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Biomedical subjects

W von Seelen

Publications and source records attributed to W von Seelen.

At least 19 recordsLinked to original sources
PubMed2000

Self-organizing maps for visual feature representation based on natural binocular stimuli.

We model the stimulus-induced development of the topography of the primary visual cortex. The analysis uses a self-organizing Kohonen model based on high-dimensional coding. It allows us to obtain an arbitrary number of feature maps by defining different operators. Using natural binocular stimuli, we concentrate on discussing the orientation, ocular dominance, and disparity maps. We obtain orientation and ocular dominance maps that agree with essential aspects of biological findings. In contrast to orientation and ocular dominance, not much is known about the cortical representation of disparity. As a result of numerical simulations, we predict substructures of orientation and ocular dominance maps that correspond to disparity maps. In regions of constant orientation, we find a wide range of horizontal disparities to be represented. This points to geometrical relations between orientation, ocular dominance, and disparity maps that might be tested in experiments.

Algorithms↗
PubMed1999

Complex behavior by means of dynamical systems for an anthropomorphic robot.

We present an architecture to generate behavior for an anthropomorphic robot. The goal is to equip the robot with the capacity to interact with a human. Motivated by the research on biological systems, our basic assumption is that the behavior to perform determines the external and internal structure of the behaving system. We describe the anthropomorphic design of our robot and present a distributed control system that generates human-like navigation and manipulation behavior. As the mathematical framework for this purpose we have developed a control system which is entirely based on dynamical systems in the form of instantiated dynamics and neural fields. We also present a dynamic scheme for the behavioral organization based on competitive dynamics.

Journal Article↗
PubMed1997

Optical imaging of cat auditory cortex cochleotopic selectivity evoked by acute electrical stimulation of a multi-channel cochlear implant.

We measured reflectance changes by means of optical imaging of intrinsic signals to study the effects of acute electrical cochlear stimulation on the topography of the cat auditory cortex. After single-pulse electrical stimulation at selected sites of a multichannel implant device, we found topographically restricted response areas representing mainly the high-frequency range in AI. Systematic variation of the stimulation pairs and thus of the cochlear frequency sites revealed a systematic and corresponding shift of the response areas that matched the underlying frequency organization. Intensity functions were usually very steep. Increasingly higher stimulation currents evoked increasingly larger response areas, resulting in decreasing spatial, i.e. cochleotopic, selectivity; however, we observed only slight positional shifts of the focal zones of activity. Electrophysiological recordings of local field potential maps in the same individual animals revealed close correspondence of the locations of the cortical response areas. The results suggest that the method of optical imaging can be used to map response areas evoked by electrical cochlear stimulation, thereby maintaining a profound cochleotopic selectivity. Further experiments in chronically stimulated animals will shed more light on the degree of functional and reorganizational capacities of the primary cortex and could be beneficial for our understanding of the treatment of profound deafness.

Animals↗
PubMed1995

Optical imaging of rat somatosensory cortex reveals representational overlap as topographic principle.

We measured reflectance changes by means of optical imaging of intrinsic signals to study the topography of the paw representations in rat somatosensory cortex. Following circumscribed tactile stimulation of single digits or pads, we found large and partially overlapping areas of reflectance changes (delta R). The diameters of their focal zones defined at 75% maximal delta R were in the range of 150 microns and preserved all details of the underlying maps. Zones of overlap were in the range 15-25% measured at half-maximal delta R. In contrast, we found sharp boundaries with no overlap between the fore- and hindpaw representations. The data suggest that large and overlapping cortical maps constitute a normal type of neural representation supporting the idea of a distributed neural processing scheme.

Animals↗
PubMed1993

Variations in acoustical beam properties of intracoronary Doppler catheters.

The limitations of coronary angiography in assessing the functional significance of coronary obstructions is well known. While the critical variable of coronary blood flow cannot be readily measured, intraluminal Doppler sonography offers useful related functional information on blood flow velocity. In order to fully evaluate Doppler signals it is essential to have exact knowledge of the transducer transmission characteristics and of the ultrasound beam topology. In an experimental set-up, the transmitter-receiver characteristics of five commonly used Doppler catheters were investigated. In comparing the beam characteristics we found inhomogeneities in the lateral beam spread. At a penetration depth of 3.0 mm the beam shape varied from a minimum of 1.25 mm up to a maximum of 3.5 mm. The mean was 2.25 mm. The different beam profiles of the investigated Doppler transducers cause an error in measuring the blood flow velocity. The blood flow velocity tends to be underestimated the more the vessel diameter and the blood flow velocity increase. Contrary to transducer design optimized for imaging, for spectral analysis of the Doppler signal it would be advantageous to have as broad a beam as possible in order to illuminate the entire vessel lumen.

