Testing metal micro-electrodes

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M. A. KINNARD AND P. D. MACLEAN

satisfactory amplifying system makes possible a protracted study of the activity of individual units revealed by small spikes (e.g., Fig. 3, ,4, B).

REFERENCES BAK, A. F, Testing metal micro-electrodes. Electroeneeph. clin. Neurophysiol., 1967, 22: 186-187. MACLEAN, P. D. A chronically fixed stereotaxic device for intracerebral exploration with macro- and microelectrodes. Electroenceph. clin. NeurophysioL, 1967, 22: 180-182. Wn~SKA, A. Aktionspotentialentladungen einzelner Netzhautelemente des Froscbes. Acta Soc. Med. Fenn. "Duodecim", 1940, 22: 63-75. WOLBARSHT, M. L., MAcNICHOL JR., E. 1:'. and WAGNER, H. G. Glass insulated platinum microelectrode. Science, 1960, 132: 1309-1310.

SUMMARY A. platinum-iridium micro-electrode for intracerebral exploration with a self-contained stereotaxic device is described. The substitution of a durable paint for glass insulation previously recommended for such electrodes eliminates the need of a glass micro-forge. An analysis is given of the amplitude and sign of a sample of spikes successively recorded from 5000 units. Eighty-five per cent were negative in sign. The low noise level achieved with this electrode and a satisfactory amplifying system makes possible an extensive study of individual units of small amplitude.

Reference: KINNARD, M. A. and MACLEAN, P. D. A platinum micro-electrode for intracerebral exploration with a chrorfically fixed stereotaxic device. Electroenceph. clin. Neurophysiol., 1967, 22: 183-186.

TESTING METAL MICRO-ELECTRODES ANTHONY F. BAK

Laboratory of Neurophysiology, N.I.M.H. and N.LN.D.B., National Institutes of Health, Bethesda, Md. (U.S.A.) (Accepted for publication: July 27, 1966)

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Fig. 1 The electrode characterized by curve A recorded the activity of a total of 62 units, 26 on one experiment and 36 on another. The other electrode (curve B) successfully recorded from 103 units (33, 57, and 13 on three different experiments) even though the tip area is small. Note, however, that curve B indicates two possible recording areas.

Electroenceph. el&. Neurophysiol., 1967, 22: 186-187

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MICRO-ELECTRODE TESTING The most widely used electrode for recording extracellular potentials is the metal micro-electrode. A simple and reliable procedure for testing the recording capabilities of these electrodes has not been developed. The following account describes such a test and discusses its interpretation. A metal micro-electrode tip may be considered an ultramicro-electrolytic condenser with an apparent tip capacitance inversely proportional to frequency. It has a condenser surface area approximating that of a right circular cone and offers high polarization resistance to DC current flow. Since the resistance of the microelectrode is very high, the effective capacitance can be measured with a very high frequency. PROCEDURE The electrodes are supported by a micro-positioner with a resolution of 2#. t Chilled saline agar is used as the volume conductor (0.5% agar in 0.9% NaC1). The capacitance of the micro-electrode tip is measured with a direct-reading capacitance meter at a frequency of 135 kc. ~ The first measurement is taken with the microelectrode just touching the surface of the agar. Successive readings are made at small increments of penetration. The point where the curve reaches a plateau (see Fig. 1) indicates the locus at which the recording tip ends and effective insulation begins. The capacitance meter is also capable of detecting insulation breaks along the shank of the micro-electrode because a defect in the insulation on the shank will give further increase in capacitance. The magnitude of the increase depends on the size of the break (Fig. 1, B). An increase of 2 pF or more may in effect become another recording area. The distance from this new locus to the micro-electrode tip can be measured on the graph (Fig. l ). If the break in the insulation increases the capacitance to a value greater than 400 pF, 1 W. R. Prior and Co., Ltd., Bishop's Stortford, Herts.,'Great Britain. 2 Tektronix type 130 L-C Meter.

the meter may return to a zero reading because this is exceeding the upper limits of the meter. Insulation breaks, not observable with a microscope, can be easily detected on the direct-reading capacitance meter. The capacitance meter should have a sensitivity range of about 0.5-100 pF and the minimum oscillator frequency should be 100 kc. The characteristics of platinum and stainless steel micro-electrodes have been tested with a capacitance meter in this laboratory for more than 3 years. This simple test has been reliable for selecting good metal micro-electrodes. Ideal micro-electrodes should have maximum capacitance per unit of volume. With the platinum-iridium electrode described by Kirmard and MacLean (1967), the desired exposed tip length given by the point of plateau of the curve (Fig. 1) ranges from 15 to 25 #. The capacitance at the point of first contact for a properly shaped recording electrode is about 1-2 pF (Fig. 1), and the maximum capacitance of the entire recording tip measures about 10-15 pF. The voltage required at 135 kc does not have any noticeable effect. Allowing the micro-electrode to remain in the volume conductor for a long period of time does not change its characteristics. For an average electrode the apparent capacity of the uninsulated tip measures about 12 pF at 135 kc and about 600 p F at 1 kc. In our experience, all electrodes with these characteristics are good recording electrodes. SUMMARY Description of a simple and reliable procedure for testing the recording capabilities of metal micro-electrodes. REFERENCE KINNARD, M. A. and MACLEAN, P. D. A platinum microelectrode for intracerebral exploration with a chronically fixed stereotaxic device. Electroenceph. din. Neurophysiol., 1967, 22: 183-186.

Reference: BAK,A. F. Testing metal micro-electrodes. Electroenceph. clin. Neuropt~ysiol., 1967, 22: 186-187.