Measurement of microelectroretinogram by photodiode stimulation

Systems and Computers in Japan, Vol. 23, No. 9, 1992 Tranelated from Denshi Jdro Tsushin Gakkai Ronbudi , Vol. 74-DII. No. 5 , May 1991, pp. 630437

Measurement of Microelectroretinogram by Photodiode Stimulation

Xin Jiang, Nonmember, and Tatsuya Morita, Member

Faculty of Engineering, Osaka University, Suita, Japan 565

SUMMARY

The electroretinogram (ERG) is considered interesting not only as a diagnostic information in the ophthalmological clinics but also in biological engineering as a clue to the optoelectric conversion mechanism in the retina. Conventionally, ERG is observed using the whole field stimulation of the retina by a strong flash, where the information concerning the local activities of the retina cannot be obtained. Another point in ERG is that the analysis is difficult due to the nonlinearity and the ambiguity of the stimulus waveform. This paper proposes a new method which analyzes the frequency chacteris- tics of the micro-ERG @-ERG), evoked by the stimulation using the lightemitting diode (LED).

This method has the following advantages: the stimulating waveform can be controlled arbitrarily; the method of linear system analysis can be applied; the local stimulation can be realized; and the color sensation can be examined.

This paper discusses two basic techniques in the detection of the p-ERG. One is the Fourier analysis of the synchronized ERG evoked by the sine wave stimulus, and the other is the correlation analysis using the random stimulus. The results of the two methods are compared, and the usefulness of the proposed method is indicated.

Key words: Microelectroretinogrm; light-emitting diode stimulation; linear system analysis; FFT and correlation technique; sinusoidal and random stimulation.

1. Introduction

The electroretinogram (ERG) is the potential generated from the cell layer composing the retina. It is well known as one of the objective electrophysiological phenomena of the retina and has been utilized clinically as a of diagnosing retinal diseases and evaluating the curative effect [l, 21. ERG is also interesting from the viewpoint of biological engineering as a powerful means of analyzing the initial stage of visual information pro- -Sing [3-51.

Conventionally, the flash or checkerboard pattern has been used in the measurement of ERG. A problem in this method is that the entire retina is stimulated. This makes it difficult to examine the local retinal response, e.g., the functional difference between the central and the peripheral arm. Although the retinal function based on the result of ERG measurement has been analyzed, it is difficult to determine quantitatively the transfer characteristics from the light stimulus to ERG since the time waveform of the light stimulus is not defined precisely.

To solve those problems, this paper proposes a new method of measurement for the frequency characteristics of p-ERG, using the lightemitting diode (LED) as the light source [6-81.

This method has the following advantages.

(1) The stimulus light of arbitrary time waveform can be realized.

ISSN0882-1666/92/0009-0032$7 SOJO 32 1992 Scripta Technica, Inc.

Fig. 1. Typical flash ERG (after [2]).

(2) Since the source has a low intensity, it will be possible to examine the characteristics in the linear range.

(3) The retina can be stimulated locally.

(4) The characteristics concerning the color sensation can be examined.

On the other hand, ERG evoked by the LED stimulation is very weak and difficult to detect due to other bioelectric phenomena and external noises, which are superposed. Consequently, the proposed method mea- sures the weak p-ERG by the following two methods. It is verified first that the ERG component synchronized to the stimulus can be detected by Fourier analysis. Then the frequency characteristics of ERG are examined by the correlation analysis using the random signal stimulation. The frequency characteristics obtained by the two methods are compared and the usefulness of the p-ERG measuring method is indicated.

2. Status of Studies of Retinal Potential

The (ERG) is the action potential of the retinal cells to the optical stimulus and can be recorded m u - rately by inserting an electrode into the retina if possible. However, such a measurement is difficult in the clinical environment. For the ophthalmological purpose, a contact lens electrode usually is applied, and the ERG for the flashlight is recorded as the potential change between the cornea and the forehead.

I I I I

1 , I I

10 10' 10 '

F r e q u e n c y ( 1 1 z )

Fig. 2. Frequency charcteristics of the ERG (after r31).

