Self-Organisation in the Nervous System

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The term self-organization refers to the process by which individuals organize their communal behavior to create global order by interactions amongst themselves rather than through external intervention or instruction. As a highly complex and dynamic system involving many different elements interacting with each other, the nervous system displays many features of self-organization. This chapter discusses three forms of neural self-organization namely self-organization in development, self-organization as a complement to experiential changes, and self-organization as a complement to damage. Self-organization in development is concerned the development of the nervous system. Since a key challenge in our understanding of the nervous system is to comprehend how such a highly structured yet complex system can emerge from a single fertilized egg. Self-organization as a complement to experiential changes refers to later stages in development, when self-organization plays a role along with other mechanisms such as those involving external signals arising from the sensory environment. Self-organization as a complement to damage is attributed to the adult nervous system that can respond to surgical or accidental damage. The facility for damaged brain to regenerate is either minimal or non-existent, which implies that the brain can self-organize, allowing healthy regions to take over functions previously carried out by other regions.

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... ANNs have gained popularity among other models to solve pattern recognition and classification problems due to their ability to learn noncomplex linear input and output relationships (Willshaw, 2006) and their ability to learn from sequences and self-adaptation to input data. ...
The development of exoskeletons has contributed to the rehabilitation of the population with different degrees of disability. These devices contemplate some feedback signals for their control; these signals can come from a user control or be taken from the brain’s or muscles’ user electrical response. In the specific case of exoskeletons of the lower extremities, they generally depend only on control algorithms to develop the trajectories of the user’s lower extremities. In general, they also implement electromyography (EMG) interfaces as separate systems for measuring patient activity or improvements in the rehabilitation of the musculoskeletal system that carry their devices. For this reason, a human in the loop scheme was proposed in this work; a combination of a recurrent neural network (RNN) and an adaptive non-singular fast terminal sliding mode controller (ANFTSMC) strategy is employed to classify the user’s movements and control the trajectories of an exoskeleton. This paper presents the construction of the electromyographic signals (EMGS) database, containing data acquired from brachii biceps and sternocleidomastoid muscles. Then, the training and validation of the RNN for the classification of EMGS. The mathematical approach of the ANFTSMC and the stability analysis of this control scheme for the trajectory tracking problem of the Sit-to-Stand task in four degrees of freedom exoskeleton. Finally, the complete human-in-the-loop system working with the RNN classifier and the ANFTSMC is established, with the advantage of being an effective, precise, and intelligent system that can be used by people with high degrees of motor disability.
... Nevertheless, more that ten years earlier von der Malsburg (1973) had proposed the first mathematical simulation of the development of orientation sensitive cells in the striate cortex by self-organization. Since then, the simulation of cortical development by self-organization become a well established research direction (Miikkulainen et al. 2005;Willshaw 2006), but no fruitful interaction with the canonical circuit research ever took place. ...
This paper addresses a fundamental line of research in neuroscience: the identification of a putative neural processing core of the cerebral cortex, often claimed to be “canonical”. This “canonical” core would be shared by the entire cortex, and would explain why it is so powerful and diversified in tasks and functions, yet so uniform in architecture. The purpose of this paper is to analyze the search for canonical explanations over the past 40 years, discussing the theoretical frameworks informing this research. It will highlight a bias that, in my opinion, has limited the success of this research project, that of overlooking the dimension of cortical development. The earliest explanation of the cerebral cortex as canonical was attempted by David Marr, deriving putative cortical circuits from general mathematical laws, loosely following a deductive-nomological account. Although Marr’s theory turned out to be incorrect, one of its merits was to have put the issue of cortical circuit development at the top of his agenda. This aspect has been largely neglected in much of the research on canonical models that has followed. Models proposed in the 1980s were conceived as mechanistic. They identified a small number of components that interacted as a basic circuit, with each component defined as a function. More recent models have been presented as idealized canonical computations, distinct from mechanistic explanations, due to the lack of identifiable cortical components. Currently, the entire enterprise of coming up with a single canonical explanation has been criticized as being misguided, and the premise of the uniformity of the cortex has been strongly challenged. This debate is analyzed here. The legacy of the canonical circuit concept is reflected in both positive and negative ways in recent large-scale brain projects, such as the Human Brain Project. One positive aspect is that these projects might achieve the aim of producing detailed simulations of cortical electrical activity, a negative one regards whether they will be able to find ways of simulating how circuits actually develop.
