in simulierten Bedingungen, am Kilimanjaro und im Karakorum

Christoph Guger, Markus Tannheimer, Stefan Krausert, Wolfgang Domej, Günter Edlinger

Veränderungen des EEG, EKG und der Konzentrationsfähigkeit mit der Höhe in simulierten Bedingungen, am Kilimanjaro und im Karakorum

Changes in EEG, ECG and mental concentration with altitude in simulated conditions, on Kilimanjaro and in the Karakorum

SUMMARY

With increasing altitude, the concentration of oxygen in the air remains constant but its partial pressure drops. In order to describe how this affects the body, experiments were performed in a hypobaric chamber (4.000 m), on Kilimanjaro (5.896 m) and in the Karakorum (6.304 m). The subjects had to perform a reaction time task at different altitudes. The EEG and ECG were recorded simultaneously. Additionally, the oxygen saturation of the blood was measured at different altitudes. The subjects filled out a Lake Louise questionnaire that describes the degree of altitude mountain sickness (AMS). On Kilimanjaro, the group also performed the d2-test of attention to quantify processing speed and concentration at different altitudes. This work explains the study design, the performed measurements and the planned signal analysis.

Keywords: high altitude medicine, EEG, ECG, Lake Louis Score, d2-test

ZUSAMMENFASSUNG

Mit dem Anstieg der Höhe bleibt die Konzentration des Sauerstoffs in der Luft konstant, allerdings sinkt der Sauerstoffpartialdruck ab. Um den Einfluss dieses Effektes auf den menschlichen Körper zu untersuchen wurden Experimente in einer Unterdruckkammer (4.000 m), am Kilimanjaro (5.896 m) und im Karakorum (6.304 m) durchgeführt. Die Versuchspersonen führten in verschiedenen Höhen einen Reaktionstest durch. Simultan wurde das EKG und

das EEG gemessen. Zusätzlich wurde die Sauerstoffsättigung des Blutes auf verschiedenen Höhen erhoben. Die Versuchspersonen füllten auch einen LakeLouise-Fragebogen aus, um die Höhensymptomatik zu beschreiben. Am Kilimanjaro führte die Gruppe zusätzlich einen d2-Aufmerksamkeits- und Belastungstest durch. Diese Arbeit beschreibt das Studiendesign, die durchgeführten Messungen und die geplanten Auswertungen der Daten.

Schlüsselwörter: Höhenmedizin, EEG, EKG, Lake-Louis-Score, d2-Test

INTRODUCTION

Even with little subjective awareness of the reduced amount of oxygen at an altitude of 2.700 m, the cardiovascular and central nervous system are already affected (1). A study on the Dachstein showed that the heart rate (HR) increased from 69,1 bpm at 990 m altitude to 80,4 bpm at 2.700 m altitude in a group of 10 subjects (1). Additionally, heart-rate variability (HRV) parameters were decreased. The RMSSD (root mean square of squared differences of adjacent heart beat intervals) changed from 33,0 to 28,5 ms; the pNN50 (the number of heart beat intervals which differ by more than 50 ms divided by the total amount of heart beats) from 9,6 % to 0,9 % and the SDNN index (standard deviation of successive heart beat interval differences) from 58,1 ms to 41,1 ms. All changes were significant. The HRV parameters in the frequency domain can be used to describe the parasympathetic and sympathetic nervous systems. The normalized low frequency component (LFnorm), which describes the activity around 0,04–0,15 Hz, represents the activity of the sympathetic system. The normalized high frequency component (HFnorm) describes variations between 0,15 –0,4 Hz and mainly represents the parasympathetic system. In the Dachstein study, LFnorm activity increased from 51,1 to 65,4 and HFnorm decreased from 35,1 to 25,0. The LF/HF ratio also increased from 2,1 to 4,4. This clearly shows that with the increase in altitude, the sympathetic system becomes more active compared to the parasympathetic system. These effects were also shown in a study in a hypobaric chamber at 5.000 m altitude (2) in long-term exposure studies to altitudes above 4.000 m (3,4,5) and in mountaineers at 2.700 m and 3.700 m (6). It is well known that the execution of movement is accompanied by a desynchronization of mu and central beta rhythms over the corresponding cortical representation areas (7). The movement-related power decrease in a specific frequency band (event-related desynchronization, ERD) can be found in EEG traces