Figure 1 shows the ERG waveform of a normal subject [2]. Figure 2 shows the power spectrum of ERG [3]. The ERG waveform is divided into the smooth and negative a wave, the following positive b wave, and the oscillatory small wave (oscillatory potential, OP wave) appearing in the transient from the a wave to the b wave. The diagnosis of various diseases can be made based on the shapes of those components [l]. The pigmentary retinal denaturalization and siderosis bulbi can be diagnosed by the abnormal a wave. The circulatory disorder of the inner layer of the retina (such as central retinal artery occlusion and retinal detachment) cau be diagnosed by the abc9rmal b wave. Diabetic retinopathy and the light retinal -ircuitry disorder can be diagnosed as the abnormal OP wave.

The origins of those waves are considered as follows. The a wave originates from the receptors, the b wave originates from the Miiller cells (i.e., the cells concerned with the mechanical support and the metabo- lism of the retina). The OP wave originates from the amacrine cells.

Studies of ERG by stimulations other than the flash stimulus have been made. The pattern ERG method is used to measure the retinal potential evoked by the rever- sal checkerboard pattern on the TV monitor. It is considered interesting since the function of the internal retinal layer can be examined, which is not possible by the flash ERG. Studies have also been made on the flicker ERG which uses the flickering light.

From the viewpoint of engineering studies, Usui et al. proposed a method to separate the OP wave camp nents using a retinal model constructed based on physio- logical knowledge. The a and b waves were synthesized

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3. Method of Experiment I ’ C - 9 8 0 1 3

: S H I E L D Lw I

Fig. 3. Micro-ERG measurement system.

by the model; and by subtracting the result from the ERG waveform, the OP wave was separated. Kanata et al; presented a simulation for ERG using an electronic circuit model [5].

Thus, the traditional studies of ERG from the engineering viewpoint were based on the flash ERG, In this wee, however, the flashlight from the source stimulates the whole retina, and the detected ERG is the retinal potential over a wide range. In other words, one can consider that the traditional retinal cell model simulates the average of the whole retinal characteristics. Another point is that the time waveform of the flash is not defined clearly. Consequently, at present, ERG is analyzed by assuming that the light stimulus is an impulse.

In this study, LED is used as the light source. By locally stimulating the retina, the input light and the induced p-ERG are measwed. First, the Fourier analysis is applied and it is verified that the activity component synchronized to the stimulus can be discriminated as p- ERG. Then, using the correlation analysis by the random signal, the frequency characteristics of ERG are determined. The usefulness of theERG measuring method by LED stimulation is indicated after comparing the frequency chorpcteristics obtained by the two stimulations.

3.1. Method of stimulation

Figure 3 shows the method of experiment by the block diagram. In this study, the light-emitting diode (LED) is used as the stimulus source. Since LED has the nonlinear diode characteristics, the light is produced by the following FM modulation. Also, to realize a highly accurate on-line measurement, the light generation is controlled by a computer. The stimulus waveform is either a sine wave or a random signal generated from the computer, depending on the purpose of experiment. The sine wave of up to 200 Hz is used. Components above 250 Hz are eliminated from the random signal. The stimulus signal converted into the FM modulated signal by the V-F converter which corresponds linearly to the voltage of 0-10 V to 0-10 kHz. The pulse train after V-F conversion is shaped into the pulse of voltage approximately 5 V and width 0.1 ms by the monostable multi- vibrator. Then LED is activated through the driver.

It is designed that the sine wave generated from the computer never goes below 1 V. Consequently, the FM modulation frequency never goes below 1 kHz. Since the retina cannot respond to such a high-frequency component, one can consider that the retina is stimulated by the intensity proportional to the stimulation signal generated by the computer. The pulsewidth is set so that there is no gap between the FM modulated pulses and LED is light- ed continuously when the computer generates a constant voltage of 10 V.

