Two distinct minima can be seen in the a-wave from a normal adult, separated by an early oscillation. The ascending limb also contains oscillations (bumps) which, when isolated, comprise the oscillatory potentials.

3.1.1.3  Oscillatory Potentials

Oscillatory potentials [9] are obtained from the darkadapted eye immediately after recording the maximal response and with the same stimulus conditions. The only difference is the high-pass filter, which should be set to 75–100 Hz. Alternately, digital filtering can be used to extract the oscillatory components after testing is completed. To optimize the oscillatory potentials, a “conditioning flash” can be presented approximately 15–30 s prior to data acquisition. Responses obtained under these conditions are from the cone system, since the conditioning flash adapts the rod system. The oscillatory potential generally contains at least two prominent peaks termed 01 and 02. The summed amplitude of the components (“caliper-square” method or “oscillatory index”) is often used as an index of inner retinal activity [10]. Oscillatory potentials are generated by cells of the inner retina, including amacrine and interplexiform cells, and tend to be sensitive to retinal ischemia in diseases such as diabetic retinopathy and sickle cell retinopathy. They will also be reduced by diseases of the outer retina that reduce input to the oscillatory potentials.

3.1.1.5  Light-Adapted Flicker Response

Light-adapted flicker responses should also be recorded in the presence of a 34 cd/m2 (10 ft.-L) background and after at least 10 min of light adaptation. The response is elicited by the standard stimulus flickering at 30 Hz (Technically, the rate is set to 29 or 31 Hz to avoid locking onto harmonics of 60 Hz noise.). Generally, the first few responses are larger than subsequent responses, so recording should not begin until steady-state conditions have been reached. The amplitude is measured from response minimum to response maximum. B-wave implicit time is measured from stimulus onset to b-wave peak (Fig. 3.2) and typically measures 30 ms or less in normal adults.

3.1.2  Repeat Variability

Unlike psychophysical procedures, which must be dramatically altered for testing infants, the methodology for full-field ERG testing is comparable across ages. As a result, it is possible to monitor disease progression from infancy to adulthood in patients with progressive retinal diseases such as retinitis pigmentosa. The repeat variability of amplitude measures is comparable in patients of different ages above the age of 4 years. Figure 3.3, for example, shows repeat variability measures for 30 Hz flicker responses in patients tested twice over a short-time interval. Variability is comparable across ages and also (not shown) comparable among

3.1.1.4  Single-Flash Cone Response

“Pure” cone responses can be obtained by suppressing rod activity with a white background light. The luminance of the background should be 34 cd/m2 (10 ft.-L) and the patient should be light-adapted for 10 min before recording. This period of light adaptation is necessary because the cone signal grows during the first few minutes of light adaptation [11]. The stimulus used for recording the light-adapted cone response is the same as that used for the maximal response. The single-flash cone response in normal adults generally contains a prominent a-wave with an implicit time of approximately 15 ms, a b-wave with an implicit time of approximately 30 ms, and at least two subsequent oscillations after the b-wave.

Fig. 3.3  Test–retest variability measures based on three groups of patients: young males with X-linked retinitis pigmentosa tested twice in 2 months (triangle), older patients with retinitis pigmentosa tested twice in 2 months (diamond), and older controls tested twice in 1 year (circle). Dashed lines indicate 2 SDs. Test–retest differences are independent of age in patients older than 4 years of age

patients with varying response amplitudes [12]. The consensus among a number of centers is that rod and cone b-waves have to decrease by greater than approximately 0.25 log unit (40%) for progression to be considered significant at the 95% confidence level. At ages less than 4 years, repeat variability measures are not generally available and are more complicated, since maturation and reliability affect test–retest differences and the ability to monitor disease progression.

3.1.3  Maturation of Full-Field

ERG Responses

Full-field ERGs to ISCEV standard maximum intensity bright flashes of white light have been recorded from preterm infants as early as 30 weeks after conception [13], and several authors have documented the early maturation of responses in preterm infants [13–16]. As shown in Fig. 3.4, the rod response shows considerable immaturity at birth [17, 18], both in terms of amplitude (which is a factor of 10 lower than adult) and implicit time (which is 23 ms slower than adult). The standard combined response rapidly matures so that, by 4 months postterm, amplitude is within 50% of adult values and implicit time is within 12 ms of adult values. Cone responses also mature early so that, by 4 months postterm, amplitude is

within 25% of adult values. ERG norms for different ages have been published by several authors [17, 19, 20] and show general agreement. Prediction intervals for the ISCEV responses can be found at http://www. infantvision.org.