measured over the sensorimotor areas during e.g. finger movements. A right hand finger movement produces an ERD in the left hemisphere close to electrode position C3 of the international 10/20 electrode system. Similarly, a left hand movement results in an EEG desynchronization over the right hemisphere close to C4. The recovery phase from the movement starts with movement offset and typically lasts between 1 and 2 seconds. In this phase, the mu rhythm slowly returns to its resting state while bursts of short-lasting oscillations in the beta band occur after the movement offset. The peak of such a postmovement beta event-related synchronization (ERS) occurs at about 0,6 to 1 second after movement offset (7). In contrast to an ERD, an ERS indicates a power increase in a specific frequency band. The post-movement beta ERS (beta rebound) occurs mainly on the contralateral side (with respect to the movement side) but also with a smaller amplitude on the ipsilateral side. It displays a strict somatotopic organization and coincides with reduced corticospinal excitability (8). An ERD in the alpha band is characteristic of perceptual, judgment and memory tasks. The power decrease is greater with increased task complexity or attention (9, 10). Event-related EEG changes were also analyzed in the Dachstein study. The subject had to press a button as fast as possible after a green light flashed. This button press was analyzed with the ERD and ERS method. The study showed that the ERD in the alpha range stayed almost constant between 990 m and 2.700 m, but the ERS (beta rebound) decreased from +42 % at 990 m to +12 % at 2.700 m. This study showed for the first time that the post-movement beta ERS (beta rebound) is significantly attenuated at the high altitude compared to the low altitude measurement (1). Both self-paced finger movement and electrical median nerve stimulation are terminated by a beta rebound of similar magnitude and latency (11). Corticospinal excitability was shown in the reaction time movement tasks to be significantly reduced in the first second after EMG offset (12). These findings support the hypothesis that beta ERS could be related to an idling or deactivated state (7,13), or even active immobilization of the motor cortex (14). Such a beta rebound originates mainly in the pre-central localized motor cortex and is attenuated or even suppressed during activation of the motor cortex (8,15). The beta ERS can therefore be seen as an inverse marker for the excitability level of motor cortex neurons with an attenuation when the excitability level is increased. The suppressed post-movement beta ERS at the altitude of 2.700 m may therefore be interpreted as a result of an increased cortical excitability level when compared with the reference altitude of 990 m (1). At an altitude of 2.700 m, essential sensory/motor functions are not inhibited, but the time to learn a new task is already increased (16).

The ERD indicates the activated cortical areas involved in the processing of cognitive information and production of motor behaviour. An increased ERD can be the result of a larger neural network involved in the task of information processing. Such an enhancement of ERD can be found in Parkinson patients (17), in subjects with lower IQs (18) or during increased task complexity (19). ERD changes in the alpha band, however, were not significant when the results from 2.700 m and 990 m in the Dachstein study were compared. One explanation can be the relatively low altitude of 2,700 m. The present study therefore investigates the changes of ERD at higher altitudes. A higher ERD could be expected with increasing altitude. Furthermore, the changes of the beta-rebound will be investigated with the increasing altitude above 2.700 m. An interesting question is if the high altitude acclimatization also affects the ERD and the ERS. It can be expected that with successful adaptation, the beta-rebound will return to the values found at sea level.

Studies also showed decreased finger-tapping speed, reduction of short-term memory and aphasic deficits (20) after hypoxic exposure to an altitude of 8.850 m in a chamber simulation. Mountaineers who had climbed above 8.500 m also showed a decrease in performance in a neuropsychometric test two to ten months after having returned to low altitude. The study showed EEG and magnetic resonance imaging (MRI) abnormalities (20). The findings support the presumption that non-life-threatening hypoxia may damage the brain at extreme altitudes.

