- Email: [email protected]
Attempt to record electrical cerebral activity
Brain Research Bulletin 77 (2008) 323–326
Contents lists available at ScienceDirect
Brain Research Bulletin journal homepage: www.elsevier.com/locate/brainresbull
Commentary
Attempt to record electrical cerebral activity Pierrre Pouget Center for Integrative & Cognitive Neuroscience, Vanderbilt Vision Research Center, Department of Psychology, Vanderbilt University, Nashville, TN 37203, United States
a r t i c l e
i n f o
Article history: Received 23 May 2008 Accepted 26 August 2008 Available online 24 September 2008 Keywords: EEG History Neuroscience Cerebral activity 20th century
Commentary on “Ein Vesuch der Registrierung der elektrischen Gehirnerscheinungen” In July 1979 a special issue of the Journal “Neirofiziologiya” was devoted to the centenary of the birth of an eminent Russian scientist, the Professor Vladimir Vladimirovich Pravdich-Neminski. Vladimir Pravdich-Neminski was a representative of the Kazan’ and Kiev Physiological Schools. The range of his scientific interests was very wide. However, his attention was concentrated mainly on electrical phenomena in living organisms, the first information of which was to be found in publications by R. Caton (1875). Although the importance of Caton’s discovery cannot be overemphasized it was almost completely ignored by the English-speaking physiologists of his time. Fortunately, Caton’s work did capture the attention of the growing neurophysiology community in Eastern Europe. In, 1883, Ernst Fleischl von Marxow (1846–1892), an Austrain physiologist wrote to the university of Vienne his famous sealed letters. In these letters he described visual evoked potential in response to light recorded from the skull, showing that the electrical potential can be measured not only from exposed cortex. The series of two letters were opened at a session of the imperial academy of science on November 1890. That same year in Poland, Adolf Beck (1863–1942) was successful in replicating Caton’s experiments. This is in this scientific context that in 1908, V.V. PravdichNeminski started to work in the physiology laboratory of Kiev University. He quickly undertook an intensive study of electrical
E-mail address: [email protected]. 0361-9230/$ – see front matter © 2008 Elsevier Inc. All rights reserved. doi:10.1016/j.brainresbull.2008.08.024
activity of the central nervous system of animals in particular dogs and he started to use a low-inertia string galvanometer, an apparatus for making string and a double commutator switch of his own invention. If Caton, Fleischl von Marxow and others physiologists did observe the electrical activity of cortex before Vladimir Neminski; he was the first record of these electrical activities. The work of Neminski is certainly one among many other important steps that did prepare the way application of the method of recording brain activity to human has been developed in the early part of the 20th century. In fact, when Berger (1929) began his investigation of the human ECG (EEG), he confirmed the correctness of the classification of ECG rhythms suggested by Pravdich-Neminskii in 1913 in his report. Although Berger replaced the Roman numerals denoting the categories of the ECG waves by the Greek letters “alpha,” “beta,” and so on. During the last years of his life, Pravdich-Neminskii also suggested a new method of judging the electrical activity of the brain in phases of diastole and systole, that he named tonoelectrocerebrography (1951). In 1958, a collection of selected papers from his works was entitled “Electrocerebrography, Electromyography, and the Importance of Ammonium Ions in the Vital Processes of the Organism”. These works reflect the many-sidedness of his scientific activity and establishes beyond question his role in a number of discoveries and investigations of the functions of the nervous system. Among other findings, he showed that ammonia is liberated from nerve tissue at the cathode of a stimulating current and he also obtained some original data on electromyography. Besides his scientific research, V.V. Pravdich-Neminskii devoted much time teaching and lecturing. The scientific studies of Pravdich-Neminski’s are known the world over, although it has to
324
P. Pouget / Brain Research Bulletin 77 (2008) 323–326
Fig. 1. On the photographs, numbers indicate the following: I the time line in steps of 200 ms; II the trace of cerebral pulsation; III the movement of the trace; IV the trace of blood pressure in the arteria femoralis; V the moment of N. ischiadicus stimulation. The ischiadic nerve was stimulated with an intermittent current at a coil distance of 19 cm. Positive fluctuations of the trace were noted.