The subject is placed in a shield to prevent the external noise. The LED driving circuit, however, generates an electromagnetic induction noise due to the high pulse frequency. Then an ERG-like response may some- times be observed even if the stimulus light is shut off (wnstimulus). To avoid this kind of noise, LED is placed outside the shield and the stimulus light is guided into the shield through the optical fiber. The lightemitting end of the fiber is 5 X 6 mm, at the distance of 110 mm from the eye of the subject. If the light scatters in the eye are neglected, the irradiated diameter of the spot on the retina is approximately 3 deg.

To measure the noise level in the nonstimulus condition, a shutter is provided between the LED and the optical fiber. The reason for not using the switch but the mechanical shutter to shut off the LED electrically is that there is a possibility that the effect of the forementioned high-frequency noise from the LED driver cannot be eliminated completely even if the optical fiber is used. In

34

other words, by subtracting the activity for the nonstimulus condition from that for the stimulus condition, the noise is eliminated leaving the ERG component.

In the nonstimulus condition, there is not any target to eye fixation; and, consequently, an involuntary eye movement is produced in the subject. The contact resis- tance change of the contact lens electrode and EMG noise due to this eye movement may affect the measured potential. To avoid such effects, a fixation point is provided by a weak green light near the lightemitting end of the optical fiber in the nonstimulus condition. This fixation light is on continuously and is not supposed to correlate to the frequency component of the stimulus light. Consequently, one can assume that there is no serious effect 011 the ERG response. As a result of this fixation point, the drift-like fluctuation is reduced, result- ing in the stable measurement.

3.2. Measurement and processing

As the measurement electrode, the most usual ERG contact lens electrode is attached to the cornea of the test eye (right eye). The disposable electrodes are attached to the center of the forehead as the reference electrode, and the earlobe of the same side of the test eye as the ground. The nontest eye is shielded optically.

The signal from the electrode is amplified by a preamplifier by 10 times. It goes through the bandpass filter of 0.1 to 200 Hz to eliminate the high-frequency components. Then it is amplified by the main amplifier and given to the computer. The gain of the main amplifier is variable in the range of 0 to 1OOO. Consequently, the gain of the overall measuring system is adjustable up to 10,Ooo.

3.2.1. Sine wave stimulus

For the sine wave stimulus, ERG response is acquired to the computer by the sampling frequency of 2 Wz, in sytlchronization with the D/A conversion for stimulus generation. The sample data are extracted effectively by a sampling frequency of 400 Hz. The measurement period for one observation is 12 s, and 10 times averaging is executed to eliminate the noise. To determine the frequency characteristics, the data for 1 s after the onset of stimulus are discarded, which may contain a transient response. FFT analysis is applied to the 4096 point of data for 10 s that follows. The amplitude and the phase of the ERG component corresponding to the stimulus frequency are determined. The frequency resolution of the FFT analysis is

The frequency of the stimulus generated from the computer is preset as the integer multiple of the forementioned resolution. Consequently, the quantization error in regard to the FFT analysis can be ignored.

The stimulus frequency quoted in the later result of meawrement is the nominal value, which is the integer value closest to the accurate stimulus frequency. Using the result of FFT analysis, the difference between the stimulus condition and the nonstimulus condition is calculated, and the amplitude and the phase of the result are considered as the ERG component.

3.2.2. Random stimulus

In the case of random stimulation, the ERG response is A/D converted by the sampling frequency of 1 kHz, in synchronization with the D/A conversion of the stimulus generation. The measurement period for an observation is 6 s, and approximately 20 times averaging is executed to eliminate the noise. An arbitrary interval of 4096 points is extracted from the measured data of 6OOO points, and the correlation analysis is applied.

