Sleep telemetry in the rat: I. A miniaturized FM-AM transmitter for EEG and EMG

Sleep telemetry in the rat: I. A miniaturized FM-AM transmitter for EEG and EMG

112 Electroencephalography and Clinical Neurophysiology, 1978, 4 4 : 1 1 2 - - 1 1 4 © Elsevier/North-Holland Scientific Publishers, Ltd. Technical ...

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112

Electroencephalography and Clinical Neurophysiology, 1978, 4 4 : 1 1 2 - - 1 1 4 © Elsevier/North-Holland Scientific Publishers, Ltd.

Technical contribution SLEEP TELEMETRY EEG AND EMG

IN

THE

RAT:

I. A M I N I A T U R I Z E D

FM--AM

TRANSMITTER

FOR

P. RUEDIN, J. BISANG, P.G. WASER and A.A. BORBELY *

Institute of Pharmacology, University of Ziirich, CH-8006 Ziirich (Switzerland) (Accepted for publication: May 16, 1977)

The use of telemetry permits recording from cornpletely unrestrained, behaving animals. In the past few years multichannel telemetry of EEG and EMG has been extensively used in our laboratory to study sleep in rats. These studies were performed with a 4-channel transmitter that could be plugged onto a socket on the rat's head and thus connected to chronically implanted electrodes (Voegeli and Kraft 1972; Kraft and Voegeli 1973: Borb61y et al. 1973; Moser et al. 1974). However, in several respects this transmitter proved to be inadequate: first, due mainly to the pulse interval modulation technique, the receiver was too susceptible to external pulses which often arose in the laboratory during switching operations of electrical circuits (light sources, computer, etc.). Such pulses could severely interfere with the channel synchronization of the receiver and block signal transmission for a relatively long time. Secondly, due to the size of the transmitter we had difficulty in fixing the electrode assembly with the socket firmly to the animal's head for long-term studies. A considerable torque could be generated during rapid, violent head movements, particularly when plugging-on or removing the transmitter or the battery, and thereby the assembly could be loosened or ripped off entirely. These as well as other minor shortcomings prompted us to develop the FM-AM transmitter which is described in the present report. In this new system, signal transmission by frequency modulation, as well as careful selection of the transmitting frequency, minimized the interference by external noise sources. Since 2 channels are sufficient for the analysis of the vigilance states in sleep studies, we were able to reduce considerably the size and weight of the transmitter as compared to the previous model, and thus achieve more reliable anchoring to the skull.

The typical data of the telemetry system are shown in Table I, and the dimensions of the transmitter, and its mode of fixation are presented in Fig. 1. (A block diagram is available upon request.) The input signals modulate the amplitudes of the subcarriers (2 and 5 kc/sec) which are then added together to modulate the frequency of the main carrier (between 88 and 108 Mc/sec). The transmitter is assembled on two printed circuits using 67 discrete components, and then embedded in epoxy resin (Araldite). The exact transmitting frequency is specified during the assembly of the oscillator. The oscillator coil constitutes the transmitter antenna. The battery is enclosed in the narrow end of the transmitter (Fig. 1) to ensure good and clean battery contacts. The brain

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The study has been supported by the Swiss National Science Foundation, grant nr. 3.624-0.75. * Author to whom requests for reprints should be addressed,

Fig. 1. Dimensional drawing of transmitter with screws and cylinders (D) for fixation on the skull, battery (B) and its contact plate (A), and cover (C) of the battery case.