be mentioned that priority for results obtained by this scientist is not always mentioned in all publications. Today for all these reasons, his work is not only of historical, but also of great scientific interest. By translating his original article entitled “Ein Vesuch der Registrierung der elektrischen Gehirnerscheinungen” published in 1913; I simply wanted to share the opportunity to non-German readers to access and to appreciate the work and challenges facing the pioneers of the field of neuroscience. The only modification of the photographs was made to increase their contrast. All figures are original figures from the manuscript published in 1913.
3.
4.
Translation of the article “Ein Vesuch der Registrierung der elektrischen Gehirnerscheinungen” As we know, in 1875, Caton, established the fundaments to study the correlates between the physiological changes in cortex conditions and its variations of electrical activity. Later, Wwdenski, but also Gotch and Horsely (1888), Beck (1896, 1904–1905), Fleisch v. Marxow (1890–1883), Danilewski (1891), Beck and Cybylski (1892), Larionow (1899), Tschirjew (1904), Kaufman (1912) conducted studies on the currents measured in the cortex of warmblooded animals. In fact, most of the researchers at that time saw in these changes of electrical activity of the cortex one expression of the internal physiology of the mind. It is particularly surprising that despite these numerous observations any recordings have been realized yet. These recordings would have been possible using the existing technology of a regular capillary electrometer. This possibility to record electrical activity of the cortex has been offered to us by the honored professor Tshagowetz, who did permit us to use a large Einthoven galvanometer. The photographic recordings of the electrical activity of the brain were realized during periods of spontaneous activity or after electrical stimulations of the ischiadic nerve. During the course of some of the experiments, the oscillatory activity of the brain was recorded simultaneously to the blood pressure measured in the femoral artery. I do present here the results from experiments realized on nine dogs. Some of the results presented here are in accordance with those of previous authors: 1. Spontaneous fluctuations of electrical current can be observed simply after making contact with the bones of the skull. Specifically the recordings were made on top of the motor and visual areas of the brain. The frequency of the oscillatory activity for the two regions was calculated to be between 12 and 14 cycles/s. At the surface of the cortex (and on top of the dura) the spontaneous fluctuations considerably vary from 12 to 20 to 35-cycle/s. During the period of suffocation, after the artificial respiratory system was stopped, the frequency of the larger fluctuations is reduced and varies around 4–7 cycles/s. 2. The contra-lateral stimulation of the ischiadic nerve produced a positive oscillation but also a negative oscillation in the occipital region depending on the animals. The polarity of the stimulated activity varied as function of the polarity of the current but did not produce an alteration of the oscillations. When
5.
6.
there are large spontaneous oscillations before the stimulation, the dampening effect of the stimulus can be clearly observed. The complete stop of the oscillatory activity was not commonly observed (Figs. 1 and 3). The positive fluctuations (and respectively the negative fluctuation) sometimes are preceded by negative (and respectively positive) fluctuations, which then together form biphasic waves. During repeated stimulation of the ischiadic nerve using an alternative current, we can observe that the two consecutive stimulations generated a second fluctuation of the current in the brain that has smaller amplitude and shorter duration compared to the first fluctuation. In this particular example, the response latency to the stimulation was faster in particular for the frontal response (0.15 s compared to 0.075 s). This effect was particularly clear for coils being placed 3 cm apart. During simultaneous recordings of the brain’s pulsation, the oscillatory electrical activity of the brain, and the arterial pressure it was not observed that the negative (or positive, respectively) fluctuation of the brain’s currents, which were triggered by the stimulation of the ischiadic nerve with a relatively short latency (less than a second), was synchronous to any change in cerebral pulsation or amplitude of blood pressure (Fig. 2). The latter changes were delayed. In some of these cases, the relation between the largest artery pressure and increased pulsation of the brain and the preceding increase in amplitude of the oscillatory activity of the brain, while showing reduced frequency, was clearly visible. In these cases, the oscillations appeared as a second, later response to the stimulation of the ischiadic nerve where the negative deflections were observed first. During the beginning of suffocation of the animal the stimulation of the ischiadic nerve produced a reduced oscillatory response compared to the one observed later. At the same time, it was important to note that the changes in fluctuation were observed in the opposite direction from the one observed ear-
Fig. 2. Coil distance 10 cm.