The small amplitudes in infants under 6 weeks of age require considerable caution in interpretation. Normal limits for rod b-wave amplitudes, for example, include nondetectable responses until late infancy. One way to deal with this limitation is to record intensity– response functions in infants, using stimulus intensities that exceed the ISCEV standard for the rod response. For these brighter stimuli, white flashes cannot be used since they would simultaneously elicit cone responses. A useful procedure is to use blue flashes to help isolate the rods. Responses to cone-matched red stimuli can be subtracted at the highest intensities to provide further rod isolation [21]. Representative responses from normal preterm infants tested at 36 weeks, 40 weeks, and 57 weeks postconception are shown in Fig. 3.5a. Retinal illuminances of the stimulus flashes are shown in log scotopic troland-seconds, where 0.1 corresponds to the retinal illuminance of the ISCEV standard rod ERG. Peak-to-peak b-wave amplitudes are plotted as a function of retinal illuminance in Fig. 3.5b. Solid curves are best-fits of the so-called Naka–Rushton function, showing the maturation in sensitivity (horizontal shift – log k) and maximum response (vertical

Fig. 3.4  ISCEV standard responses from representative normal infants at different ages

30-Hz Flicker ERGs

Single-flash

Cone ERG

100 ms

100 V

b

1000

10

6

5

4

3

2

1

V 50 ms20

Adult

57 weeks

40 weeks

36 weeks

shift – log Vmax) for normal infants in comparison to the adult function. Using intensity–response functions over a range of stimulus intensities to determine rod and cone thresholds, it was found that rod ERG thresholds drop by an average of 100-fold over the 5-month- period between 36 weeks postconception and 4 months postterm age (57 weeks postconception). This change

reflects maturation of both the sensitivity (log k) and the maximum response (log Vmax) of the rod-mediated responses [15].

Large increases in rod sensitivity have been observed over the first postnatal months of normal human development using behavioral measures [22–26] Darkadapted visual thresholds, for example, are elevated by

about 1.2 log units at 1 month-of-age, but improve rapidly and are within 0.6 log unit of that of adults by about 3 months-of-age [23]. This rapid maturation parallels that seen in the rod ERG b-wave. The b-wave, however, is the sum of two different potentials: a potential generated within the rod photoreceptor and a potential of opposite polarity generated by the cells of the inner retina, presumably the Muller and bipolar cells of the INL. Hood et al. [27] have argued that immaturities in b-wave parameters cannot locate the retinal site responsible for the observed maturational changes, since either or both photoreceptor and INL immaturities could have the same effects on measured b-wave parameters.

It is well-established that the leading edge of the a-wave of the ERG provides a measure of the electrical activity of the rod receptors [28–31]. In humans, the flash-induced ERG a-wave can be fit with a computational model of the activation phase of transduction that provides estimates of transduction parameters that are analogous to those described for single primate rods [32–34]. The Hood and Birch model [29] provides estimates for three parameters: RmP3, the maximum saturated amplitude; S, a sensitivity parameter that can be related to gain or amplification of the transduction mechanism; and td, a brief delay. Thus, critical immaturities in infant vision that originate at the level of the rod photoreceptor can be studied using the leading edge of the ERG a-wave. Figure 3.6 shows a-waves from infants at 6 weeks and 4 months postterm in comparison to adult. The phototransduction model is fit to the ensemble of responses from four stimulus illuminances ranging from 3 to 4.4 log scotopic troland-seconds. The maximum saturated amplitude, RmP3, in infants is considerably lower than that in adults. At 6 weeks of age, RmP3 is on average 0.47 log unit less than that in adults, or about 34% of adult values. By 4 months, log RmP3 is 0.29 log unit less, or about 51% of adult values.

Within the context of the rod model, RmP3 is proportional to the number of GMP-gated channels in the outer segment membrane that are closed during activation phase of phototransduction [28–31]. Prior studies have documented the changes in retinal anatomy during normal development [35, 36]. Rod outer segments increase in length in the human and primate retina. For example, at birth, rod outer segments in the human midperipheral retina are approximately 30–50% the length of that of a normal adult, but by 13 months- of-age, are relatively mature-appearing [35]. Rod photoreceptor cross-sectional area and packing density

+100.0 # 4917 - 6 weeks

log S = 0.83

0.0log RmP3 = 1.74

−100.0

# 4878 - 4 months

log S = 1.23

log RmP3 = 1.88

amplitude ( V)

# 5494 - adult

log S = 1.21

log RmP3 = 2.49

time (ms)

Fig. 3.6  Representative ERG recordings to a range of flash intensities for 6-week-old and 4-month-old infants, and a normal adult. Retinal illuminance varied from approximately 3.1 to 4.0 log scot troland-seconds in the infants and from 3.5 to 4.4 log scot troland-seconds in the adult. The irregular wavy curves are the raw data and the smooth curves are the fit of the phototransduction model. The parameters shown were obtained from each series of responses by estimation of a single set of parameters (ensemble fit). In fitting the computational model, td, the brief delay before response onset, was held constant at 3.2 ms

also increase with age. Moreover, there are regional variations in the anatomical changes that occur in the developing eye; photoreceptors in the rod-ring develop earlier than those of the more central retina [37].

Estimates for the parameter, S, derived from the fit of the rod phototransduction model, are also smaller in infants than in adults. Within the context of the rod model, S is a sensitivity parameter that scales flash energy. Thus, any factor that decreases quantal catch