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TABLE I Typical data of the telemetry system Transmitter Dimension 19.5 x 16 × 6.3 mm Weight (including battery) 3.0 g Battery: 1 mercury cell (Varta 7107) 50 mAh, 1.35 V Operating time with 1 battery: 200 h Frequency of main carrier (FM): selectable between 88 and 108 MHz Frequency of subcarriers (AM): 2 and 5 kc/sec Omnidirectional transmission sphere of 2 m diameter (could be increased by antennae amplifiers) Frequency response (6 dB points = half amplitude): < 1 Hz--500 Hz Input: 2 channels, asymmetric Input impedance: approx. 2 × 106 ohms; for EEG and EMG recordings internally shunted to 6.8 x 104 ohms Input signal range (linear): 0--1.5 mV (channel 1); 0--2.5 mV (channel 2) Temperature stability of gain: better than 1%/°C Total system Gain: 2000--4000 (adjustable) Noise (input impedance 6,8 × 104 ohms; 1--300 c/sec): <5 pV pp Interchannel crosstalk: < 1%

and muscle electrodes are soldered to 3 hollow goldplated brass cylinders which are cemented to the skull at predetermined positions. The transmitter is fastened by screws onto these cylinders, which thus have the dual function of providing electrical contact and mechanical fixation. The exchange of the battery can be easily performed on the animal, since only the front screw has to be removed to open the battery case and to lift out the battery with a magnet. The radio signal is received by 3 standard FM radio anten-

Fig. 2. Rat with transmitter on its head.

nae (orthogonal to each other at a distance of approximately 1 m from the cage), connected to 3 synchronized FM receivers with automatic frequency control (AFC; range _+400 kc/sec). The present telemetry system permits also semiquantitative recording of head movements. This is achieved by summing the differentiated and rectified outputs of the field detectors of the 3 receivers. The signal thus obtained reflects the field changes occurring during movement of the transmitter relative to the antennae. Although a linear recording of movements is not possible by this method, the integrated signal is similar to that obtained from a force recorder under the animal's cage (Fig. 3). Thus the integrated head movement signal can provide a relatively faithful measure of the animal's m o t o r activity. The recording of motor activity by radiotelemetry may be particularly useful to obtain activity records of individual animals in a group situation. The telemetry system has been continuously used for several months in a series of experiments in which the vigilance states were automatically determined on the basis of the telemetric EEG and EMG signals (Neuhaus and Borb~ly 1977). Artifacts due to shifts in the transmitting frequency were observed only during episodes of rigorous head grooming. We believe therefore that the reliability, the small size of the transmitter and the extended operating time make the present system suitable for a wide range of applications.

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Fig. 3. Integrated movement signals in arbitrary units obtained concomitantly by telemetric head movement detection (HM) and by a force recorder under the cage (FR). Integration over 10 sec period. Onset of dark period at 18h.

Summary

References

The article describes a miniature 2-channel FM-AM transmitter for recording EEG and EMG in unrestrained, small animals. Field changes during head movements yield a signal which can serve as a measure of motor activity.

Borb~ly, A., D~iniker, M., Moser, R. and Waser, P.G. Multiparameter telemetry in neuropharmacological research. AGEN-Mitteilungen, 1973, 15: 29--34. Kraft, W. und Voegeli, F. 4-Kanal Miniatursender zur Uebertragung des Elektroencephalogramms von Kleintieren. AGEN-Mitteilungen, 1973, 15" 19-24. Moser, R., D~niker, M. and Borb~ly, A.A. EEG-telemerry in the rat: selective recording from 5 out of 12 chronically implanted electrodes. Biotelemetry II. 2nd Int. Syrup., Davos 1974, Karger, Basel, 1974: 182--184. Neuhaus, H.U. and Borb~ly, A.A. Sleep telemetry in the rat. II. Automatic identification and recording of vigilance states. Electroenceph. clin. Neurophysiol., 1977, 44: 115--119. Voegeli, F. and Kraft, W. Multichanne] telemetry of physiological parameters (body temperature, ECG, EEG) in the rat. I. Design and methods. In H.P. Kimmich and J.A. Vos (Eds.), Biotelemetry, Meander, Leiden, 1972: 371--380.

R~sum~

Tdldmesure du s o m m e i l chez le rat: I. E m e t t e u r FMAM miniaturisdpourEEG et EMG

Un ~metteur FM-AM miniaturis~ fi 2 voies permettant d'enregistrer I'EEG et I'EMG chez de petits animaux libres, est d~crit dans cet article. Des changements de champs pendant des mouvements de t~te fournissent un signal qui permet d'enregistrer l'activit~ motrice,