Available online 4 March 2010
Contents lists available at ScienceDirect
Journal of Economic Psychology journal homepage: www.elsevier.com/locate/joep
Editorial
Contributions to decision neuroscience Scanning methodologies such as functional Magnetic Resonance Imaging (fMRI), electroencelography (EEG) and others have allowed neuroscientists to focus on brain activations during complex real life behaviours. Researchers have now moved beyond the clinical setting to study a wide range of basic processes in healthy subjects. This emerging discipline is broadly referred to as cognitive neuroscience. Within this field several groups have focussed on the study of decision-making, giving rise to the field of decision neuroscience. Decision neuroscience encompasses the behaviours of the individual in society in a wide range of decision-making contexts from economic decisions to health issues, communication and purchasing of goods; whether to purchase a product or not is a basic unit of economic and consumption behaviour. Decision neuroscience has thus initiated a multidisciplinary research agenda for economics, psychology and consumer behaviour. Within the nascent field of decision neuroscience several special interest groups have formed that focus for example on economic and public policy questions (referred to as neuroeconomics) or on marketing questions (referred to as consumer neuroscience). All the papers in this special issue of the Journal of Economic Psychology: Decision Neuroscience originate from the Conference on Neuroeconomics (‘‘ConNecs”) held at the Copenhagen Business School on May 15–16, 2008, which was attended by a large group of researchers involved in various aspects of decision neuroscience. From the many valuable contributions submitted, a number have been selected for this special issue to demonstrate the variety of the research carried out. From important theoretical contributions like that of Bechara and Reimann to more applied examples such as the paper by Ohme et al., the papers vary in the methodology used, ranging from fMRI over EEG to psycho-physiological procedures, and in their area of application, from general decision theory in economics to marketing and communication. The editors hope that this selection will provide readers with a thorough and varied introduction to the fast growing field of decision neuroscience. In ‘‘The somatic marker framework as a neurological theory of decision making”, Bechara and Reimann review the findings on what is probably the most important contemporary theory of emotional impact on decision-making. The theory originated with Damasio’s and Bechara’s joint work in the early 90s and has found confirmation in numerous studies since then. The Iowa gambling task, in particular, has been a major instrument in this research work. Essentially, the theory posits that decision makers store the consequences of alternative choices effectively. It is based upon observations with patients with lesions on the frontal lobes of the brain and in studies with healthy individuals. fMRI scanning has been a critical instrument in this development and the following areas in the brain have been identified as critical for emotional influences in accordance with the somatic marker theory (see Fig. 1): the medial prefrontal cortex with a particular focus on its ventromedial and orbitofrontal parts, the dorsal lateral prefrontal cortex and the amygdala. While Bechara and Reimann examine theoretical formulations of decision neuroscience, Ramsøy and Skov expand on this by reviewing genetically-driven changes in neurotransmitter levels as a factor influencing human decision-making. In ‘‘How genes make up your mind: Individual biological differences and value-based decisions” they depart from classical neurological work on the role of dopamine and serotonin in Parkinson’s and other mental diseases. They point out that variations in biological factors may account for individual differences in the processes underlying value-based decision-making. They argue that emotions are associated with neural processes whereby neural transmitters such as dopamine and opioid are important; these play a particular role for computing expected rewards. On the other hand, negative emotions are particularly related to brain areas where the neurotransmitter serotonin is important. Following a general discussion, the authors point to the role of ordinary-level variations in dopamine and serotonin in human decision-making. Specifically, they demonstrate that dopamine levels influence risk-taking in gambling tasks and that serotonin levels influence moods that may in turn influence decision-making. They argue that increased levels of serotonin influence anxiety and may generate an increased fear of losing, of risk or even ambiguity. Conversely, low levels of serotonin are related to depression, in which responses to reward and losses are lower and thus decisions will be less influenced by emotional factors. They then review a range of evidence suggesting that drug-induced alterations in dopamine and serotonin levels may influence decisionmaking. In a final section they argue that genetic differences may cause individual differences in dopamine and serotonin and other neurotransmitter levels. As a consequence they underline the need to test models of decision-making where 0167-4870/$ - see front matter Ó 2010 Published by Elsevier Ltd. doi:10.1016/j.joep.2010.03.001
Editorial / Journal of Economic Psychology 31 (2010) 764–766
765
