1.2Focal Macular ERGs

The design and development of the instrument required for recording focal macular ERGs from normal subjects and patients with macular diseases have been major accomplish- ments in my life. I first took part in the study of focal macular ERGs in 1976 with Tatsuo Hirose in Boston [1] and have continued to refine the various aspects of this technique up to the present.

The principle of recording a focal macular ERG includes presenting a small stimulus to the macula and recording the response from the stimulated area. Many investigators have tried to obtain reliable responses from the human macula, but the results have not been satisfactory routine clinical examinations [2–6]. To eliminate contaminating stray light

responses, background illumination must be used to depress the sensitivity of the area sur- rounding the stimulus [7, 8]. By combining the focal stimulus with background illumination, focal responses can be recorded. It is also essen- tial to monitor the location of the stimulus on the fundus during the recordings, particularly in eyes with a central scotoma, to be certain that only the fovea is stimulated [1, 9].

In 1981, we succeeded in building an instru- ment for recording focal macular ERGs [10, 11], and more than 3500 patients with various macular diseases have been examined to date [12]. The results have been informative, and valuable data have been obtained on the normal and abnormal physiology of the macular area of the retina.

1.2 Focal Macular ERGs

To develop an instrument to record focal macular ERGs with the capability of monitor- ing the location of the stimulus on the fundus, we modified an infrared television fundus camera (Canon CR-45NM). An overall view and diagram of the system are shown in Figs. 1.20 and 1.21, respectively. The light for viewing the fundus (2, in Fig. 1.21) passes through an infrared filter (4) before entering the eye (11).

The fundus image is reflected into the television camera (22) and is viewed on a television screen with an overall field of view of 45° (24).

The light for this viewing system is obtained from a tungsten light bulb (27), and the light beam passes through a fixation plate (26) with 16° of arc fixation target. By moving the fixation plate (26), the fixation point can be moved over 25° of the central fundus. Another target, attached to the side of the fundus camera, is used for fixation by the fellow eye when a large central scotoma is present in the eye being examined.

A 200-watt halogen lamp (33, in Fig. 1.21) is used as the source for the light stimulus. A rotating chopper blade (35) driven by pulses from an electronic stimulator (31) controls the frequency and duration of the light stimuli. The rise and decay time of the stimulus chopped by the shutter is 4.2 ms. The stimulus light is carried to the fundus camera by a fiberoptic cable (36). The light is made homogeneous by

a diffuser (37), and the spot size is varied by adjusting the aperture (38) on a movable plate (39). By moving this plate (39), the stimulus spot can be moved over 25° of the central fundus, and its position can be monitored on the television screen. The intensity and color of the light stimulus can be changed by inserting neutral density and colored filters into the filter holder (40). Photographs of the stimulus spot on the fundus can be taken with a 35-mm camera (28) or a Polaroid camera (30).

The light source for the background illumi- nation is another tungsten lamp (50, in Fig.

1.21). The light passes through a diffuser (48) to give homogeneous background illumination.

The intensity of the background illumination is controlled by neutral density filters (49), and the light is transmitted into the eye at a visual angle of 45°. Additional background illumina- tion is used for the peripheral retina outside the central 45°. A plastic hemisphere, 10 cm in diameter, is attached to the top of the fundus camera (46). Miniature lamps (47) are installed on the inner wall of the hemisphere and are covered by a diffuser. The intensity of the peripheral background illumination is equal- ized subjectively to that obtained from the fundus camera. Thus, homogeneous back- ground illumination of nearly the entire visual field is obtained.

1.2.1 The System

1.2.1.1 Observation and Stimulation Systems

Fig. 1.20. Overall view of the observation and stimu- lation systems for focal macular ERG and visually evoked response (VER) recordings. The examiner records the ERGs while monitoring the stimulus on the fundus by the infrared television fundus camera (A). A plastic hemisphere with miniature lamps is attached to the top of the camera to obtain background illumi- nation for the peripheral retina (B). A Burian-Allen bipolar contact lens is used to record the ERGs (C).

(From Miyake et al. [10])

Fig. 1.21. Optical components of the observation and stimulation systems. 1, reflector; 2, observation lamp; 3, condenser lens; 4, infrared filter; 5, flash tube; 6, condenser lens; 7, mirror; 8, annulus; 9, relay lens; 10, relay lens; 11, patient’s eye; 12, objective lens; 13, beam splitter; 14, mirror with an aperture; 15, focusing lens; 16, imaging lens; 17, beam splitter;

18, movable lens; 19, field lens; 20, beam splitter; 21, imaging lens; 22, infrared television

camera; 23, cable; 24, television monitor; 25, relay lens; 26, fixation plate; 27, lamp; 28, 35-

mm film camera; 29, relay lens; 30, Polaroid camera; 31, controller; 32, reflector; 33, exciting

light source; 34, motor; 35, chopper; 36, optic fiber; 37, diffuser; 38, aperture plate; 39,

movable plate; 40, filter; 41, mirror; 42, mirror; 43, mirror; 44, projection lens; 45, reflector; 46,

diffuser; 47, lamp; 48, diffuser; 49, filter; 50, lamp. (From Miyake et al. [10], with permission)

