Clinical electrooculography requires a patient to track two points in the visual
field ... The electrooculogram (EOG), a useful clinical test of retinal function, is not.
COMPUTERS
AND
BIOMEDICAL
RESEARCH
Computer
5,654-658
Automated
(1972)
Electrooculography
J. R. BOURNE, P. R. HUDAK, Electrical
AND J. L. DUKE
and Biomedical Engineering Program, I/cmdeubilt Nashville, Tennessee 37235
Universit?,
Received June 19,1972
Clinical electrooculography requires a patient to track two points in the visual field separated by at least 30” visual angle in a horizontal plane while recording the concomitant change in potential from electrodes placed at the external canthi of the two eyes. This test is relatively lengthy, requiring long periods of light and dark adaptation. A minicomputer program has been written to automate the test and to analyze and reduce the data. Time required to administer the test, as well as time needed to analyze the data, has been significantly reduced by computer automation.
The electrooculogram (EOG), a useful clinical test of retinal function, is not
in widespread use due to tedious recording and analysis procedures. This paper describesan automated technique, using a minicomputer system, to simplify both recording and analysis of the EOG. The standing corneofundal potential of the eye can be modeled as an electric dipole with the positive segmentof the dipole located anteriorly. As the eye rotates in a horizontal plane, potential field changescan be easily recorded from electrodes placed at the external canthi of the two eyes. These potentials are thought to be generated by the pigment epithelium and, as discussedby Arden, Barrada, and Kelsey (I), can be used to diagnose such abnormalities as retinitis pigmentosa, retinal detachment, myopia, choroidal lesionsand vascular lesions. The standing potential of the eye varies asa function of light or dark adaptation. The magnitude of the potential change under theseconditions forms the basisfor a clinical assessmentof retinal abnormalities. Typically, clinical EOG’s are obtained by requiring a patient to voluntarily track between two points separated by at least 30” visual angle during light and dark adaptation. The variation of the EOG peak-to-peak amplitude produced during eye movement is normally measuredrather than the standing potential of the eye. Kelsey (1966) provides the following protocol for clinical EOG measurements. During each minute of a 30-min test period, five to six successiveeye movements are measuredin a IO-set interval. The average peak-to-peak EOG amplitude, obtained from three eye movements between the targets is plotted during dark adaptation Copyright All rights
0 1972 by Academic Press, Inc. of reproduction in any form reserved.
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ELECTROOCULOGRAPHY
(first 13 min) and during light adaptation (13-30 min). A typical EOG light-dark adaptation curve is shown in Fig. 1 (after Imaizuma, 1966). The maximum value of the curve is often called the light peak (Lp) and the minimum value is called the dark trough (Dt). Normal values of the Lp/Dt ratio are greater than 1.85 and the magnitude of the Lp-Dt difference is normally greater than 100 ~LV. The latencies at which Dt and Lp occur after light and dark adaptation onset, the Lp/Dt ratio and the Lp-Dt difference are the values most often used in clinical assessments.
10
FIG.
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30
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I. Normal EOG during light and dark adaptation. Light adaptation onset is at 30 min.
A major drawback to the useof the EOG asa routine clinical evaluation of retinal function is that rather time-consuming, although simple, procedures are needed to record and processthe EOG. Typically, acquisition of data from a patient requires at least 1 hr becauseof the preparation time and repeated measurementsduring both light and dark adaptation. Additional time is required to reduce the data to the form shown in Fig. 1. Data reduction is usually carried out by reading EOG magnitudes directly from a strip chart record. This procedure, both tedious and time consuming, was improved by Henkes, van der Gon, van Marle, and Schreinemachers(2), who designeda semiautomatic method for EOG measurement. Their method consisted of having subjects track fixation lights, separated by 30”, alternating at 0.5 Hz for 15-set periods in each minute of a 25-min recording session.The EOG was recorded from the external canthi of the two eyes, amplified, fed through a bandpass filter centered at 0.5 Hz and displayed on a cathode ray oscilloscope. The 0.5-Hz filter was included to insure that variations in the EOG due only to the 0.5-Hz eye movements would be recorded. By photographing vertical bars corresponding to
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BOURNE, HUDAK,
AND DUKE
EOG amplitudes, sequentially produced on the oscilloscope during a slow sweep, these authors were able to obtain a record of the EOG light-dark adaptation curve. METHOD
The method used for EOG automation described in this paper is similar to the method described by Henkes et al. (2) but relies on a digital minicomputing system. SUBIECT
ROOM I
FIG. 2. System used for EOG acquisition. SCR, Strip chart recorder; ST I, Schmitt trigger 1; AI, amplifier I. 10 -DI
SEC
4
b
* *
EoG
I II I
1 I I
I
I
I
I
I
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TRACKING SIGNAL
I
II !‘DI I BUZZER AND LIGHTS COME ON
FIG.
