This file was created by the Typo3 extension sevenpack version 0.7.14 --- Timezone: CEST Creation date: 2013-05-24 Creation time: 03-03-32 --- Number of references 4 article 1571 Experimental Brain Research 2002 8 145 3 407-410 Abstract Transcranial magnetic stimulation applied over the occipital pole is able to induce the perception of light flashes called phosphenes. For phosphene detection and threshold measurement subjects are usually blindfolded or investigated in the dark. The question that we posed here is whether phosphene thresholds change with variations in ambient light. In six subjects we measured thresholds under four different conditions: closed eyes (5 minutes of adaptation) and 0.5, 100, and 3200 cd/m2 background illumination. No systematic change in phosphene thresholds was observed with different lighting conditions. In three subjects we repeated the measurements after one week and again found similar values with no systematic modulation. Our data show that cortical excitability does not change with different light adaptation levels. This confirms that the main adaptation to light takes place at subcortical levels, namely at the retina. The practical conclusion is that it is unnecessary to blindfold subjects when determining phosphene thresholds. http://www.kyb.tuebingen.mpg.de http://www.kyb.tuebingen.mpg.de http://www.kyb.tuebingen.mpg.de Department Kirschfeld http://link.springer.com/content/pdf/10.1007%2Fs00221-002-1160-3 Biologische Kybernetik Max-Planck-Gesellschaft 10.1007/s00221-002-1160-3 kammerTKammer sbeckSBeck article 949 Clinical Neurophysiology 2001 11 112 11 2015-2021 Objectives: To quantify phosphene thresholds evoked by transcranial magnetic stimulation (TMS) in the occipital cortex as a function of induced current direction. Methods: Phosphene thresholds were determined in 6 subjects. We compared two stimulator types (Medtronic-Dantec and Magstim) with monophasic pulses using the standard figure-of-eight coils and systematically varied hemisphere (left and right) and induced current direction (latero-medial and medio-lateral). Each measurement was made 3 times, with a new stimulation site chosen for each repetition. Only those stimulation sites were investigated where phosphenes were restricted to one visual hemifield. Coil positions were stereotactically registered. Functional magnetic resonance imaging (fMRI) of retinotopic areas was performed in 5 subjects to individually characterize the borders of visual areas; TMS stimulation sites were coregistered with respect to visual areas. Results: Despite large interindividual variance we found a consistent pattern of phosphene thresholds. They were significantly lower if the direction of the induced current was oriented from lateral to medial in the occipital lobe rather than vice versa. No difference with respect to the hemisphere was found. Threshold values normalized to the square root of the stored energy in the stimulators were lower with the Medtronic-Dantec device than with the Magstim device. fMRI revealed that stimulation sites generating unilateral phosphenes were situated at V2 and V3. Variability of phosphene thresholds was low within a cortical patch of 2×2 cm2. Stimulation over V1 yields phosphenes in both visual fields. Conclusions: The excitability of visual cortical areas depends on the direction of the induced current with a preference for latero-medial currents. Although the coil positions used in this study were centered over visual areas V2 and V3, we cannot rule out the possibility that subcortical structures or V1 could actually be the main generator for phosphenes. http://www.kyb.tuebingen.mpg.de http://www.kyb.tuebingen.mpg.de http://www.kyb.tuebingen.mpg.de Department Kirschfeld http://www.sciencedirect.com/science/article/pii/S1388245701006733 Biologische Kybernetik Max-Planck-Gesellschaft 10.1016/S1388-2457(01)00673-3 kammerTKammer sbeckSBeck MErb WGrodd article 40 Clinical Neurophysiology 2001 2 112 2 250-258 Objectives: To evaluate the stimulation effectiveness of different magnetic stimulator devices with respect to pulse waveform and current direction in the motor cortex. Methods: In 8 normal subjects we determined motor thresholds of transcranial magnetic stimulation in a small hand muscle. We used focal figure-of-eight coils of 3 common stimulators (Dantec Magpro, Magstim 200 and Magstim Rapid) and