... An analogy is found from studies of insect colonies, in which specific groups of insects each have specific roles to perform, and these different groups interact in particular ways. The functioning of the colony as a whole is then an emergent property of the multiple, interacting parallel activities, as indeed is the appearance of executive control (e.g., Willshaw, 2006). One formal model of working memory built broadly on this concept was proposed as consisting of interacting cognitive subsystems (Barnard, 1999). ...
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More than 40 years ago, Baddeley and Hitch (1974) published an article with a wealth of experimentation and theorization on working memory, the small amount of information held in mind and often used within cognitive processes such as language comprehension and production, reasoning, and problem solving. We honor this seminal accomplishment in the present special issue, and take this opportunity to provide an introduction to our perspectives on the origin of the theory of working memory, how it has affected our work, what may be coming in the near future, and how the research articles in the present issue contribute to several related themes within the clearly thriving field of working memory.
... In the UK a foresight project is underway which already has produced a series of reviews of the state of the art in cognitive systems. These include topics like sensory processing including speech recognition, auditory processing, vision, representations, and learning (Hughes 2003); action specification and selection (Barnard et al. 2003); learning and memory aspects like working, long-term, episodic, semantic, value, emotional, spatial and event memory (); representation and learning (Walsh and Laughlin 2003); self-organisation and pattern formation (Willshaw 2003); speech and language processing (Marslen-Wilson 2003); social cognition including distinguishing the self and other agents, imitation, deception, complex emotions, empathy, morality, cultural evolution, but also communication failures like autism, psychopathy and others (Frith and Blakemore 2003); and finally advanced Neuroscience technologies such as brain imaging, single-cell and multiple single-unit recording, optical imaging, and combinations thereof (Ahmed et al. 2003). ...
There are few examples of an extended adversarial collaboration, in which investigators committed to different theoretical views collaborate to test opposing predictions. Whereas previous adversarial collaborations have produced single research articles, here, we share our experience in programmatic, extended adversarial collaboration involving three laboratories in different countries with different theoretical views regarding working memory, the limited information retained in mind, serving ongoing thought and action. We have focused on short-term memory retention of items (letters) during a distracting task (arithmetic), and effects of aging on these tasks. Over several years, we have conducted and published joint research with preregistered predictions, methods, and analysis plans, with replication of each study across two laboratories concurrently. We argue that, although an adversarial collaboration will not usually induce senior researchers to abandon favored theoretical views and adopt opposing views, it will necessitate varieties of their views that are more similar to one another, in that they must account for a growing, common corpus of evidence. This approach promotes understanding of others’ views and presents to the field research findings accepted as valid by researchers with opposing interpretations. We illustrate this process with our own research experiences and make recommendations applicable to diverse scientific areas.
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Reasoning, problem solving, comprehension, learning and retrieval, inhibition, switching, updating, or multitasking are often referred to as higher cognition, thought to require control processes or the use of a central executive. However, the concept of an executive controller begs the question of what is controlling the controller and so on, leading to an infinite hierarchy of executives or ‘homunculi’. In what is now a QJEP citation classic, Baddeley (1996) referred to the concept of a central executive in cognition as a ‘conceptual ragbag’ that acted as a placeholder umbrella term for aspects of cognition that are complex, were poorly understood at the time, and most likely involve several different cognitive functions working in concert. He suggested that with systematic empirical research, advances in understanding might progress sufficiently to allow the executive concept to be ‘sacked’. This article offers an overview of the 1996 article and of some subsequent systematic research, and argues that after two decades of research, there is sufficient advance in understanding to suggest that executive control might arise from the interaction among multiple different functions in cognition that use different, but overlapping brain networks. The article concludes that the central executive concept might now be offered a dignified retirement.