Let the stimulus be x(t), the electrode potential be fit), the noise be n(t), and the transfer characteristics of the ERG generation mechanism be g(t). Then the electrode potential is given by

Multiplying x(t - A ) by both sides and averaging,

is obtained. Since the random stimulus and the noise are uncorrelated, their crosscorrelation +JA) is expected to converge to zero. Then the cross correlation between the stimulus and the electrode potential UUI be approx- imated by the convolution integral of the autocorrelation of the stimulus and the ERG generation mechanism. By applying the Fourier transform to Eq. (2), the transfer characteristics W'o) of the ERG generation mechanism can be determined as

Using this relation, the frequency characteristics of ERG are determined as the ratio of the cross-spectral density

35

function 4Jjo) between the random stimulus and ERG to the autospectral density function #,(jo) of the random stimulus. In this case also, the effect of the noise is eliminated by forming the difference between the stimulus condition and the nonstimulus condition.

3.3. Measurement procedure

Thirty minutes before the start of measurement, the mydriatic drug is given so that the pupil of the test eye does not vary during photostimulation. Then the subject wearing electrodes enters the shield for dark adaptation and is instructed to lie down on a bed in the supine position and gaze at the stimulus light. The stimulus period for one observation is 12 s in the case of sine wave, but effectively about 1 min is required since a decision procedure is required to confirm whether or not the measurement is successful.

The result of measurement is averaged for 10

2 C V l -

," 10 a 0

* . . . . . . . . 1 1 I... . - - I

( a ) C P v 3

times. Consequently, the measurement can be made only for five or six frequencies in the experiment for one day (about an hour). Considering the fatigue of the subject and the effect of the mydriatic drug, a longer time measurement is impossible. Thus, four to five days (actually two weeks, since an interval of three days is set considering the pharmacological effect of the mydriatic drug) are needed to determine all the frequency characteristics.

Fig. 4. Examples of micro-ERG: stimulus (a), response of micro-ERG (b) and (c), and

background (d).

For random stimulation, one stimulation period is 6 s. Consequently, even if 30 trials are made, the data needed to draw all the frequency characteristics can be acquired within an hour.

corresponding to the dc bias, and the sine wave is superposed at the start of the stimulus.

As an example, Fig. 4 shows the response waveform together with the stimulus waveform for the stimu-

4. Result of Experiment lation with frequency 3 Hz and 10 times averaging as well as for the nonstimulus condition. Figure qa ) is the stimulus waveform, Figs. 4(b) and (c) are the waveforms for the stimulus condition, and Fig. 4(d) is the waveform in the nonstimulus condition. The abscissa is the time [s] from the onset of the stimulus, and the coordinate is the potential [pV] in terms of the voltage at the mearmre ment electrode.

Using the red LED (660 nm and 3000 mcd) and two subjects (male), the frequency characteristics of the meesured ERG are obtained as follows for the cases of sine wave and random stimulation.

4.1. Response to sine wave stimulation

The stimulus voltage is constructed by superposing the sine wave of amplitude 4 V on the 6-V dc bias. When the LED is "dark," it still emits light with one- fifth the intensity of "light." It should be noted that when LED suddenly is made on in the dark environment, the step response may be produced in the ERG. Conse- quently, LED is made stationary on with the intensity

In either of the response waveforma in Fig. 4 the slow potential change covering the whole scale may be the mixed potential of the biological potential, e.g., the eye movement and the drift of the electrode. In Fig. 4(b), a ripple response of amplitude of some 10 pV is superposed on the slow potential change. This ripple can be discriminated as ERG because it is synchronized with the stimulus shown in Fig. 4(a).

36

1 0 0

P)

C 0 a a L..

a 1

P, 0 . 1

0 . 0 1 0 5 1 0 1 5 2 0 2 5

f requency [Hzl (a )

100

m

rt = 10

U

: * c 0 a a L P) 0 . 1

0 . 0 1 0 5 1 0 1 5 2 0 2 5

( b ) f r equency CHzl

Fig. 5 . Examples of FFT analysis of micro-ERG at 3-Hz stimulation (a) and background (b).

However, as in Fig. 4(c), it may be difficult to observe a remarkable difference from the waveform in the nonstimulus condition. In the waveform of Fig. 4(c), an ERG-like waveform is observed shortly before the end of the stimulus. However, the amplitude of the ERG component is small over the whole response period and the discrimination by observation is difficult.

