The effect of gold foil electrode position on the electroretinogram in human subjects

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Vision Rex Vol. 29, No. 9, pp, 1085-1087,1989 Printed in Great Britain. All rights reserved

Copyright Q 1989Maxwell Pergamon Macmillan plc

RESEARCH NOTE

THE EFFECT OF GOLD FOIL ELECTRODE POSITION ON THE ELECTRORETINO~~M IN HUMAN SUBJECTS A. MCALLAN, J. SINN and G. W. AYLWARD Save Sight and Eye Health Institute, Department of Ophthalmology, Sydney Eye Hospital, Sir John Young Crescent, Woolloomootoo, NSW 201 I, Australia (Received 19 December 1988) Abstract-Photopic ERGS were recorded in two human subjects using gold foil electrodes mounted at various positions in the lower fornix. The peak to peak amplitude of the b-wave was reduced in both subjects when the centre of the electrode was placed beyond I5 mm from the medial side of the limbus. The implicit time of the a-wave and the b-wave was not dependent on electrode position. These results emphasize the importance of correct placement of gold foil electrodes. Gold foil electrode

Ampljtude

Electroretinography for research and clinical use in human subjects was facilitated by the introduction of contact lens electrodes (Riggs, 1941; Karpe, 1946). The need for optical clarity and greater comfort led to the development of other forms including the gold foil electrode (Borda, Gillam & Coats, 1977; Arden, Carter, Hogg, Siegal & Margolis, 1979) which has gained widespread acceptance. The gold foil electrode is more comfortable than contact lens types but results in a greater degree of variability (Gjotterberg, 1986). This limits its value particularly in the recording of small signals such as the pattern electroretinogram (Trick & Trick, 1984). Recent work has drawn attention to the variation in the electroretinogram (ERG) with electrode position (Cringle, Alder & Yu, 1988). A feature of the gold foil electrode is the lack of stabilisation on the cornea as compared with contact lens electrodes. The present study was carried out in order to determine the effect of gold foil electrode position on the ERG. We used electrodes manufactured in our laboratory from our own gold plated plastic. 19 pm foil plated with gold on one side by ion bombardment was cut into 20 x 5 mm strips and attached to thin wire using electroconductive tape (Vaegan, 1984). Subjects were two of the authors (AM & JS) who are both healthy males and normal ophthalmolo~~ally. Stimulation was by a Grass PS22 photic stimulator (Grass Instruments, Mass., U.S.A.) mounted on a 4Oem Ganzfeld bowl (LKC systems, MD, 1085

Human subjects

U.S.A.). The stimulator was used on the lowest intensity setting throughout these experiments. The background illumination was set to l/4 maximum, approx. 35 cd me2 measured using a digital photometer (Macam L103). The gold foil electrodes were inserted into the lower fornix of both eyes. Silver/silver chloride (3M Red Dot) electrodes mounted on the ipsilateral temple were used as a reference and another on the forehead served as an earth. Signals were amplified 10,000 times and filtered between 2 and 350 Hz before being fed to a PDPl l/23 computer via an analogue to digital converter. Software incorporating automatic artifact rejection was used to average and store the signals. 64 sweeps were averaged for each recording condition. Photopic ERGS were recorded binocularly for 8 different positions of the gold foil electrode along the lower lid. The electrode position was assessed with the use of a photograph taken after the ERG was recorded. Position was expressed as the distance from the centre of the electrode to a vertical line passing tangential to the limbus on the medial side. The reference and earth electrodes remained unchang~ in each subject for all recordings. The amplitude and implicit time of the u-wave and b-wave were determined for each ERG. b-wave amplitude was measured from the trough of the a-wave to the peak of the b-wave. The results are shown in Table 1, The b-wave amplitude data was scaled according to V/V,,,,, for each subject so that both sets of data could

1086

Research Note Table 1. Peak to peak amplitude and implicit times for various electrode positions Subiect

Distance (mm)

Amplitude (UV)

Implicit time a-wave (msec)

Implicit time b-wave (msec)

