Eye movements are a powerful tool to examine cognitive processes. However, in most paradigms little is known about the dynamics present in sequences of saccades and fixations. In particular, the control of fixation durations has been widely neglected in most tasks. As a notable exception, both spatial and temporal aspects of eye-movement control have been thoroughly investigated during reading. There, the scientific discourse was dominated by three controversies, (i), the role of oculomotor vs. cognitive processing on eye-movement control, (ii) the serial vs. parallel processing of words, and, (iii), the control of fixation durations. The main purpose of this thesis was to investigate eye movements in tasks that require sequences of fixations and saccades. While reading phenomena served as a starting point, we examined eye guidance in non-reading tasks with the aim to identify general principles of eye-movement control. In addition, the investigation of eye movements in non-reading tasks helped refine our knowledge about eye-movement control during reading. Our approach included the investigation of eye movements in non-reading experiments as well as the evaluation and development of computational models. I present three main results : First, oculomotor phenomena during reading can also be observed in non-reading tasks (Chapter 2 & 4). Oculomotor processes determine the fixation position within an object. The fixation position, in turn, modulates both the next saccade target and the current fixation duration. Second, predicitions of eye-movement models based on sequential attention shifts were falsified (Chapter 3). In fact, our results suggest that distributed processing of multiple objects forms the basis of eye-movement control. Third, fixation durations are under asymmetric control (Chapter 4). While increasing processing demands immediately prolong fixation durations, decreasing processing demands reduce fixation durations only with a temporal delay. We propose a computational model ICAT to account for asymmetric control. In this model, an autonomous timer initiates saccades after random time intervals independent of ongoing processing. However, processing demands that are higher than expected inhibit the execution of the next saccade and, thereby, prolong the current fixation. On the other hand, lower processing demands will not affect the duration before the next saccade is executed. Since the autonomous timer adjusts to expected processing demands from fixation to fixation, a decrease in processing demands may lead to a temporally delayed reduction of fixation durations. In an extended version of ICAT, we evaluated its performance while simulating both temporal and spatial aspects of eye-movement control. The eye-movement phenomena investigated in this thesis have now been observed in a number of different tasks, which suggests that they represent general principles of eye guidance. I propose that distributed processing of the visual input forms the basis of eye-movement control, while fixation durations are controlled by the principles outlined in ICAT. In addition, oculomotor control contributes considerably to the variability observed in eye movements. Interpretations for the relation between eye movements and cognition strongly benefit from a precise understanding of this interplay. / Blickbewegungen stellen ein wichtiges Instrument dar, um kognitive Prozesse zu untersuchen. In den meisten Paradigmen ist allerdings wenig über die Entstehung von Sakkaden und Fixationen bekannt. Insbesondere die Kontrolle der Fixationsdauern wurde häufig außer acht gelassen. Eine wesentliche Ausnahme stellt die Leseforschung dar, in der sowohl zeitlichliche als auch räumliche Aspekte der Blickbewegungssteuerung im Detail betrachtet wurden. Dabei war der wissenschaftliche Diskurs durch drei Kontroversen gekennzeichnet, die untersuchten, (i), welchen Einfluss okulomotorische bzw. kognitive Prozesse auf die Blicksteuerung haben, (ii), ob Worte seriell oder parallel verarbeitet werden und, (iii), wie Fixationsdauern kontrolliert werden. Die vorliegende Arbeit zielt im wesentlichen darauf ab, die Dynamik von Fixationssequenzen zu erforschen. Ausgehend von den Erkenntnissen beim Lesen untersuchten wir Blickbewegungen in Nichtlese-Aufgaben, mit dem Ziel allgemeine Prinzipien der Blicksteuerung zu identifizieren. Zusätzlich versuchten wir mit Hilfe dieser Aufgaben, Erkenntnisse über Prozesse beim Lesen zu vertiefen. Unser Vorgehen war sowohl von der Durchführung von Experimenten als auch der Entwicklung und Evaluation computationaler Modelle geprägt. Die Hauptbefunde zeigten: Erstens, okulomotorische Phänomene des Lesens lassen sich in Suchaufgaben ohne Wortmaterial replizieren (Kapitel 2 & 4). Dabei bestimmen okulomotorische Prozesse die Fixationsposition innerhalb eines Objektes. Diese wiederum beeinflusst das nächste Sakkadenziel sowie die Fixationsdauer. Zweitens, wesentliche Vorhersagen von Modellen, in denen Blickbewegungen von seriellen Aufmerksamkeitsverschiebungen abhängen, konnten falsifiziert werden (Kapitel 3). Stattdessen legen unsere Erkenntnisse nahe, dass die Blicksteuerung von der parallelen Verarbeitung mehrerer Objekte abhängt. Drittens, Fixationsdauern werden asymmetrisch kontrolliert (Kapitel 4). Während hohe Verarbeitungsanforderungen Fixationsdauern unmittelbar verlängern können, führen niedrige Verarbeitungsanforderungen nur zeitlich verzögert zu einer Reduktion. Wir schlagen ein computationales Modell ICAT vor, um asymmetrische Kontrolle zu erklären. Grundlage des Modells ist ein autonomer Zeitgeber, der unabhängig von der momentanen Verarbeitung nach zufälligen Zeitintervallen Sakkaden initiiert. Unerwartet hohe Verarbeitungsanforderungen können die Initiierung der nächsten Sakkade hinauszögern, während unerwartet niedrige Verarbeitungsanforderungen den Beginn der nächsten Sakkade nicht verändern. Der Zeitgeber passt sich allerdings von Fixation zu Fixation neuen Verarbeitungsanforderungen an, so dass es zu einer zeitlich verzögerten Reduktion der Fixationsdauern kommen kann. In einer erweiterten Version des Modells überprüfen wir die Kompatibilität ICATs mit einer realistischen räumlichen Blicksteuerung. Die Ähnlichkeit von Blickbewegungsphänomenen über Aufgaben hinweg legt nahe, dass sie auf allgemeinen Prinzipien basieren. Grundlage der Blicksteuerung ist die verteilte Verarbeitung des visuellen Inputs, während die Kontrolle der Fixationsdauer auf den Prinzipien von ICAT beruht. Darüber hinaus tragen okulomotorische Phänomene wesentlich zur Variabilität der Blicksteuerung bei. Ein Verständnis dieses Zusammenspiels hilft entscheidend den Zusammenhang von Blickbewegungen und Kognitionen besser zu verstehen.

