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R Kern

Publications and source records attributed to R Kern.

At least 19 recordsLinked to original sources
PubMed2006

Encoding of naturalistic optic flow by a population of blowfly motion-sensitive neurons.

In sensory systems information is encoded by the activity of populations of neurons. To analyze the coding properties of neuronal populations sensory stimuli have usually been used that were much simpler than those encountered in real life. It has been possible only recently to stimulate visual interneurons of the blowfly with naturalistic visual stimuli reconstructed from eye movements measured during free flight. Therefore we now investigate with naturalistic optic flow the coding properties of a small neuronal population of identified visual interneurons in the blowfly, the so-called VS and HS neurons. These neurons are motion sensitive and directionally selective and are assumed to extract information about the animal's self-motion from optic flow. We could show that neuronal responses of VS and HS neurons are mainly shaped by the characteristic dynamical properties of the fly's saccadic flight and gaze strategy. Individual neurons encode information about both the rotational and the translational components of the animal's self-motion. Thus the information carried by individual neurons is ambiguous. The ambiguities can be reduced by considering neuronal population activity. The joint responses of different subpopulations of VS and HS neurons can provide unambiguous information about the three rotational and the three translational components of the animal's self-motion and also, indirectly, about the three-dimensional layout of the environment.

Animals↗
PubMed2006

[Asymptomatic carotid stenoses. Course and discussion of their treatment].

Asymptomatic carotid stenoses are common. They are associated with the risk factors for arteriosclerosis and are a specific risk factor for coronary artery disease, with an annual morbidity/mortality (MM) rate of 8-10%. Detection of a carotid stenosis should therefore be followed by a complete risk factor analysis and modification, if appropriate; a thorough cardiological diagnostic investigation is particularly important. Individual risk indicators for stroke (annual MM 1-2%) are a rapid progression of the degree of stenosis, certain morphological patterns and the presence of extra- and intracranial multi-vessel disease. Trials available so far on the treatment of asymptomatic carotid stenosis have not revealed any significant parameters that would justify individual decisions on whether or not surgery is appropriate. There is therefore a danger that many patients will be exposed to a severe risk of stroke, which cannot be justified unless the centres where such interventions are performed have proven MM complication rates of <3%, the life expectancy of the patients treated is not restricted as the result of other illnesses and the patients explicitly request surgery after a thorough explanation of the risks. Conservative treatment strategies provide the best protection for the majority of patients.

Carotid Stenosis↗
PubMed2006

Representation of behaviourally relevant information by blowfly motion-sensitive visual interneurons requires precise compensatory head movements.

Flying blowflies shift their gaze by saccadic turns of body and head, keeping their gaze basically fixed between saccades. For the head, this results in almost pure translational optic flow between saccades, enabling visual interneurons in the fly motion pathway to extract information about translation of the animal and thereby about the spatial layout of the environment. There are noticeable differences between head and body movements during flight. Head saccades are faster and shorter than body saccades, and the head orientation is more stable between saccades than the body orientation. Here, we analyse the functional importance of these differences by probing visual interneurons of the blowfly motion pathway with optic flow based on either head movements or body movements, as recorded accurately with a magnetic search coil technique. We find that the precise head-body coordination is essential for the visual system to separate the translational from the rotational optic flow. If the head were tightly coupled to the body, the resulting optic flow would not contain the behaviourally important information on translation. Since it is difficult to resolve head orientation in many experimental paradigms, even when employing state-of-the-art digital video techniques, we introduce a 'headifying algorithm', which transforms the time-dependent body orientation in free flight into an estimate of head orientation. We show that application of this algorithm leads to an estimated head orientation between saccades that is sufficiently stable to enable recovering information on translation. The algorithm may therefore be of practical use when head orientation is needed but cannot be measured.

Algorithms↗
PubMed2005

Stroke recurrences in patients with symptomatic vs asymptomatic middle cerebral artery disease.