systematically varied current direction (postero-anterior versus antero-posterior, perpendicular to the central sulcus) as well as pulse waveform (monophasic versus biphasic). The coil position was kept constant with a stereotactic positioning device. Results: Motor thresholds varied consistently with changing stimulus parameters, despite substantial interindividual variability. By normalizing the values with respect to the square root of the energy of the capacitors in the different stimulators, we found a homogeneous pattern of threshold variations. The normalized Magstim threshold values were consistently higher than the normalized Dantec thresholds by a factor of 1.3. For both stimulator types the monophasic pulse was more effective if the current passed the motor cortex in a postero-anterior direction rather than antero-posterior. In contrast, the biphasic pulse was weaker with the first upstroke in the postero-anterior direction. We calculated mean factors for transforming the intensity values of a particular configuration into that of another configuration by normalizing the different threshold values of each individual subject to his lowest threshold value. Conclusions: Our transformation factors allow us to compare stimulation intensities from studies using different devices and pulse forms. The effectiveness of stimulation as a function of waveform and current direction follows the same pattern as in a peripheral nerve preparation (J Physiol (Lond) 513 (1998) 571). http://www.kyb.tuebingen.mpg.de http://www.kyb.tuebingen.mpg.de http://www.kyb.tuebingen.mpg.de Department Kirschfeld http://www.sciencedirect.com/science/article/pii/S1388245700005137 Biologische Kybernetik Max-Planck-Gesellschaft 10.1016/S1388-2457(00)00513-7 kammerTKammer sbeckSBeck thielscherAThielscher ULaubis-Herrmann HTopka poster 122 2000 2 79 Transkranielle Magnetstimulation (TMS) über dem Hinterkopf führt zur Wahrnehmung von konturierten Lichtblitzen, sogenannten Phosphenen. Sie erscheinen überwiegend in den unteren Gesichtsfeldhälften, kontralateral zur Position der Spule. Unklar ist bisher, in welchen kortikalen Strukturen die Phosphene generiert werden. Bei bisher vier Versuchspersonen wurde die retinotope Repräsentation im visuellen Kortex mit Hilfe der funktionellen Magnetresonanztomographie (fMRT) gemessen. Die Exzentrizität der Retinotopie wurde durch einen langsam expandierenden flickernden Ring abgebildet, der Polarwinkel durch ein flickerndes Segment, welches durch das ganze Gesichtsfeld rotierte. Die Auswertung erfolgte mit dem Programm BrainVoyager (Rainer Goebel, Maastricht). Die Ergebnisse der einzelnen Personen wurden auf der jeweiligen individuell rekonstruierten Kortexoberfläche dargestellt und die Grenzen zwischen den visuellen Arealen V1 bis V4 ermittelt. Die gleichen Versuchspersonen dokumentierten die Lage und Form der Phosphene, die an verschiedenen Orten über dem gesamten Hinterkopf durch fokale TMS evoziert wurden. Die genaue Position derTMSSpule relativ zum Schädel der Versuchsperson wurde mit Hilfe eines stereotaktischen Positionierungssystems mit einer Präsision von ± 1,5 mm aufgezeichnet. Die Überlagerung der Phosphen-Stimulationsorte mit der funktionellen Architektur des visuellen Kortex zeigt: 1. Phosphene lassen sich sowohl von oberflächlich gelegenen Anteilen von V1 als auch von den benachbarten Arealen V2 und V3 evozieren. 2. Die Lage der wahrgenommenen Phosphene im Gesichtsfeld folgt in erster Näherung der mit fMRT gemessenen Retinotopie. 3. Phosphene, die durch Stimulation über V1 evoziert werden, unterscheiden sich nicht in Größe oder Form von Phosphenen, die durch Stimulation über V2 oder V3 evoziert werden. Die Abhängigkeit der Lage der Phosphene von der Retinotopie des visuellen Areals spricht gegen die Annahme von Marg & Rudiak (Optometry Vis Sci 71, 1994), daß Phosphene durch subkortikale Stimulation der Radiatio optici generiert werden, und bestätigt die Vermutung von Meyer et al. (Electroenceph Clin Neurophysiol Suppl 43, 1991), daß striäre und extrastriäre Areale die Generatoren der Phosphene sind. http://www.kyb.tuebingen.mpg.de http://www.kyb.tuebingen.mpg.de http://www.kyb.tuebingen.mpg.de Department Kirschfeld http://www.twk.tuebingen.mpg.de/twk00/ Biologische Kybernetik Max-Planck-Gesellschaft Tübingen, Germany 3. Tübinger Wahrnehmungskonferenz (TWK 2000) kammerTKammer MErb sbeckSBeck WGrodd