The present study introduces the d2-test as instrument to quantify the effect of altitude. It is a standard instrument for quantifying concentration, speed and attention in clinical and applied settings (21). The test was originally developed to investigate aptitude for driving a motor vehicle. Basically the subject has to differentiate similar visual symbols as fast as possible. The number of correctly processed items is a marker for individual attention and concentration performance. The test lasts approximately 5 minutes and can also be performed simultaneously by groups.

The Lake Louise Score (LLS) was created in Canada in 1991 and 1993 (22). It is a simplified and standardized scoring system that allows diagnosis and quantification of mountain sickness in altitude research. Currently, it is widely used by mountaineers and trekkers because it is short, has a simple format and is easy to complete. The LLS is sensitive enough to detect altitude mountain sickness (AMS), but the very specific scoring system also avoids over-diagnosis.

The present work describes the study design to investigate the changes of EEG, ECG, LLS, SpO2 and d2-test parameters during fast ascents and during acclimatization to high altitudes.

EXPERIMENTS AND METHODS

In summer 2006 experiments were performed at Kilimanjaro/Tanzania, in a hypobaric chamber in Königsbrück/Germany and during the first ascent to the Shimshal Whitehorn in the Karakorum/Pakistan.

Participating subjects

(i) Kilimanjaro In July 2006 eight healthy subjects (3 female, 5 male, 31–36 years) participated in the experiment on Kilimanjaro. 7 subjects participated the first time. 1 subject participated 2 years earlier in the same experiment in the Alps. 7 subjects were right handed, 1 subject was left handed.

(ii) Königsbrück In September 2006 twelve healthy subjects (all male, 21–33 years) participated in the experiment in the hypobaric chamber in Königsbrück. All subjects participated the first time in the experiment. All 12 subjects were right handed.

(iii) Shimshal In July 2006 four healthy subjects (1 female, 3 male, 31–37 years) climbed the Shimshal Whitehorn and performed the experiment. All subjects participated the first time in the experiment. All 4 subjects were right handed.

EXPERIMENTS

The experiments at Kilimanjaro, in Königsbrück and at Shimshal consisted of different parts. First EEG and ECG data was acquired to investigate the autonomous and central nervous system during a reaction time task. Afterwards the oxygen saturation was measured and a Lake Louise Score questionnaire was filled out. Additionally, on Kilimanjaro a d2-test was performed. The subjects were informed of potential risks, where after a written consent was obtained. After arrival at a new altitude, the subjects rested at least for 1 hour prior to the experiment. A detailed description of the experiments is given below:

(I) EEG, ECG, reaction time task

EEG and ECG were recorded while the subjects performed a psycho-physiological experiment (reaction time task). Each measurement lasted about 6 minutes. The subjects were sitting in front of the flashing light and were instructed not to move during the experiment so as to assure high data quality. A Pocket PC-based EEG and ECG recording system g.MOBIlab (g.tec medi-

cal engineering GmbH, Graz, Austria) was used for biosignal acquisition and to display the reaction time paradigm. One channel of ECG (Einthoven I) and two bipolar EEG channels were recorded. The EEG electrodes were mounted 2,5 cm posterior and 2,5 cm anterior to electrode positions C3 and C4 of the international 10/20 electrode system. Gold electrodes were used for a high signal quality. The EEG ground electrode was mounted on the forehead. Signals were recorded at a sampling frequency of 256 Hz and with a resolution of 16 Bits. The protocol of the reaction time experiment is illustrated in Figure 1. At second two a green or red light flashes for 200 ms. The subject's task is to press a button with the right index finger as fast as possible in response to a green flash but refrain in the case of a red flash. The duration of the trial is 5 seconds. To avoid adaptations of the subject, each trial is followed by a random interval of 0 to 2 seconds. The blinking sequence of the green light (50 times) and the red light (10 times) is randomly distributed. A complete experiment consists of 60 trials.