BACKGROUND: Although the natural history of extracranial carotid artery disease has been investigated systematically, limited data are available on the course of middle cerebral artery (MCA) disease. METHODS: The authors observed 102 consecutive patients (67 men, 35 women; mean age 61.9 years) with significant MCA stenosis or occlusion as demonstrated by transcranial Doppler and transcranial color-coded duplex ultrasonography. Forty-six patients entered the study after TIA (n = 17) or stroke (n = 29); 56 patients were asymptomatic. Neurologic and ultrasound investigations were performed at regular intervals with a mean follow-up of 31 (range 6 to 117) months. Patients were continuously treated with either platelet inhibitors (n = 75) or anticoagulation (n = 27). RESULTS: Nineteen cerebral ischemic events (11 strokes, 8 TIAs) occurred during follow-up, resulting in an overall annual rate of 7.3%. Thirteen events (8 strokes, 5 TIAs) were attributable to the vascular territory ipsilateral to MCA disease. Patients with symptomatic MCA disease at study entry had an overall stroke risk of 12.5% per year (ipsilateral: 9.1%), whereas the annual incidence in primarily asymptomatic MCA disease was only 2.8% (ipsilateral: 1.4%; p < 0.01). Symptomatic MCA disease was an independent predictor for overall (hazard ratio [HR] 7.91, 95% CI 2.03 to 30.79; p < 0.01) and ipsilateral (HR 9.66, 95% CI 1.5 to 62.25; p = 0.02) cerebrovascular events. CONCLUSIONS: Compared with asymptomatic middle cerebral artery disease, there was a high and continuous recurrence rate of ischemic events in symptomatic patients, which was even higher than in patients with symptomatic extracranial carotid artery disease.

Aged↗
PubMed2005

On the computations analyzing natural optic flow: quantitative model analysis of the blowfly motion vision pathway.

For many animals, including humans, the optic flow generated on the eyes during locomotion is an important source of information about self-motion and the structure of the environment. The blowfly has been used frequently as a model system for experimental analysis of optic flow processing at the microcircuit level. Here, we describe a model of the computational mechanisms implemented by these circuits in the blowfly motion vision pathway. Although this model was originally proposed based on simple experimenter-designed stimuli, we show that it is also capable to quantitatively predict the responses to the complex dynamic stimuli a blowfly encounters in free flight. In particular, the model visual system exploits the active saccadic gaze and flight strategy of blowflies in a similar way, as does its neuronal counterpart. The model circuit extracts information about translation velocity in the intersaccadic intervals and thus, indirectly, about the three-dimensional layout of the environment. By stepwise dissection of the model circuit, we determine which of its components are essential for these remarkable features. When accounting for the responses to complex natural stimuli, the model is much more robust against parameter changes than when explaining the neuronal responses to simple experimenter-defined stimuli. In contrast to conclusions drawn from experiments with simple stimuli, optimization of the parameter set for different segments of natural optic flow stimuli do not indicate pronounced adaptational changes of these parameters during long-lasting stimulation.

Adaptation, Physiological↗
PubMed2005

Motion adaptation leads to parsimonious encoding of natural optic flow by blowfly motion vision system.

Neurons sensitive to visual motion change their response properties during prolonged motion stimulation. These changes have been interpreted as adaptive and were concluded, for instance, to adjust the sensitivity of the visual motion pathway to velocity changes or to increase the reliability of encoding of motion information. These conclusions are based on experiments with experimenter-designed motion stimuli that differ substantially with respect to their dynamical properties from the optic flow an animal experiences during normal behavior. We analyze for the first time motion adaptation under natural stimulus conditions. The experiments are done on the H1-cell, an identified neuron in the blowfly visual motion pathway that has served in many previous studies as a model system for visual motion computation. We reconstructed optic flow perceived by a blowfly in free flight and used this behaviorally generated optic flow to study motion adaptation. A variety of measures (variability in spike count, response latency, jitter of spike timing) suggests that the coding quality does not improve with prolonged stimulation. However, although the number of spikes decreases considerably during stimulation with natural optic flow, the amount of information that is conveyed stays nearly constant. Thus the information per spike increases, and motion adaptation leads to parsimonious coding without sacrificing the reliability with which behaviorally relevant information is encoded.

Action Potentials↗
PubMed2005

Function and coding in the blowfly H1 neuron during naturalistic optic flow.

