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K. Bizheva, S. Abdallah, S. Boyd, O. Ramahi; Physiological Origins of The Functional Optical Coherence Tomography Signal: Simulation of Light Scattering From Light-Stimulated Retinal Photoreceptors. Invest. Ophthalmol. Vis. Sci. 2008;49(13):3199.
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© ARVO (1962-2015); The Authors (2016-present)
To explain the physiological origins of the optical signals recorded from the photoreceptor layer of light stimulated living retinas, acquired with Functional Ultrahigh Resolution Optical Coherence Tomography (fUHROCT).
We developed a model of light interaction with a single or photoreceptor cell or groups of photoreceptors. The model is based on the Finite-Difference Time-Domain method and allows us to investigate the contribution of light scattered from the discs in the Outer Segment (OS) and the mitochondria or other cellular organelles in the Inner Segment (IS) of the photoreceptor to the total back-reflected light signal detected by the fUHROCT system. Results from the simulations are compared to fUHROCT data acquired from excised animal retinas, and from the retinas of living rats.
Simulation results show that 5% increase in the refractive index of the membrane discs located in the photoreceptor OS can cause significant and detectable increase in the optical reflectivity of the entire OS. Smaller magnitude, but detectable changes in the OS reflectivity also resulted from 5-10% increase in the aperture (diameter) of the OS discs. No significant changes in the OS or IS reflectivity can be attributed to changes in refractive index of the photoreceptor outer cell membrane, that are believed to be a result of membrane hyperpolarisation under external light stimulation. Preliminary results from the IS show that 5% increase or decrease in the mitochondrial refractive index cannot result in detectable changes in the overall reflectivity of the photoreceptor IS.Optical recordings acquired with fUHROCT before, during and after application of a light stimulus showed time-dependent changes in the retinal reflectivity in the photoreceptor layer that correlated well with the results obtained from the photoreceptor model.
Results from the computational model suggest that conformational changes of the rhodopsin molecules that can cause increase in the refractive index of the OS disks, as well as osmotic swelling of the OS are most likely the major causes for the increase of the OS reflectivity observed in fUHROCT tomograms acquired from light stimulated living retinas. Furthermore, the simulation results suggest that hyperpolarization of the photoreceptor outer cell membrane does not have as significant contribution to the fUHROCT signal as expected.
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