Green light(50x)

Red light(10x)

Button Press

0 -2 s

0 1 2 5 6 43 7

Time [s] Fig.1: Timing of the paradigm. At 2 s of the trial a green or a red light flashes. If the green light flashes the subject has to press the button as fast as possible. The subject is not allowed to press the button if the red light flashes.

(ii) Oxygen saturation

The oxygen saturation (SpO2) was measured with a portable measurement device (SPO 5500 P, Graseby Medical Ltd, UK) which was fixed on the index finger.

(iii) Lake Louise Score (LLS)

The questionnaire contains 8 questions about headache, gastrointestinal symptoms, fatigue/weakness, dizziness, difficulty sleeping, mental status, ataxia and peripheral oedema. AMS is present if there was an altitude rise AND the per-

son has headache AND at least another symptom AND a total score equal or above 5 is reached. The minimum score 0 means that the subject is healthy; the maximum score is 25.

(iv) d2-test

The test shows 14 rows with 47 signs each (total of 658 items) as shown in Figure 2. The task is to differentiate the symbols d or p with 1 to 4 dashes. Every d with 2 dashes has to be crossed out with a pencil. For each row 20 seconds are available. All other signs should not be marked. Each row contains 21 or 22 targets in random order. Each row is a small test on its own and allows the investigation of the performance over time. The test counts the number of items processed (TN), the percentage of errors (E%), the concentration performance (CP –the number of correctly processed items minus the incorrectly crossed items), the errors (E) and the TN minus E (TN-E) as well as the fluctuation rate (FR). About 5 minutes are needed to perform the test. Important is that the test can also be performed by a group simultaneously.

Fig.2:

First 3 lines of the d2-test showing the processed items.

MEASUREMENT POINTS

(i) Kilimanjaro

The height profile of the Kilimanjaro climb is shown in Figure 3. First Mt. Meru was climbed for acclimatization and afterwards Mt. Kilimanjaro was climbed over the Machame route. The Machame route is one of the most popular routes of the mountain. In total the experiment was performed 3 times: (i) at 3,570 m at Saddle hut on the way to Mt. Meru, (ii) at 4,633 m in the Barafu camp on the way to the summit of Mt. Kilimanjaro and (iii) at sea level in Mombasa at 0 m altitude. The measurement lasted for all 8 subjects approximately for 1,5 hours at every measurement time point. Figure 4 shows the reaction time experiment performed on the way to Mt. Meru.

Fig.3: Height profile of the Mt. Meru and Mt. Kilimanjaro climb. The numbers on the x-axis indicate the day; B – breakfast, L – lunch, D – dinner. The climb started at 1.380 m in Arusha. The first measurement point (MP) was at 3.570 m in the Saddle hut on the way to Mt. Meru. The second measurement point was at 4.633 m in the Barafu camp. This is the last camp before reaching the Uhuru Peak (highest summit of Mt. Kilimanjaro). The third measurement was performed at 0 m altitude in Mombasa 7 days after the climb.

Fig.4. C. Guger during the EEG and ECG measurements at Kilimanjaro in the Saddle hut.

(ii) Königsbrück

The height profile artificially produced by the hypobaric chamber in Königsbrück is given in Figure 5. In total the experiment was performed first after 11 hours at 4,000 m and after one hour back on sea level. The 12 people were divided into 2 groups of 6 people to perform the experiment on different days because of the limited size of the hypobaric chamber and because of the handling of the experiments. The measurements lasted for 6 subjects approximately for 1 hour at every measurement time point. Figure 6 shows the subject sitting in the hypobaric chamber and performing the reaction time experiment.