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We argue that the concepts of mechanism and autonomy appear to be antagonistic when autonomy is conflated with agency. Once these concepts are disentangled, it becomes clearer how autonomy could emerge from complex forms of control — especially, homeostatic regulatory systems. While research in AI and robotics would do well to continue incorporating biomimetic strategies, we propose that invoking models of allostatic mechanisms is a better way to understand how autonomy in artificial systems can be enhanced.
Metaphor influences the construction of biological models and theories and the analysis of its use can reveal important tools of thought. Some aspects of biological organisation are investigated through the analysis of metaphors associated with treating biosystems as a kind of text. In particular, the use of glue and verbs is considered. Some of the reasons why glue is important in the construction of hierarchies are pursued in the light of specific examples, and some of the conceptual links between glue in biology and other domains is discussed. Verbs are shown to be important in the construction of networks. Some of the relations between glue, verb and text are considered and the text metaphor is placed within a much broader context of ideas associated with form, relation and system. The paper concludes with comments on the nature of biological information and the need for extending or better understanding the verbal vocabulary.
Conference Paper
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Biological neural systems are well known for their robust and power-efficient operation in highly noisy environments. Biological circuits are made up of low-precision, unreliable and massively parallel neural elements with highly reconfigurable and plastic connections. Two of the most interesting properties of the neural systems are its self-organizing capabilities and its template architecture. Recent research in spiking neural networks has demonstrated interesting principles about learning and neural computation. Understanding and applying these principles to practical problems is only possible if large-scale spiking neural simulators can be constructed. Recent advances in low-cost multiprocessor architectures make it possible to build large-scale spiking network simulators. In this paper we review modeling abstractions for neural circuits and frameworks for modeling, simulating and analyzing spiking neural networks.
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The question of how spatial structures and order are generated in the developing organism is one of the most challenging areas of biological research today. Much useful work has been done, principally by observing pattern regulation occurring after an experimental disturbance, but the underlying mechanisms — the multiplicity of molecular, cellular and tissue interactions taking place still remain obscure. This book shows which types of molecular interaction are able to account for well known and representative results documented in the literature. The mechanisms proposed are illustrated by precise, yet simply understood mathematical models, a means profitably applied for illuminating complex systems in many other disciplines. The book begins by discussing mechanisms which have the ability to generate biological patterns, and goes on to consider how these mechanisms are used to generate positional information. A detailed comparison of the general theory with insect development is presented, and further chapters review the generation of subpatterns and how cells respond. Also considered are mechanisms for the activation of particular genes, which arc seen to be compatible with developmental alteration induced by a known class of mutants. The book concludes with a collection of FORTRAN computer programs which allow a simulation of the assumed interactions. The precise mathematical treatment of biological dada presented here represents the first attempt to formulate a coherent theory of pattern formation during development. It will certainly stimulate further research, and will be read with interest by developmental biologists and those interested in mathematical modelling of complex systems.
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A nerve net model for the visual cortex of higher vertebrates is presented. A simple learning procedure is shown to be sufficient for the organization of some essential functional properties of single units. The rather special assumptions usually made in the literature regarding preorganization of the visual cortex are thereby avoided. The model consists of 338 neurones forming a sheet analogous to the cortex. The neurones are connected randomly to a retina of 19 cells. Nine different stimuli in the form of light bars were applied. The afferent connections were modified according to a mechanism of synaptic training. After twenty presentations of all the stimuli individual cortical neurones became sensitive to only one orientation. Neurones with the same or similar orientation sensitivity tended to appear in clusters, which are analogous to cortical columns. The system was shown to be insensitive to a background of disturbing input excitations during learning. After learning it was able to repair small defects introduced into the wiring and was relatively insensitive to stimuli not used during training.
