A transistor amplifier for chronic micro-electrode recording

AUTOMATIC TEMPERATURE CONTROL APPARATUS REFERENCES

CHESHIRE,F. C. and EIDELBERG,E. Thermoelectric freezing stage for serial brain sectioning. Electroenceph. clin. Neurophysiol., 1966, 21 : 85-86. HAYASHI, H. and AUSTIN, G. Temperature effects on

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neuronal membrane potentials. Physiologist, 1967, 10: 198. MtLHORN JR., H. T. The application of control theory to physiological systems. Saunders, Philadelphia, Pa., 1966: 94-112. NEWMAN, D. W. Instrumental methods of experimental biology. Macmillan, New York, 1964: 462.

Reference: HAYASHI,H. and AUSTIN,G. A n automatic temperature control apparatus for micro-electrode techniques. Electroenceph. clin. NeurophysioL, 1968, 25: 180-183.

A TRANSISTOR AMPLIFIER FOR CHRONIC MICRO-ELECTRODE RECORDING H . ROKUSHIMA, K . I. NAKA 1 AND R . K1DO

Shionogi Research Laboratory, Fukushima-ku, Osaka (Japan) (Accepted for publication: December 27, 1967)

Recently, we have developed a technique for recording spike potentials from the hypothalamus of the unrestrained and unanesthetized cats by chronically implanting tungsten micro-electrodes of less than 1 p in diameter (Naka and Kido 1967). To overcome the recording problems that are present when high impedance electrodes are used, a small high input impedance amplifier (using a field effect transistor, FET, as the input stage) has been developed which cart be attached to the head of the experimental animal. Several reports have already appeared on the application of the FET to micro-electrode experiments (Laviron and Gerin 1967; Oomura et al. 1967). However, this seems to be the first attempt to employ a transistor amplifier and chronically implanted micro-electrodes to obtain stable and noise free recording of spike potentials from intact animals (Oomura and Ooyama - - personal communication - - have succeeded in recording spike potentials from the hypothalamus of the cat by employing a FET amplifier). The present method allows us to solder the free end of the implanted electrode directly to the input of the amplifier thus minimizing the length of the input lead and, hence, the movement artifact.

METHOD Fig. 1 shows the general layout of the implantation of micro-electrodes and attachment of the amplifier. One end of the implanted wire was kept free for electrical connection direct to the input of the amplifier. Four to six micro-electrodes cart be implanted and this electrode assembly is anchored to the skull with the aid of a guard ring, screws and dental cement (see Naka and Kido 1967, for detail of implantation and electrode construction). 1 Present address: California Institute of Technology, Pasadena, Calif. 91109, U.S.A.

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MC I RO-ELECTRODES Fig. 1 Schematic diagram of implantation of micro-electrodes and attachment of the high input impedance amplifier. Upper half of the socket (male) could be removed with the amplifier and cable. Tungsten micro-electrodes have glass sleeves which cover entire length of the electrode except about 5 m m at the tip. The electrodes also have a short glass ring to help anchor the electrode to the skull with dental cement. For easier connection a short flexible wire is soldered to the free end of the tungsten wire. A 9-pin miniature socket (female) is held on the skull and serves a dual function as a connector for conventional cortical and/or depth EEG electrodes and as a base for supporting the amplifier. The circuit diagram of the amplifier with a positive feedback is shown in Fig. 2. The amplifier has a gain of 35 dB at 1000 c/see and a frequency range of 0.1-15 kc/sec at -4- 3 dB with a signal source impedance of 20 Mf~ shunted by a 20 pF capacitor. Input

Electroenceph. clin. Neurophysiol., 1968, 25: 183-184

H. ROKUSHIMA et al.

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system. A diode (SD 10l) was used as an over-voltage protector for the input FET. The whole amplifier can be constructed within a dimension of 25 × 20 × 15 m m and it weighs less than 15 g. One or two amplifiers can easily be carried by a cat. A cat can spend an ordinary life in an observation cage (60 × 60 × 50 cm) with an amplifier attached to its head. Though violent movement of the animal might result in entanglement of the cable, this can be solved by use of a swivel-type connector.

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Typical spike potentials recorded by the present methods (Fig. 3) were obtained from the mesencephalic reticular formation. No artifacts were introduced while the cat was consuming food (A, black line) or during leg withdrawal (B). However, this method failed to eliminate some of the artifacts such as those due to shock to the amplifier (when the cat bumped its head against the wall of the cage and when cat crashed or chewed fish or mouse bones; the later artifact may be due to vibration of the electrode tip).

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Fig. 3 Spike potentials recorded from the mesencephalic reticular formation of the cat. A: obtained while the cat was eating pet food (indicated by black line). Spike potentials appeared when the cat stopped eating momentarily. B: obtained 4 days later from the same electrode. Spontaneous spike potentials were induced by intravenous injection of L-DOPA. Pinching of a leg (black line) results in withdrawal of the leg, disappearance of large spike potentials and appearance of small spike potentials. impedance of the amplifier is more than 500 M~-2 at 1000 c/sec. The high input impedance of the amplifier makes it difficult to evaluate the time constant of theinput network. Time constant of the amplifier is determined by the capacity (47/~F) placed in parallel with the emitter resistance (50 k£~) of the second stage transistor and is about 2.4 sec. Equivalent input noise is less than 10 #V (P-P) for the shorted input circuit. In actual operation, however, capacitor " C " (470 pF) was added which limited the frequency range of the amplifier so that frequencies above 5 kc/sec were attenuated. As the spike potentials were recorded by a F.M. tape recorder run at either 7.5 or 15 in./sec (frequency range up to 2.5 or 5 kc/sec), this capacitor served to reduce the noise level of the amplifier without affecting the performance of the recording

A small transistor amplifier with high input impedance was constructed. This amplifier can be attached to the head of cat to obtain stable recording of single spike potentials from chronically implanted tungsten microelectrodes. R~SUM~: UN AMPLIFICATEUR FRANSISTORISE POUR ENREGISTREMENT PAR MICRO-I~LECTRODES CHRONIQUEMENT IMPLANTI~ES Un petit amplificateur transistoris6 est construit avec une imp6dance d'entr~e 61ev6e. Cet amplificateur peut 6tre fix6 & la t~te du chat pour obtenir un enregistrement stable de potentiels de pointe isol6s & partir de micro61ectrodes de tungst/~ne chroniquement implant6es. We acknowledge the technical assistance of Miss K. Kishida. REFERENCE S LAVIRON, A. et GERIN, P. l~tage d'entr~.e miniaturis6 pour enregistrements avec des 61ectrodes fines. Electroenceph. clin. Neurophysiol., 1967, 21: 565-566. NAKA, K. I. and KIDO, R. Hypothalamic spike potentials recorded by chronically implanted tungsten microelectrodes. Brain Res., 1967, 5: 422~,24. OOMURA,

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high input impedance preamplifier. Physiol. Behav., 1967, 2: 93-95.

Reference: ROKOSatMA, H., NAKA, K. I. and KrDO, R. A transistor amplifier for chronic micro-electrode recording. Electroeneeph. clin. Neurophysiol., 1968, 25: 183-184.