Fig.5: Height profile artificially produced by the hypobaric chamber in Königsbrück. The ascent started with an elevation gain of 1m/s until 1.000 m were reached. Thereafter the elevation gain was 5 m/s. Then a constant altitude of 4.000 m was kept for 12 hours. The descent was done with 5 m/s until sea level was reached again. The first measurement point (MP) was between hours 10 to 11. The second MP was 1 hour after returning to sea level between hours 13 and 14.

(iii) Shimshal

The height profile of the Shimshal climb is given in Figure 7. The experiment was performed 6 times. The measurements lasted for all subjects about 45 minutes. Figure 8 illustrates the experimental setup.

Fig.6: Measurements in the hypobaric chamber in Königsbrück

Fig.7: Height profile of the expedition in the Karakorum, Pakistan. The number on the x-axis indicates the day. After reaching Islamabad, the group took a jeep ride to Gilgit, Passu and Shimshal where the Shimshal Whitehorn was the first time visible. Therefore an initial experiment was performed at 2.500 m and at 3,100 m some days later. Then the first ascent of the Chukurrthadest Sar West

in history was performed by the group. The mountain has ice slopes up to 60 degrees. After the tour to 5.366 m the third experiment was performed. Then another first ascent in history of the Sunrise Peak east was made. The next two experiments were performed at 3.100 m and 4.480 m respectively. 4.480 m is already the altitude of the Whitehorn base camp (BC). Finally the first ascent in history was made to the Whitehorn which has an altitude of 6.304 m. The route contains ice slopes of up to 75 degrees. The last experiment was again performed in Shimshal at 3.100 m.

Fig.8: Markus Tannheimer during the experiment in front of the Chukurthadest Sar.

In all three locations first the reaction time task was performed. Then the SpO2 was determined and the LLS was filled out. At Kilimanjaro additionally the d2- test was performed.

DATA ANALYSIS

EEG The bandpower in the alpha (8-13 Hz) and in the beta (14–30 Hz) frequency ranges will be calculated for both electrode positions. This allows to investigate the mental load during the reaction time paradigm with the finger movement. The analysis will show the changes of the ERD and ERS with the ascent and acclimatization.

ECG The ECG recordings will be used to calculate the QRS complexes. Then the time interval from the R-peak of one QRS complex to that of the previous is calculated and termed the RR interval. The variations in the heart rate are quantified by HRV analysis in the time domain and the frequency domain.

(i) HRV Time Domain The simplest time-domain variable is the mean heart rate (MeanHR) in bpm. More complex measurements can be divided into two classes: (i) those derived from direct measurements of the heart rate: the SDNN index is the mean of the standard deviation of 30-second segments and describes the variability due to cycles longer than 30 seconds; (ii) those derived from the RR interval difference between consecutive intervals: the NN50 is the number of intervals which differ by more than 50 ms from the previous interval. pNN50 is the NN50 divided by the total number of NN intervals. RMSSD is calculated as the square root of the mean squared difference of successive RR intervals.

(ii) HRV Frequency Domain Power spectrum (PS) analysis provides the basic information on how power is distributed as a function of frequency. Three main spectral components can be distinguished: (i) very low frequency (VLF): < 0,04 Hz, (ii) low frequency (LF): 0,04–0,15 Hz and (iii) high frequency (HF): 0,15–0,4 Hz. The unit of these parameters is ms2. For normalization the LF (LFnorm) and HF (HFnorm) components are divided by the total power minus the VLF component. This minimizes the effect of the total power on LF and HF. The LF/HF ratio describes the balanced behavior of both components.

Oxygen saturation and LLS The oxygen saturation and the LLS can be compared for the different measurement points.

d2-test The d2-test gives several parameters (TN, E %, CP, E, TN-E and FR) which can be used to compare the different measurement points. The parameters can also be compared to group studies performed with large populations in different countries at sea level.

Correlation analysis This analysis allows to investigate the correlation of the different parameters to each other. Therefore the HR, HRV, bandpower in the alpha and beta region, oxygen saturation, LLS and the d2 test parameters can be correlated.

