In examining the first direct in vivo
images of the human cone mosaic, one of the more salient features of the appearance of individual cone photoreceptors is that they vary considerably in their reflectance [1
]. With the advent of ophthalmic adaptive optics (AO) [3
], it has become almost routine to non-invasively obtain images of the cone mosaic. Regardless of the AO imaging modality used (scanning laser ophthalmoscope, fundus camera, or optical coherence tomography), similar regional variation in the appearance of cones has been seen in the corresponding in vivo
images of the cone mosaic [5
]. By measuring the Stiles-Crawford effect of individual cones using an AO fundus camera, it was shown that this spatial variability is not caused by cone-to-cone differences in directional tuning [12
]. However despite being a universal feature of images of the cone mosaic, the origin of the cell to cell variability in cone reflectance remains unclear.
Besides exhibiting variability in reflectivity between different cones, individual cones also vary in their reflectivity over time, on scales ranging from seconds to hours [13
]. These changes occur both in the presence and absence of a stimulus, and it has been suggested that these changes reflect physiological activity within the photoreceptor. For example, using a flood-illuminated AO fundus camera, Pallikaris et al.
suggested that long-term variation in cone reflectivity could be due to the process of disc shedding [13
]. Recently, Pircher et al
] and Jonnal et al.
] provided data suggesting that the longer term temporal changes in cone reflectivity are due to the outer segment renewal process. In contrast, rapid changes in reflectivity can be seen in response to stimulation with light [17
], and it has been suggested that these rapid changes in cone reflectivity measured in vivo
are related to the phototransduction process [17
]. The clinical applications of such measurements could be substantial; with the ability to monitor cone structure and function, researchers would be positioned to elucidate more clearly the disease sequence of retinal degenerations, and also provide additional tools for assessing therapeutic efficacy in individuals receiving intervention.
The human retina has two classes of photoreceptor, cones and rods. While rods outnumber cones by nearly 20:1, cones have received considerably more attention in cellular retinal imaging, primarily due to their easy visualization, even without AO-equipped devices [1
]. This is unfortunate, given the prominent role that rods play in aging [21
] and devastating retinal degenerations [25
]. In cases where rod dysfunction precedes that of the cones, the inability to image rod structure and function represents a significant barrier in bringing high-resolution imaging tools to bear on their management. Part of the difficulty in translating previous studies on the spatial and temporal properties of cones to the rod mosaic has simply been an inability to readily resolve rods in vivo. Besides a couple reports of rod visualization in the diseased retina [27
], there had only been a single report of rod visualization in the normal retina. However, it was the result of significant image processing and enhancement, and provided only intermittent rod visualization [29
]. Recently, we developed an AO scanning ophthalmoscope (AOSO) capable of imaging the contiguous rod photoreceptor mosaic [30
]. Here we sought to investigate the spatial and temporal variation in reflectivity of the rod mosaic and compare its behavior to that previously observed for the cone photoreceptor mosaic.