Thus, it happens that ERG is much smaller than the noise and is difficult to recognize. Then FFT is applied to extract and analyze the synchronized ERG. Figures 5 and 6 show the result of FFT analysis for the stimulus with frequencies 3 and 14 Hz and nonstimulus condition, respectively. In the figures, the abscissa is the stimulus frequency [Hz] and the coordinate is the logarithmic

0 5 1 0 1 5 2 0 2 5 f r equency CHzl

( a >

100

m =- 10 a. U

: 1 e 0 a v1 f 0 . 1

0 .01 0 5 1 0 1 5 2 0 2 5

f r equency CHzl

(b)

Fig. 6. Examples of FFT analysis of micro- ERG at 14-Hz stimulation (a) and back-

ground @).

amplitude spectrum with base 10. In the nonstimulus condition, the frequency spectrum distribution decays uniformly from the low to high frequencies. By contrast, a pulsed spectral component appears at the stimulus frequency in the stimulus condition, indicating that the synchronized ERG is detected. On the other hand, in some cases it is observed that the LED driving signal leaks in the nonstimulus condition. Consequently, as pointed out in the description of the method of experiment, the ERG component is obtained by forming the difference between the stimulus and the nonstimulus condition.

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.-- 0.2 0.6 1 2 6 10 20 50 100 ZOO

frequency Hr

( a )

frequency Hr

( b )

Fig. 7. Frequency characteristics of micro-ERG measured by sinwidal (dot) and random stimuli (curve). Subjects (A) (a) and B (b).

4.2. Frequency characteristics by sine wave stimulation

Figures 7(a) and (b) show the gain and the phase characteristics, respectively, of the synchronized ERG for two subjects. The dots in the figure indicate the FFT analysis which is the result of 10 times averaged data. The curve close to those dots shows the result by the random stimulation discussed in section 4.3. The abscissa of the gain diagram is the frequency [Hz], and the coordinate is the ERG gah in regard to the ratio of the electrode potential to the oscillator sine wave voltage (4 V). The abscissa of the phase diagram is the frequency [Hz], and the coordinate is the phase difference from the sine wave stimulus [deg].

There is a fluctuation in the data for the gain characteristics. This is due to the fact that it is difficult to maintain the constant condition for several days of measurement. In either of the two subjects, it is observed that the gain tends to decease monotonically with the increase of the frequency, with the slope of approximately -20 dB/dec. Thk seems to indicate the integrating process in the ERG generation mschanism.

The phase characteristics depend somewhat on the subject, but the phase lag increases With the increase of

the frequency, nearly in proportion. This seems to indicate the silent period of the ERG generation mechanism.

4.3. Frequency response for random stimulus

Thus it is verified that ERG can be detected using the sine wave stimulus. Since few high-order harmonics are contained in ERG, one can consider that the ERG generation mechanism is a linear system for the weak LED stimulus. Therefore, the frequency characteristics of the ERG generation mechanism are determined from the input/output spectral density function using the random stimulus. Figures 7(a) and (b) show the results of measurement for the same two subjects.

Comparing the results to those (dot) by the sine wave stimulation given in section 4.2, the following observations are made. In the case of subject A, the gain characteristic by the random stimulation is approximately 6 dB lower; while in the case of subject B, the curve and the dot almost agree.

In the phase diagram, there seems to be a difference in the high-frequency range. When the phase ex- ceeding 360 deg is to be measured, however, a decision must be made based on the continuity of the phase

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around that frequency. In general, this is difficult. The phase change of 360 deg corresponds to the situation where the vector representing the frequency characteristics goes around the origin in the complex plane. When the gain is small (vector is short), the locus may or may not go around the origin depending on a small noise. From the result of measurement, the gain often decreases in the high-frequency range, which indicates the frequency occurrence of the forementioned phenomenon. Con- sidering the foregoing point and the fact that it is difficult to maintain the condition for the sine wave experiment for several days, one can assume that the results of measurement for sine wave and random stimuli agree well.