0.0

41.14 40.69 46.44 50.63 41.26 46.27 41.11 17.73 36.57 50.60 51.22 51.17 48.32 48.89 28.34 23.04 24.17 25.93 26.02 25.5 26.13 26.51 27.53 16.66 30.72 24.48 24.52 28.61 25.47 24.38 20.88 16.63

16 16 16 16 17 16 16 17 16 16 16 16 16 16 16 16 17 17 17 17 17 17 17 16 17 17 17 17 17 17 17 17

24 24 24 24 25 24 24 23 24 24 24 24 24 23 24 24 25 25 25 26 25 25 25 25 25 25 25 26 25 25 25 25

AM (RE)

2.0 3.5 6.0 9.5 12.0 17.5 21.5 - 5.0 0.0 5.5 7.0 11.5 14.0 20.0 22.0 -2.5 1.0 4.0 5.0 8.5 11.5 14.5 21.5 - 4.0 2.0 3.0 6.0 10.0 12.0 17.5 21.0

AM (LE)

JS (RE)

JS (LE)

be combined. The scaled results are shown plotted against electrode position in Fig. 1. The amplitude of the b-wave was reduced when the electrode was placed greater than

0

Subied AM- LE

0

4

I 0

I

5

Dkmce

I 10

I 15

I

M

I 25

from Medial Side of Limbus (mm)

Fig. 1. Scaled amplitude of the b-wave for both eyes of both subjects plotted against distance laterally from the medial side of the limbus.

15 mm laterally from the medial side of the limbus. However within the central area between the medial and lateral limit of the cornea there was no consistent variation with position. There was a marked reduction for displacements greater than 15 mm and this is probably due to the fact that the electrode has no direct contact with the cornea. These results are consistent with similar work carried out with contact lens electrodes (Sundmark, 1958). The implicit times did not change for the range of electrode positions that we measured. In the dog eye there was a change in implicit time of the b-wave with electrode position both in vivo and in vitro (Cringle et al., 1988), but this was using a fixed cornea1 electrode and a moveable reference. It is likely that the latter arrangement resulted in predominance of the local ERG from peripheral retina. With a temporal reference we were more likely to be recording a summed response from the entire retina. In summary we have demonstrated a variation in the amplitude of the ERG related to position of the gold foil electrode. Attention should be paid to correct placement of gold foil

Research Note

electrodes but prodding the electrode is placed near the centre of the cornea ~sitiona~ variation is not important.

REFERENCES Arden, G. B., Carter, R. M., Hogg, C., Siegel, I. M. Br Margolis, S. (1979). A gold foil electrode: Extending the horizons for clinical electroretinography. Investigative Ophf~atmotogy vtsuat Science, I&, 421-426. Borda, R. P., Gillam, R. M. & Coats, A. C. (1977). Gold-coated Mylar (GMC) dectrode for electroretinography. ~oc~e~~a a~~~h~tmotogtc~, Fr~eeedings Series, l5, 339-343. Cringle, S. J., Alder, V. A. & Yu, D, Y. (1988). The distribution of ERG potentials on the &era in vioo and in uttro. Ctintcal Vision Sciences, 2, 285-291.

1087

Gjotterberg, M. (i986). Electrodes for el~troret~no~aphy: A comparison of four different types. Archives of Qphrhalmotogy, 104, 569-570.

Karpe, G. (1946). The basis of clinical el~troret~nography. Acta Up~thuimoio~tca (Suppl.) 24, 1-I 18. Riggs, L, A. (1941). Continuous and reproducible records of the electrical activity of the human retina. Proceedings of the Society for exper~entat 204-207.

Biology and Medicine, 48,

Sundmark, 3%(19.58). fnfluence of the contact glass on the el~troretinogmm, Documenta oph~halmologtca, Advauces in Op~t~imology XVIII. ~o~citttum Belgica 627-63 1. Trick, G. L. & Trick, L. R. (1984). An evaluation of variation in pattern reversal retinal potential chamcterjs* tics. ~oc~~tu op~r~atmoIogtcu, Proceedings Series, 40, 47-67.

Vaegan (1984) An improved method of constructing pattern el~troret~nogram electrodes. ~ocumenfu ophthatmotogtea, Pro~eed~g~ Series, 40, 287-291.