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W. Suzuki, G. Hanazono, T. Nanjo, K. Ito, J. Nishiyama, M. Tanifuji, K. Tsunoda; Properties of Flash-Evoked Retinal Activity Revealed by Functional Optical Coherence Tomography (fOCT) in the Macaque Retina. Invest. Ophthalmol. Vis. Sci. 2010;51(13):1034.
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OCT can detect flash-evoked light scattering changes following neuronal activation in the neural tissues. This is called functional OCT (fOCT) technique. The basic properties of the fOCT signals, however, have not been well investigated. We have measured fOCT signals with both diffuse and focal stimuli from macaque retina to precisely investigate their properties and possible sources.
We have developed a fOCT system based on spectral domain OCT (SLD, 840 nm; band width 50 nm; Galvano Scanning mirror, 30 Hz). The intrinsic signals (Tsunoda et al, IOVS 2004, Hanazono et al, IOVS 2007) were measured by the same system with IR light (870 nm) through the same optical pathway. Following dark-adaptation, the posterior retina was stimulated by (1) diffuse flashes (duration; 1ms) with different intensities or (2) focal flashes. The evoked signals were calculated by dividing the images obtained after the stimulus by those obtained during the pre-stimulus period pixel by pixel after spatial alignment of the images.
With diffuse flash stimulation, increases in the OCT signals were observed in the outer segment of photoreceptor layer (OS). The magnitude of the fOCT signals increased (> 40 % for the strongest stimulation at fovea) with flash intensities. In the inner segment and outer segment (IS/OS) junction, fOCT signals were also observed, but with opposite polarity to those in the OS. fOCT signals were not observed either in the RPE or choroid. With focal stimulation, increases of OCT signals were observed both in the OS of the stimulated region and in the ganglion cell layer of the non-stimulated region. The polarities of the fOCT signals were not different in stimulated and non-stimulated regions, not like in the case of the intrinsic signal imaging.
The fOCT signals reflect the stimulus-evoked neural activities both in the outer and inner retinas. However, their polarities of the signals are not same as those in the intrinsic signal imaging, indicating that fOCT signals may be generated by the different mechanisms from those of the intrinsic signal imaging.
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