Flash-Evoked Inner and Outer Retinal Activity Revealed by Functional Optical Coherence Tomography (OCT) and Intrinsic Signal Imaging in Anesthetized Macaque

Abstract

Purpose: : Maheswari reported that the local light scattering changes following neural activation in the cat cerebral cortex can be detected by OCT (Maheswari 2002). This functional OCT technique has been used to detect the flash-evoked light scattering changes in a retinal slice of rabbits (Bizheva 2006) and in the retina of rats in vivo (Srinivasan 2006). However, the origin of the signal has still not been determined definitively. Stimulus-evoked light scattering changes in the intact retina have also been investigated by intrinsic signal imaging (ISI; Tsunoda 2004, Inomata 2008, Hanazono 2007 and 2008). We have measured flash-evoked functional OCT signals and intrinsic signals from the same location of the macaque retina to determine the exact properties and possible sources of the functional OCT signals.

Methods: : We have developed a spectral domain OCT system that can monitor the ocular fundus of anesthetized rhesus monkeys (SLD, 840 nm; band width 50 nm; Galvano Scanning mirror, 50 Hz; Line CCD, 1024 pixels, 12 bit). This system can also measure the intrinsic signal with infrared light (870 nm) through the same optical pathway. Following dark-adaptation, the posterior retina was stimulated by a brief diffuse flash at 3 to 30 minute intervals. The evoked signal was calculated by dividing the image obtained after the stimulus by that obtained during the pre-stimulus period pixel by pixel after spatial alignment of the images.

Results: : OCT showed that when the flash interval was 30 minutes, fast reflectance increases, reaching 80% of the maximum within 100 ms, were observed in the photoreceptor outer segments, with the strongest reflectance change at the fovea. With 3 minutes flash intervals, slow reflectance increases (peak >5 sec following flash) were observed in the retinal nerve fiber layer and the optic nerve head. Similar pattern of signal distribution and time course was obtained in the ISI although the signal polarity was reversed.

Conclusions: : The flash-evoked functional OCT signal and the ISI signal appear to have similar properties in the signal distribution and time course. Functional OCT can detect not only the light scattering changes in the photoreceptor layer, but also the flash-evoked slow metabolic changes in the inner retina.

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