A Burian–Allen bipolar contact lens electrode (Fig. 1.20C) is used to record the ERGs. This lens allows the examiner to observe the fundus through the fundus camera clearly, and it allows a sharp image of the stimulus to be formed on the retina. When the ERG and visual evoked response (VER) are recorded simultaneously, the two responses are fed to two amplifiers, and the output of the amplifiers is fed to a signal

processor for signal summation. Usually, 256 or 512 responses are summed with a stim- ulus frequency of 4.5 Hz. Using an artifact rejection system, baseline fluctuations larger than 40 mV are rejected from the summation.

The luminances of the stimulus light and back- ground illumination are 29.46 and 2.84 cd/m

, respectively.

1.2.1.2 Recording System

To prove that the responses recorded by our system are really focal, a 5° diameter stimulus spot was moved in 7.5° steps from the optic disk through the fovea to 15° temporal to the fovea.

The ERGs and VERs recorded simultaneously at each position from a normal subject are shown in Fig. 1.22 [13]. The mean amplitudes of the responses from four normal subjects are shown at the bottom of Fig. 1.22.

The amplitudes of the ERG and VER were largest when the stimulus spot was on the fovea,

and they became smaller as the spot was moved away from the fovea. Most importantly, a response was not present when the stimulus spot was on the optic disk, indicating that the responses were not contaminated by stray light responses.

Fundus photographs from three patients with unilateral differently sized macular colobomas are shown in Fig. 1.23 [13]. The sizes of the macular coloboma were approximately 8°–10°, 11°–12°, and 17°–20° in diameter in

1.2.2 Proof of Focal Responses

Fig. 1.22. Left: A 5° diameter stimulus spot was moved in 7.5° steps from the optic disk through

the fovea to 15° temporal to the fovea. A small spot on the fovea is a fixating target, which is

useful for keeping the examining eye stable during each recording. Right, top: ERGs and VERs

recorded simultaneously at each position from a normal subject. Right, bottom: Relative ampli-

tudes of ERGs and VERs from four normal subjects. The ERG and VER amplitudes are maximum

in the fovea and are absent at the optic disk. Filled circles, ERG; open circles, VER. (From Miyake

[13])

cases 1, 2, and 3, respectively. The full-field pho- topic ERGs and full-field 30-Hz flicker ERGs of the affected and normal fellow eyes are shown in Fig. 1.24. The amplitudes of full-field pho- topic ERGs of the affected eye are within the normal range but are smaller than those of the normal fellow eye in cases 2 and 3. However, the 30-Hz flicker ERGs from the two eyes did not differ significantly in any of the patients.

These results indicate that the full-field cone- mediated ERGs are of limited value but may be better than the 30-Hz flicker ERGs for evaluating macular function.

Additional information can be obtained by studying focal macular ERGs. The simultane- ously recorded focal macular ERGs and VERs elicited from the affected and normal fellow eyes in these three cases are shown in Fig. 1.25

Fig. 1.23. Fundus photographs showing differ- ent sizes of macular colobomas. The sizes of the colobomas are approximately 8°–10° (case 1), 11°–12° (case 2), and 17°–20° (case 3). (From Miyake et al. [13], with permission)

Fig. 1.24. Full-field photopic ERGs (left) and full-field 30-Hz flicker

ERGs (right) from the affected eye and the normal fellow eye in

the three patients with macular colobomas shown in Fig. 1.23.The

full-field photopic ERGs are smaller in the eye with the coloboma

in cases 2 and 3, although the 30-Hz flicker ERGs from the two

eyes did not differ significantly in any of the patients. (From

Miyake [13], with permission)

[13]. The size of the stimulus spot was adjusted so it was approximately the same as that of the colobomas, and the stimulus spot was placed exactly on the coloboma by monitoring the fundus.

The ERGs and VERs were unrecordable in all cases, indicating that the stimuli for these recordings were stimulating the retina only underneath the spot, and the “responses” were indeed focal.

Fig. 1.25. Focal macular ERGs and VERs recorded simultaneously from the eyes with macular colobo- mas and normal fellow eyes in the three patients shown in Figs. 1.23 and 1.24. The diameters of the stimulus spots were 6° (case 1), 10° (case 2), and 15°

(case 3). The ERGs and VERs are unrecordable in all cases, indicating that the stimuli were stimulating the retina only underneath the spot, and the

“responses” were indeed focal. (From Miyake [13],

with permission)

As described in the previous section on full- field ERGs, oscillatory potentials (OPs) are wavelets superimposed on the ascending slope of the b-wave of the conventional ERG and are generated independently of the a-waves and b- waves. The site of generation of the OPs is not yet fully known, but experimental evidence indicates that OPs reflect the activity of inhibitory feedback synaptic circuits in the retina. Although many studies have investi- gated their physiological properties and clinical value, the OPs in humans were evaluated as

components of the total ERGs elicited by ganzfeld stimuli until we succeeded in record- ing the OPs from the human macula using the focal macular ERG recording system in 1988 [13, 14].

