Optical coherent tomography of intrinsic optical signal response at photoreceptor outer segments

Abstract

 

Purpose

 

This study was designed to validate en face optical coherence tomography (OCT) mapping of stimulus-evoked intrinsic optical signal (IOS) response at photoreceptor outer segments, and to assess the effect of spatial resolution on IOS sensitivity and signal-to-noise ratio (SRN).

 

Methods

 

Isolated frog eyecups were selected for this study. Animal handling was approved by the Institutional Animal Care and Use Committee of the University of Alabama at Birmingham. A rapid line-scan OCT (LS-OCT), which provided high-spatial (micrometer level) and high-temporal (millisecond level) resolution, was employed to map IOS dynamics at the photoreceptor outer segment layer. A near infrared (800-860 nm) superluminescent diode (SLD) was used for OCT imaging, and a green light-emitting diode (LED) was used for retinal stimulation. IOS images were reconstructed through dynamic OCT data processing. Spatial and temporal characteristics of the IOS response were quantitatively analyzed. A digital filtering strategy was employed to assess the effect of spatial resolution on IOS sensitivity and SNR.

 

Results

 

Robust IOS response was observed at the photoreceptor outer segment layer. Both positive and negative IOSs were observed within retinal stimulation area. The positive and negative IOSs had onset-times of 2.5 ms and 2 ms, and reached half-peak magnitudes at 7 ms and 6 ms, respectively. With virtually decreased spatial resolution, the IOS sensitivity dropped rapidly. In contrast, the IOS SNR reached a peak value at ~10-mirometer resolution.

 

Conclusions

 

It is feasible to map transient IOS at photoreceptor outer segments using the LS-OCT. High-spatial resolution is important for ensuring IOS sensitivity; while high-temporal resolution is essential for characterizing onset process of the fast IOS. Slight difference of the positive and negative IOSs suggests that multiple aspects, such as G-protein binding and releasing processes within photoreceptors, may affect the IOS imaging.

 

 

A. IOS images acquired at 200 frames/s. White circle shows a 10 ms stimulus; B. IOS images in the red rectangle area in A with resolution digitally decreased. Resolutions from left to right are (in μm) 2, 5, 10, 20 and 50; C. The effect of spatial resolution on IOS magnitude ΔI/I and SNR. Red color: positive IOS, blue color: negative IOS; D. Time characteristics of positive and negative IOSs at photoreceptor outer segment.

 

Keywords: 552 imaging methods (CT, FA, ICG, MRI, OCT, RTA, SLO, ultrasound) • 648 photoreceptors • 688 retina