Abstract
Purpose: :
A theoretical model has been proposed to explain the rapid fluctuation in the reflectance of single cones (scintillation) during video–rate imaging and bleaching. The model hypothesizes that scintillation is caused by interference between reflections from the inner/outer segment junction and those near the tip of the outer segment. This suggests that the coherence length of the imaging light source and refractive index dynamics due to bleaching of the outer segment play key roles in the observed scintillation. Thus, positive correlation of the observed cone scintillation with photopigment bleaching and the imaging light source’s coherence length should provide strong confirmation of the model. To this end, we developed a series of experiments to test this possibility.
Methods: :
Imaging of cone photoreceptors in subjects was achieved with an adaptive optics (AO) illuminated retina camera. The AO system dynamically corrected the most significant ocular aberrations across a 6 mm pupil. For bleaching, a 670 nm light source flood illuminated a 1 deg retinal patch at 1.4 deg eccentricity using 2 msec exposures at a repetition rate of 30 Hz over 3 sec. Each flash delivered 78x106 trolands. Interleaved with the bleaching flashes were 8 msec flashes from an 840 nm SLD. The 840 nm light permitted imaging of the cone photoreceptor mosaic without photopigment bleaching. The spectral bandwidth of the 840 nm light was tuned to produce temporal coherence lengths of 12 µm and 44 µm in the cone outer segments. These values straddled the ∼37 µm length of the outer segments as measured with OCT. Four sets of data were collected on each subject using the two coherence lengths with and without bleaching.
Results: :
With bleaching, slight scintillation was observed for the shorter coherence length and significant scintillation for the longer one. Without bleaching, even less scintillation was observed.
Conclusions: :
Experiments correlate well with model predictions providing substantive support for the two–surface interference model.
Keywords: imaging methods (CT, FA, ICG, MRI, OCT, RTA, SLO, ultrasound) • photoreceptors • retina