Kreiter, Andreas
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Kreiter, Andreas
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Kreiter, Andreas
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Item-typ:Veröffentlichung, Über die Detektion visueller Geschwindigkeitsänderungen in Primaten : Neuronale Repräsentation im Areal MT des Makaken und Modellierung der perzeptuellen Verhaltensleistung des Menschen(2017-04-25); ; ; Neurons in the mediotemporal area of the macaque (MT) respond to accelerations of visual stimuli with a high transient increase in firing rate that decays to a markedly lower reponse plateau after a few hundred milliseconds. The latency of this transient response correlates with reaction time in tasks requiring the detection of such changes. However, it remains unknown how the sign, the amplitude and the latency of transients in MT generally depend on the sign and the size of a speed change, and how this interacts with a neuron's tuning to absolute speeds. Moreover, it has never been adressed whether such transients may explain speed change detection at a behavioral level. By characterizing the representation of a wide range of positive and negative speed changes in MT of awake and behaving macaques, this thesis obtains three major findings: First, neuronal transients do not only depend on the absolute speed after a speed change, but are scaled in such a way that their relative amplitudes systematically reflect the size of the speed change itself. For each neuron, this scaling increases with the distance of a speed change from its preferred speed, and depends on its transient-sustained-ratio as a measure of the extent to which it is subject to short-term adpatation within the local ciruitry of MT. Second, simulations in the framework of signal-detection-theory demonstrate that the detection rates of human observers under similar conditions can be reproduced by a physiological detection model that applies an upper and lower threshold, respectively, to positive and negative population transients of a moderate number of MT neurons. Third, the latency of the transient responses is found to mirror the pattern of reaction times in detection tasks, and fullfills predictions of a psychophysical model of motion detection by its dependence on the the absolute value of a speed change and an incomplete normalization for higher base speeds. Because of this close relation between the neuronal representation and measures of detection behavior, this thesis draws the conclusion that neuronal transients in MT are systematically shaped to facilitate the detection of speed changes by the application of a simple threshold mechanism to the population response. It highlights yet another functional purpose of adaptation and presents a framework for future analyses of the detection of changes in other motion or stimulus domains.Dissertation425 139 - Some of the metrics are blocked by yourconsent settings
Item-typ:Veröffentlichung, Untersuchung des Einflusses von Aufmerksamkeit auf die Interaktion der Areale V1 und V4 des Rhesusaffen bei einem Formidentifikations-Paradigma(2014-03-04); ; ; To select behavioral relevant information from irrelevant, a mechanism is needed to change the strength of coupling between related neuronal populations. The hypothesis of communication through coherence (CTC) provides a possible mechanism. This was tested by having a macaque monkey solve a shape-identification-task. Intracortical extracellular recording of neuronal activity was made simultaneously in the visual areas V1 and V4. Results show modulation of coupling even on almost single cell level in V4. Coupling is strongly enhanced by attention between recorded cells in V4 and neuronal populations in V1 representing and processing the relevant stimulus, whereas coupling with V1 population representing and processing the irrelevant stimulus is much weaker. The hypothesis of CTC is strongly supported by the new findings presented in this thesis.Dissertation299 131 - Some of the metrics are blocked by yourconsent settings
Item-typ:Veröffentlichung, Dynamics of Neuronal Interactions in the Visual Cortex of Macaca Mulatta(2018-05-25); ; ; This work is dedicated to studying the effects of attention which result in measurable changes in the functioning of single neurons and neuronal populations. A special emphasis lies on establishing a suitable method of recording from multiple neuronal populations in the visual cortex of a behaving monkey, as well as on analyzing the acquired signals. A chronically implanted large-area electrode array was used for epidural recording of brain potentials. Additionally, two electrode arrays for large-area chronic intracortical recording were designed, manufactured, and successfully implemented on a macaque monkey. By this means, local field potentials were recorded from different neuronal populations in two visual areas of a macaque monkey as he performed a shape-tracking task presented in front of him on a screen. Two kinds of stimulus-driven neuronal response in areas V1 and V4 were acquired: with the stimulus being behaviorally relevant and hence lying within the locus of the monkeya s attention, and with the stimulus being ignored and hence lying outside the locus of the monkeya s attention. Comparing the two response types provided a clue to the differences in neuronal processing of attended and non-attended stimuli. According to the working hypothesis, oscillatory responses generated in different neuronal populations by an attended stimulus would be synchronized in the gamma frequency band. To test this hypothesis, the acquired brain potentials were wavelet transformed and oscillatory power of different neuronal populations as well as phase coherence between them was computed. The results revealed a high degree of phase coherence in the gamma frequency band between oscillatory responses of neuronal populations in areas V1 and V4 representing the attended stimulus. A non-attended stimulus, however, produced non-phase-locked oscillatory responses in areas V1 and V4. The results presented here supported the working hypothesis, and the gamma-band synchronization was suggested to provide the mechanism of attention which enhances the processing of behaviorally relevant stimuli.Dissertation364 188 - Some of the metrics are blocked by yourconsent settings
Item-typ:Veröffentlichung, Exploring Cognitive States of the Macaque Monkey with fMRI(2013-08-20); ; ; This thesis describes experiments with macaque monkeys that were trained to perform several variants of a Delayed match to sample task inside a 3T Siemens Allegra scanner while functional whole brain images were acquired. In these tasks, a stream of shapes is shown in the lower left or right visual hemifield. The monkeys had to memorise the initial shape and give a manual response when a matching shape occurred. Different task variants were used that manipulated the demands for memory and attention. A GLM analysis was applied to identify brain regions with an increased BOLD signal during the delay period. The response profile of selected regions of interest was further characterised in more detail by using an approach based on extensive trial averaging. A distributed network of brain regions was robustly found across paradigms and monkeys. The results demonstrate the feasibility of fMRI experiments with primates performing cognitive tasks, but also yielded some unexpected findings.Dissertation308 239 - Some of the metrics are blocked by yourconsent settings
Item-typ:Veröffentlichung, Neuronale Grundlagen objektbasierter Aufmerksamkeit(2006-12-19); ; ; In visual attention research, the identification of neural substrates of attention has proven to be one of the most basic questions. Psychophysical studies suggest attention to operate on spatial locations, on single stimulus features and on whole objects. While receiving increasing interest, still little is known about the neural principles of object-based attention. Recently, an explicit distinction between space-based and object-based mechanisms of attention could be achieved by directing subjects to attend to one of two spatially overlapping, transparent random dot surfaces. The study at hand used three variations of this paradigm to identify a neural correlate of object-based attention on the basis of single unit responses in the macaque brain. Changes in firing rates of single neurons in visual area MT, ranging from 12-74%, were shown to provide such a correlate. Moreover, the results presented suggest that both coherent motion criteria and surface color contribute to an attentional selection of surfaces in area MT.Dissertation341 195