individual differences are taken into consideration. As areas for future research they point to the interplay between genetic and experience-based factors in decision-making, and the role of genes/neurotransmitters in determining choices. Risk plays a crucial role in (economic) decision-making and has thus received considerable attention in the decision neuroscience literature. The same holds for the phenomenon of intertemporal choice. The paper by Jamison and Wegener that has won the best paper award during the ConNecs focuses on these phenomena. They point out that the terms ‘‘intertemporal choice”, ‘‘delay discounting”, and ‘‘time preference” refer to the same construct, namely individuals trading off outcomes to be received at different points in time. This is seen as fundamentally different from the idea of ‘‘prospection”, where the respondent imagines himself at some future point in time. The paper claims that the way in which people make evaluations when faced with a choice of outcomes to appear at different times is different from the form that prospection normally assumes, when ‘‘an agent is simply asked to imagine him or herself at some future point in time”. In the authors’ view, when people compare alternatives that occur at different times they apply evaluation procedures similar to those that would occur if they were asked to evaluate the future mental state of others; the processes encompassed by the term ‘‘theory of mind” are relevant here. The brain systems involved when individuals attempt to empathise with others are basically the same as when they empathise with themselves in the future. The areas of the brain that they involve are the temporal poles in the medial prefrontal cortex and areas both within and in proximity to the tempo-parietal junction. In reviewing the available literature, the authors find evidence for their proposition and report findings from a study where these areas are activated when people evaluate immediate monetary rewards compared with delayed monetary rewards varying from one week to one month and one year. In discussing future research, they stress the need to explain the extent to which individuals, who are more or less proficient in predicting the behaviour of others, are good at predicting their own future actions. The authors discuss the social implications of temporal choice. Many conflicting situations – healthwise and otherwise – in which the individual is placed involve the comparison of immediate rewards with future rewards. If the way in which future alternatives are evaluated is better understood and parameters are identified that can improve this evaluation procedure and make it more efficient, it could have considerable social implications. DaSilva, Moreira and Matsushita focus on risk-seeking behaviour among pre-school children. Based on research with monkeys, which were found to prefer risky choices in a gambling task, they propose that humans initially prefer risky choices but eventually learn by experience to avoid them. To explain this they conducted a study with 100 4–6-year-olds, finding a similar tendency to prefer risky choices but also an ability to learn to avoid risky choices in subsequent tasks. In a more general sense the findings suggest that individual differences in the preference for risky choices may depend upon previous opportunities (and ability) to learn about the respective consequences of risky vs. non-risky choices. Thus, differences in this respect may relate to pathological gambling and similar behaviours. In the subsequent paper, ‘‘Celebrities and shoes on the female brain: The neural correlates of product evaluation in the context of fame”, Stallen, Smidts et al. focus on the neural correlates of product evaluations in situations where celebrities are associated with the products to be evaluated. The authors depart from general consumer research findings that celebrity endorsements influence the effects of advertising, resulting in preferred brands and advertisements. They point out that whereas earlier work demonstrated that the pairing of a positively valence stimuli – a celebrity – with a product resulted in a conditioning process where effect is transferred from the neutral stimuli to the product, little is known about why this result occurs. The authors offer two possible explanations. First, they suggest that the perception of the celebrity may bring to mind facts about them, or suggest a personal relationship with them. Alternatively, the celebrity may trigger retrieval of implicit memories which are not dependent on conscious recall. In both cases, emotions may be influenced in terms of how positive or negative they are and/or in terms of how far they are aroused. To explore these processes, the authors report findings from an experiment where young female subjects were presented with photographs of equally attractive famous and non-famous faces together with photographs of products. In a preliminary behavioural test the authors found that shoes presented with famous faces were better recalled. Conversely, the purchase intention was not significantly different between shoes presented with or without famous faces. In the fMRI part of the experiment it was found that the perception of celebrity faces generated activity in the right medial orbitofrontal cortex when the combination of famous face/shoes was compared with non-famous face/shoes. The authors interpret this as being due to the generation of associations between famous faces and the product. The influence on the evaluation of shoes, however, was less certain. Moreover, the number of subjects in this condition was too small for a significant testing. The authors emphasise the need for further research, and to