S-cone ERG recording is performed following the specifications of the current ISCEV standard for full-field ERG [
1]. Recordings of the S-cone ERG can be embedded within the standard protocol [
1] or conducted as a separate protocol. Additional factors are outlined below:
a.Spectral characteristics and luminance of the background
The photopic background for the recording of the S-cone ERG must be of spectral composition and strength that will achieve two goals: (1) saturate the rod system and (2) minimize the contributions of the L/M-cones systems. The initial studies used a white background, but later studies and the developments of new electrophysiology of vision systems indicated that bright LEDs with spectral peaks between 570 and 620 nm (yellow, amber, orange) were more suitable for the purpose of obtaining an S-cone ERG (Table
1). A high background luminance of appropriate wavelength may optimize isolation of S-cone-mediated responses, but patient comfort, tolerance and the possibility of photophobia must also be considered.
Table 1Stimulus and background conditions for recording S-cone ERG
| 441 nm (interference filter) | Not stated | Not stated | Yellow (> 530 nm) | 3.0–6.0 log Td |
| 440 nm (interference filter) | 0.63 W/m2 | Not stated | Yellow | 13 Wm2 |
| 440 nm (interference filter) | 7.58 μW/cm2 | 3 | Yellow | 204 μW/cm2 |
| 450 nm (Wratten 98) | approximately 0.02 μW | 5.1 | White | >5000–9000 Td (> 100 cd/m2) |
| 450 nm (Wratten 98) | Xenon flash of 5.0 cd s m−2 and Wratten 98 | 5 | White | 1.7 log cd m−2 (50 cd m−2) |
| 450 nm (LED) | Long flash of 2.8 log Td (12.6 cd m−2) | 3 | Wratten 12 (Yellow) | 4.8 log Td (= 1262 cd m−2) |
| 440 nm (Wratten 98) | Xenon flash of 2.1 cd s m−2 and Wratten 98 | 4 | Yellow | 100 cd m−2 |
| 440 nm (LED) | 0.3–1.5 log Td–s (= 0.04–0.6 cd s m−2) | Not stated | 610 nm (orange) | 18,900 Td (= 378 cd m−2) |
| 450 nm (Wratten 98) | Xenon flash of 5.0 cd s m−2 and Wratten 98 | 5 Hz | White | 1.7 logcd m−2 (50 cd m−2) |
| Combination of two filters of 430 and 450 nm | 0.034 cd s m2 (= 1.71 Td–s) | 2 | Yellow | 10,807 Td (= 215 cd m−2) |
| 440 nm (LED) | 0.03–0.05 cd s/m−2 | 4 | 590 nm (orange) | 300 cd m−2 |
| 430 nm (LED) | 0.03 cd s m−2 (− 1.5 log cd s m−2) | 1–5 | White | 2.0 log cd m−2 (100 cd m−2) |
| 445 nm (LED) | 0.08–0.8 cd s m−2 (80 cd m−2 with 1–10 ms flashes) | Not stated | 620 nm (orange) | 560 cd m−2 |
| 449 nm (Wratten 47B) | 0.016–0.125 cd s m−2 | 2 | 594 nm (amber) | 100 cd m−2 |
| 440 nm | 65 cd m−2, 10 ms | Not stated | 660 nm (orange) | 350 cd m−2 |
| 449 nm (Wratten 47B) | 0.016 cd s m−2 | 2 | 594 nm (amber) | 100 cd m−2 |
| 470 nm (LED) | 0.1–0.5 cd s m−2 | Not stated | 594 nm (amber) | 500 cd m−2 |
| blue 445 nm | 0.4 cd s m−2 | 1 | 600 nm (orange) | 560 cd m−2 |
| Blue (445 nm) (LED) | 2.0 cd s m−2 | Not stated | 594 nm (amber) | 15 cd m−2 |
b.Spectral characteristics of the test flash
Short-wavelength (blue) stimuli may be generated using Xenon light in combination with a suitable optical filter (Kodak #98, having a peak transmission at 450 nm) or by using a light emitting diode (LED) with a peak output of short wavelength, in the region of 440–470 nm (Table
1). Wavelengths longer than 470 nm typically generate ERGs that are dominated by the L/M-cones, and the contribution of the S-cones to the ERG may not be detectable. Light sources with wavelength shorter than 440 nm may not have sufficient strength to elicit a reliable S-cone ERG because the S-cone ERG action spectrum peaks at 450 nm [
3].
c.Strength of the test flash
Laboratories that use backgrounds of 300 cd m
−2 have used flashes of low strength, 0.03–0.05 cd s m
−2, while laboratories using bright backgrounds of about 500 cd m
−2 use stronger flashes, 0.1–0.5 cd s m
−2, in order to elicit robust S-cone mediated responses (Table
1). Weak flashes may be more selective but elicit responses of relatively low amplitude (Fig.
1), whereas stronger stimuli may be difficult for some patients to tolerate.
d.Units of flash strength and luminance
It is acknowledged that the strength of the colored flash requires consideration of the spectral sensitivity of the eye and is accurately described by radiant energy (Watts/steradian/m
2/nm), but for uniformity of clinical use and consistency with other brief flash stimuli, strength is defined in this protocol in units of cd s m
−2. For stimulus durations > 10 ms, stimulus strength is given in units of luminance (cd m
−2).