Abstract
Purpose: :
For very short duration increment flashes the human photopic electroretinogram (ERG) is composed of the b-wave response, but for flashes in excess of about 50 ms the d-wave emerges at the termination of the flash. The b- and d-waves are generated by the summed activity from photoreceptors, depolarizing and hyperpolarizing bipolar cells as well as a small contribution from more proximal inner-retinal networks. The activity generated by these various cell classes differs, often in a nonlinear fashion, in response to increments and decrements of light. We assessed the contribution of nonlinear response components to the ERG by performing a temporal frequency analysis of the b- and d-waves to positive and negative flashes.
Methods: :
ERGs were recorded from human subjects with normal vision using DTL fiber electrodes. Pupils were dilated and ERGs were filtered from 0.1-300 Hz. Stimuli were positive or negative square-wave flashes generated using light emitting diodes mounted in a ganzfeld stimulator. Flashes varied in duration from 10-150 msec, presented at 1) a mean luminance of 150 cd/m2 and 2) over a range of luminances to control for changes in retinal adaptation due to longer duration flashes. Response amplitudes were measured from baseline to peak of each waveform. Waveform subtraction and Fourier analysis were used to quantify the contribution of nonlinear response components.
Results: :
The amplitude of b-waves and d-waves differed depending on flash polarity. The d-wave was typically smaller than the b-wave even using negative flashes. However, b- and d-waves elicited to positive and negative flashes, respectively, were similar in shape when stimuli were presented under constant adaptation conditions. Waveform subtraction revealed brief oscillations with temporal frequencies between 25 and 40 Hz for both the b- and d-waves.
Conclusions: :
The response characteristics of the b- and d-waves are influenced by the prior adaptation state of the retina, which should be considered when using these stimuli. Frequency analysis of subtracted waveforms reveals responses that are consistent with nonlinear activity from more proximal inner-retina networks.
Keywords: electroretinography: non-clinical • retina: distal (photoreceptors, horizontal cells, bipolar cells) • retina: proximal (bipolar, amacrine, and ganglion cells)