Functional Imaging of Single Cone Photoreceptors Using an Adaptive Optics Flood Illumination Camera

Abstract

Purpose:: Phototransduction initiates the neural visual process and represents a key facet in our understanding of vision and ocular disease. Existing methods for in vivo measurement of phototransduction, however, have limited spatial resolution, allowing only macroscopic measurements, or limited temporal resolution, leading to undersampling of fast dynamics that occur over several tens of milliseconds. Recently, we have shown that scintillation in the reflectance of individual cones, i.e. a rapid oscillation in the cone’s reflectance, emerges under certain imaging conditions [1,2]. We hypothesize that this phenomenon is linked to specific stages in phototransduction. As such, we investigate scintillation of single cones using a high speed adaptive optics retina camera.

Methods:: Videos of the cone mosaic at 1.4 degrees eccentricity were acquired at 192 Hz and over 2 s intervals using an adaptive optics retina camera (similar to [2]). The imaging source was a superluminescent diode ( = 835 nm, Δ = 18.7 nm) whose temporal coherence length was adjusted using spectral filters. This permitted control of the interference and therefore amplification of weak scattering changes that occur in the outer segments. A single stimulus flash (670 nm) of 4 to 8 ms (3.4 x 10⁵ Td s to 1.3 x 10⁶ Td s) was presented during imaging.

Results:: Stimulus was necessary to elicit scintillation. The effect of stimulus was assessed by comparing the scintillation contrast before and after stimulus. Averaged over 400 randomly chosen cones, the stimulus increased the scintillation by a factor of 0.6 (p<10^-8 with α = .01). The effect was considerably more significant when examining individual cones. Scintillation appears to stop after 100 ms, with a minimum time-to-peak of 20-25ms.

Conclusions:: Results support the link between phototransduction and cone scintillation. Average time course of the scintillation is consistent with cone ERG and suction pipette measurements. Variations between cones may be attributable in part by spectral class differences.[1] R. S. Jonnal et al., Invest. Ophthalmol. Vis. Sci. 47: E-Abstract 1806 (2006).[2] J. Rha et al., Opt. Express 14, 4452-4569 (2006).

Keywords: imaging/image analysis: non-clinical • retina • photoreceptors