Naturalistic stimuli, reconstructed from measured eye movements of flying blowflies, were replayed on a panoramic stimulus device. The directional movement-sensitive H1 neuron was recorded from blowflies watching these stimuli. The response of the H1 neuron is dominated by the response to fast saccadic turns into one direction. The response between saccades is mostly inhibited by the front-to-back optic flow caused by the forward translation during flight. To unravel the functional significance of the H1 neuron, we replayed, in addition to the original behaviorally generated stimulus, two targeted stimulus modifications: (1) a stimulus in which flow resulting from translation was removed (this stimulus produced strong intersaccadic responses); and (2) a stimulus in which the saccades were removed by assuming that the head follows the smooth flight trajectory (this stimulus produced alternating zero or nearly saturating spike rates). The responses to the two modified stimuli are strongly different from the response to the original stimulus, showing the importance of translation and saccades for the H1 response to natural optic flow. The response to the original stimulus thus suggests a double function for the H1 neuron, assisting two major classes of movement-sensitive output neurons targeted by H1. First, its strong response to saccades may function as a saccadic suppressor (via one of its target neurons) for cells involved in figure-ground discrimination. Second, its intersaccadic response may increase the signal-to-noise ratio (SNR) of wide-field neurons involved in detecting translational optic flow between saccades, in particular when flying speeds are low or when object distances are large.

Action Potentials↗
PubMed2004

Microbial monitoring of spacecraft and associated environments.

Rapid microbial monitoring technologies are invaluable in assessing contamination of spacecraft and associated environments. Universal and widespread elements of microbial structure and chemistry are logical targets for assessing microbial burden. Several biomarkers such as ATP, LPS, and DNA (ribosomal or spore-specific), were targeted to quantify either total bioburden or specific types of microbial contamination. The findings of these assays were compared with conventional, culture-dependent methods. This review evaluates the applicability and efficacy of some of these methods in monitoring the microbial burden of spacecraft and associated environments. Samples were collected from the surfaces of spacecraft, from surfaces of assembly facilities, and from drinking water reservoirs aboard the International Space Station (ISS). Culture-dependent techniques found species of Bacillus to be dominant on these surfaces. In contrast, rapid, culture-independent techniques revealed the presence of many Gram-positive and Gram-negative microorganisms, as well as actinomycetes and fungi. These included both cultivable and noncultivable microbes, findings further confirmed by DNA-based microbial detection techniques. Although the ISS drinking water was devoid of cultivable microbes, molecular-based techniques retrieved DNA sequences of numerous opportunistic pathogens. Each of the methods tested in this study has its advantages, and by coupling two or more of these techniques even more reliable information as to microbial burden is rapidly obtained.

Adenosine Triphosphate↗
PubMed2003

Visually guided orientation in flies: case studies in computational neuroethology.

To understand the functioning of nervous systems and, in particular, how they control behaviour we must bridge many levels of complexity from molecules, cells and synapses to perception behaviour. Although experimental analysis is a precondition for understanding by nervous systems, it is in no way sufficient. The understanding is aided at all levels of complexity by modelling. Modelling proved to be an inevitable tool to test the experimentally established hypotheses. In this review it will by exemplified by three case studies that the appropriate level of modelling needs to be adjusted to the particular computational problems that are to be solved. (1) Specific features of the highly virtuosic pursuit behaviour of male flies can be understood on the basis of a phenomenological model that relates the visual input to the motor output. (2) The processing of retinal image motion as is experienced by freely moving animals can be understood on the basis of a model consisting of algorithmic components and components which represent a simple equivalent circuit of nerve cells. (3) Behaviourally relevant features of the reliability of encoding of visual motion information can be understood by modelling the transformation of postsynaptic potentials into sequences of spike trains.

Animals↗
PubMed2003

FliMax, a novel stimulus device for panoramic and highspeed presentation of behaviourally generated optic flow.

A high-speed panoramic visual stimulation device is introduced which is suitable to analyse visual interneurons during stimulation with rapid image displacements as experienced by fast moving animals. The responses of an identified motion sensitive neuron in the visual system of the blowfly to behaviourally generated image sequences are very complex and hard to predict from the established input circuitry of the neuron. This finding suggests that the computational significance of visual interneurons can only be assessed if they are characterised not only by conventional stimuli as are often used for systems analysis, but also by behaviourally relevant input.

Animals↗
PubMed2001

Chaperone-like properties of lysophospholipids.