P. Pouget / Brain Research Bulletin 77 (2008) 323–326
325
Fig. 3. (a, b and c) Stimulation of the ischiadic nerve in three stages of suffocation: at the beginning, i.e. directly after stopping the artificial respiration. Positive deflections of small amplitude. Coil distance 8 cm. After a few (3–4) minutes the positive deflections are replaced by a negative fluctuation. Afterwards a reduction of spontaneous fluctuations is seen. Coil distance as before. Towards the end, i.e. shortly before the disappearance of the large fluctuations. The negative fluctuations show larger amplitude in comparison to the positive fluctuations at the beginning of the suffocation. Coil distance as above.
lier. For example, the small positive oscillations at the beginning of the suffocation (Fig. 3a) were replaced by a major series of negative deflections during later stages. 7. Stopping the artificial respiratory system on animal that previously did receive curare did not produce a noticeable and immediate effect (between 0.5 and 1 min) on the spontaneous oscillatory activity of the brain (Fig. 4a). Later considerable oscillations were observed. Grouped or single large fluctuations appeared, on which the previous small oscillations are superimposed (Fig. 4b and c). After the stop of these long and irregular traces, the reduced oscillatory activity noted previously is no longer present (Fig. 4c). The tracing is linear as shown in Fig. 4d and the trace only shows a slight and moderate change in slope in the direction of the first phase of increase or decrease of the initial current. Later, when large but rhythmic oscillations reappeared which were oriented in a different direction, no more superimposed fluctuations were observed. For all the animals the manifestation of all the oscillatory activities of the brain disappears 4–6 min after the terminal phase of the suffocation (which was elicited at the end of long hours of recording). The last oscillations are present for all animals but differ from one animal to another. Notable is the increase in amplitude of oscillations mentioned above while showing reduced frequency. This increase in amplitude appears in coincidence with the defecation of the animals. 8. The simultaneous recordings of the electrical fluctuations of the brain and of the blood pressure show slow increases of the isolated pulsation (as illustrated photogram NR4) which, have their highest amplitude at the appearance of the largest oscillatory activity of the brain. But even after the disappearance of the oscillatory activity (∼5 min) the amplitude of the pulsations remain extensive and can even be observed for several more minutes. The tracing of the oscillatory activity of the
brain then shows a slope change towards the direction of the reduction, and later towards an increase from the initial resting current. The slope of the oscillatory activity changes direction repeatedly. 9. We’ve already noticed during the stimulation of the ischiadic nerve generating oscillatory activities, that the cessation of the stimulation was not immediately followed by a return to the initial baseline of activity. The direction of these negative deviations was opposite to the fluctuations seen at the beginning of the suffocation (cf. point 6). It should be noted here that a slow drop of activity was observed at this time, which could not be reproduced after 1 or 2 min. The inversion of the polarity of the injected current into the nerve did not produce an inverse activation of the oscillatory current. 10. The potential difference observed between signals recorded above the motor or the visual area of the same hemisphere did not perdure during the entire recording. Its amplitude varied around 8 mV, or more often around 2 mV. The anterior part of the lobe being usually observed as more positive. The size of the spontaneous oscillatory activity did not exceed 1 mV. The electrical currents in cortex were recorded using nonpolarized electrodes (with long cotton sheaths) that were inserted in the brain through craniotomies above the left-hemispheric motor and visual area. Animals were paralyzed with curare. The ischiadic nerve was stimulated contro-laterally using an induction system (one accumulator in the primary circuit) after du BoisRaymond. In most cases the two inducting coils were not placed closer than 7 cm to each other. The gold thread of the galvanometer had a resistance of 3340 . There were 2, 4, or 5 accumulators in the electromagnet of the galvanometer. They provided an output current of 1–4 to 3–5 A.
