Ocular dynamics and visual tracking performance after Q-switched laser exposure

Abstract

ABSTRACT In previous investigations of q-switched laser retinal exposure in awake task oriented non-human primates (NHPs), thethreshold for retinal damage occurred well below that of the threshold for permanent visual function loss. Visual functionmeasures used in these studies involved measures of visual acuity and contrast sensitivity. In the present study, we examinethe same relationship for q-switched laser exposure using a visual performance task, where task dependency involves moreparafoveal than foveal retina. NHPs were trained on a visual pursuit motor tracking performance task that requiredmaintaining a small HeNe laser spot (0.3 degrees) centered in a slowly moving (0.5deg/sec) annulus. When NHPs reliablyproduced visual target tracking efficiencies > 80%, single q-switched laser exposures (7 nsec) were made coaxially with theline of sight of the moving target. An infrared camera imaged the pupil during exposure to obtain the pupillary response tothe laser flash. Retinal images were obtained with a scanning laser ophthalmoscope 3 days post exposure under ketamineand nembutol anesthesia. Q-switched visible laser exposures at twice the damage threshold produced small (about 50 m)retinal lesions temporal to the fovea; deficits in NHP visual pursuit tracking were transient, demonstrating full recovery tobaseline within a single tracking session. Post exposure analysis of the pupillary response demonstrated that the exposureflash entered the pupil, followed by 90 msec refractory period and than a 12 % pupillary contraction within 1.5 sec from theonset of laser exposure. At 6 times the morphological threshold damage level for 532 nm q-switched exposure, longer termlosses in NHP pursuit tracking performance were observed. In summary, q-switched laser exposure appears to have a higherthreshold for permanent visual performance loss than the corresponding threshold to produce retinal threshold injury.Mechanisms of neural plasticity within the retina and at higher visual brain centers may mediate the stability of visualfunction and performance metrics. Long term repeated exposure to the retina, however, may eventually dampen the ability ofhigher visual brain centers to detect declining retinal neural output from cumulative retinal damage. Individuals chronicallyexposed to such laser sources should have more frequent ophthalmic retinal surveillance.Keywords: Laser bioeffects, laser safety, visual function, visual performance, q-switched laser, diagnostics, treatmentI. INTRODUCTIONThe increased utilization of q-switched laser sources in industry, academia, and the military requires that our understandingof how q-switched time domain laser induced retinal damage differs in its relationship to visual function vs. that for longertime domains (msec), where mechanisms of damage are primarily thermal