5. Discussions

ERG evoked by LED stimulation is much weaker thantthe ERG generated by the traditional flash and the checkerboard pattern stimulation. On the other hand, the technique of system analysis can be applied since the stimulus waveform can be controlled precisely. This paper showed only by the basic experiment that the weak ERG can be m. However, the data obtained so far are not sufficient; and it is still difficult to make any conjecture from the data about the generation mechanism for ERG. The method seems promising since the weak potential of the order of pV can be detected from the noise.

5.1. Stimulation method

In this study, two stimulation methods are used and compared. In other words, the sine wave stimulation is used to verify that ERG is induced correctly and to verify the reliability of the measurement through comparison with the case of random stimulation. However, it is not a practical method since a long time is needed for the measurement and it is difficult to maintain the same measurement condition. The random stimulation is sim- ple in the measuremeat, although the procsdure is slight- ly more complex in the data processing. It is verified also that the result with smaller fluctuation than the sine wave stimulation is obtained.

It is difficult to compare the ERG frequency characteristics obtained in this study to the time waveform of flash ERG shown in Fig. 1. However, it is seen that a and b waves are generated at approximately 30 and 80 ms, respectively, after the stimulus, which corresponds

to 10 to 50 Hz in the frequency characteristics. Conse- quently, the small peaks in this frequency range in the gain diagram of Fig. 7 seem to correspond to the a and b waves.

The OP wave has the frequency component around 150 Hz, but the corresponding peak is not observed in the result of measurement in this study. This seems to be due to the difference of stimulation. In other words, it may be possible that the OP wave is not generated by the weak stimulation such as LED or by the stimulation of the central area of the retina. Consequently, further data should be collected for comparison with the conventional ERG waveform.

On the other hand, the obtained result is similar to the conventional lowpass characteristic shown in Fig. 2, which suggests that there is a correspondence. Figure 2, however, is based on ERG obtained by the flash stimulus, covering the frequency range of 10 Hz to 1 Wz, while Fig. 7 is the result of examining one-tenth of the frequency band. Consequently, the direct comparison is impossible. Further studies must be made such as one where the method of this study is applied to the peripheral area of the retina.

5.2. Measuring system

ERG generated by LED stimulus is very weak, especially above 100 Hz. The signal must be amplified in the measuring system so that the output of the amplifier exceeds the quantization voltage 5 mV of the A/D converter. On the other hand, ERG is superposed on the potentials due to other biological activities such as eye movement. If the drift potential due to such phenomena exceeds the measuring range of f 10 V, ERG cannot be measured. If such an incident occurs during the measurement, the measurement is made less efficient.

One way to reduce the drift-like potential with a large low-frequency component and to increase the gain at higher frequencies is to use the differential amplifier. It is seen from the result of Fig. 7 that ERG has the inte- gration characteristics of approximately -20 dB/dec, and it is expected that an almost flat characteristic can be realized over a wide frequency range by applying the differential circuit. The frequency characteristics of the original ERG can be calculated by multiplying the in- verse characteristic of the differential circuit by the data in the computer.

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The stimulus waveform is generated by the computer and can be reprodud even for the random stimulation. Consequently, it is expected that the noise due to the driving current can be eliminated by taking the difference between the stimulus condition and the nonstimulus condition.

. -- 0.2 0.6 1 2 I l o 20 so 100 200 6. Conclusions

0.2 as 1 2 I 10 20 SO 100 200

frequency Hz

Fig. 8. Frequency characteristics ERG measured by differential

applied to subject B.

of micro- amplifier

Figure 8 is an example of measurement by the random stimulation using the forementioned improved measuring system. The result of measurement is the same as the result before the improvement. Due to the effective increase of the high-frequency gain by a factor of 10, however, the peaks that seem to correspond to a and b waves are seen more clearly than in Fig. 7, which indicates an improvement of the accuracy.