326
P. Pouget / Brain Research Bulletin 77 (2008) 323–326
Fig. 4. (a, b and c) The succession of effects of suffocation on one paper strip (cut in three parts a, b and c). The photogram is downsized. Sensitivity of the trace: 1 × 10−8 . Photograph a, b, c, and d show parts of the photograph, in its actual size.
The sensitivity of the trace depended on these parameters. During the suffocation, during which large fluctuations in the trace were observed, the sensitivity never went over 1–10/s. The craniotomy for the recording of cerebral pulsations was between 2 and 4 cm in diameter and was placed just between the craniotomies over motor and visual cortex. The recordings of the pulsatory activity of the cortex were realized using a cylinder of 4.2 cm length (Plethysmograph) with a diameter of 1.6 cm. On the lower part of the cylinder a small windbag was attached and a small amount of physiologic serum (sodium chloride solution) was placed inside the cylinder. The apparatus was attached to the craniotomy using a tightly fitting rubber seal. Air fluctuations in the cylinder were recorded using a thin 80 cm long
rubber tube of 6 mm diameter, which, was connected to a piston tape system. The arterial pressure was measured in the femoral artery using a spring-manometer after Fick or Frey. To record the time of the stimulation the electromagnetic signal (Petzold) was used. The time was recorded using a Jacquet’s counter running at 250 Hz. All mechanical movements were registered with an Edelmann’s recording system using a paper strip of 75 m length. The artificial respiration was stopped during the recording of the cortical activity in order to eliminate the mechanical noise produced by the respiratory system. Such a brief interruption of the breathing had no impact on the recorded traces, as long as the lungs were well ventilated beforehand.
Contents lists available at ScienceDirect
Brain Research Bulletin journal homepage: www.elsevier.com/locate/brainresbull
Commentary
Attempt to record electrical cerebral activity Pierrre Pouget Center for Integrative & Cognitive Neuroscience, Vanderbilt Vision Research Center, Department of Psychology, Vanderbilt University, Nashville, TN 37203, United States
a r t i c l e
i n f o
Article history: Received 23 May 2008 Accepted 26 August 2008 Available online 24 September 2008 Keywords: EEG History Neuroscience Cerebral activity 20th century
Commentary on “Ein Vesuch der Registrierung der elektrischen Gehirnerscheinungen” In July 1979 a special issue of the Journal “Neirofiziologiya” was devoted to the centenary of the birth of an eminent Russian scientist, the Professor Vladimir Vladimirovich Pravdich-Neminski. Vladimir Pravdich-Neminski was a representative of the Kazan’ and Kiev Physiological Schools. The range of his scientific interests was very wide. However, his attention was concentrated mainly on electrical phenomena in living organisms, the first information of which was to be found in publications by R. Caton (1875). Although the importance of Caton’s discovery cannot be overemphasized it was almost completely ignored by the English-speaking physiologists of his time. Fortunately, Caton’s work did capture the attention of the growing neurophysiology community in Eastern Europe. In, 1883, Ernst Fleischl von Marxow (1846–1892), an Austrain physiologist wrote to the university of Vienne his famous sealed letters. In these letters he described visual evoked potential in response to light recorded from the skull, showing that the electrical potential can be measured not only from exposed cortex. The series of two letters were opened at a session of the imperial academy of science on November 1890. That same year in Poland, Adolf Beck (1863–1942) was successful in replicating Caton’s experiments. This is in this scientific context that in 1908, V.V. PravdichNeminski started to work in the physiology laboratory of Kiev University. He quickly undertook an intensive study of electrical
E-mail address: [email protected]. 0361-9230/$ – see front matter © 2008 Elsevier Inc. All rights reserved. doi:10.1016/j.brainresbull.2008.08.024