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Retinal cells have been induced to project into the medial geniculate nucleus, the principal auditory thalamic nucleus, in newborn ferrets by reduction of targets of retinal axons in one hemisphere and creation of alternative terminal space for these fibers in the auditory thalamus. Many cells in the medial geniculate nucleus are then visually driven, have large receptive fields, and receive input from retinal ganglion cells with small somata and slow conduction velocities. Visual cells with long conduction latencies and large contralateral receptive fields can also be recorded in primary auditory cortex. Some visual cells in auditory cortex are direction selective or have oriented receptive fields that resemble those of complex cells in primary visual cortex. Thus, functional visual projections can be routed into nonvisual structures in higher mammals, suggesting that the modality of a sensory thalamic nucleus or cortical area may be specified by its inputs during development.
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Complex networks are studied across many fields of science. To uncover their structural design principles, we defined “network motifs,” patterns of interconnections occurring in complex networks at numbers that are significantly higher than those in randomized networks. We found such motifs in networks from biochemistry, neurobiology, ecology, and engineering. The motifs shared by ecological food webs were distinct from the motifs shared by the genetic networks of Escherichia coli and Saccharomyces cerevisiae or from those found in the World Wide Web. Similar motifs were found in networks that perform information processing, even though they describe elements as different as biomolecules within a cell and synaptic connections between neurons in Caenorhabditis elegans. Motifs may thus define universal classes of networks. This approach may uncover the basic building blocks of most networks.
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We have determined how most of the transcriptional regulators encoded in the eukaryote Saccharomyces cerevisiaeassociate with genes across the genome in living cells. Just as maps of metabolic networks describe the potential pathways that may be used by a cell to accomplish metabolic processes, this network of regulator-gene interactions describes potential pathways yeast cells can use to regulate global gene expression programs. We use this information to identify network motifs, the simplest units of network architecture, and demonstrate that an automated process can use motifs to assemble a transcriptional regulatory network structure. Our results reveal that eukaryotic cellular functions are highly connected through networks of transcriptional regulators that regulate other transcriptional regulators.
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Adult bone marrow stem cells seem to differentiate into muscle, skin, liver, lung, and neuronal cells in rodents and have been shown to regenerate myocardium, hepatocytes, and skin and gastrointestinal epithelium in humans. Because we have demonstrated previously that transplanted bone marrow cells can enter the brain of mice and differentiate into neurons there, we decided to examine postmortem brain samples from females who had received bone marrow transplants from male donors. The underlying diseases of the patients were lymphocytic leukemia and genetic deficiency of the immune system, and they survived between 1 and 9 months after transplant. We used a combination of immunocytochemistry (utilizing neuron-specific antibodies) and fluorescent in situ hybridization histochemistry to search for Y chromosome-positive cells. In all four patients studied we found cells containing Y chromosomes in several brain regions. Most of them were nonneuronal (endothelial cells and cells in the white matter), but neurons were certainly labeled, especially in the hippocampus and cerebral cortex. The youngest patient (2 years old), who also lived the longest time after transplantation, had the greatest number of donor-derived neurons (7 in 10,000). The distribution of the labeled cells was not homogeneous. There were clusters of Y-positive cells, suggesting that single progenitor cells underwent clonal expansion and differentiation. We conclude that adult human bone marrow cells can enter the brain and generate neurons just as rodent cells do. Perhaps this phenomenon could be exploited to prevent the development or progression of neurodegenerative diseases or to repair tissue damaged by infarction or trauma.
This is the first book that attempts to bring together what is known about the fundamental mechanisms that underlie the development of the cerebral cortex in mammals. Ranging from the emergence of the forebrain from the neural plate, to the functioning adult form, the book draws on evidence from several species to provide a detailed description of processes at each stage. Where appropriate, evidence is extrapolated from non-mammalian species to generate hypotheses about mammalian development. In contrast to other texts of developmental biology, this book integrates information on regulatory processes at the levels of molecules, cells, and networks. It draws together an extensive literature on cellular development and structural morphology, biochemical and genetic events, and hypotheses that have been subject to mathematical modelling. Important methodologies such as transgenics and formal modelling, are explained for the non-specialist. Major future challenges are clearly identified. The book combines the fundamentals of experimental developmental neurobiology with accessible neural modelling. © David J. Price and David J. Willshaw 2000. All rights reserved.