include male respondents. Two other papers also investigate the neuropsychology of advertising effects. In the first, entitled ‘‘Using the P3a to gauge automatic attention to interactive television advertising”, Treleaven et al. demonstrate the use of a special version of eventrelated potential obtained with EEG labelled P3a, which focuses on the occurrence of particularly strong effects in the frontal central region of the brain. They argue for the use of this methodology rather than fMRI, since it is more cost efficient and can be applied relatively easily to a larger number of subjects. It is argued that the occurrence of the P3a reflects attention. The authors report on a project where they compared the ERP data with self reports on attention, preferences and other behavioural and cognitive measures. In the study, a 32-channel EEG cap was used while respondents were exposed to brand logos and advertisements. The exposure situations were manipulated so that some were presented as interactive television and others as regular advertising. Basically, they found that the first of two (the most demanding) forms of interactive advertising generated more attention, whereas neither of them had any impact on preferences for the advertised items. This brings
766
Editorial / Journal of Economic Psychology 31 (2010) 764–766
to mind a somewhat similar finding in the aforementioned paper by Stallen, Smidts et al., where the presence of celebrities together with selected shoes influenced attention but not preferences for the shoes. The second paper by Ohme et al. also applies frontal EEG to investigate neural responses to television advertising. Fundamental to their procedure is the assumption made by Davidson (2004) that asymmetries in frontal EEG reflect more or less emotional processing, with left-brain activities reflecting positive approach tendencies and the right brain processing negative avoidance-related activity. In the same context, Ohme et al. report findings elsewhere confirming his asymmetry model with measures testing facial muscle movements. In their study they compared responses to three 30-s commercials for the same brand, where the first part contained an emotional element in communicating the value of colours on a TV screen and the last part concentrated on product claims, brand name and product category information. Ohme et al. demonstrated that one of the three ads gave rise to significantly more left-brain dominance than the two others. They also showed a tendency for the processing of the ensuing product information to be biased in the same direction. However, it is noteworthy that the effects of the emotional part were significantly larger than those identified for the product-related part of the commercials. Their observation is in line with Stallen, Smidts et al. findings of limited purchase intent communicated by celebrities, and the less extreme purchase intent effect Treleaven et al. found in their P3a experiment. One of the topics of interest in the field of consumer neuroscience is to study the effectiveness of alternative communication, be it advertising or more complex messages. The paper by Ohme et al. falls into this category, and those by Treleaven et al. and by Stallen, Smidts et al. could also be seen in this light. They all represent intelligent attempts to test the situation that occurs when people are exposed to complex messages over an extended duration in time. While they suggest paths that future developments may take, neither provides the final answer to the ultimate question: What market communication has more impact than others? The papers discussed here have one thing in common: They all call for further research in the field of decision neuroscience. Summing up the challenges of this enterprise, the final submission by Hubert reviews the state of existing research and suggests directions for the future. Firstly, she looks at the current state of ‘neuroeconomics’ and discusses how the findings in this area may influence more general studies within cognitive neuroscience and individual decision-making in economics and other areas. She asserts that recent decision neuroscience is primarily explorative, striving to add a new theoretical perspective. Moreover, her thesis is that neuroeconomics research will move from mainly inductive studies to more deductive experimental approaches. She also points out a need for a revision of the concept of emotions in the light of physiological measures of the same, calling for research to be interdisciplinary. Finally she argues that future acceptance of neuroeconomics and consumer neuroscience within the scientific community will depend on the success of neurophysiological descriptions of classical theories, models and well-known phenomena. Taken together, these contributions demonstrate the wide spectrum of issues of interest in the nascent field of decision neuroscience. We hope that this wide range encourages the reader to gain more insights into this fascinating field of research. If it does so, this special issue will have fulfilled its goal. Copenhagen, Friedrichshafen, Fontainebleau, Spring 2010 Flemming Hansen Copenhagen Business School, Solbjerg Plads 3, CMC/Afsætningsøkonomi, 2000 Frederiksberg, Denmark E-mail address: [email protected] (F. Hansen) Peter Kenning Chair of Marketing, Zeppelin University, Am Seemooser Horn 20, 88045 Friedrichshafen, Germany E-mail address: [email protected] (P. Kenning) Hilke Plassmann Marketing Area, INSEAD, Boulevard de Constance, 77300 Fontainebleau, France E-mail address: [email protected] (H. Plassmann) Available online 4 March 2010