activity of the central nervous system of animals in particular dogs and he started to use a low-inertia string galvanometer, an apparatus for making string and a double commutator switch of his own invention. If Caton, Fleischl von Marxow and others physiologists did observe the electrical activity of cortex before Vladimir Neminski; he was the first record of these electrical activities. The work of Neminski is certainly one among many other important steps that did prepare the way application of the method of recording brain activity to human has been developed in the early part of the 20th century. In fact, when Berger (1929) began his investigation of the human ECG (EEG), he confirmed the correctness of the classification of ECG rhythms suggested by Pravdich-Neminskii in 1913 in his report. Although Berger replaced the Roman numerals denoting the categories of the ECG waves by the Greek letters “alpha,” “beta,” and so on. During the last years of his life, Pravdich-Neminskii also suggested a new method of judging the electrical activity of the brain in phases of diastole and systole, that he named tonoelectrocerebrography (1951). In 1958, a collection of selected papers from his works was entitled “Electrocerebrography, Electromyography, and the Importance of Ammonium Ions in the Vital Processes of the Organism”. These works reflect the many-sidedness of his scientific activity and establishes beyond question his role in a number of discoveries and investigations of the functions of the nervous system. Among other findings, he showed that ammonia is liberated from nerve tissue at the cathode of a stimulating current and he also obtained some original data on electromyography. Besides his scientific research, V.V. Pravdich-Neminskii devoted much time teaching and lecturing. The scientific studies of Pravdich-Neminski’s are known the world over, although it has to
324
P. Pouget / Brain Research Bulletin 77 (2008) 323–326
Fig. 1. On the photographs, numbers indicate the following: I the time line in steps of 200 ms; II the trace of cerebral pulsation; III the movement of the trace; IV the trace of blood pressure in the arteria femoralis; V the moment of N. ischiadicus stimulation. The ischiadic nerve was stimulated with an intermittent current at a coil distance of 19 cm. Positive fluctuations of the trace were noted.
be mentioned that priority for results obtained by this scientist is not always mentioned in all publications. Today for all these reasons, his work is not only of historical, but also of great scientific interest. By translating his original article entitled “Ein Vesuch der Registrierung der elektrischen Gehirnerscheinungen” published in 1913; I simply wanted to share the opportunity to non-German readers to access and to appreciate the work and challenges facing the pioneers of the field of neuroscience. The only modification of the photographs was made to increase their contrast. All figures are original figures from the manuscript published in 1913.
3.
4.
Translation of the article “Ein Vesuch der Registrierung der elektrischen Gehirnerscheinungen” As we know, in 1875, Caton, established the fundaments to study the correlates between the physiological changes in cortex conditions and its variations of electrical activity. Later, Wwdenski, but also Gotch and Horsely (1888), Beck (1896, 1904–1905), Fleisch v. Marxow (1890–1883), Danilewski (1891), Beck and Cybylski (1892), Larionow (1899), Tschirjew (1904), Kaufman (1912) conducted studies on the currents measured in the cortex of warmblooded animals. In fact, most of the researchers at that time saw in these changes of electrical activity of the cortex one expression of the internal physiology of the mind. It is particularly surprising that despite these numerous observations any recordings have been realized yet. These recordings would have been possible using the existing technology of a regular capillary electrometer. This possibility to record electrical activity of the cortex has been offered to us by the honored professor Tshagowetz, who did permit us to use a large Einthoven galvanometer. The photographic recordings of the electrical activity of the brain were realized during periods of spontaneous activity or after electrical stimulations of the ischiadic nerve. During the course of some of the experiments, the oscillatory activity of the brain was recorded simultaneously to the blood pressure measured in the femoral artery. I do present here the results from experiments realized on nine dogs. Some of the results presented here are in accordance with those of previous authors: 1. Spontaneous fluctuations of electrical current can be observed simply after making contact with the bones of the skull. Specifically the recordings were made on top of the motor and visual areas of the brain. The frequency of the oscillatory activity for the two regions was calculated to be between 12 and 14 cycles/s. At the surface of the cortex (and on top of the dura) the spontaneous fluctuations considerably vary from 12 to 20 to 35-cycle/s. During the period of suffocation, after the artificial respiratory system was stopped, the frequency of the larger fluctuations is reduced and varies around 4–7 cycles/s. 2. The contra-lateral stimulation of the ischiadic nerve produced a positive oscillation but also a negative oscillation in the occipital region depending on the animals. The polarity of the stimulated activity varied as function of the polarity of the current but did not produce an alteration of the oscillations. When
5.
