An interesting issue is why the PhNR and its alterations in the
photopic ganzfeld flash ERG of POAG patients were not described
previously. Part of the reason may lie in the stimulus conditions that
were used. Whereas ERG studies often use broadband white test stimuli
on white backgrounds, we used red test flashes on a rod-saturating blue
background. We initially selected these conditions simply to ensure
photopic stimulation. However, in our macaque studies, we discovered
that the conditions elicited a prominent PhNR.
8 This does
not mean that stimuli
must be red flashes on a blue
background to produce PhNRs. In another study in macaques, 1.7 Hz
modulation of a diffuse white field on an RGB monitor (42° × 37°,
mean luminance of 45 cd/m
2) was found to be
adequate, although not optimal for producing PhNRs.
9 Further, in human subjects, Colotto et al.
36 recently
described PhNRs in normal and glaucomatous eyes in response to 2-Hz
modulation (92% contrast) of a 12° × 12° field on a computer
monitor (mean luminance 78 cd/m
2). Although PhNR
amplitudes for their normal subjects were quite small (approximately 2μ
V on average) compared to our present finding of approximately 20μ
V, PhNRs in patients with open angle glaucoma were reduced
significantly.
36 Finally, North et al.
37 recently reported that a PhNR that can be elicited with stimuli that
selectively produce S-cone driven responses is reduced significantly in
POAG patients. Our stimuli in the present study would have missed this
S-cone driven response.
Monochromatic full-field test stimuli may produce more obvious PhNRs
than broadband stimuli because they provide less opportunity for
inhibitory center-surround interactions in the responses of spectrally
opponent retinal ganglion cells. This could enhance ganglion cell
responses, and increase the PhNR. Further, when both background and
flash are both spectrally broadband, more opportunity exists for light
adaptation of the cone pathways that produce responses to the test
flashes. If inner retinal signals are adapted by backgrounds weaker
than those affecting outer retinal signals (e.g., Ref.
38 ), then signals originating from hyperpolarizing
bipolar cells, photoreceptors, and perhaps horizontal cells rather than
from ganglion cells would provide the dominant negative potentials in
the ERG. Supporting this suggestion is the pharmacological evidence
that distally generated negative potentials dominate in macaque
photopic ERGs when full field white flashes on white backgrounds are
used.
6
The recording conditions in our study also might have facilitated
detection of the PhNR. For instance, we did not filter low temporal
frequencies as is commonplace in ERG recordings in humans; we made DC
recordings that would not distort slower contributions to the ERG than
the a- and b-waves. With regard to electrode placement, whereas it is
quite common to make bipolar recordings of ERGs from one eye, we
recorded differentially across the eyes. This recording configuration
might be particularly good for PhNR recording. Consistent with this
idea, in their study of the optic nerve head component in the
multifocal ERG, Sutter and Bearse
39 pointed out that
placing a reference electrode on the nonstimulated eye provides a
conducting pathway for the optic nerve head component.