This article traces the author's personal history in neuroanatomical studies of the cerebral cortex including his father's pioneering studies using electrophysiology and early neuroanatomical methods. By providing the background context for the observations reported in the original heavily cited Brain Research article, the author describes how his methods of investigation with celloidin embedded material prepared with the Golgi method and Nissl staining revealed for the first time the “barrel fields” of the mouse cerebral cortex that are activated by stimulation of the facial vibrissae (whiskers). The original article reviewed the series of studies published on the murine cortical field in what later was termed a Brain Research Review. Those original observations opened an entire field of cortical organization and function that has remained an active and fertile field of neuroscience investigation.
An important problem in biology is to explain how patterned neural connections are set up during ontogenesis. Topographically ordered mappings, found widely in nervous systems, are those in which neighbouring elements in one sheet of cells project to neighbouring elements in a second sheet. Exploiting this neighbourhood property leads to a new theory for the establishment of topographical mappings, in which the distance between two cells is expressed in terms of their similarity with respect to certain physical properties assigned to them. This topographical code can be realized in a model employing either synchronization of nervous activity or exchange of specific molecules between neighbouring cells. By means of modifiable synapses the code is used to set up a topographical mapping between two sheets with the same internal structure. We have investigated the neural activity version. Without needing to make any elaborate assumptions about its structure or about the operations its elements are to carry out we have shown that the mappings are set up in a system-to-system rather than a cell-to-cell fashion. The pattern of connections develops in a step-by-step and orderly fashion, the orientation of the mappings being laid down in the earliest stages of development.
Ocular dominance columns were examined by a variety of techniques in juvenile macaque monkeys in which one eye had been removed or sutured closed soon after birth. In two monkeys the removal was done at 2 weeks and the cortex studied at 11/2 years. Physiological recordings showed continuous responses as an electrode advanced along layer IVC in a direction parallel to the surface. Examination of the cortex with the Fink-Heimer modification of the Nauta method after lesions confined to single lateral-geniculate layers showed a marked increase, in layer IVC, in the widths of columns belonging to the surviving eye, and a corresponding shrinkage of those belonging to the removed eye. Monocular lid closures were made in one monkey at 2 weeks of age, for a period of 18 months, in another at 3 weeks for 7 months, and in a third at 2 days for 7 weeks. Recordings from the lateral geniculate body showed brisk activity from the deprived layers and the usual abrupt eye transitions at the boundaries between layers. Cell shrinkage in the deprived layers was moderate - far less severe than that following eye removal, more marked ipsilaterally than contralaterally, and more marked the earlier the onset of the deprivation. In autoradiographs following eye injection with a mixture of tritiated proline and tritiated fucose the labelling of terminals was confined to geniculate layers corresponding to the injected eye. Animals in which the open eye was injected showed no hint of invasion of terminals into the deprived layers. Similarly in the tectum there was no indication of any change in the distribution of terminals from the two eyes. The autoradiographs of the lateral geniculates provide evidence for several previously undescribed zones of optic nerve terminals, in addition to the six classical subdivisions. In the cortex four independent methods, physiological recording, transneuronal autoradiography, Nauta degeneration, and a reduced-silver stain for normal fibres, all agreed in showing a marked shrinkage of deprived-eye columns and expansion of those of the normal eye, with preservation of the normal repeat distance (left-eye column plus right-eye column). There was a suggestion that changes in the columns were more severe when closure was done at 2 weeks as opposed to 3, and more severe on the side ipsilateral to the closure. The temporal crescent representation in layer IVC of the hemisphere opposite the closure showed no obvious adverse effects. Cell size and packing density in the shrunken IVth layer columns seemed normal. In one normal monkey in which an eye was injected the day after birth, autoradiographs of the cortex at 1 week indicated only a very mild degree of segregation of input from the two eyes; this had the form of parallel bands. Tangential recordings in layer IVC at 8 days likewise showed considerable overlap of inputs, though some segregation was clearly present; at 30 days the segregation was much more advanced. These preliminary experiments thus suggest that the layer IVC columns are not fully developed until some weeks after birth. Two alternate possibilities are considered to account for the changes in the ocular dominance columns in layer IVC following deprivation. If one ignores the above evidence in the newborn and assumes that the columns are fully formed at birth, then after eye closure the afferents from the normal eye must extend their territory, invading the deprived-eye columns perhaps by a process of sprouting of terminals. On the other hand, if at birth the fibres from each eye indeed occupy all of lay IVC, retracting to form the columns only during the first 6 weeks or so, perhaps by a process of competition, then closure of one eye may result in a competitive disadvantage of the terminals from that eye, so that they retract more than they would normally. This second possibility has the advantage that it explains the critical period for deprivation effects in the layer IV columns, this being the time after birth during which retraction is completed. It would also explain the greater severity of the changes in the earlier closures, and would provide an interpretation of both cortical and geniculate effects in terms of of competition of terminals in layer IVC for territory on postsynaptic cells.
