Optical properties of retinal tissue and the potential of adaptive optics to visualize retinal ganglion cells in vivo

Many efforts have been made to improve the diagnostic tools used to identify and to estimate the progress of ganglion cell and nerve fibre degeneration in glaucoma. Imaging by optical coherence tomography and measurements of the dimensions of the optic nerve head and the nerve fibre layer in central retinal areas is currently used to estimate the grade of pathological changes. The visualization and quantification of ganglion cells and nerve fibres directly in patients would dramatically improve glaucoma diagnostics. We have investigated the optical properties of cellular structures of retinal tissue in order to establish a means of visualizing and quantifying ganglion cells in the living retina without staining. We have characterized the optical properties of retinal tissue in several species including humans. Nerve fibres, blood vessels, ganglion cells and their cell processes have been visualized at high image resolution by means of the reflection mode of a confocal laser scanning microscope. The potential of adaptive optics in current imaging systems and the possibilities of imaging single ganglion cells non-invasively in patients are discussed.

Figural relationship effects and mechanisms of visual masking

In each of three visual pattern masking experiments, four curved letters (C, O, Q, S) and four angular letters (E, I, L, T) served as targets preceded or followed by either a curved mask (Q,S, and C superimposed) or an angular mask (T and E superimposed). With a dark fixation and interstimulus interval field and target-mask engergies that produce clearly identifiable targets, the following figural relationship effects were found. At stimulus onset asynchronies (SOAs) from 0 to 20 msec (in both forward and backward masking), target recognition was more accurate when targets and masks overlapped exactly (same features) than when they did not (different features). At backward masking SOAs beyond 20 msec, this pattern was reversed, but there was no such reversal in forward masking. Such results indicate that the dominant mechanism of masking at SOAs from 0 to 20 msec is luminance summation over time but that luminance summation gives way to feature-specific interference at longer SOAs. Subsequent experiments demonstrate that (a) luminance summation effects are reduced by using bright fixation and interstimulus fields and (b) feature-specific interference is eliminated by using low-energy (and, therefore, less than perfectly identifiable) targets and masks.

Figural relationship effects and mechanisms of visual masking

In each of three visual pattern masking experiments, four curved letters (C, O, Q, S) and four angular letters (E, I, L, T) served as targets preceded or followed by either a curved mask (Q,S, and C superimposed) or an angular mask (T and E superimposed). With a dark fixation and interstimulus interval field and target-mask engergies that produce clearly identifiable targets, the following figural relationship effects were found. At stimulus onset asynchronies (SOAs) from 0 to 20 msec (in both forward and backward masking), target recognition was more accurate when targets and masks overlapped exactly (same features) than when they did not (different features). At backward masking SOAs beyond 20 msec, this pattern was reversed, but there was no such reversal in forward masking. Such results indicate that the dominant mechanism of masking at SOAs from 0 to 20 msec is luminance summation over time but that luminance summation gives way to feature-specific interference at longer SOAs. Subsequent experiments demonstrate that (a) luminance summation effects are reduced by using bright fixation and interstimulus fields and (b) feature-specific interference is eliminated by using low-energy (and, therefore, less than perfectly identifiable) targets and masks.