151 research outputs found

    A survey of visual preprocessing and shape representation techniques

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    Many recent theories and methods proposed for visual preprocessing and shape representation are summarized. The survey brings together research from the fields of biology, psychology, computer science, electrical engineering, and most recently, neural networks. It was motivated by the need to preprocess images for a sparse distributed memory (SDM), but the techniques presented may also prove useful for applying other associative memories to visual pattern recognition. The material of this survey is divided into three sections: an overview of biological visual processing; methods of preprocessing (extracting parts of shape, texture, motion, and depth); and shape representation and recognition (form invariance, primitives and structural descriptions, and theories of attention)

    Toward a further understanding of object feature binding: a cognitive neuroscience perspective.

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    The aim of this thesis is to lead to a further understanding of the neural mechanisms underlying object feature binding in the human brain. The focus is on information processing and integration in the visual system and visual shortterm memory. From a review of the literature it is clear that there are three major competing binding theories, however, none of these individually solves the binding problem satisfactorily. Thus the aim of this research is to conduct behavioural experimentation into object feature binding, paying particular attention to visual short-term memory. The behavioural experiment was designed and conducted using a within-subjects delayed responset ask comprising a battery of sixty-four composite objects each with three features and four dimensions in each of three conditions (spatial, temporal and spatio-temporal).Findings from the experiment,which focus on spatial and temporal aspects of object feature binding and feature proximity on binding errors, support the spatial theories on object feature binding, in addition we propose that temporal theories and convergence, through hierarchical feature analysis, are also involved. Because spatial properties have a dedicated processing neural stream, and temporal properties rely on limited capacity memory systems, memories for sequential information would likely be more difficult to accuratelyr ecall. Our study supports other studies which suggest that both spatial and temporal coherence to differing degrees,may be involved in object feature binding. Traditionally, these theories have purported to provide individual solutions, but this thesis proposes a novel unified theory of object feature binding in which hierarchical feature analysis, spatial attention and temporal synchrony each plays a role. It is further proposed that binding takes place in visual short-term memory through concerted and integrated information processing in distributed cortical areas. A cognitive model detailing this integrated proposal is given. Next, the cognitive model is used to inform the design and suggested implementation of a computational model which would be able to test the theory put forward in this thesis. In order to verify the model, future work is needed to implement the computational model.Thus it is argued that this doctoral thesis provides valuable experimental evidence concerning spatio-temporal aspects of the binding problem and as such is an additional building block in the quest for a solution to the object feature binding problem