This paper examines the idea that ordered patterns of nerve connections are set up by means of markers carried by the individual cells. The case of the ordered retinotectal projection in amphibia and fishes is discussed in great detail. It is suggested that retinotectal mappings are the result of two mechanisms acting in concert. One mechanism induces a set of retinal markers into the tectum. By this means, an initially haphazard pattern of synapses is transformed into a continuous or piece-wise continuous projection. The other mechanism places the individual pieces of the map in the correct orientation. The machinery necessary for this inductive scheme has been expressed in terms of a set of differential equations, which have been solved numerically for a number of cases. Straightforward assumptions are made as to how markers are distributed in the retina; how they are induced into the tectum; and how the induced markers bring about alterations in the pattern of synaptic contacts. A detailed physiological interpretation of the model is given. The inductive mechanism has been formulated at the level of the individual synaptic interactions. Therefore, it is possible to specify, in a given situation, not only the nature of the end state of the mapping but also how the mapping develops over time. The role of the modes of growth of retina and tectum in shaping the developing projection becomes clear. Since, on this model, the tectum is initially devoid of markers, there is an important difference between the development and the regeneration of ordered mappings. In the development of duplicate maps from various types of compound-eyes, it is suggested that the tectum, rather than the retina, contains an abnormal distribution of markers. An important parameter in these experiments, and also in the regeneration experiments where part-duplication has been found, is the range of interaction amongst the retinal cells. It is suggested that the results of many of the regeneration experiments (including apparently contradictory ones) are manifestations of a conflict between the two alternative ways of specifying the orientation of the map: through the information carried by the markers previously induced into the tectum and through the orientation mechanism itself.
Since the auditory system has evolved together with the vocalization system, the auditory system is specialized for receiving and processing acoustic signals that are frequently used by an animal and are important for the animal's survival. As the number of acoustic signals in the environment increased, animal sounds became more stereotyped and the auditory system evolved a) a larger filter bank; b) higher sensitivity, narrower tuning curves and larger number of neurons for processing more important signals; and c) more neural networks acting as filters to extract particular types of signals. Specialized neurons, which express the output of such neural networks, should show a level-tolerant specificity and rejection mode. The mustache bat Pteronotus parnellii rubiginosus offers a prime example of the specialized auditory system. For the reception and fine frequency analysis of species-specific orientation sounds and echoes, its peripheral auditory system has a big filter bank that contains a group of unusually sharply tuned neurons. The primary auditory cortex shows tonotopic organization that is disproportionate, reflecting the amplitude spectrums of biologically significant sounds, and it consists of functional divisions that are organized for information processing in different modes depending on the biological significance of each of the frequency bands in the sounds. The functional organization of the auditory cortex is discussed in relation to both the detector and spatiotemporal pattern theories.