6.
there are large spontaneous oscillations before the stimulation, the dampening effect of the stimulus can be clearly observed. The complete stop of the oscillatory activity was not commonly observed (Figs. 1 and 3). The positive fluctuations (and respectively the negative fluctuation) sometimes are preceded by negative (and respectively positive) fluctuations, which then together form biphasic waves. During repeated stimulation of the ischiadic nerve using an alternative current, we can observe that the two consecutive stimulations generated a second fluctuation of the current in the brain that has smaller amplitude and shorter duration compared to the first fluctuation. In this particular example, the response latency to the stimulation was faster in particular for the frontal response (0.15 s compared to 0.075 s). This effect was particularly clear for coils being placed 3 cm apart. During simultaneous recordings of the brain’s pulsation, the oscillatory electrical activity of the brain, and the arterial pressure it was not observed that the negative (or positive, respectively) fluctuation of the brain’s currents, which were triggered by the stimulation of the ischiadic nerve with a relatively short latency (less than a second), was synchronous to any change in cerebral pulsation or amplitude of blood pressure (Fig. 2). The latter changes were delayed. In some of these cases, the relation between the largest artery pressure and increased pulsation of the brain and the preceding increase in amplitude of the oscillatory activity of the brain, while showing reduced frequency, was clearly visible. In these cases, the oscillations appeared as a second, later response to the stimulation of the ischiadic nerve where the negative deflections were observed first. During the beginning of suffocation of the animal the stimulation of the ischiadic nerve produced a reduced oscillatory response compared to the one observed later. At the same time, it was important to note that the changes in fluctuation were observed in the opposite direction from the one observed ear-
Fig. 2. Coil distance 10 cm.
P. Pouget / Brain Research Bulletin 77 (2008) 323–326
325
Fig. 3. (a, b and c) Stimulation of the ischiadic nerve in three stages of suffocation: at the beginning, i.e. directly after stopping the artificial respiration. Positive deflections of small amplitude. Coil distance 8 cm. After a few (3–4) minutes the positive deflections are replaced by a negative fluctuation. Afterwards a reduction of spontaneous fluctuations is seen. Coil distance as before. Towards the end, i.e. shortly before the disappearance of the large fluctuations. The negative fluctuations show larger amplitude in comparison to the positive fluctuations at the beginning of the suffocation. Coil distance as above.