Angioplasty, Balloon, Coronary↗
PubMed1993

The role of the lateral suprasylvian visual cortex of the cat in object-background interactions: permanent deficits following lesions.

The contribution of the lateral suprasylvian cortex to pattern recognition was studied by behavioural detection experiments in combination with bilateral lesions of different parts of the lateral suprasylvian areas (LSA) and area 7 in seven cats. In a two-alternatives forced choice task the cats had to discriminate simple outline patterns which were additively superimposed on a structured visual background made up of broadband Gaussian noise. For various stimulus conditions (moving or stationary patterns and/or background) the detection probability (PD) of the cats was measured as a function of the signal-to-noise ratio (S/N). Each cat was tested before and after the lesion. Four different types of lesion could be distinguished depending on their extent: (1) lesion of parts of the (LSA); (2) lesion of parts of the LSA with undercutting of areas 17, 18 and 19; (3) lesion of area 7; (4) lesion of area 7 and parts of the LSA. 1. We found that a large bilateral lesion of the LSA led to significant deficits in all test situations which were dependent on the existence of relative velocity of moving patterns against a structured background. The ability of the cats to discriminate simple outline patterns which were kept stationary was not reduced. On the contrary, when they were tested with stationary and moving patterns on unfocused (empty) backgrounds, we found, to our great surprise, that the performance of the lesioned cats was significantly improved compared with intact animals. As these lesioned cats had no deficits with moving patterns on a uniformly grey background, we conclude that the deficits with the moving patterns must have been caused by interactions between patterns and background, and not by movement of a pattern per se. 2. As soon as the lesion of the LSA was extended by a bilateral undercutting of areas 17, 18 and 19 we found very severe deficits in all test situations, regardless of whether the patterns were moving or kept stationary, or whether they were superimposed on a background or not. The most substantial deficits occurred when the patterns were moving on a stationary background. In these situations the cats were no longer able to reach the 84% correct criterion. Again, the cats were able to reach criterion with moving patterns on a uniformly grey background indicating that this deficit is probably caused by the interaction of patterns and background and not by motion of the patterns per se.(ABSTRACT TRUNCATED AT 400 WORDS)

Animals↗
PubMed1987

Characteristics of neuronal systems in the visual cortex.

The coupling complexity of cortical areas makes it very difficult to analyse them experimentally. Studies of model systems provide the possibility of adapting the analysis to the available data base and elaborating the fundamental properties that depend on the structure of the system. We propose a model system of variable complexity that is spatially two-dimensional and time-dependent, uses feedback for iteration and smoothing, includes the mapping of the cortical networks and can be nonlinear as the case requires. Combining such elementary systems on the basis of neuroanatomical findings enables us to simulate cortical mappings and to interpret neurophysiological data. The decisive factor is that the dynamics of the system and the neuroanatomically based spatial coupling are closely connected with each other.

Animals↗
PubMed1987

[Initial clinical results of tissue characterization by T1, T2 and proton density in nuclear magnetic resonance tomography].

The NMR parameters (proton density, relaxation times T1 and T2) have been assessed by Carr-Purcell-Meiboom-Gill (CPMG) spin echo sequences. A computer assisted analysis of the data of 21 patients with cerebral tumours allowed a classification of tumour tissue in different tumours. The use of quantitative procedures for tissue characterisation allows the differentiation of benign and malignant brain tissue by characteristic colour coding demonstrating morphological details like tumour, edema and necrosis as well as indicating the histological types of the tumours of the central nervous system.

Brain↗
PubMed1987

[Reference substance for initialling the marker space in NMR tomography].

Dimethylsiloxane polymers are tested to serve as a reference medium in quantitative MR imaging. Simultaneously measured during the patient's examination, the reference data are used to normalize the intrinsic MR tissue parameters. This method helps to achieve interindividual comparability of tissue vectors which are defined by proton density, spin-spin and spin-lattice relaxation times, thus leading to a remarkable improvement in tissue classification.