It is suggested that a system of chemical substances, called morphogens, reacting together and diffusing through a tissue, is adequate to account for the main phenomena of morphogenesis. Such a system, although it may originally be quite homogeneous, may later develop a pattern or structure due to an instability of the homogeneous equilibrium, which is triggered off by random disturbances. Such reaction-diffusion systems are considered in some detail in the case of an isolated ring of cells, a mathematically convenient, though biologically unusual system. The investigation is chiefly concerned with the onset of instability. It is found that there are six essentially different forms which this may take. In the most interesting form stationary waves appear on the ring. It is suggested that this might account, for instance, for the tentacle patterns onHydra and for whorled leaves. A system of reactions and diffusion on a sphere is also considered. Such a system appears to account for gastrulation. Another reaction system in two dimensions gives rise to patterns reminiscent of dappling. It is also suggested that stationary waves in two dimensions could account for the phenomena of phyllotaxis. The purpose of this paper is to discuss a possible mechanism by which the genes of a zygote may determine the anatomical structure of the resulting organism. The theory does not make any new hypotheses; it merely suggests that certain well-known physical laws are sufficient to account for many of the facts. The full understanding of the paper requires a good knowledge of mathematics, some biology, and some elementary chemistry. Since readers cannot be expected to be experts in all of these subjects, a number of elementary facts are explained, which can be found in text-books, but whose omission would make the paper difficult reading.
The problem of pattern is considered in terms of how genetic information can be translated in a reliable manner to give specific and different spatial patterns of cellular differentiation. Pattern formation thus differs from molecular differentiation which is mainly concerned with the control of synthesis of specific macromolecules within cells rather than the spatial arrangement of the cells. It is suggested that there may be a universal mechanism whereby the translation of genetic information into spatial patterns of differentiation is achieved. The basis of this is a mechanism whereby the cells in a developing system may have their position specified with respect to one or more points in the system. This specification of position is positional information. Cells which have their positional information specified with respect to the same set of points constitute a field. Positional information largely determines with respect to the cells' genome and developmental history the nature of its molecular differentiation. The specification of positional information in general precedes and is independent of molecular differentiation. The concept of positional information implies a co-ordinate system and polarity is defined as the direction in which positional information is specified or measured. Rules for the specification of positional information and polarity are discussed. Pattern regulation, which is the ability of the system to form the pattern even when parts are removed, or added, and to show size invariance as in the French Flag problem, is largely dependent on the ability of the cells to change their positional information and interpret this change. These concepts are applied in some detail to early sea urchin development, hydroid regeneration, pattern formation in the insect epidermis, and the development of the chick limb. It is concluded that these concepts provide a unifying framework within which a wide variety of patterns formed from fields may be discussed, and give new meaning to classical concepts such as induction, dominance and field. The concepts direct attention towards finding mechanisms whereby position and polarity are specified, and the nature of reference points and boundaries. More specifically, it is suggested that the mechanism is required to specify the position of about 50 cells in a line, relatively reliably, in about 10 hours. The size of embryonic fields is, surprisingly, usually less than 50 cells in any direction.
In a series of Xenopus larvae (stages 28 to 35), the left eye cup was dorsoventrally and anteroposteriorly inverted. After metamorphosis, the retinotectal projection was mapped by recording action potentials evoked in the tectum by a small spot of light projected on the retina. Normal retinotectal projection was found following rotation of the eye cup at stage 28 to 29. Rotation of the eye cup at stage 30 resulted in anteroposterior inversion of the retinotectal projection; rotation at stages 31, 32, and 35 resulted in inversion of the projection in anteroposterior and dorsoventral axis of the retina. Therefore the retinal ganglion cells were unspecified at stage 28 to 29; spatial specification of ganglion cells occurred in the anteroposterior axis of the eye cup at stage 30 and in the dorsoventral axis between stages 30 and 31.
We have anatomically analyzed retinotopic organization using the 14C-labeled 2-deoxy-D-glucose method. The method has several advantages over conventional electrophysiological mapping techniques. In the striate cortex, the anatomical substrate for retinotopic organization is surprisingly well ordered, and there seems to be a systematic relationship between ocular dominance strips and cortical magnification. The 2-deoxyglucose maps in this area appear to be largely uninfluenced by known differences in long-term metabolic activity. This method should prove useful in analyzing retinotopic organization in various visual areas of the brain and in different species.
