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Primate photopic sine-wave flicker ERG: vector modeling analysis of component origins using glutamate analogs
1Department of Ophthalmology and Visual Sciences, University of Michigan, Ann Arbor, MI 48105, USA.
Purpose:
To study how the photoreceptoral and postreceptoral ON- and OFF-components contribute to the photopic sine-wave flicker ERG in the monkey by isolating the components with glutamate analogs.
Methods:
Monkey photopic flicker ERGs were elicited with sine wave stimuli (mean luminance, 2.66 log cd/m(2); 80% modulation depth, on a 40 cd/m(2) white background) and were recorded for stimulus frequencies of 4 Hz to 64 Hz, before and after intravitreal injection of DL-2-amino-4-phosphonobutyric acid (APB) and cis-2, 3-piperidinedicarboxylic acid (PDA) that block ON- and OFF-bipolar activity, respectively. The amplitude and phase of the fundamental component were analyzed.
Results:
The flicker response amplitudes increased after APB, for frequencies of 6 Hz to 32 Hz. The further addition of PDA to isolate the photoreceptor component resulted in a relatively small residual response that decreased monotonically from 4 Hz to 32 Hz. The postsynaptic APB (ON-) and PDA (OFF-) sensitive components were isolated by subtraction and were characterized by amplitude and phase vectors. The ON- and OFF-components were larger than the initial control responses for stimuli of 8 Hz to 40 Hz. These two components had a frequency-dependent phase difference of 160 degrees to 230 degrees; normally, they interfere with each other and reduce their net contribution. The phase difference between ON- and OFF-components was nearly 180 degrees for a 10-Hz stimulus, and the phase cancellation caused a prominent dip in amplitude at this frequency.
Conclusions:
These results indicate that postreceptoral ON- and OFF-components contribute substantially to the sine-wave flicker ERG, especially at higher stimulus frequencies. Because of phase cancellation, they mask each other in the net response in a frequency dependent fashion. The photoreceptor contribution is greater than the net postsynaptic component only for frequencies of approximately less than or equal to 10 Hz. These results can be summarized by a vector model that may be useful for interpreting changes resulting from retinal disease.