Brain↗
PubMed1986

New balloon catheter for prolonged percutaneous transluminal coronary angioplasty and bypass flow in occluded vessels.

A new balloon catheter was developed for continuous perfusion of coronary arteries during angioplasty (CPC catheter). Steerable Grüntzig balloon catheters (3.7 mm) with two lumina were formed. The first lumen was used for balloon inflation. Side holes to the second lumen proximally and distally to the balloon were created for coronary perfusion even during inflation phase. At a perfusion pressure of 120 mmHg, a flow rate of 63 +/- 3 ml/min with 0.9% saline and 43 +/- 1 ml/min with plasma expander were measured. In experiments on five dogs, dilation time until appearance of signs of ischemia could be prolonged in three of five dogs from 30 to 40 s, 120 to 203 s, and 180 to 420 s comparing conventional and CPC balloon catheters. In 11 patients with proximal lesions, dilation time could be increased from 39.5 +/- 23.9 s to 81.1 +/- 36.3 s (p less than 0.01) until appearance of angina pectoris. ST segment changes were observed in 10/11 patients using conventional catheters. Using CPC catheters, no ST segment changes were observed in four patients; time until appearance of ST segment changes was delayed in the other seven patients. The CPC catheter seems to be an alternative catheter in proximal lesions of the left and right coronary artery, allowing the possibility of prolonged dilation and increased safety to the patient. In case of dissection or perforation, the CPC catheter can be used for perfusion of the distal part of the coronary vessel until emergency bypass surgery.

Adult↗
PubMed1986

[Value of physical parameters and digital signal processing in tissue differentiation].

It was the aim of this study to improve tissue characterization using ultrasonic scatter signals and image processing procedures by computer. Analysis of various tissue probes and phantoms showed that structures can be differentiated by scatter as a function of frequency and angle of scatter. The application of this method as an imaging system was realised by a reconstruction algorithm to compute scatter images. By means of analysing images of the thyroid and prostate we found that the detection rate of computerised texture analysis is much higher than that of visual evaluation.

Diagnosis, Differential↗
PubMed1985

[Quantitative technics in ultrasonic diagnosis].

This paper presents techniques for computer-aided analysis of medical ultrasonic images. Methods of image processing, feature extraction and a classification algorithm for tissue characterization are described. Based on these techniques a pattern recognition system for semi-automatic analysis of ultrasonic images has been developed and proved within three clinical studies, which aimed to improve the ultrasound diagnosis of the prostate, the thyroid and the female breast. The results show that computer analysis of ultrasonic images is capable of providing a quantitative diagnosis as well as of differentiating tumor lesions by sonography.

Breast Neoplasms↗
PubMed1984

[Computer-assisted evaluation of ultrasound images of the prostate].

The prostate was examined by sonography to diagnose tumourous changes in the organ. Several visual parameters were found to interpret the pictures. The ultrasonic images can be prepared and improved by means of image-processing algorithm were computed while evolving a method for the automatic evaluation of ultrasonic images. The value of this method and its results are discussed.

Computers↗
PubMed1983

On the analysis of the cat's pattern recognition system.

The objective of the paper is to determine in abstract terms the algorithms used by the cat detecting simple patterns and to quantify the contributions of the visual areas 17, 18, 19 for this task. The data incorporated in the algorithm are collected from behavioral experiments where the animals had to distinguish between two patterns. The patterns were superimposed with gaussian noise and the detection probability was measured. The resulting model describes pattern recognition in two steps: first extraction of features and second classification. The test of the validity of the model system was to predict the outcome of similar experiments but with different patterns. With the help of the model it is possible to describe the effect of a lesion in parametric form. This in turn gives some hints about the functions of the visual areas 17, 18, 19 for the specific fask, tested in the experiments.

Animals↗
PubMed1982

[Clinical testing and improvement of an ultrasound method for detection of pathologic changes in the breast].

Recent years have seen rapid advances in ultrasound techniques, increasing the chances of early diagnosis of changes in the female breast, irrespective of the presence of signs or symptoms. Sonography is excellent for the observation of breast tissue changes. However in common with other medical imaging techniques, it does not provide qualitative data--it is only a diagnostic guide. The article, therefore, includes a list of criteria which are useful in differential diagnosis of benign and malignant tissue changes.

Breast Neoplasms↗