In the establishment of connections between nerve and muscle there is an initial stage when each muscle fibre is innervated by several different motor axons. Withdrawal of connections then takes place until each fibre has contact from just a single axon. The evidence suggests that the withdrawal process involves competition between nerve terminals. We examine in formal models several types of competitive mechanism that have been proposed for this phenomenon. We show that a model which combines competition for a presynaptic resource with competition for a postsynaptic resource is superior to others. This model accounts for many anatomical and physiological findings and has a biologically plausible implementation. Intrinsic withdrawal appears to be a side effect of the competitive mechanism rather than a separate non-competitive feature. The model's capabilities are confirmed by theoretical analysis and full scale computer simulations.
Brain damage, brain repair
  • J W Fawcett
  • A E Rosser
  • S B Dunnet
Fawcett, J. W., Rosser, A. E., and Dunnet, S. B. (2001). Brain damage, brain repair. Oxford University Press.
  • G M Shepherd
Shepherd, G. M. (1994). Neurobiology. Oxford University Press, 3rd edition.
The developing brain
  • M Brown
  • R Keynes
  • A Lumsden
Brown, M., Keynes, R., and Lumsden, A. (2001), The developing brain, Oxford University Press.
The Triumph of the Embryo
  • L Wolpert
Wolpert, L. (1991). The Triumph of the Embryo. Oxford University Press.
is a classic treatise on the nervous system, first published in 1977 which has undergone continual revision since then
  • Nervous System Nicholls
Nervous system Nicholls et al. (2001) is a classic treatise on the nervous system, first published in 1977 which has undergone continual revision since then. Shepherd (1994);
is a collection of papers reviewing recent research in this field
  • Brain Fawcett
Brain damage and repair Fawcett et al. (2001) is a collection of papers reviewing recent research in this field. References
is a text at the molecular biology level and Wolpert (1991) is a readable introduction to embryonic development for non-specialists
  • Development Alberts
Development Alberts et al. (1994) is a text at the molecular biology level and Wolpert (1991) is a readable introduction to embryonic development for non-specialists.
are research level texts on the development of the nervous system, the latter two being more recent. The recent research monograph by Price and Willshaw (2000) describes genetic, molecular, systems and modelling approaches to understanding neocortical development
  • Sanes
Development of the nervous system Purves and Lichtman (1985); Sanes et al. (2000); Brown et al. (2001) are research level texts on the development of the nervous system, the latter two being more recent. The recent research monograph by Price and Willshaw (2000) describes genetic, molecular, systems and modelling approaches to understanding neocortical development. Elman (1996) takes a connectionist approach to development.
The organisation of behavior
  • D Hebb
Hebb, D. (1949). The organisation of behavior. Wiley, New York.
From Neuron to Brain
  • J G Nicholls
  • P A Fuchs
  • A R Martin
  • B G Wallace
Nicholls, J. G., Fuchs, P. A., Martin, A. R., and Wallace, B. G. (2001). From Neuron to Brain. Sinauer Associates, Sunderland, Mass., 4th edition.
Chemoaffinity in the orderly growth of nerve fiber patterns and connections
  • R W Sperry
Sperry, R. W. (1963). Chemoaffinity in the orderly growth of nerve fiber patterns and connections. Proc. Nat. Acad. Sci., USA, 50:703-710.
are research level texts on the development of the nervous system, the latter two being more recent. The recent research monograph by Price and Willshaw (2000) describes genetic, molecular, systems and modelling approaches to understanding neocortical development
  • Brown
Brown et al. (2001) are research level texts on the development of the nervous system, the latter two being more recent. The recent research monograph by Price and Willshaw (2000) describes genetic, molecular, systems and modelling approaches to understanding neocortical development. Elman (1996) takes a connectionist approach to development.
Principles of Neural Development
  • D Purves
  • J W Lichtman
Purves, D. and Lichtman, J. W. (1985). Principles of Neural Development. Sinauer Associates, Sunderland, MA.