    Probing for local synaptic connectivity in the adult mouse auditory cortex

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    Die grundlegenden Funktionsweisen wie unser Gehirn neue Informationen speichert beruht auf zwei Theorien. Erstens, Neuronen verbinden sich zu einem Netzwerk mit unterschiedlichen starken Verbindungen zueinander. Zweitens, Ă€ußere EinflĂŒsse können diese Verbindungen verĂ€ndern. Dadurch können sich neue Neuronen dem Netzwerk anschließen oder sich auch die StĂ€rke der Verbindungen von bereits im Netzwerk vorhandenen Neuronen Ă€ndern. Um mehr ĂŒber die Funktionsweise unseres Gehirns zu erfahren ist es absolut notwendig ein Schaltdiagramm corticaler Netzwerke zu haben der alle Verbindungen der Neuronen zueinander enthĂ€lt. In dieser Arbeit untersuchten wir die synaptischen Verbindungen im auditorischen Cortex, eine Hirnregion wichtig fĂŒr die Prozessierung von Tönen in verschiedenen assoziativen Lernparadigmen. Wir verwendete coronale Hirnschnitte von erwachsenen (8-14 Wochen alten) C57Bl6/6J MĂ€usen. Wir machten gleichzeitig ganz-Zell Ableitungen von vier Pyramidenzellen der Schicht 2/3 und der Schicht 5. Diese Methode erlaubt es die synaptische VerbindungsstĂ€rke zwischen diesen vier Neuronen zu messen. Wir fanden eine niedrige Verbindungswahrscheinlichkeit zwischen gleichzeitig gemessen Neuronen und weiters dass die Wahrscheinlichkeit einer bidirektionalen Verbindung zwischen zwei zufĂ€llig ausgewĂ€hlte Neuronen höher war als erwartet. Die Verteilung der StĂ€rken der synaptischen Verbindungen (der höchste Punkt der Amplitude des postsynaptischen Potentials (EPSP)) zeigte wenige starke Verbindungen. Dies deutet darauf hin, dass synaptische Verbindungen in lokalen Netzwerken von seltenen aber dafĂŒr starken Verbindungen dominiert werden. Wir fanden diese Verbindungen in beiden untersuchten Hirnschichten was darauf hindeutet, dass diese seltenen aber starken Verbindungen die Grundlage der Informationsverarbeitung in corticalen Netzwerken sein könnte. Wir fanden auch, dass die VariabilitĂ€t der EPSP Amplitude entweder durch die verĂ€nderte Wahrscheinlichkeit der Neurotransmitterfreisetzung oder durch eine verĂ€nderte Anzahl der freigesetzten Neurotransmittervesikel auf der praesynaptischen Seite entstehen kann. Dies deutet darauf hin, dass beide Parameter Wahrscheinlichkeit und Anzahl unabhĂ€ngig voneinander sind. Im zweiten Teil untersuchten wir den relativen Anteil des erregenden und inhibierenden Stroms zu Schicht 2/3 Neuronen mit prĂ€ziser zeitlicher Auflösung. Die strikte Balance zwischen diesen Strömen ist kritisch fĂŒr die Funktion corticaler Netzwerke und fĂŒr die Anpassung der Eigenschaften corticaler Neuronen. Daher ist es notwendig herauszufinden, wie diese Balance aufrechterhalten wird. Wir stimulierten extrazellulĂ€r zwei unabhĂ€ngige von einander zu Schicht 2/3 fĂŒhrende Nervenbahnen und maßen die synaptisch erregende und inhibierende LeitfĂ€higkeit von Schicht 2/3 Neuronen. Wir fanden das die Balance zwischen erregenden und inhibierenden Strömen gleich groß fĂŒr beide Nervenbahnen war und weiters, dass das Eintreffen der inhibierenden Ströme 2ms schneller war als das der erregenden Ströme. Dies deutet darauf hin, dass diese intercorticalen Nervenbahnen monosynaptisch mit Schicht 2/3 Neuronen verbunden sind. Interessanterweise fanden wir, dass fast 50 Prozent aller erregenden Ströme gleichzeitig mit zwei inhibitorischen Strömen eintrafen. Dies wurde vorher nicht beschrieben und könnte auf ein feedback oder feedforward Netzwerk lokaler Interneuronen zurĂŒckzufĂŒhren sein. In dritten Teil untersuchten wir ob optogenetische Manipulation wĂ€hrend eines Verhaltensexperiments die Eigenschaften lokaler corticaler Netzwerke verĂ€ndert. Wir expremierten Channelrhodopsin in Pyramidenzellen des auditorischen Cortex und photostimulierten diese Zellen wĂ€hrend eines Verhaltensexperiments. Dadurch ist es uns Möglich festzustellen ob diese Neuronen ein bestimmtes Verhalten auslösen können. Wir untersuchten auch, ob sich die spezifischen Verbindungen dieser Nervenzellen wĂ€hrend des Lernens einer Verhaltensaufgabe Ă€ndern. Wir fanden, dass die Stimulation durch Channelrhodopsin dazu verwendet werden kann, den Einfluss prĂ€zise getimter Aktionspotentiale auf das erlernen einer Verhaltensaufgabe zu untersuchen. Weiters fĂŒhrten wir ganz-Zell Ableitungen an Schicht 2/3 Neuronen von MĂ€usen die die Verhaltensaufgabe gelernt hatten durch. Wir fanden heraus, dass sich die Erregbarkeit von Neuronen in diesen MĂ€usen nicht von der Erregbarkeit von Neuronen in wildtyp MĂ€usen unterscheidet. Wir konnten keine Unterschiede in den EPSP Amplituden verbundenen Neuronen feststellen. Dies deutet darauf hin, dass die durch Channelrhodopsin ausgelöste Depolarisation nicht zu einer stĂ€rkeren Verbindung zwischen diesen Neuronen fĂŒhrt.The current mechanistic view on how the brain is able to store memories over long periods of time is based on two key concepts. The first is that memories are stored in the configuration of the connectivity of neurons in an assembly and in the set of synaptic weights of those connections; the second being that experience can mold and rewire the network connectivity and its synaptic weights. It becomes clear that the understanding of cortical function will always require the unraveling of synaptic connectivity in cortical circuits, that is, establishing the wiring diagrams between individual neurons. In the present work, a first effort was made in order to investigate the excitatory synaptic local circuitry in the adult mouse auditory cortex, a brain area critically involved in sound encoding required for proper associative motivational leaning. For this purpose, coronal whole-brain slices from adult (8-14 weeks old) C57Bl6/6J mice were used. Several simultaneous quadruple whole-cell recordings from layer 2/3 and layer 5 pyramidal neurons were made, a method that allows for quantitative functional measures of synaptic connectivity at the level of individually indentified neurons. It was observed that local circuitry is characterized by low connection probabilities between pairs of neurons, and that bidirectional connections are more common than expected in a random network. The distribution of synaptic connections strengths (defined as the peak of excitatory postsynaptic potential (EPSP) amplitude), has a heavier tail and implies that synaptic weight is concentrated among few synaptic connections. In both layers it was found the existence of rare but reliable large-amplitude synaptic connections, which are likely to contribute strongly to reliable information processing. Moreover, another central finding is that the EPSP amplitude variability can be ascribed to changes in the number of release presynaptic sites, or due to the probability of neurotransmission release, implying that modulations in synaptic transmission can be described by changes in both parameters independently. In the second part, the relative contribution, with precise temporal resolution, of excitatory and inhibitory drives that impinge onto layer 2/3 pyramidal neurons was investigated. The strict balance of these two synaptic conductances plays a critical role in cortical function and in the shaping of the tuning properties of cortical neurons. It is of utmost importance to describe how this balance is achieved and maintain. By means of intracortical extracellular stimulation of two independent but convergent input pathways into layer 2/3 neurons, synaptic conductances could be recorded and decomposed into their excitatory and inhibitory components. It was observed that excitatory/inhibitory balance is of equal magnitude in both stimulated pathways, and that on average a time difference less than 2 ms between the arrival of inhibition compared with the excitation favors for a monosynaptic nature of the stimulated intracortical projections that synapses onto the recorded layer 2/3 pyramidal neurons. On the other hand, it was observed that on almost half of the recorded neurons, the excitation conductance was flanked by two inhibitory barrages, a phenomenon never described so far. A possible feedback or feedforward inhibitory circuitry made by local interneurons could explain this observation. In the third part, one final question was posed: are the features that describe local synaptic circuitry changed upon optogenetic manipulation in a behavioural task? By means of combining expression of channelrhodopsin in auditory cortex pyramidal neurons, with their direct photostimulation in the context of a behaviour task, it was possible to assess the role of a subset of neurons in driving behaviour. Possible changes in their intrinsic interconnectivity were also studied upon learning. Though extremely labour intense, it was concluded that ChR2-based optical microstimulation can be used to dissect the impact of precisely timed action potentials in a subset of neurons in driving behaviour. Whole-cell recordings from layer 2/3 neurons from the subset of mice that reached correct performance levels were performed as before. It was observed that ChR2-expressing neurons in trained mice had similar intrinsic excitability features when compared with non-trained mice. The recorded EPSP amplitudes from pairs of connected neurons had similar rages among both groups of mice, indicating that periodic depolarizations of ChR2-positive neurons does not induce any synaptic scaling effect in these neurons

