Characterizing the early response of the optoretinogram

Abstract

Purpose : The optoretinogram (ORG) refers to the optical signature of stimulus-induced retinal activity. In cones, it includes a rapid reduction in the outer segment optical path length (OPL) followed by a slower expansion. Here, we characterize this response using phase-resolved OCT.

Methods : ORGs were obtained from healthy volunteers with a multimodal imager whose key constituents were a high-speed line-scan SD-OCT, a line-scan ophthalmoscope, and a stimulus channel (528nm LED). OCT volumes were acquired at 5 to 7 deg temporal eccentricity after 3-4 min dark adaptation, followed by a brief (1 to 5 ms) flash stimulus of varying bleach strength (0.5%–32%). Maximum volume rate was 324 Hz (16.2 kHz B-scan rate). The recorded OCT volumes were reconstructed, registered and segmented for cone outer segment tips (COST) and inner-outer segment junction (ISOS). Dynamics of the OPL changes between the ISOS and COST was computed for various bleach strengths.

Results : The magnitude and shape of the ORG was very repeatable: the average variability across six recordings with and without stimulus was 8 nm and 2.5 nm, respectively. The light-driven response consisted of a rapid (<5 ms) reduction in OPL after stimulus onset, followed by a slower (>100 ms) increase, defined as the “early” and “late” ORG responses, respectively. The response onset latency with respect to stimulus was similar for 3% and 32% bleach - 0.4 ms and 0.54 ms, respectively. Without AO, the early response was robust when averaged across retinal areas as small as 0.07 deg² encompassing ≈10 cones. With AO, the early response was observable in individual cones. Amplitude of the early response increased along a logarithmic saturating function from 5 nm to 40 nm, with the bleach strength rising from 0.5% to 32%. A small fraction (~4%) of the cones had early response amplitudes less than 5 nm at 32% bleach, and may represent S-cones.

Conclusions : The ORG early response was characterized by the following key features: sub-ms onset of response, with its peak occurring as early as 2.5 ms for intense stimuli and at ~5 ms for lower intensities, the required stimulus intensity was sufficiently high to bleach cone opsins, and the amplitude saturated with increasing stimulus intensity. These characteristics indicate that the early ORG response is attributable to the early receptor potential (ERP) – a fast hyperpolarization observable via electrophysiology in cones during the isomerization of opsins.

This is a 2020 ARVO Annual Meeting abstract.