Abstract
Purpose :
The light-evoked response in the cone iORG is attributed to the self-interference of reflections arising from the outer segment. Coherence length of the imaging light and imaging speed are hypothesized to play an important role in the fidelity and characteristics of iORG. In this study, we evaluated the role of these two parameters on the iORG using an adaptive optics line-scan ophthalmoscope(AO-LSO).
Methods :
An AO-LSO was used to image two healthy subjects at 1.5° and 10° eccentricity. A spectral band centered at 820 nm was obtained from a supercontinuum light source. Band-pass filters of varying spectral widths were introduced in illumination, and provided a range of coherence lengths for imaging, equal to 2.5, 4.3, 8.6 & 17.8 µm in air. The field-of-view was set to 0.9°x0.8°, and imaging speed was varied : 17, 30, 60 & 90 Hz. Each recording included 1 min. of dark adaptation followed by 5-7 seconds of imaging. After 1s, a stimulus (528±5nm) illuminated the retina. For each coherence length, speed and eccentricity, five trials with stimulus and 2 trials without stimulus were recorded. iORG signals were extracted using published methods from a population of cones (Cooper et al. 2017) and studied for their dependence on coherence length, imaging speed and eccentricity.
Results :
Cone iORGs were measurable for each condition in both subjects. The response amplitude was greater by 1.95 - 2.4x on average at 1.5° compared to 10° eccentricity across all coherence lengths. At both locations, increasing coherence length from 2.5 to 17.8 µm improved the response amplitude (1.9x at 1.5° and 1.6x at 10° on average). The rate of response amplitude increase vs. coherence length was greater by 2.6x on average at 1.5°. Repeatability improved with increasing coherence length at 1.5°; lower coefficients of variation between repeat measurements were observed for longer coherence lengths, with a maximum mean improvement of 3.6x between 2.5 to 17.8 µm. However, at 10°, this trend did not hold, and all coherence lengths had similar repeatability. Throughput of signal extraction expectedly improved with imaging speed, but the response amplitude remained similar.
Conclusions :
Higher speed and longer coherence length of illumination benefit iORG response amplitude, repeatability and throughput, however, the advantages are more pronounced at a near-foveal eccentricity for cones with longer outer segments.
This abstract was presented at the 2024 ARVO Annual Meeting, held in Seattle, WA, May 5-9, 2024.