lier. For example, the small positive oscillations at the beginning of the suffocation (Fig. 3a) were replaced by a major series of negative deflections during later stages. 7. Stopping the artificial respiratory system on animal that previously did receive curare did not produce a noticeable and immediate effect (between 0.5 and 1 min) on the spontaneous oscillatory activity of the brain (Fig. 4a). Later considerable oscillations were observed. Grouped or single large fluctuations appeared, on which the previous small oscillations are superimposed (Fig. 4b and c). After the stop of these long and irregular traces, the reduced oscillatory activity noted previously is no longer present (Fig. 4c). The tracing is linear as shown in Fig. 4d and the trace only shows a slight and moderate change in slope in the direction of the first phase of increase or decrease of the initial current. Later, when large but rhythmic oscillations reappeared which were oriented in a different direction, no more superimposed fluctuations were observed. For all the animals the manifestation of all the oscillatory activities of the brain disappears 4–6 min after the terminal phase of the suffocation (which was elicited at the end of long hours of recording). The last oscillations are present for all animals but differ from one animal to another. Notable is the increase in amplitude of oscillations mentioned above while showing reduced frequency. This increase in amplitude appears in coincidence with the defecation of the animals. 8. The simultaneous recordings of the electrical fluctuations of the brain and of the blood pressure show slow increases of the isolated pulsation (as illustrated photogram NR4) which, have their highest amplitude at the appearance of the largest oscillatory activity of the brain. But even after the disappearance of the oscillatory activity (∼5 min) the amplitude of the pulsations remain extensive and can even be observed for several more minutes. The tracing of the oscillatory activity of the
brain then shows a slope change towards the direction of the reduction, and later towards an increase from the initial resting current. The slope of the oscillatory activity changes direction repeatedly. 9. We’ve already noticed during the stimulation of the ischiadic nerve generating oscillatory activities, that the cessation of the stimulation was not immediately followed by a return to the initial baseline of activity. The direction of these negative deviations was opposite to the fluctuations seen at the beginning of the suffocation (cf. point 6). It should be noted here that a slow drop of activity was observed at this time, which could not be reproduced after 1 or 2 min. The inversion of the polarity of the injected current into the nerve did not produce an inverse activation of the oscillatory current. 10. The potential difference observed between signals recorded above the motor or the visual area of the same hemisphere did not perdure during the entire recording. Its amplitude varied around 8 mV, or more often around 2 mV. The anterior part of the lobe being usually observed as more positive. The size of the spontaneous oscillatory activity did not exceed 1 mV. The electrical currents in cortex were recorded using nonpolarized electrodes (with long cotton sheaths) that were inserted in the brain through craniotomies above the left-hemispheric motor and visual area. Animals were paralyzed with curare. The ischiadic nerve was stimulated contro-laterally using an induction system (one accumulator in the primary circuit) after du BoisRaymond. In most cases the two inducting coils were not placed closer than 7 cm to each other. The gold thread of the galvanometer had a resistance of 3340 . There were 2, 4, or 5 accumulators in the electromagnet of the galvanometer. They provided an output current of 1–4 to 3–5 A.
326
P. Pouget / Brain Research Bulletin 77 (2008) 323–326
Fig. 4. (a, b and c) The succession of effects of suffocation on one paper strip (cut in three parts a, b and c). The photogram is downsized. Sensitivity of the trace: 1 × 10−8 . Photograph a, b, c, and d show parts of the photograph, in its actual size.
The sensitivity of the trace depended on these parameters. During the suffocation, during which large fluctuations in the trace were observed, the sensitivity never went over 1–10/s. The craniotomy for the recording of cerebral pulsations was between 2 and 4 cm in diameter and was placed just between the craniotomies over motor and visual cortex. The recordings of the pulsatory activity of the cortex were realized using a cylinder of 4.2 cm length (Plethysmograph) with a diameter of 1.6 cm. On the lower part of the cylinder a small windbag was attached and a small amount of physiologic serum (sodium chloride solution) was placed inside the cylinder. The apparatus was attached to the craniotomy using a tightly fitting rubber seal. Air fluctuations in the cylinder were recorded using a thin 80 cm long
rubber tube of 6 mm diameter, which, was connected to a piston tape system. The arterial pressure was measured in the femoral artery using a spring-manometer after Fick or Frey. To record the time of the stimulation the electromagnetic signal (Petzold) was used. The time was recorded using a Jacquet’s counter running at 250 Hz. All mechanical movements were registered with an Edelmann’s recording system using a paper strip of 75 m length. The artificial respiration was stopped during the recording of the cortical activity in order to eliminate the mechanical noise produced by the respiratory system. Such a brief interruption of the breathing had no impact on the recorded traces, as long as the lungs were well ventilated beforehand.