Spike detection and sorting using PARAFAC2 method.

In this contribution we introduce the Parallel Factor 2 (PARAFAC2) analysis as a novel method for the simultaneous detection and classification of neural action potentials. In order to measure these action potentials (spike signals), stem cell derived neuronal cells are cultivated on the surface of a Micro Electrode Array (MEA). Here, the neuronal cells produce ion currents, which can be measured as extracellular electric potentials. Whenever a cell or a group of cells produces ion currents, either spontaneously or evoked by a stimulus, a spike signal can be measured by the electrodes of the MEA. Stimulated cells produce spikes and groups of spikes (bursts) which propagate in space over the MEA. In the recorded data, different source types (e.g., cells which respond directly to external stimuli and cells which are triggered by other neural cells) are characterized by different spike shapes. The proposed PARAFAC2 method is able to separate these spike shapes (sources) in time, frequency and space (channels) enabling an improved performance in noisy scenarios. Furthermore, PARAFAC2 allows for a causality analysis on the measured spike signals (i.e. the identification of different signal paths). Thereby, the PARAFAC2 decomposition is able to exploit the multi-dimensional structure of the MEA data.

In-vivo signal transmission using an intra-corporal RF transmitter.

In clinical routine, measurements of human physiological parameters are very important. In this paper, a study of RF transmission from the inside to the outside of a biological body is described. In the course of this work, an overview of the state of the art of wireless biotelemetry and the basics of biological tissue attenuation are given. In addition, several prototype transmitters were designed and developed with frequencies ranging from 50 to 700 MHz. With these transmitters a study of an in-vivo transmission was run to measure realistic attenuation values of a living biological subject. In the evaluation phase, a prototype transmitter was placed in the esophagus, near the heart, of a narcotized living pig. This allows demonstrating the transmission out of an animal with human-like tissue properties. The results show a possible transmission at 58, 119, 240, 418 and 672 MHz with acceptable loss.

[Improving SNR (signal to noise ratio) in multichannel EEG recording]. / Verbesserung des SNR bei mehrkanaligen EEG-Ableitungen.

The main problem in measurements of the focal VEP (Visual Evoked Potential) is its weak SNR (Signal-to-Noise Ratio). The most common method for enhancement of the SNR is the stimulus synchronized averaging. For study of single responses other ways in SNR improvement are needed. In this contribution a new method based on space-time selective measurement is introduced, which can be interpreted as beaming a signal source. Since the anatomical structures of sources generating the focal VEP are known in general and if the electrode positions are of sufficient density over the visual cortex, a source beamer can be realized by controlling the channels' delays.

Spectral characteristics of light sources for S-cone stimulation.

PURPOSE: Electrophysiological investigations of the short-wavelength sensitive pathway of the human eye require the use of a suitable light source as a S-cone stimulator. Different light sources with their spectral distribution properties were investigated and compared with the ideal S-cone stimulator. METHODS: First, the theoretical background of the calculation of relative cone energy absorption from the spectral distribution function of the light source is summarized. From the results of the calculation, the photometric properties of the ideal S-cone stimulator will be derived. The calculation procedure was applied to virtual light sources (computer generated spectral distribution functions with different medium wavelengths and spectrum widths) and to real light sources (blue and green light emitting diodes, blue phosphor of CRT-monitor, multimedia projector, LCD monitor and notebook display). The calculated relative cone absorbencies are compared to the conditions of an ideal S-cone stimulator. RESULTS: Monochromatic light sources with wavelengths of less than 456 nm are close to the conditions of an ideal S-cone stimulator. Spectrum widths up to 21 nm do not affect the S-cone activation significantly (S-cone activation change < 0.2%). Blue light emitting diodes with peak wavelength at 448 nm and spectrum bandwidth of 25 nm are very useful for S-cone stimulation (S-cone activation approximately 95%). A suitable display for S-cone stimulation is the Trinitron computer monitor (S-cone activation approximately 87%). The multimedia projector has a S-cone activation up to 91%, but their spectral distribution properties depends on the selected intensity. LCD monitor and notebook displays have a lower S-cone activation (< or = 74%). CONCLUSION: Carefully selecting the blue light source for S-cone stimulation can reduce the unwanted L-and M-cone activation down to 4% for M-cones and 1.5% for L-cones.

A periodogram-based method for the detection of steady-state visually evoked potentials.

The task of objective perimetry is to scan the visual field and find an answer about the function of the visual system. Flicker-burst stimulation--a physiological sensible combination of transient and steady-state stimulation--is used to generate deterministic sinusoidal responses or visually evoked potentials (VEP's) at the visual cortex, which are derived from the electroencephalogram by a suitable electrode array. In this paper we develop a new method for the detection of VEP's. Based on the periodogram of a time-series, we test the data for the presence of hidden periodic components, which correspond to steady-state VEP's. The method is applied successfully to real data.