Lysophospholipids are metabolic intermediates in phospholipid turnover, detergent molecules with membrane-modulating effects, and multifunctional cellular growth factors in eukaryotic cells. In bacterial cells, lysophospholipids are mostly found in the form of lysophosphatidylethanolamine. We show that a heat shock from 30 to 42 degrees C increases four-fold the Escherichia coli pool of lysophosphoethanolamine and that lysophospholipids display chaperone-like properties. Lysophosphatidylethanolamine, like molecular chaperones such as DnaK, promotes the functional folding of citrate synthase and alpha-glucosidase after urea denaturation. Like chaperones, lysophophatidylethanolamine, lysophosphatidylcholine, lysophosphatidylinositol and lysophosphatidic acid prevent the aggregation of citrate synthase at 42 degrees C. The renaturation and solubilisation of proteins by lysophospholipids occur at micromolar concentrations of these compounds, close to their critical micellar concentration. Furthermore, lysophosphatidylethanolamine is much more efficient than other detergents tested for the renaturation and solubilisation of citrate synthase. In contrast with lysophospholipids, phosphatidylethanolamine and phosphatidylcholine are not able to promote citrate synthase folding nor to prevent its aggregation at 42 degrees C. The chaperone-like properties of lysophospholipids suggest that, in addition to their known functions, they might affect the structure and function of hydrophilic proteins.

Bacterial Proteins↗
PubMed2001

Neural processing of naturalistic optic flow.

Stimuli traditionally used for analyzing visual information processing are much simpler than what an animal sees in normal life. When characterized with traditional stimuli, neuronal responses were found to depend on various parameters such as contrast, texture, or velocity of motion, and thus were highly ambiguous. In behavioral situations, all of these parameters change simultaneously and differently in different parts of the visual field. Thus it is hardly possible to predict from traditional analyses what information is encoded by neurons in behavioral situations. Therefore, we characterized an identified neuron in the optomotor system of the blowfly with image sequences as they were seen by animals walking in a structured environment. We conclude that during walking, the response of the neuron reflects the animal's turning direction nearly independently of the texture and spatial layout of the environment. Our findings stress the significance of analyzing the performance of neuronal circuits under their natural operating conditions.

Animals↗
PubMed2001

Outdoor performance of a motion-sensitive neuron in the blowfly.

We studied an identified motion-sensitive neuron of the blowfly under outdoor conditions. The neuron was stimulated by oscillating the fly in a rural environment. We analysed whether the motion-induced neuronal activity is affected by brightness changes ranging between bright sunlight and dusk. In addition, the relationship between spike rate and ambient temperature was determined. The main results are: (1) The mean spike rate elicited by visual motion is largely independent of brightness changes over several orders of magnitude as they occur as a consequence of positional changes of the sun. Even during dusk the neuron responds strongly and directionally selective to motion. (2) The neuronal spike rate is not significantly affected by short-term brightness changes caused by clouds temporarily occluding the sun. (3) In contrast, the neuronal activity is much affected by changes in ambient temperature.

Adaptation, Ocular↗
PubMed2001

Molecular microbial diversity of a spacecraft assembly facility.

In ongoing investigations to map and archive the microbial footprints in various components of the spacecraft and its accessories, we have examined the microbial populations of the Jet Propulsion Laboratory's Spacecraft Assembly Facility (JPL-SAF). Witness plates made up of spacecraft materials, some painted with spacecraft qualified paints, were exposed for approximately 7 to 9 months at JPL-SAF and examined the particulate materials collected for the incidence of total cultivable aerobic heterotrophs and heat-tolerant (80 degrees C for 15-min.) spore-formers. The results showed that the witness plates coated with spacecraft qualified paints attracted more dust particles than the non-coated stainless steel witness plates. Among the four paints tested, witness plates coated with NS43G accumulated the highest number of particles, and hence attracted more cultivable microbes. The conventional microbiological examination revealed that the JPL-SAF harbors mainly Gram-positive microbes and mostly spore-forming Bacillus species. Most of the isolated microbes were heat resistant to 80 degrees C and proliferate at 60 degrees C. The phylogenetic relationships among 23 cultivable heat-tolerant microbes were examined using a battery of morphological, physiological, molecular and chemotaxonomic characterizations. By 16S rDNA sequence analysis, the isolates fell into seven clades: Bacillus licheniformis, B. pumilus, B. cereus, B. circulans, Staphylococcus capitis, Planococcus sp. and Micrococcus lylae. In contrast to the cultivable approach, direct DNA isolation, cloning and 16S rDNA sequencing analysis revealed equal representation of both Gram-positive and Gram-negative microorganisms.

Bacillus↗
PubMed2001

Acute stroke patterns in patients with internal carotid artery disease: a diffusion-weighted magnetic resonance imaging study.