The aim of thesis is acquaint with possibilities of fixation biological tissues. Choose a suitable tissue and fix it with formaldehyde and another fixator. Last but not least make measurements which determine effects of formaldehyde on the relaxation time of biological tissues. Mice brains were chosen as a suitable for tissues fixation. Measuring effects of formaldehyde and another fixator to relaxation time carried out at the Institute of Scientific Instuments Academy of Sciences of the Czech Republic.

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A native desert Rhizobium, AZ-M1, was isolated from a nodulated mesquite tree (Prosopis juliflora var. velutina) following inoculation of mesquite seed with a desert soil. This strain and a selected commercial strain (31A5) were used in a greenhouse study to determine their N fixation efficiency against applied N fertilizer. Strain AZ-M1 was a more efficient N fixer than 31A5. The survival rate of the two strains was tested in three different desert soils in a controlled laboratory study. The native strain AZ-M1 grew and survived in the soils over a period of one month. The commercial strain did not grow and the population decreased from 10⁸ cells gm⁻¹ of dry soil to below 10⁴ cells after 14 days. Soil factors affected survival of both strains. The competitiveness of the two strains was compared in a greenhouse experiment. The native isolate out competed strain 31A5 in nodule occupancy regardless of cell number when applied as a mixed inoculant. A high incidence of double occupancy was found in the root nodules when double strain inoculants were applied. The two rhizobia were fast growing rhizobia, lowering the pH of a defined medium. Strain AZ-M1 showed a high intrinsic resistance to 3 antibiotics among 12 tested. Strain AZ-M1 has been shown to be highly effective, fairly competitive and survive better in desert soils than strain 31A5.

One low salt tolerant alfalfa (Medicago sativa L.) cultivar and two germination salt tolerant alfalfa selections were compared for growth and N fixation at four salinity levels (0, -0.3, -0.6 and -1.2 Mpa). In the first experiment a Hoagland's solution at 5 ppm-N was used; in the second experiment the solution had a 1 ppm-N concentration and supplemental light was used. No significant differences were found among the cultivars. This provides additional support that germination salt tolerance is not necessarily related to salt tolerance for growth. Nitrogen fixed to the first harvest was 61, 48, 49, and 27% of the total shoot N for plants in the control, -0.3, -0.6, and -1.2 Mpa solutions, respectively. At the second harvest, N fixation percentages were 94, 89, 80, and 57% for the corresponding salinity levels which showed significant reduction in N fixation at -0.3 Mpa. The evaluation of N fixation was by 15N techniques.

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<p>The microbial life of sub-Antarctic islands plays a key role in the islands ecosystem, with microbial activities providing the majority of nutrients available for primary production. Knowledge of microbial diversity is still in its infancy and this is particularly true regarding the diversity of micro-organisms in the Antarctic and sub-Antarctic regions. One particularly important functional group of micro-organisms is the diazotrophs, or nitrogen-fixing bacteria and archaea. This group have not been well studied in the sub-Antarctic region, but play an important role in the nutrient cycling of the island. This thesis explored the diversity of nitrogen-fixing organisms in the soil of different ecological habitats on the sub-Antarctic Marion Island.</p>