* ; :E%2 TRACKING
1,1 p TR 1 1 1
;:I ‘R
, s
2 2
1’1 TR J
3 3
3. Timing diagram. Dl, delay 1; D2, delay D2; TRI, trigger 1; Sl, stimulus 1.
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Figure 2 is a diagram of the system used to collect light-dark adaptation EOG’s. Two miniature lamps are mounted at eye level, 30” apart, 3 ft in front of the patient. The patient is asked to rest his chin in a chin cup and to track the lights as they alternate at a rate of 0.5 Hz. A buzzer is provided which signals the patient to begin tracking. After this signal occurs, the patient tracks the lights for 10 set until the
-1 TAKE
20
DATA POINTS
ADD
T O SUM1
ADD
T O SUM*
AVERAGE SUMI. SUM2, SURTRACT AND
STORE
FIG. 4. Flowchart of computer program.
lights are extinguished. This cycle is repeated at I-min intervals. Figure 3 is a timing diagram relating the tracking signal and the resulting EOG. After a delay Dl set by the operator, to allow the patient to begin tracking, and a delay D2, a set of 20 samples are taken, averaged and stored. D2, a delay from the light onset until the beginning of sampling, allows the patient’s eye to settle on the target before any data is taken. The average of the 20 samples is carried out to compensate for any deviation of the patient’s eye during fixation of the light. Next, the tracking signal triggers the opposite light and sampling occurs again after a delay of D2 seconds. Peak-to-
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HUDAK,
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DUKE
peak values of the EOG are determined by subtracting the maximum and minimum values calculated during the nasal and temporal deviations of the eyes. Figure 4 shows a flow chart of the program written for the Digital Equipment Corporation Lab 8/e that was used to carry out EOG automation. The program is written in PAL III machine language and utilizes two Schmitt trigger inputs to signal the beginning of a data acquisition period and the onset of each light. At present, only one a/d channel is used. However, it would be simple to add a second data channel if it was desired to monitor the two eyes simultaneously. The Lp/Dt ratio and the Lp-Dt difference plus the latencies of Lp and Dt are printed at the end of the test period. A plot of the light-dark adaptation curve is also plotted on the teletype for a permanent record. The PAL III program listing and binary source tape will be submitted to the Digital Equipment Computer Users Society (DECUS) in order that interested members can obtain copies. DISCUSSION
In operation, the EOG automation program has been successful in significantly reducing operator time. Once the delay parameters are initialized, the system can run untended for the entire test if the EOG waveform does not drift appreciably. Little drift is encountered when electrodes are attached so that a minimum resistance is obtained. A possible simplification of the system would be to use ac amplification to counter dc drift problems. Such a modification would require the program to detect maximum and minimum values of the EOG waveform rather than averaging samples during the period when the eyes fixate on one target. Another probable future inclusion in the program is an artifact detector. For example, it would be desirable to determine if the patient had actually started tracking during data collection. Presently, we rely on the patient obeying instructions and faithfully performing the required eye movements. However, no significant problems of this type have been encountered. Clinical electrooculography can be easily automated with a small and reiatively inexpensive minicomputer. Similar automation could be obtained with an on-Iine time-sharing terminal or with intermediate digital or analog tape recording. llse of computers to automate little-used and time-consuming tests, such as the EOG, may make it feasible to conduct such tests more routinely. REFERENCES
, AND KELSEY, J H. New clinicaltestof the retinalfunctionbased uponthe standingpotentialof the eye. Brit. J. Ophthal. 46,449-467 (1962). 2. HENKES, H. E., VAN DER GON, J. J. D., VAN MARLE, G. W., AND SCHREINMACHERS, H. P. Electra-oculography: A recording semi-automatic procedure. Brit. J. Ophthal. 52, 122-126 (1968). 3. IMAIZUMA, K. The clinical application of electro-oculography (EOG). In “Clinical Electroretinography. Proc 3rd Int. Symp.” (H. M. Burian and J. H. Jacobson, Eds.), pp. 311-326. New York, Pergamon, 1966. 4. KELSEY, J. H. Clinical electrooculography. Brit. J. Ophthal. 50, 438-440 (1966). I
ARDEN,
G
B
, BARRADA,
A