BACKGROUND AND PURPOSE: Diffusion-weighted (DW) MRI is a sensitive method that facilitates early stroke pattern identification. There are limited data about the influence of stenosis grade on the development of particular stroke patterns in internal carotid artery (ICA) disease. We therefore investigated the lesion patterns on DW MRI in acute stroke patients with ICA disease. METHODS: DW MRI was analyzed in 102 consecutive acute stroke patients with different degrees of ipsilateral ICA disease. Patients were assigned to 1 of 5 observed lesion patterns: (1) territorial ischemia, (2) subcortical ischemia without or (3) with embolus fragmentation, (4) disseminated lesions in distal cortical regions, and (5) multiple lesions in hemodynamic risk zones. In addition, perfusion-weighted (PW) MRI and MR angiography information was included in the assessment. RESULTS: All patterns were observed in the different stages of ICA disease. Half of the patients with high-grade or subtotal stenosis had lesions in hemodynamic risk zones. Territorial stroke occurred in 47.6% of patients with ICA occlusion. Statistical analysis showed a significant relationship between the degree of stenosis and the observed stroke pattern (P=0.001). In 77.8% of patients exhibiting high-grade ICA stenosis, subtotal stenosis, or occlusion, the perfusion lesion was larger than the diffusion lesion (PW/DW mismatch). CONCLUSIONS: Although in the individual patient any of the infarct patterns may occur, in statistical terms the incidence of a particular stroke pattern is clearly dependent on the degree of stenosis.

Aged↗
PubMed2001

Neuronal processing of behaviourally generated optic flow: experiments and model simulations.

The stimuli traditionally used for analysing visual information processing are much simpler than what an animal sees when moving in its natural environment. Therefore, we analysed in a previous study the performance of an identified neuron in the optomotor system of the fly by using as visual stimuli image sequences that were experienced by the animal while walking in a structured environment. These electrophysiological experiments revealed that the fly visual system computes from behaviourally generated optic flow a rather unambiguous representation of the animal's self-motion. In contrast to conclusions based on simple stimuli, the directions of turns are represented by an interneuron, the HSE cell, quite independent of the spatial layout of the environment and its textural properties when the cell is stimulated with behaviourally generated optic flow. This conclusion is substantiated here by further experimental evidence. Moreover, it is shown that the largely unambiguous responses of the HSE cell to behaviourally generated optic flow can be replicated to a large extent by a network model of the fly's visual motion pathway. These results stress the significance of naturalistic stimuli for analysing what is encoded by neuronal circuits under natural operating conditions.

Algorithms↗
PubMed2000

Assessment of acute cyto- and genotoxicity of corrosion eluates obtained from orthodontic materials using monolayer cultures of immortalized human gingival keratinocytes.

Whilst a patient is undergoing orthodontic treatment, dental appliances based on non-precious metals or titanium remain in the oral cavity for up to several years. Throughout this period the appliance is in either direct or indirect contact with the oral mucosa. To investigate the possibility of cell damage occurring as a result of appliance corrosion, monolayer cultures of immortalized human gingival keratinocytes were assessed for acute cyto- and genotoxicity using the hexosaminidase assay and the Comet assay respectively. The materials tested included 1. a nickel-free wire, 2. a UK-1 bond, 3. nickel-free as well as nickel-containing brackets with and without color signature and 4. a titanium expansion screw. Each of the test materials was corroded in a solution consisting of equal amounts of lactic acid and sodium chloride (0.1 M) for 1, 3, 7 and 14 days. The cell cultures were then exposed to eluates exhibiting the highest ion concentrations. None of the eluates was found to exhibit acute cytotoxicity, regardless of the type of test system used. Qualitative assessment using neutral red dye for live cells and either trypan blue or propidium iodide to disclose dead cells failed to reveal any significant increase in cell damage when exposed cells were compared to control cultures. Unrestricted cell vitality was confirmed by quantifying viable cells through measurement of hexosaminidase enzyme activity. Furthermore, assessment of genotoxicity revealed no apparent DNA damage to immortalized gingival keratinocytes following exposure to the test eluates. Because the materials tested in this study were corroded using the exacting methods normally applied to precious metals or gold-containing alloys, the lack of either acute cyto- or genotoxic effects following exposure to the test eluates indicates that the materials tested exert no adverse effects on cells similar to those of the target tissue exposed to the materials in situ.

Cell Culture Techniques↗