Macular scotomas, affecting visual functioning, characterize many eye and neurological diseases like AMD, diabetes
mellitus, multiple sclerosis, and macular hole. In this work, foveal visual field defects were modeled, and their effects
were evaluated on spatial contrast sensitivity and a task of stimulus detection and aiming. The modeled occluding scotomas, of different size, were superimposed on the stimuli presented on the computer display, and were stabilized on the retina using a mono Purkinje Eye-Tracker. Spatial contrast sensitivity was evaluated using
square-wave grating stimuli, whose contrast thresholds were measured using the method of constant stimuli with "catch
trials". The detection task consisted of a triple conjunctive visual search display of: size (in visual angle), contrast and
background (simple, low-level features vs. complex, high-level features). Search/aiming accuracy as well as R.T.
measures used for performance evaluation. Artificially generated scotomas suppressed spatial contrast sensitivity in a size dependent manner, similar to previous studies. Deprivation effect was dependent on spatial frequency, consistent with retinal inhomogeneity models. Stimulus detection time was slowed in complex background search situation more than in simple background. Detection speed was dependent on scotoma size and size of stimulus. In contrast, visually guided aiming was more sensitive to scotoma effect in simple background search situation than in complex background. Both stimulus aiming R.T. and accuracy (precision targeting) were impaired, as a function of scotoma size and size of stimulus. The data can be explained by models distinguishing between saliency-based, parallel and serial search processes, guiding visual attention, which are supported by underlying retinal as well as neural mechanisms.
The severity and characteristics of retinal injury following laser radiation derived from laser and tissue related factors.
We have previously shown that retinal damage following Nd:YAG Q-switched laser radiation in rabbits was related to
physical parameters, i.e. energy levels and number of pulses. Yet, an extremely large variability in the severity of the
damage was found under similar exposure paradigms, even within the same retina. This emphasizes the role of the
biological variables in the pathological mechanism of laser-induced retinal damage.
The aim of the present study was to further study histological parameters of the injury in relation to retinal site and to
elucidate their role in the initiation and characteristics of the damage, following various energy levels (10-50 &mgr;J) and
number of pulses (1-4).
Pigmented rabbits were exposed to Nd:YAG laser radiation (532nm, pulse duration: 20ns). Exposures were conducted in
retina tissue, adjacent to the optic nerve, with a total of 20 exposures per retina. Animals were sacrificed 15 min or 24
hours post exposure, eyes enucleated and processed for paraffin embedding. 4&mgr;m thick serial sections, stained with
hematoxylin and eosin, were examined under light microscopy.
Two major types of retinal damage were observed: focal edema confined to the pigmented epithelium and the
photoreceptor cells, and hemorrhages, associated with destruction of retinal tissue. While focal edema associated with
slight elevation of the photoreceptor layer seems to depend on the pigmented epithelium, hemorrhages were related also
to the choroid vasculature at the site of radiation. It is suggested that a thermo-mechanical mechanism is involved in laser
induced retinal hemorrhages at energies above 10-30&mgr;J (2-1 pulses, respectively).
An optical system designed for exposure of rabbit eyes to laser radiation and in-situ retinal damage assessment is presented. The laser radiation is of 2nd harmonic Q-switched Nd:YAG laser at 532 nm. The system is designed for multiple exposures at a regular grid array within a pre-determined region of the retina. Damage assessment is done in real time parallel to the exposure process. We present experimental results that demonstrate the versatility of the system for the determination of the threshold for laser-induced retinal damage in rabbit eye.