Remote detection of human electroencephalograms using ultrahigh input
impedance electric potential sensors
C. J. Harland, T. D. Clark,
and R. J. Prance
Centre for Physical Electronics, School of Engineering and Information Technology, University of Sussex,
Brighton, Sussex, BN1 9QT, United Kingdom
~Received 29 July 2002; accepted 4 September 2002!
In this letter, we demonstrate the use of very high performance, ultrahigh impedance, electric potential probes in the detection of electrical activity in the brain. We show that these sensors, requiring no electrical or physical contact with the body, can be used to monitor the human electroencephalogram ~EEG! revealing, as examples, the a and b rhythms and the a blocking phenomenon. We suggest that the advantages offered by these sensors compared with the currently
used contact ~Ag/AgCl! electrodes may act to stimulate new developments in multichannel EEG monitoring and in real-time electrical imaging of the brain. © 2002 American Institute of Physics.
Electrical activity in the human brain was ﬁrst reported
in 1929 by Hans Berger
1. who recorded the electrical potential variations from the scalp and introduced the term electroencephalogram ~EEG! to describe the graphical time domain signal resulting from these changes in electrical potential. Since Berger’s seminal work, great advances have been made in the ampliﬁcation, electronic processing, and real-time display of EEG signals. These advances, aided by the recent introduction of computer-based techniques for signal recognition and manipulation, have led to the EEG becoming an invaluable tool in the diagnosis of many neurological illnesses.
2. Despite these advances in signal quality,
the practical convenience of collecting EEGs is still limited
by the traditional sensor techniques used to detect brain electrical activity. In special cases, this activity has been recorded by means of electrodes in contact with the surface of
the exposed brain ~i.e., with a portion of the skull bone removed and by using depth electrodes where subcutaneous
needles are inserted into the exposed brain tissue!. However,
for conventional clinical analysis, the EEG is recorded using
electrodes that are in real charge current contact with the
scalp tissue. In practice, this is provided by an Ag/AgCl electrode used in conjunction with an electrolytic paste. These
act together to form an electrical transducer to convert the
ionic current ﬂow in the skin into an electron ﬂow which can
then be detected by an electronic ampliﬁer.
3. In this, the electrolyte performs the dual role of a conducting paste and a glue to anchor the electrode to the scalp. In preparation, the scalp has to be cleaned which usually involves the shaving of hair and abrasion of the skin—a process that is both uncomfortable to the subject and which also has a tendency to lead to unreliable electrical contact.
In the past these problems have been circumvented, at
least at the research level, by the use of superconducting
quantum interference device ~SQUID! magnetometers
4. to detect the magnetic rather than the electric component of the ﬁelds generated by the ﬂow of currents in the brain. While SQUID magnetometers can have quite sufﬁcient sensitivity to follow these magnetic signals ~magnetoencephalograms—
MEGs!—and remotely, off head—they carry with them certain drawbacks. The most obvious is cryogenic operation.
However, there is also the general requirement for magnetically shielded environments, always a very expensive consideration. In addition, the need to run in feedback lock due to the inherent piece-wise linear response of SQUIDs, and
the difﬁculty of summing signals electronically from two or
more SQUID systems, are also factors which must be considered. These problems can be avoided, while remote operation is maintained, if we make use of the recently developed
room temperature, ultrahigh input impedance, electric potential sensor for EEG
5. rather than MEG detection. This sensor operates using electric displacement, rather than real charge, current. It therefore does not require real electrical contact to the signal source ~e.g., the human body! in order to function. This overcomes the disadvantages of conventional Ag/AgCl electrodes while still maintaining suf-ﬁcient sensitivity ~70 nV/AHz at 1 Hz noise ﬂoor!
6. to detect body electrical signals off body. This development clearly can be very advantageous in detecting the electrical activity within the brain since, being truly noninvasive, it should be possible to operate this sensor without removing scalp hair and without any direct electrical contact to the scalp. We have previously reported on the use of such low noise, ultrahigh impedance, electric potential sensors in the detection and recording of very high quality electrocardiograms ~ECGs! with no electrical contact to the body.
6 . Furthermore, using similar techniques, we have been able to record the human heartbeat at distance up to 1 m off body with no electrical connection between the sensor and the body. In this letter, we demonstrate that we now have the sensitivity to detect EEGs through the scalp hair with no electrical contact to the scalp. We also show that it is now possible to detect brain electrical activity with an air gap between the scalp hair and the sensor.
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