The focal macular ERGs seen in Fig. 1.26 were elicited by five different-diameter stimu- lus spots in 2.5° steps projected on the macula in a normal subject. The a-waves and b-waves of the ERGs were recorded with a time constant (TC) of 0.03 s and a 100-Hz high-cut filter, and the OPs were recorded with a TC of 0.003 s and

1.2.3 Macular Oscillatory Potentials

Fig. 1.26. Focal macular ERGs recorded simultaneously with two time constants (right)

from a normal subject.The stimuli were differently sized spots centered on the macula

(left). A time constant (T.C.) of 0.03 s with a 100-Hz high-cut filter was used to record

a-waves and b-waves; and a T.C. of 0.003 s with a 300-Hz high-cut filter was used to

recorded oscillatory potentials (Ops). (From Miyake [13])

a 300-Hz high-cut filter. The OPs consisted of three or four wavelets (O1–O4) and were clearly observed in the responses to all stimulus spot sizes.

To investigate the distribution of OPs in the human macular area, we used ring or annular stimuli, as shown in Fig. 1.27. We adjusted the stimulus conditions so the amplitudes of the a- waves and b-waves were essentially the same for the circular stimuli and annular stimuli (Fig. 1.28). Under these conditions, the OPs were significantly larger with the annular stimuli than with the circular stimuli, suggest- ing that the distribution of OPs is different from those of the a-waves and b-waves in the human macular region [13, 14]. The changes in the amplitude of response to the spot sizes and annuli indicated that the distribution of the neural elements generating the OPs is relatively sparse in the fovea. However, they become rel- atively more dense than those generating the a- waves and b-waves in the parafovea and even more dense in the perifovea.

Another unique property of the macular OPs is the nasotemporal asymmetry [15]. Semi- circular stimuli were used to compare the ERGs elicited by stimulating the temporal and nasal macula, as shown in Fig. 1.29. Focal ERGs elicited by stimulating the temporal and nasal retina with semicircular stimuli and circular stimuli (15° in diameter) are shown in Fig. 1.30.

The amplitudes and implicit times of the a- waves and b-waves in the nasal retina are almost identical to those from the temporal retina, whereas the amplitudes of the OPs are much larger in the temporal retina than in the nasal retina. The amplitude of the focal ERGs recorded with the circular stimulus was approximately the same as the sum of the amplitudes of the temporal and nasal ERGs.

These new properties of the macular OPs, as distinct from the a-waves and b-waves, have been confirmed by others recently with multi- focal ERGs [16, 17]. The asymmetrical ampli- tudes of the OPs in the nasal and temporal retina may have resulted from the various

Fig. 1.27. Circular (top) and annular (bottom) stimuli on the macula

retinal elements contributing to the OPs. The OPs reflect neuronal activity in the inner nuclear layer of the retina and are probably mediated by the amacrine cells or the inter- plexiform cells. It is interesting that there is some correlation between the distribution of dopamine-containing amacrine cells in macaque retina and that of OPs in the human macular region [18].

The significant temporal and nasal asym- metry only in OPs was surprising (Figs. 1.29, 1.30). Some reports have suggested that this asymmetry may be related to a nasotemporal difference in the number of cones and ganglion cells or to asymmetry of the optical density of photopigments in the foveal cones. However, we have not found any reports that provide evi- dence for asymmetry of the OPs.

Fig. 1.28. Comparison of focal macular ERGs elicited by a circular and an annular stimulus on the macula in four

normal subjects. It was adjusted so there was little difference in the amplitudes of the a-waves and b-waves between

the two recording conditions, but the OPs are much larger with the annular stimuli. (From Miyake et al. [14])

Fig. 1.29. Semicircular stimuli with the edge of the semicircle passing through the verti- cal axis

Fig. 1.30. Comparison of focal ERGs using semicircular stimuli on the nasal and temporal macular areas and a circu- lar stimulus (15°). The OPs in the tempo- ral macula are significantly larger than those in the nasal macula, and only the OPs show this significant asymmetry.

(From Miyake et al. [15], with permission)

1.2.3.1 Components of Focal Macular ERG in Humans

Focal macular ERGs recorded from a normal human subject demonstrating the various com- ponents are shown in Fig. 1.31. The a-waves and b-waves, OPs, on and off components, and flicker responses are shown. Because these

components originate from the neural activity of different retinal neurons in different retinal layers, a layer-by-layer analysis of macular function can be performed objectively by analyzing the components.

Fig. 1.31. Components of the focal macular ERG

recorded from a normal subject. ON and OFF

responses recorded with 1-Hz stimulus frequency

(top); a-wave, b-wave, and OPs recorded with 5-Hz

stimulus frequency (middle); and 30-Hz flicker

responses (bottom) are shown

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