Reaction time The individual reaction time from the light flash until the button press can be analyzed at the different altitude levels.

DISCUSSION

During the study data was acquired at very high altitudes in real life conditions and in simulated conditions. The experiment in the hypobaric chamber allows to quantify the effects of a fast ascent to 4.000 m. The Kilimanjaro study investigates the ascent to 3.570 m within 3 days and to 4.633 m within 8 days. Additionally it will show the effects of the acclimatization tour to Mt. Meru (4.566 m) before reaching Mt. Kilimanjaro. The expedition to Shimshal enables the comparison at altitudes of 2.500 m, 3.100 m and 4.480 m. The measurements at 3.100 m were several times repeated to investigate the effect of climbs to altitudes above 4.000 m during the acclimatization process.

It is the first time that EEG and ECG activity was acquired during a well defined work load at altitudes above 2,700 m. This will allow to investigate the changes of heart-rate, heart-rate variability including sympathetic and parasympathetic activity at different altitudes. The EEG parameters ERD and ERS in the alpha and beta ranges allow to quantify changes of the central nervous system.

Of special interest is the relation of the EEG and ECG changes to the LLS and to the oxygen saturation. LLS and oxygen saturation are widely used methods

to describe high altitude effects. This will allow to investigate the EEG and ECG changes of subjects with high LLS and low oxygen saturation.

It was also the first time that the d2-test was used at altitude. The concentration performance might be an easy to determine and important parameter for AMS scoring.

ACKNOWLEDGEMENT

We are grateful to Eva Hornbachner, Elisabeth Pranzl, Nicole Guger-Ulrich, Mike Gruber, Hannes Stiebitzhofer, Robert Leeb, and Herbert Ramoser for their participation in the Kilimanjaro study, to Matthias Braun, Matthias Robl and Alexandra Robl for their participation in the Shimshal expedition and to all the participants of the German Bundeswehr in the study in Königsbrück, Germany. We are also grateful to Dr. Welsch and to the Bundeswehr for the possibility to perform the study in the modern high altitude chamber in Königsbrück in Germany. The project was partially funded by the European Union project PRESENCCIA and the FFG in Austria.

REFERENCES

(1) Guger, C., Domej, W., Lindner, G., Pfurtscheller, K., Pfurtscheller, G.,

Edlinger, G.: Effects of a fast cable car ascent to an altitude of 2.700 meters on EEG and ECG. Neurosci. Lett. 377 (1), 53–58 (2005);

(2) Bernardi, L., Passino, C., Wilmerding, V., Dallam, G.M., Parker, D.L.,

Robergs, R.A., Appenzeller, O.: Breathing patterns and cardiovascular autonomic modulation during hypoxia induced by simulated altitude. J.

Hypertens. 19, 947–958 (2001)

(3) Bernardi, L., Passino, C., Spadacini, G., Calciati, A., Robergs, R., Greene, R., Martignonni, E., Anand, I., Appenzeller, O.: Cardiovascular autonomic modulation and activity of carotid baroreceptors at altitude. Clin.

Sci. (Lond.) 95, 565–573 (1998)

(4) Farinelli, C.C., Kayser, B., Binzoni, T., Ceretelli, P., Girardier, L.: Autonomic nervous control of heart rate at altitude (5.050 m). Eur. J. Appl.

Physiol. 69 (6), 502–507 (1994)

(5) Hughson, R.L., Yamamoto, Y., McCullough, R.E., Sutton, J.R., Reeves,

J.T.: Sympathetic and parasympathetic indicators of heart rate control at altitude studied by spectral analysis. J. Appl. Physiol. 77(6), 2537–2542 (1994)

(6) Kanai, M., Nishihara, F., Shiga, T., Shimada, H., Saito, S.: Alterations in autonomic nervous control of heart rate among tourists at 2.700 and 3.700 m above sea level. Wilderness Environ. Med. 12 (1), 8–12 (2001)

(7) Pfurtscheller, G., Stancak, A. Jr., Neuper, C.: Post-movement beta synchronization. A correlate of an idling motor area?, Electroencephalogr.