    Neural plasticity and the limits of scientific knowledge

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    Western science claims to provide unique, objective information about the world. This is supported by the observation that peoples across cultures will agree upon a common description of the physical world. Further, the use of scientific instruments and mathematics is claimed to enable the objectification of science. In this work, carried out by reviewing the scientific literature, the above claims are disputed systematically by evaluating the definition of physical reality and the scientific method, showing that empiricism relies ultimately upon the human senses for the evaluation of scientific theories and that measuring instruments cannot replace the human sensory system. Nativist and constructivist theories of human sensory development are reviewed, and it is shown that nativist claims of core conceptual knowledge cannot be supported by the findings in the literature, which shows that perception does not simply arise from a process of maturation. Instead, sensory function requires a long process of learning through interactions with the environment. To more rigorously define physical reality and systematically evaluate the stability of perception, and thus the basis of empiricism, the development of the method of dimension analysis is reviewed. It is shown that this methodology, relied upon for the mathematical analysis of physical quantities, is itself based upon empiricism, and that all of physical reality can be described in terms of the three fundamental dimensions of mass, length and time. Hereafter the sensory modalities that inform us about these three dimensions are systematically evaluated. The following careful analysis of neuronal plasticity in these modalities shows that all the relevant senses acquire from the environment the capacity to apprehend physical reality. It is concluded that physical reality is acquired rather than given innately, and leads to the position that science cannot provide unique results. Rather, those it can provide are sufficient for a particular environmental setting
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