Grating visual evoked potentials in the evaluation of laser bioeffects: twenty nanosecond foveal ruby exposures

A series of experiments was performed to determine the parameters necessary to produce immediate measurable changes in the visual system of nonhuman primates after exposure of the fovea to laser radiation. The parameters investigated were stimulus spatial frequency (1.6 and 2.8 c/deg), size of the visual field stimulated (30 and 3.6 degrees), retinal diameter of the laser exposure (50 and 500 micron), and total intraocular energy. Steady-state visual evoked potentials (VEP's) were recorded from cynomolgus monkeys in response to an oscillating grating. Single 20 ns Q-switched ruby laser pulses (694.3 nm) were directed into the fovea of the experimental eye during the pattern stimulation. No immediate effects on the VEP were noted. Strong delayed effects occurred 45 to 120 s postexposure and were characterized by large phase shifts in the response signal as referenced to the stimulus, magnitude decreases, variance increases, and a loss of waveform correlation with pre-exposure baseline signals. These effects persisted for approximately 30 s before the VEP again became normally re-entrained.