Reflectance Speckle of Retinal Nerve Fiber Layer Reveals Axonal Activity

Bascom Palmer Eye Institute, University of Miami Miller School of Medicine, 1638 NW Tenth Avenue, Miami, FL, 33136, United States.
Investigative ophthalmology & visual science (Impact Factor: 3.66). 03/2013; 54(4). DOI: 10.1167/iovs.12-11347
Source: PubMed

ABSTRACT PURPOSE: This study investigated the retinal nerve fiber layer (RNFL) reflectance speckle and tested the hypothesis that temporal change of RNFL speckle reveals axonal dynamic activity. METHODS: RNFL reflectance speckle of isolated rat retinas was studied with monochromatic illumination. A series of reflectance images were collected every five seconds for about 15 min. Correlation coefficients (CC) of selected areas between a reference and subsequent images were calculated and plotted as a function of the time intervals between images. An exponential function fit to the time course was used to evaluate temporal change of speckle pattern. To relate temporal change of speckle to axonal activity, in vitro living retina perfused at a normal (34°C) and a lower (24°C) temperature, paraformaldehyde-fixed retina and retina treated with microtubule depolymerization were used. RESULTS: RNFL reflectance was not uniform; rather nerve fiber bundles had a speckled texture that changed with time. In normally perfused retina, the time constant of the CC change was 0.56 ± 0.26 min. In retinas treated with lower temperature and microtubule depolymerization, the time constants increased by 2 - 4 times, indicating that the speckle pattern changed more slowly. The speckled texture in fixed retina was stationary. CONCLUSIONS: Fixation stops axonal activity; treatments with either lower temperature or microtubule depolymerization are known to decrease axonal transport. The results obtained in this study suggest that temporal change of RNFL speckle reveals structural change due to axonal activity. Assessment of RNFL reflectance speckle may offer a new means of evaluating axonal function.

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