A system
6 with a full-field bowl (58 cm in diameter) and a
white flicker light was used. The test was performed under photopic
conditions and required no fixation by the subject. The flicker
threshold was determined at a constant frequency of 37.1 Hz at a
time-average luminance of 10 candelas [cd]/m
2.
The mean luminance of the full-field bowl was corrected by taking into
account the pupil diameter and the Stiles–Crawford effect. The
contrast sensitivity was assessed using a staircase tracking procedure.
The mean value of at least six threshold crossings entered the
evaluation.
Both electrophysiological tests were performed with a two-channel
Maxwellian view system with a Xenon arc lamp as the light source. The
circular field was 32° in diameter in all recordings. With this
stimulus system, retinal illuminance is independent of pupil width;
therefore, no correction of pupil width was necessary. For peak latency
of the blue-on-yellow onset visual evoked potential
(VEP),
7 one channel provided a high-contrast, 0.88-cyc/deg
square-wave stripe pattern of blue light (460 nm, 3.3 ×
10
2 trolands [td]), the other channel provided
a homogeneous yellow adaptation light (570 nm, 1.3 ×
10
4 td) that was superimposed on the stripe
pattern. Stimulation was in the onset (200 msec)–offset (500 msec)
mode. Recording was monopolar from the inion against the left ear lobe
while the right ear lobe was grounded. After amplification (EMP
88 [Electronic Medicine Technique, Pölzl, Munich, Germany],
filter: 0.5–70 Hz), 150 sweeps (400 msec in length) were averaged
(500-Hz sampling rate). Peak time measurements of the onset responses
were made from the moment of pattern onset to the peak of the main
negative wave (N1). For amplitude of the black-and-white
pattern-reversal electroretinogram (ERG), only one channel of the
viewing system was used. The stimulus was a vertical, high-contrast
(0.93), black-and-white square-wave stripe pattern with a spatial
frequency of 0.88 cyc/deg. The pattern reversal was square wave and
occurred at a frequency of 7.8 Hz. The mean luminance was 4263 photopic
td. The responses were recorded with a carbon glide electrode hooked
over the subject’s lower eye lid. After amplification (EMP 88, filter:
0.5 Hz-70 Hz, no notch filter) the responses were averaged and stored
in a digital computer (IBM-AT, Armonk, NY; sampling rate 1000 Hz, 256
msec sweep,
n = 30). Four pattern-reversal
responses, and therefore eight amplitudes, were analyzed within one
sweep. A subsequent fast Fourier analysis evaluated the amplitude of
the second harmonic component of a total of 240 pattern-reversal
responses. In both procedures, ERG and VEP, two recordings were made to
check for reproducibility.