Assessment of human sleep depth is being de-standardized by recently advised EEG electrode locations.

Human sleep depth was traditionally assessed by scoring electro-encephalographic slow-wave amplitudes at the globally standardized C4-M1 electrode derivation. Since 2007, the American Association of Sleep Medicine (AASM) has accepted three additional derivations for the same purpose. These might well differ in slow wave amplitudes which would bias the scorings. Some derivations might also introduce large inter-individual variability. We compared mean and variability of slow wave amplitudes between six derivations including the four AASM ones. Slow wave amplitudes in those derivations were simultaneously measured using automated analysis in 29 patients. Each amplitude was divided by the average from the six derivations, thus removing shared factors such as age, gender and sleep depth while retaining factors that differ between the derivations such as caused by local skull characteristics, electrode distance and neuronal dipole orientation. The remaining inter-individual variability differed significantly and up to a factor of two between the AASM derivations. The amplitudes differed significantly and up to 60% between the AASM derivations, causing substantial scoring bias between centres using different derivations. The resulting de-standardization most likely affects any patient group because the amplitude differences were consistent over diagnoses, genders, and age. Derivation-dependent amplitude thresholds were proposed to reduce the scoring bias. However, it would be better to settle on just one derivation, for instance Cz-Oz or Fpz-Cz because these have lowest variability while matching the traditional C4-M1 amplitudes.

A DC attenuator allows common EEG equipment to record fullband EEG, and fits fullband EEG into standard European Data Format.

OBJECTIVE: Traditional electroencephalogram (EEG) recorders reject low frequencies and DC and therefore cannot handle fullband EEG. Dedicated fullband recorders use non-standard file formats, because the standard format (EDF) cannot handle large DC electrode offset voltages. Both facts limit the development and use of fullband EEG. We developed a modification that allows conventional equipment to record fullband EEG, and adapts both types of recorders to EDF. METHODS: The modification is a simple filter that attenuates the DC component and thus makes the EEG fit within traditional equipment limitations and EDF. The review software automatically 'de-attenuates' the DC component, without loss of information. RESULTS: DC attenuation by a factor of 10 made both types of recorders store DC attenuated fullband EEG into EDF files. Recordings were made during 0.5-24h in 46 subjects. The DC de-attenuator automatically reconstructed the original fullband EEG within an amplitude range of ±100mV and with a resolution of 0.3µV. Using sintered Ag-AgCl electrodes attached with common procedures, reconstructed DC EEG in spontaneously moving subjects ranged between ±32mV. CONCLUSIONS: The modification works. Fullband recordings can now be analyzed by independent software, archived and exchanged. SIGNIFICANCE: Any EEG system can be made to record fullband EEG into standard EDF.

Easy listening to sleep recordings: tools and examples.

BACKGROUND AND PURPOSE: To specify a simple conversion of neurophysiological signals contained in sleep recordings into standard audio files and to illustrate how our cerebral audio processor can then detect specific signal characteristics. METHODS: A software package (freely accessible from the Internet) has been developed that converts signals from standard EDF (or EDF+) format to standard audio (WAV) format, a process usually called audification. The software has been applied to sleep EEG, EOG and ECG. The software is easy to apply. RESULTS: A wide range of audified signals is described, stressing the analogy with familiar sounds. Audio properties of EEG in different sleep stages, EOG, ECG and respiration signals are discussed. Auditive presentation of the signals invokes brain processes that differ essentially from the commonly applied visual interpretation, including physiological frequency analysis. CONCLUSIONS: Such auditive interpretation may complement the visual one. The widespread use of EDF+ and multimedia computers makes such audification simple and straightforward.