Full-field ERGs were performed with a computer-based portable
electrodiagnostic system (EPIC-3000; LKC Technologies, Gaithersburg,
MD) and Burian–Allen bipolar contact lens electrodes (Hansen
Ophthalmics, Iowa City, IA) in 16 homozygotes, 2 heterozygotes, and 5
normal subjects (ages, 22–35 years). A handheld Ganzfeld (Kurbisfeld)
with stroboscope and background light was used to deliver the following
four stimuli: dim white flash, dark-adapted (>2 hours), to elicit a
rod ERG b-wave (minimum normal amplitude, 160 μV; maximum normal
implicit time, 92 msec); bright white flash, dark-adapted, to elicit a
mixed rod and cone ERG a-wave (minimum normal amplitude, 112 μV) and
b-wave (minimum normal amplitude, 310 μV); and light-adapted (>9
minutes) cone ERGs elicited with 1 Hz bright white flashes on a white
background (minimum normal b-wave amplitude, 70 μV); and 29 Hz bright
white flashes on the same background (minimum normal peak-to-peak
amplitude, 65 μV; maximum normal timing, 29.5 msec). In patients with
no recordable responses to these stimuli, a special protocol was used
to resolve smaller signals. In this protocol, the stimulus was a 29-Hz
bright white flash presented without background (after approximately 10
minutes of light adaptation). Responses were recorded with an amplifier
bandwidth of 1 to 70 Hz, digitized at 5 kHz, and averaged over >200
cycles. Off-line, ERGs were detrended by subtracting a third-order
polynomial fit to data; peak-to-peak amplitude and timing of the small
signals in patients were estimated by fitting a 29-Hz sinusoid to the
data and allowing the amplitude and the phase to vary. Photoelectric
artifact was ignored in all data manipulations by discarding regions in
the immediate neighborhood of the stimulus.