5.3. Shield

It is well known that the shielding is important in the biological measurement. It is found in LED stimulation that the noise with unexpectedly large amplitude is superposed on the measured waveform, which is due to the high-frequency driving current. There is no effective means to cope with this noise, which includes the use of the shielded cable; and the noise is conjectured to be due to the current flowing in the body of the subject. Conse- quently, to suppress the noise, the LED element as well as the driving circuit are placed far away from the shield encasing the subject, and the light is directed to the subject through the optical fiber.

This paper proposed a measurement method for ERG by applying LED. Although the external noise and biological activities make the measurement of weak ERG difficult, the method of measurement for the weak ERG is established as is shown in this paper.

The LED stimulation has remarkable features compared to the traditional flash stimulation whereby an arbitrary stimulus waveform c8n be constructed accurate- ly and the system analyzing technique can be applied to the result of measurement. In this study, the resulks of measurement by the sine wave and random stimulations are compared, and the effectiveness of the system analyzing technique is demonstrated. The feature of random stimulation is that the frequency characteristics can be measured in a short time, and it is planned by the authors to analyze the functional difference among various areas of the retina by this method. It is expected that this method will be applied to diagnoses in clinical ophthal- mology.

Acknowledgement. This study was initiated and executed by Mr. K. Hori (presently with NTT) and Mr. H. Akiyama (presently with Hitachi, Ltd.) in Prof. Fujii’s (presently with Kobe Steel Indust.) laboratory and the authors are grateful to them. The study is being supported by a Sci. Gr. from the Min. Educ. (Gen. B 02452181), for which the authors also are grateful.

REFERENCES

1. E. Adachi. Limit of electrophysiological examina- tions. Ophthal., 28, 10, 1009-1019 (1986).

2. I. Watanabe and Y. Miyake. Clinics of ERG and EOG. Igakushoin (1984).

3. S. Usui, E. Nagasaka, and H. Suga. Automatic measurement of electroretinogram. Trans. (D-11) I.E.I.C.E., Japan, J73-D-n, 6, pp. 914-922 (1990)-

4. S. Usui, E. Nagasaka, andY. Miyake. Compo- nent analysis of electroretinogram. Proc. 28th Ann. Meeting JSMEBE, 1-F-5, 221 (1989).

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5. M. Takada, T. Ito, and Y. Kanata. Stimulation of flicker ERG by electronic circuit model. Papem of Technical Group on Medical and Biological Engi- neering, I.E.I.C.E., Japan, MBE86-104, 77-84 (1987).

6. K. Hori, T. Morita, and K. Fujii. Frequency analysis of ERG by LED stimulus light. Proc. 28th AM. Meting JSMEBE, 1-F4, 220 (1989).

7. X. Jiang, H. Akiyama, andT. Morita. Frequency analysis of ERG by LED stimulation. Papers of Technical Group on Medical and Biological Engi- neering, I.E.I.C.E., Japau, MBE89-124, 81-86

T. Morita, X. Jiang, and H. Akiyama. Frequency analysis of ERG by LED stimulation (2). Proc.

(19% 8.

29th Ann. MWhg JSMEBE, 2-C-12 (1990).

AUTHORS (from left to right)

Xin Jiang graduated in 1983 from the Sch. Med., Beijing 2nd Med. Coll., China. He was a Research Associate in 1989 in the Dept. of Electrical Eng., Fac. Eng., Osaka University, where he is currently in the doctoral program. He is engaged in research on the measurement of microelectroretinogram. He is a member of JSMEBE.

Tatsuya Morita graduated in 1963 from the Dept. Electrical Eng., Fac. Eng., Osaka University, where he obtained a Dr. of Eng. degree in 1969. He then became 811 Assistant in 1969 and an Assoc. Prof. in 1974 in the Dept. Electrical Eng., Fac. Eng., Osaka University. In the past he engaged in mearch on visual and auditory information processing. Presently, he is engaged in research on the measurement of microelectrorethogr, analysis of speech utterance mechanism, synthesis of natural speech, and the location/separation of several speakers. He is a member of IEW; JSMEBE; Sw. Instr. Contr. Eng.; and SOC. System Contr. Inf.

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Measurement of microelectroretinogram by photodiode stimulation