ADAPTIVE OPTICS

Adaptive optics scanning laser ophthalmoscopy (AOSLO) allows non-invasive, in vivo visualisation of the retinal microstructures of a living human eye and may help to pave the way to earlier detection and better management of a range of retinal pathologies, according to Wolf M Harmening PhD.

“Adaptive optics compensate for ocular blur and create access to clinically relevant retinal structures in the living eye. But we can go a step further with the AOSLO by delivering light to microscopic cellular targets in order to conduct single cell psychophysics. This gives us the opportunity to correlate retinal structure and visual function directly,” he said.

Addressing delegates attending the 13th EURETINA congress in Hamburg, Dr Harmening, a postdoctoral fellow at the University of Bonn, Germany, said that adaptive optics imaging techniques allow for direct visualisation of cellular intraretinal structures that were previously inaccessible with traditional imaging systems.

“Utilising adaptive optics really brings us forward and allows us a deeper view into microscopic structure of the retina in living eyes. We are now able to see rods and cones in vivo and with a little bit of tweaking we can even see the retinal pigment epithelial cells. We can also observe blood flow and analyse it in the smallest capillaries around the fovea and also focus on the retinal nerve fibre layer if required,” he said. Dr Harmening explained that the utility of the AOSLO moving towards functional tests derives from three core technologies incorporated in one device.

“First, adaptive optics allows us to compensate for aberrations in the eye, giving cell-level access to the retina. Second, the effects of chromatic aberration can be measured and corrected with sub-cellular resolution, and finally to be able to target and hit single cells repeatedly, AOSLO imaging enables us to correct for eye motion with high-speed, real-time eye tracking functionality,” he said.

Much of Dr Harmening’s research has focused on what he terms “Single-cell psychophysics,” studying the relationship between physical stimuli delivered to individual cells in the retina and the sensations and perceptions they result in. By targeting individual cones and neighbouring cells with multiple scans, the AOSLO enables researchers to classify and map complex retinal circuitry, said Dr Harmening. In a recent study, for instance, AOSLO was used to distinguish L-cones from M-and S-cones using psychophysical testing.

“Using this approach the cones can be mapped and classified. Of course there is still a lot of work to be done on understanding why the map looks a particular way, but we are making progress in this direction,” he said. Another interesting application of AOSLO technology lies in trying to understand more about colour perception and the role of surrounding cones in the living human retina.

“Once you have mapped out a particular area of cones, then you can start to ask about retinal circuitry and how these signals from single cones integrate in a specific observer,” said Dr Harmening. He cited in particular the work in this field of Ramkumar Sabesan et al who showed how it is possible to directly test colour perception from single cone stimulation. “He found that the colour perceived can be quite different and is a function of the position where the stimulus actually landed on the cone. So this can tell us something about the receptive field properties of these cells and the ganglion cells behind them,” he said. Stimulating two cones at the same time is a useful way of studying how signal integration works on single-cell level, concluded Dr Harmening.