Clin. Neurophysiol. 98 (4), 281–293 (1996)

(8) Schnitzler, A., Salenius, S., Salmelin, R., Jousmäki, V., Hari, R.: Involvement of primary motor cortex in motor imagery: a neuromagnetic study.

Neuroimage 6 (3), 201–208 (1997)

(9) Boiten, F., Sergeant, J., Geuze, R.: Event-related desynchronization: the effects of energetic and computational demands. Electroencephalogr. Clin.

Neurophysiol. 82 (4), 302–309 (1992)

(10) Klimesch, W., Pfurtscheller, G., Schimke, H.: Pre- and poststimulus processes in category judgement tasks as measured by event-related desynchronization (ERD). Int. J. Psychophysiol. 6, 186–203 (1992)

(11) Pfurtscheller, G., Lopes da Silva, F.H.: Event-related EEG/MEG synchronization and desynchronization: basic principles, Clin. Neurophysiol. 110 (11), 1842–1857 (1999)

(12) Chen, R., Yaseen, Z., Cohen, L.G., Hallett, M.: The time course of corticospinal excitability in reaction time and self-paced movements. Ann.

Neurol. 44 (3), 317–325 (1998)

(13 Pfurtscheller, G., Woertz, M., Müller, G.,Wriessnegger, S., Pfurtscheller,

K.: Contrasting behavior of beta event-related synchronization and somatosensory evoked potential after median nerve stimulation during finger manipulation in man, Neurosc. Lett. 323 (2), 113–116 (2002)

(14) Salmelin, R., Hamalainen, M., Kajola, M., Hari, R.: Functional segregation of movement-related rhythmic activity in the human brain. Neuroimage 2 (4), 237–243 (1995)

(15) Grafton, S.T, Woods, R.P., Mazziotta, J.C., Phelps, M.E.: Somatotopic mapping of the primary motor cortex in humans: activation studies with cerebral blood flow and positron emission tomography. J. Neurophysiol. 66 (3), 735–742 (1991)

(16) Townes, B.D., Hornbein, T.F., Schoene, R.B., Sarnquist, F.H., Grant, I.:

Human cerebral function at extreme altitude. In: J.B. West, S. Lahiri (Ed.),

High Altitude in Man. American Physiological Society, Bethesda, MD, pp. 31–36 (1984)

(17) Defebvre, L., Bourriez, J.L., Destee, A., Guieu, J.D.: Movement-related desynchronization pattern preceding voluntary movement in untreated

Parkinson’s disease. J. Neurol. Neurosurg. Psychiatry 60 (3), 307–312 (1996)

(18) Neubauer, A.C., Sange, G., Pfurtscheller, G.: Psychometric intelligence and event-related desynchronization during performance of a letter matching task. In: G. Pfurtscheller, F.H. Lopes da Silva (Ed.), Event-related desynchronization and related oscillatory phenomena of the brain. Handbook of electroencephalography and clinical neurophysiology, vol. 6, Elsevier, Amsterdam, pp. 219–231 (1999)

(19) Sterman, M.B., Kaiser, D.A., Veigel, B.: Spectral analysis of event-related EEG responses during short-term memory performance. Brain

Topogr. 9, 21–30 (1996)

(20) Hornbein, T.F.: The high-altitude brain, J. Exp. Biol. 204 (Pt 18), 3129–3132 (2001)

(21) Brickenkamp, R., Zillmer, E.: The d2-Test of Attention. (1st US ed.). Seattle,

WA: Hogrefe & Huber Publishers (1998)

(22) Roach, R., et al.: The Lake Louise acute mountain sickness scoring system, in Hypoxia and mountain medicine: proceeding of the international hypoxia symposium., J. Sutton, C. Houston, and G. Coates, Editors., Queen

City Printers: Burlington VT. p. 272–274 (1993)